DRY ANAEROBIC DIGESTION OF MUNICIPAL SOLID
WASTE AND DIGESTATE MANAGEMENT STRATEGIES
A dissertation submitted in partial fulfillment of the requirements for
the degree of Doctor of Philosophy in
Environmental Engineering and Management
Examination Committee: Prof. Chettiyappan Visvanathan(Chairperson)
Prof. Ajit P. Annachhatre
Dr. P. Abdul Salam
External Examiner: Dr. Yasumasa Tojo
Laboratory of Solid Waste Disposal
Division of Environmental Engineering
Hokkaido University, Japan
Previous Degree: Master of Science (Honors) Agriculture
(Soil and Environmental Sciences)
University of Agriculture, Faisalabad
Scholarship donor: Higher Education Commission (HEC)
Asian Institute of Technology
School of Environment, Resources and Development
First of all, the author would like to thank Allah, the most gracious, the most beneficent,
for giving him the opportunity to achieve higher education at this level and giving him the
courage and the patience during the course of his Ph.D. study at AIT.
The author would like to express his profound gratitude to his adviser Prof. C. Visvanathan
for kindly giving valuable guidance, stimulating suggestions and ample encouragement
during the study at AIT. The author is deeply indebted to Prof. Ajit Annachhatre and Dr.
Abdul Salam for their valuable comments, suggestions and support and serving as
members of the examination committee.
A special thank is addressed to Dr. Yasumasa TOJO for kindly accepting to serve as the
external examiner. His constructive and professional comments will be highly appreciated.
A special note of appreciation is extended to Dr. Obuli P. Karthikeyan for his help and
great interest in this research including valuable comments and suggestions during various
phases. Special thanks to Dr. Romchat Rattanaoudom for her help and guide as a senior
and a colleague.
Sincere thanks are given to Ms. Phonthida Sensai, Mr. Supawat Chaikasem, Mr.
Muhammad Zeeshan Ali Khan and Mr. Amila Abeynayaka as helping friends. The author
would also like to thank all friends, EEM staff, laboratory colleagues and technicians for
their help, moral support and cooperation which contributed in various ways to the
completion of this dissertation.
The author gratefully acknowledges Higher Education Commission (HEC) of Pakistan and
AIT for the joint scholarship for the Ph.D. study at AIT.
The author would like to dedicate this piece of work to his beloved brother who passed
away during the course of this study. His long lasting love and prayers always inspired and
encouraged author to fulfill his desires.
Deepest and sincere gratitude goes to his beloved parents (Mr. and Mrs. Sheikh
Muhammad Ramzan) for their endless love, encouragement and prayers. The author
wishes to express his deepest appreciation to his siblings for their prayers, patience and
understanding throughout the entire period of this study.
Global solid waste generation is continuously rising. Improper disposal of the gigantic
amount of solid waste seriously affects the environment and contributes to climate change
by release of green house gases (GHGs). Practicing anaerobic digestion for organic
fraction of municipal solid waste (OFMSW) can reduce emissions to environment and
thereby alleviate the environmental problems together with production of biogas, an energy
source, and digestate, a soil amendment. Dry anaerobic digestion has gained much
attention because of its advantages of lesser water addition, lower reactor volume and
higher volumetric biogas production than wet digestion. However, one of its problems is
accumulation of ammonia which is more common in digesters fed with improper C/N ratio
wastes and needs to be corrected.
This study was carried out to evaluate the performance of a pilot-scale thermophilic dry
anaerobic reactor for biogas production and to analyze the management options for the
digestate. This was achieved by investigating substrates of different C/N ratio to get a
correct feedstock for dry anaerobic digestion (to minimize ammonia accumulation) and by
investigating different organic loading rates (OLRs) of the correct feedstock. Moreover,
GHG emission potential of digestate was calculated (based on its characteristics) with and
without storage and curing and different digestate management options were analyzed.
In first experiment, the effect of C/N ratio and total ammonia-N accumulation in a dry
anaerobic digestion was studied effectively. Two simulations of OFMSW were prepared to
attain C/N ratio 27 and C/N ratio 32 using biodegradable feedstocks such as food waste,
fruit and vegetable waste, leaf waste and paper waste. Results showed that the simulation
with C/N ratio 32 had about 30% less ammonia-N in digestate as compared to that with
C/N ratio 27. Moreover, a free ammonia accumulation/inhibition effect was documented
and methods to overcome the adverse effects were discussed.
In another experiment, correct feedstock from the first experiment (C/N ratio 32) was used
as substrate to improve the performance of the same reactor. The effect of different OLRs,
such as 4.55, 6.30 and 8.50 kg VS/m 3 d, was studied on the parameters like biogas
production, VS removal and VFA accumulation. Results showed that increase in OLR
proportionally increased the gas production rate (5, 6.37 and 7.55 m 3 /m 3 reactor vol/d for three
OLRs respectively) of reactor, but the specific methane production reduced (330, 320 and
266 L CH4/kg VS). Similarly, VS removal also reduced (78, 75 and 67%) with increase in
OLR. The system performed well at OLR and RT of 6.40 kg VS/m 3 d and 24 days
respectively, however, purpose of treatment also determines the optimum operating
Digestate from the reactor was characterized and its C/N ratio and GHG emission potential
was calculated. It was found that the C/N ratio of digestate was 15-20 for most of the study
period, which is safe range for its application to agricultural land without further treatment.
The GHG potential calculation shows that storage of the digestate for 2 months decreased
its GHG potential by 10%, hence, storage was found to be a source of GHG emission.
Moreover, application of digestate directly to land has minimum net GHG emission (i.e. -
11 gCO2-eq/kg digestate). Therefore, digestate should be applied to land immediately after
digestion to minimize GHG emission from the storage system.
Table of Contents
Chapter Title Page
Title page i
Table of Contents iv
List of Tables vii
List of Figures viii
List of Abbreviations x
1 Introduction 1
1.1 Background 1
1.2 Objectives of the Study 2
1.3 Scope of the Study 3
2 Literature Review 4
2.1 Introduction of Dry Anaerobic Digestion 4
2.2 Process of Anaerobic Digestion: The Fundamentals 5
2.2.1 Hydrolysis 5
2.2.2 Acidogenesis 6
2.2.3 Acetogenesis 6
2.2.4 Methanogenesis 7
2.3 Inhibition of Dry Anaerobic Digestion 8
2.3.1 Volatile fatty acids (VFA) 8
2.3.2 Ammonia 9
2.4 Optimization of Factors Affecting Dry Anaerobic Digestion 11
2.4.1 pH 11
2.4.2 Solids content 11
2.4.3 C/N ratio 13
2.4.4 Temperature 14
2.4.5 Mixing 16
2.4.6 Retention time 17
2.4.7 Organic loading rate 18
2.5 Other techniques to Optimize Dry Anaerobic Digestion 19
2.5.1 Physical pretreatment 19
2.5.2 Chemical pretreatment 19
2.5.3 Biological pretreatment (inoculation) 20
2.5.4 Co-digestion 20
2.6 Reactor Design for Dry Anaerobic Digestion 21
2.6.1 Single-stage batch systems 22
2.6.2 Single-stage continuous systems 23
2.6.3 Multi-stage continuous systems 23
2.6.4 Design of available technologies for dry anaerobic digestion 25
2.7 Research Progress and Research Needs of Dry Anaerobic Digestion 27
2.8 Anaerobic Digestion and Digestate Management 30
2.8.1 Need of digestate management and digestate utilization 30
2.8.2 Effect of prior digestion on properties of digestate 31
2.9 Characteristics of Digestates 33
2.9.1 Characteristics of solid digestates 33
2.9.2 Characteristics of liquid digestates 35
2.9.3 Presence of organic pollutants 37
2.9.4 Presence of heavy metals 38
2.9.5 GHG emission potential of digestate 38
2.10 Management Aspects of Anaerobic Digestate 39
2.10.1 Separation of liquid and solid digestate 39
2.10.2 Direct land application of liquid digestate 40
2.10.3 Aerobic post-treatment of solid digestate and its effects on 40
2.10.4 Digestate storage and its effects on characteristics 41
2.11 Post Utilization Monitoring Issues of Anaerobic Digestate 42
2.11.1 Effect of digestate application on soil 42
2.11.2 Influence of digestate application on plant growth and health 42
2.12 Research Needs for the Dissertation 43
3 Methodology 44
3.1 Inoculum and Simulations of Waste 45
3.1.1 Inoculum for anaerobic digestion experiments 45
3.1.2 Simulations of waste 45
3.2 Experimental Set-up 46
3.2.1 Experimental set-up for gas formation potential test 46
3.2.2 Experimental set-up for pilot-scale experiments 46
3.3 Experimental Conditions 47
3.3.1 Experimental conditions for gas formation potential test 47
3.3.2 Experimental conditions for Phase I pilot experiment 48
3.3.3 Experimental conditions for Phase II pilot experiment 50
3.4 Digestate Management and GHG Emissions Estimation (Phase III) 51
3.4.1 Storage of digestate 52
3.4.2 Dewatering of digestate 52
3.4.3 Curing of dewatered digestate 53
3.4.4 Estimation of GHG emissions in the digestate management
3.5 Analytical Methods 58
4 Results and Discussion 60
4.1 Gas Formation Potential of Waste 60
4.2 Effect of C/N Ratio and Ammonia-N Accumulation on ITDAR
(Results of Phase I Pilot Experiment)
4.2.1 Performance of ITDAR during start-up and continuous
4.2.2 Effect of C/N ratio and ammonia-N accumulation in ITDAR 65
4.2.3 Summary of the effect of ammonia-N accumulation in ITDAR 68
4.2.4 Energy balance of ITDAR in Phase I pilot experiment 70
4.3 Optimization of a Pilot-Scale Thermophilic Dry Anaerobic Digester
(Results of Phase II Pilot Experiment)
4.3.1 Start-up of ITDAR in phase II pilot experiment 71
4.3.2 Stability parameters of ITDAR: Effect of organic loading rate 74
4.3.3 Effect of organic loading rate on performance parameters of
4.4 Digestate Management and GHG Emissions (Phase III) 79
4.4.1 Characteristics of raw digestate 79
4.4.2 Characteristics of stored, dewatered and cured digestate 81
4.4.3 Digestate management from perspectives of GHG emissions 83
4.5 Decentralized Dry Anaerobic Digestion of OFMSW for a Community
of 5000 People
4.5.1 Design of the decentralized AD system 86
4.5.2 Preparation of feedstock for dry AD (Pre-treatment) 86
4.5.3 Operation of decentralized AD system 87
4.5.4 Generation of methane and energy 88
4.5.5 Digestate management 88
4.5.6 Reduction of GHG emissions 90
4.5.7 Material flow (VS balance) 90
5 Conclusions and Recommendations 91
5.1 Conclusions 91
5.2 Recommendations 93
Appendix A 110
Appendix B 114
Appendix C 122
Appendix D 127
Appendix E 130
Appendix F 134
List of Tables
Table Title Page
2.1 Biomethanization Inhibitors and their Inhibitory Concentration 8
2.2 Change in TAN Inhibitory Concentration with Feed TS and Temperature 10
2.3 Typical C/N Ratios of Different Materials 13
2.4 High Gas Production Rate in Relation to High Organic Loading Rate in
Dry Anaerobic Digestion 18
2.5 Performance of Various Kinds of Dry Anaerobic Digesters 28
2.6 Effect of Digestion on Properties of Waste 32
2.7 Characteristics of Solid Digestate in Dry Anaerobic Digestion Systems 34
2.8 Characteristics of Separated Liquid Digestates from Different Digestion
2.9 Concentration of Organic Pollutants in Digesates and Composts (µg/kg 37
2.10 Heavy Metal Content in Different Types of Digestates (mg/kg DM) 38
2.11 Regulations of Nutrient Loading on Agricultural Land 40
3.1 Composition and Characteristics of Simulated Feedstock 45
3.2 Characteristics of Substrate and Inoculum Used in Gas Formation Potential
3.3 Operating Conditions of ITDAR for Phase I Pilot Experiment 49
3.4 Forms and Sources of GHG Contributed and GHG Avoided 56
3.5 Analytical Methods for Various Parameters of Anaerobic Digestion of
4.1 Digestion Parameters and Methane Yield of ITDAR 66
4.2 Surplus Energy of ITDAR During Various Runs 71
4.3 Percentage of VS Removal and Specific Methane Production in ITDAR 77
4.4 Comparison of Digestate Characteristics and Guidelines 82
4.5 Characteristics of Digestate at Different Stages of Management 82
4.6 Net GHG Emissions from All Scenarios of Digestate Management 85
4.7 Technical Details of Proposed AD Plant and its Comparison to Pilot Plant 87
4.8 Technical Data of Sand Drying Bed for Digestate Dewatering 88
List of Figures
Figure Title Page
2.1 Trend of low solids and high solids anaerobic digestion plants in Europe 5
2.2 Main stages of anaerobic digestion process 6
2.3 Graphical representation of temperature ranges for anaerobic digestion 14
2.4 Capacity of mesophilic versus thermophilic digestion operation in Europe 15
2.5 General methods of mixing in dry anaerobic digestion, a) digestate
recirculation, b) biogas recirculation and c) mechanical mixer 16
2.6 Classes of dry anaerobic digestion by operational criteria 22
2.7 Comparison between one stage and two stage process in Europe 24
2.8 Designs of single-stage dry anaerobic digesters 26
2.9 Emissions from soil applied digestate to environments 30
2.10 Liquid-solid separation of digestate with production of useful products 39
2.11 Changing parameters during aerobic post-treatment 41
3.1 Phases of overall research study 44
3.2 Experimental set-up for gas formation potential test 46
3.3 Pilot-scale experimental setup of inclined thermophilic dry anaerobic
3.4 Method steps for gas formation potential test 48
3.5 Operating conditions of ITDAR for Phase II pilot experiment 51
3.6 Possible unit processes of digestate management system 52
3.7 Plastic drums for storage of digestate 53
3.8 Sand drying bed: Top view 54
3.9 Sand bed for digestate dewatering, A-A cross-sectional view 54
3.10 Comparative scenarios of digestate management 56
4.1 Cumulative and specific biogas production by feedstock 1 61
4.2 Cumulative and specific biogas production by feedstock 2 62
4.3 Time course of dry anaerobic digestion with various parameters in
4.4 Interaction of ammonia and VFA in ITDAR 67
4.5 Variation of total ammonia-N concentration and TAN/TKN ratio with
feed C/N ratio in ITDAR 69
4.6 pH profile of ITDAR during start-up 72
4.7 Profile of VFA and VFA/Alk ratio during start-up 73
4.8 CH4, CO2 and GPR fluctuation during start-up phase 74
4.9 Evolution of pH in ITDAR during continuous loading 75
4.10 Concentration of VFA in ITDAR during continuous loading 75
4.11 VFA/Alk ratio in ITDAR during continuous loading 76
4.12 Gas production rate of ITDAR during different OLRs 77
4.13 Cumulative methane per liter of reactor volume in ITDAR 78
4.14 Selection of operating conditions based on purpose of waste treatment 78
4.15 Comparison of feed and digestate regarding total solids in phase I
4.16 TKN and C/N ratio of the digestate in phase I experiment 80
4.17 TS and VS content of digestate in phase II experiment 81
4.18 GHG emission potential of OFMSW and digestates 83
4.19 Net GHG emissions from all scenarios of digestate management 86
4.20 Layout of conceptual decentralized AD plant for a community 89
4.21 Conceptual mass balance for VS of the proposed decentralized system 90
List of Abbreviations
AD Anaerobic Digestion
AIT Asian Institute of Technology
APHA American Public Health Association
BVS Biodegradable Volatile Solids
COD Chemical Oxygen Demand
DAD Dry Anaerobic Digestion
DEHP Di-2-ethylhexyl Phthalates
Digrr Digestate Recirculation Rate
DM Dry Matter
DOC Dissolved Organic Carbon
DRANCO Dry Anaerobic Compostig
EU European Union
FID Flame Ionization Detector
FM Fresh Matter
GC Gas Chromatography
GHG Greenhouse Gas
GP Gas Potential
HRT Hydraulic Retention Time
MS-OFMSW Mechanically Separated Organic Fraction of Municipal Solid Waste
MSW Municipal Solid Waste
NP Nonyl Phenol
OFMSW Organic Fraction of Municipal Solid Waste
OLR Organic Loading Rate
OM Organic Matter
ORP Oxidation Reduction Potential
PAH Polycyclic Aromatic Hydrocarbon
PBDE Polybrominated Diphenyl Ethers
PCB Polychlorinated Biphenyl
PCDD Polychlorinated Dibenzo-p-Dioxin
PCDF Polychlorinated Dibenzo Furans
RT Retention Time
RVS Refractory Volatile Solids
SDB Sand Drying Bed
SEBAC Sequential Batch Anaerobic Composting
SSHS Single Stage High Solid
SS-OFMSW Source Separated OFMSW
SRT Solids Retention Time
STP Standard Temperature and Pressure
TCD Thermal Conductivity Detector
TS Total Solids
UASB Upflow Anaerobic Sludge Blanket
VFA Volatile Fatty Acid
VS Volatile Solids
WM Wet Mass
Global solid waste generation is increasing day by day, not only because of growing
population, but also due to improved standard of living. About 40-60% of the household
waste is biodegradable in Asia, most of which is usually disposed by landfilling or open
dumping. Improper handling and disposal of the gigantic amount of solid waste seriously
affects the air, land and water environments and human health. It is also contributing to
climate change by releasing methane and carbon dioxide. There is a pressing need to
manage it from the time of creation to its safe disposal. Incineration may also not be
suitable for organic waste because of low calorific value due to its high moisture content.
Moreover, it causes air pollution and also requires high capital and operating cost.
Therefore anaerobic digestion and composting could be alternative options to treat organic
Anaerobic digestion is widely being practiced as major treatment option for disposal of
organic municipal solid waste on par with composting technology. Anaerobic digestion
mainly combines with the energy recovery benefits, green house gas mitigation and
produces stable end products, which can be further upgraded as compost for land
application (Forster-Carneiro et al. 2008; Walker et al. 2009). In general, anaerobic
digestion systems are broadly categorized under wet (20 %
total solids), mesophilic (35 -40 o C) or thermophilic (> 55 o C), batch or continuous and
single or two stage systems (Fdez-Guelfo et al. 2010; Forster-Carneiro et al. 2008; Yabu et
Dry anaerobic digestion offers several advantages over wet digestion process like, lesser
water addition, smaller reactor volume, technical simplicity in design due to plug flow
movement of substrate and no mechanical devices required inside the reactor for mixing
and easy handling of digested residues (Guendouz et al. 2010; Yabu et al. 2011). Similarly,
better process conversion efficiency and maximum net energy gains are reported especially
with the thermophilic operations of dry anaerobic digestions systems (Fdez-Guelfo et al.
2010; Forster-Carneiro et al. 2008). For example, the Dranco and Kompogas processes are
single stage, dry, thermophilic systems, which have been commercialized in Europe and
other parts of the world.
Ammonia-N accumulation is, however, identified as a major issue with dry thermophilic
anaerobic digestion systems, which can affect the overall methane yield. Generally, the
OFMSW is characterized with the average of 4% of protein content, a major source of
nitrogen, which is removed via ammonification process and accumulated as ammonia-N
(Jokela and Rintala, 2003). Also, the chances of ammonia-N accumulation are higher, if
the feedstock is mixed up with the large portions of food processing waste and animal
waste from slaughter houses. Even though, the protein degradation process is found to be
very slow, the released ammonia-N tend to accumulate in anaerobic digesters because of
leachate recycling and there is no mechanism to remove it except by leachate removal or
Thus, the leaching is the only mechanism proposed to overcome this issue, through which
the ammonia-N concentration is reduced with the external water addition. But water
addition is not desirable in the case of dry anaerobic digestion systems. Also, ammonia-N
stripping is not considered as good option for organic wastes with the high solid contents
due to their poor fluidity and difficulty in handling. On the other hand, it can be governed
possibly through adjusting the carbon to nitrogen (C/N) ratio of the feedstock, which
determines the overall ammonia-N concentration within the digester (Straka et al. 2007).
Albeit, the optimum C/N ratio for anaerobic digestion is agreed to be in the range of 20 to
30 (Li et al. 2011), with a higher C/N ratio, the release of lower concentrations of
ammonia-N can be expected within the anaerobic systems.
But, the adjustment of feedstock C/N ratio, especially in a large scale centralized systems,
is one of the major problems besides getting the insufficient feedstocks to operate at their
full capacity throughout the year ( Siles et al. 2010). In addition, the overall net energy
gained from the centralized system is mainly balanced with the waste collection,
transportation and segregation costs. On the other hand, decentralized small scale
anaerobic systems are more attractive and easy to manipulate with the feedstock
characteristics and quantity to attain maximum net energy gain. In addition, these can
minimize the waste handling and associated pollution emissions, along with the overall
waste management costs. Hence, more detailed research in decentralized system will be
more appropriate and requisite at this juncture.
Similarly, most of the previously performed research studies on dry anaerobic digestion
use lab-scale reactor with synthetic and well-homogenized feed having the particle size of
around 10 mm. Thus there is a need of research on full-scale or pilot-scale dry anaerobic
digesters operating at conditions closer to the field conditions and optimizing their
continuous operation with practicable organic loading rates.
Digestate from anaerobic digester has been found to contain a considerable amount of
nutrients and organic matter and is useful for agriculture. However, still it has certain
residual GHG emission potential, and may contain organic pollutants, heavy metals and
pathogens. Therefore, the stored digestate tends to emit methane to the atmosphere and
hence can contribute to the climate change. Thus, it is necessary to carefully analyze
various digestate management options in terms of net GHG emissions, so that digestate can
be managed with maximum possible GHG reduction.
Therefore, this research work is intended to optimize dry anaerobic digestion in terms of
feed C/N ratio, associated ammonia-N accumulation and organic loading rates in a pilotscale
thermophilic system designed for decentralized applications. Moreover, various
options of digestate management have been analyzed from perspective of GHG emissions
for improved digestate management.
1.2 Objectives of the Study
The main objective of this research is to optimize operation and performance of dry
anaerobic digestion process treating OFMSW by testing feed C/N ratio, investigating
ammonia-N accumulation and conducting different organic loading rates. This leads to
promote dry anaerobic digestion technology, which will enable recovery of valuables
(energy and digestate) from municipal solid waste, minimize the amount of waste going to
landfill and protect the environment.
The specific objectives of this research are given in the following:
Optimization of the methane yield of OFMSW by using different feed C/N ratio and
organic loading rates under dry and thermophilic conditions;
To characterize the digestate of anaerobic digestion and analysis of digestate
management options in terms of GHG emissions.
1.3 Scope of the Study
To accomplish the above objectives, scope of the study is given as follows:
1. The methane yield optimization of dry digestion was conducted in a pilot-scale
thermophilic anaerobic digester, whereas gas formation potential test was conducted in
lab-scale reactors with eudiometer set-up.
2. Food waste, leaf waste and waste paper were collected from restaurants, fields and
offices of AIT respectively. Vegetable and fruit wastes were collected from a
vegetable and fruit market (Tallad Thai) situated nearby. Waste simulations with
different C/N ratio were prepared at Environmental Research Station of AIT.
3. Inoculum to be used for gas formation potential test consisted of anaerobic sludge
from wastewater treatment plant of Singha Beer Factory, Bangkok. Inoculum to be
used for pilot experiment was composed of anaerobic sludge, cow dung and digestate
of anaerobic digestion of municipal solid waste (MSW).
4. Solid-liquid separation of digestate was done in a pilot-scale sand drying bed.
5. Characteristics of waste and digestate and operational parameters of digestion all were
analyzed in EEM laboratory, AIT.
6. The GHG emitted from stored digestate, cured digestate and land applied digestate
were theoretically calculated based on their characteristics (C and N content), which
were analyzed in EEM Lab.
Anaerobic digestion is considered as an alternative option to manage and treat the organic
fraction of municipal solid waste (OFMSW). This process not only treats the organic waste
but also produces clean energy (biogas). The digestion residues (digestate) obtained from
the process can be used as soil amendment or even nutrient rich organic fertilizer
depending on its final quality. Based on the solid content of waste used in the process,
anaerobic digestion is of two types which are dry and wet anaerobic digestion. Dry
anaerobic digestion has got much attention due to its advantages of smaller reactor volume
requirement (higher organic loading rate), lesser water addition and lesser pretreatment
needed with higher volumetric biogas production rate as compared to wet digestion.
Moreover, high solid content of the digestate makes it simpler and easier to handle as
compared to liquid digestate of wet digestion that adds dewatering cost as well. Due to low
water content and small reactor volume, energy requirement for heating is less for dry
In this chapter, the process and problems of dry anaerobic digestion have been discussed.
Moreover, optimization of factors affecting dry anaerobic digestion has been discussed and
based on this, solutions of some problems in dry anaerobic digestion have been analyzed as
well as the developments in the process have been reviewed. Furthermore, characteristics
of digestate have been presented and the present management strategies for digestate have
2.1 Introduction of Dry Anaerobic Digestion
In dry anaerobic digestion (high -solids digestion), the feedstock to be digested has total
solids (TS) content more than 15 %. In contrast, wet anaerobic digestion (low-solids
digestion) deals with diluted feedstock having TS content less than 15% (Li et al., 2011).
Dry anaerobic digestion technology emerged from research performed in 1980s that
documented higher biogas production rates by high-solid wastes fed without dilution.
Conventionally (1990 and earlier), wet anaerobic digestion used to be the main anaerobic
digestion technology for digestion of manures in vertical reactors requiring feed material
with less than 10% TS content (Forster-Carneiro et al., 2009). But then the trend of dry
anaerobic digestion technology increased so quickly that in late 1990s, total anaerobic
digestion capacity in Europe for treating OFMSW was equally divided between the wet
and dry anaerobic digestion as shown in Figure 2.1. The trend further changed and in 2006,
dry anaerobic digestion and wet anaerobic digestion provided 56% and 44% of the
capacity respectively (De Baere, 2006) . It became > 60% for dry digestion in 2010 as
shown in the same figure.
Dry anaerobic digestion is performed with organic fraction of municipal solid waste
(OFMSW) in both horizontal and vertical plug flow reactors. Apart from OFMSW, it can
also be conducted with straws and residues of crops, solid livestock waste (e.g. cow dung,
horse dung), food waste and dewatered sewage sludge as substrates (Mumme et al., 2010;
Kusch et al., 2008; Kim and Oh, 2011; Duan et al., 2012). According to Luning et al.
(2003), both the wet and dry anaerobic digestion processes can be considered as a proven
technology for the treatment of the OFMSW because the specific gas production by the full
scale plants of the two processes is almost similar. Some available technology examples of
dry anaerobic digestion are BIOCEL, DRANCO, KOMPOGAS, Valorga, Linde-BRV and
SUBBOR, whereas those of wet digestion are BTA, KCA, BIOSTAB, and WAASA.
Cumulative capacity (kton/year)
Figure 2.1 Trend of low solids and high solids anaerobic digestion plants in Europe
2.2 Process of Anaerobic Digestion: The Fundamentals
Anaerobic digestion of organics is a complex process under both dry and wet conditions,
which can be divided into 4 biodegradation stages. The microbes involved in these
processes need different environmental conditions and have synergism. The four basic
steps of the process have been explained in Figure 2.2.
Hydrolysis of the complex organic matter is an important step of the anaerobic
biodegradation process. It is the first and often rate-limiting step during the anaerobic
digestion of complex organic matter. During this step, the complex organic matter (lipids,
proteins and carbohydrates, etc) are hydrolyzed into simple compounds (amino acids,
sugars, fatty acids, etc.) by hydrolytic microorganisms.
An approximate chemical formula for the mixture of organic waste is C6H10O4 (Ostrem,
2004). A hydrolysis reaction where organic waste is broken down into a simple sugar
(glucose) can be represented by the Eq. 1.
Wet operation Dry operation
C6H10O4 + 2H2O C6H12O6 + 2H2
Figure 2.2 Main stages of anaerobic digestion process (Modified from Appels et al.,
During acidogenesis, fermentation of hydrolyzed compounds into volatile fatty acids like
butyric acid, propionic acid, acetic acid, valeric acid, etc. happens which causes the drop of
pH. Moreover, neutral compounds i.e., methanol and ethanol, and ammonia are also
formed. Furthermore, catabolism of carbohydrates causes the evolution of CO2 and H2.
Acidogens consisting of facultative microbes and obligate anaerobes run this step of
The specific concentrations of products formed in this stage vary with the type of microbes
as well as with culture conditions such as temperature and pH. Typical reactions in the
acid-forming stages are shown below. In Eq. 2, glucose is converted to ethanol and in Eq. 3
glucose is transformed to propionate.
H 2, CO 2
30% CH 4
(Carbohydrates, Proteins, Lipids)
Acetogenesis is the third stage in which the products of acidogenesis undergo further
digestion and form acetic acid, hydrogen and carbon dioxide. Obligate microbes called
(Simple sugars, Amino acids, Fatty acids)
Volatile Fatty Acids
(Propionate, Butyrate, etc), Ethanol
CH 4 + CO 2
70% CH 4
C6H12O6 2CH3CH2OH + 2CO2 Eq. 2
C6H12O6 + 2H2 2CH3CH2COOH + 2H2O Eq. 3
acetogens play their part to run this step. Environmental conditions play their part and
affect the formation of different by-products.
Acetogenesis occurs through carbohydrate fermentation in which acetate is the main
product and other metabolic processes also occur. The result is a combination of acetate,
CO2 and H2. In anaerobic reactions, role of H2 as an intermediate compound is critically
important. The H2 gas formation occurs when oxidation of long chain fatty acids into
propionate or acetate happens. However, this oxidation is inhibited by H2 in the solution
under standard conditions. Hence, if partial pressure of H2 is sufficiently low, the above
mentioned oxidation can thermodynamically proceed. Thus, for the conversion of all acids,
low partial pressure of H2 gas is needed, which can be ensured if hydrogen consuming
microbes are present in large number. Conversion of propionate into acetate has been
shown in Eq. 4. From above discussion, it can be concluded that level of H2 (measured by
partial pressure) is also an indicator of digester’s health (Mata-Alvarez, 2003).
CH3CH2COO - + 3H2O CH3COO - + H + + HCO3 - + 3H2 Eq. 4
This is the last step of anaerobic digestion process in which methane formation happens
from the material produced in the previous step. Methane formation can happen from
methanol, acetic acid or hydrogen and carbon dioxide. Based on the raw material used,
groups of microbes (called methanogens) responsible for this step are of two types: (i)
acetoclastic methanogens which consume acetic acid mainly (Eq. 5&6, and consume
methyl alcohol as well, Eq. 7) and contribute to 2/3 rd of total methanation, and, (ii)
hydrogenotrophic methanogens which consume carbon dioxide and hydrogen (Eq. 8) and
are responsible for 1/3 rd of total methane production (Ostrem, 2004). The growth rate of
methanogens is however slower than that of organisms responsible for other steps of
2CH3CH3OH+ CO2 2 CH3 COOH + CH4 Eq. 5
CH3COOH CH4 + CO2 Eq. 6
CH3OH + H2 CH4 + H2O Eq. 7
CO2 + 4H2 CH4 + 2H2O Eq. 8
It has been found by Montero et al., (2010) that consumption of butyric acid, the main
precursor of methane, is related to hydrogenotrophic methanogens during start-up phase
and to acetotrophic methanogens during stabilization phase. It was concluded that
methanogenic population dynamics depends on the concentration of VFA (specifically
butyric acid). Thus if concentration of VFA is high, hydrogenotrophic methanogens will
2.3 Inhibition of Dry Anaerobic Digestion
Dry anaerobic digestion of organic solid waste faces many inhibition problems which are
harder to control (Guendouz et al., 2010). These include inhibition by volatile fatty acids
(VFA), ammonia, heavy metals and metals, as given in Table 2.1 with their inhibitory
concentrations. The main inhibitors of dry anaerobic digestion process (ammonia and
VFA) have been discussed in detail as under.
2.3.1 Volatile fatty acids (VFA)
Methanogens are susceptible to high concentrations of acids in reactor, so acid conditions
can inhibit their growth. Volatile fatty acids are intermediate compounds of methanation
and their high concentration can cause stress to microbes. The mainly produced
intermediates during anaerobic digestion of organics are acetic, propionic, butyric and
valeric acids (Buyukkamaci and Filibeli, 2004) whereas the concentration of acetic and
propionic acid is a useful measure for performance of digester.
Table 2.1 Biomethanization Inhibitors and their Inhibitory Concentration
Parameter Concentration of inhibition (g/L)
Volatile fatty acids >2 (as acetic acid)
> 6-8 (as overall volatile acids)
Total ammonia nitrogen 1.5-3 (at pH>7.6)
Free ammonia 0.6
Sulfide 0.25 (as H2S at pH 6.4-7.2)
0.09 (as H2S at pH 7.8-8.0)
Sulfide >0.1 (as soluble sulfide)
8 (strongly inhibitory)
3 (strongly inhibitory)
12 (strongly inhibitory)
8 (strongly inhibitory)
Chromium (Cr 3+ )
Chromium (Cr 6+ )
0.0005 (soluble metal)
Source: Chen et al., 2008; Polprasert, 2007; Dong et al., 2010
a mole/kg dry solids
However, if the ammonia concentration in the medium is very high or substrate contains
high concentrations of proteins, accumulation of VFA will not lead to acidification due to
buffer capacity provided by ammonia (Angelidak i and Sanders, 2004). Ammonia
maintains a high level of bicarbonate to do that (Cho et al., 1995).
In dry anaerobic digestion, recycling of digestate or leachate is needed to control the solid
content and to inoculate fresh waste. It causes the accumulation of VFA as well (El-Hadj et
al., 2009; Li et al., 2011). Moreover, loading of reactor over its capacity also causes the
accumulation of VFA in the digester. Furthermore, during start-up of dry anaerobic
digestion, it is highly possible that VFA accumulation happens. Angelidaki et al., (2006a)
concluded that inhibition could happen initially at higher TS and the magnitude of
inhibition would increase with increase in TS.
Presence of different volatile fatty acids is also affected by pH of reactor medium. Hu and
Yu, (2006) reported that there was not effect of pH on propionic acid production, whereas
with increase in pH, the yield of acetic acid slightly increased while that of butyric acid
increased with decrease in pH.
Proper inoculation of fresh feed ( to avoid overloading), mixing of certain percentage of
paper waste or some other slowly degradable waste (to slow down the production of
volatile acids) and controlling the loading rate of feed, are some of strategies to minimize
the problem of VFA accumulation. For further detailed discussion, please follow sections
namely inoculation, co-digestion and OLR later in this chapter.
Ammonia is produced by the biological degradation of the nitrogenous matter, mostly in
the form of proteins and urea. About 60-80% of total nitrogen (especially the proteins and
other organic nitrogen compounds) is converted to ammonia during anaerobic digestion of
organic waste (Bujoczek et al., 2000; Angelidaki et al., 2006 b; Yabu et al., 2011).
Microorganisms responsible for anaerobic digestion need low concentration of ammonia
for their growth and multiplication. However, the excess ammonia accumulates in the
digester and hinders the process. A substrate or part of substrate (in case of co-digestion)
with feed C/N ratio lower than 27 causes ammonia accumulation (Kayhanian, 1999) and
pH values exceeding 8.5, which is toxic to methanogens. In dry anaerobic digestion, apart
from low C/N ratio of the feedstock, recycling of a fraction of leachate or digestate
(intended to optimize solid contents and inoculate fresh waste) has also been found to
increase the ammonia concentration (Li et al., 2011).
Ammonia inhibits the digestion process by change in the intracellular pH through its
diffusion into cells and causing proton imbalance, increase of maintenance energy
requirement, and inhibition of a specific enzyme reaction. Inhibition happens at total
ammonia nitrogen (TAN) concentration range of 1200-6000 mg/L or more depending on
TS content, pH, temperature and degree of acclimation of reactor medium. Table 2.2
shows the effect of feed TS content and temperature on inhibitory concentration of TAN
(NH3+NH4 + ).
It is clear from Table 2.2 that at low TS content, inhibition occurs at a higher TAN
concentration and vice versa. It is also supported by other researchers (Poggi-varaldo et al.,
1997) that inhibition by TAN is expected to occur at a lower TAN concentration in dry
anaerobic digestion process as compared to semi-dry and wet digestion. Similarly if
temperature is low, inhibition happens at a higher TAN concentration and vice versa. Thus,
if TS content is very low and temperature is mesophilic, inhibition happens at very high
TAN concentration (i.e. 6000 mg/L). If TS is very high and temperature is thermophilic,
TAN can be inhibitory at very low concentration (i.e. 1200 mg/L).
Free ammonia (NH3) has been suggested to be the main cause of inhibition (Kayhanian,
1999) since it is freely membrane-permeable. Free ammonia concentration of about 600
mg/L is inhibitory for thermophilic dry digestion (Gallert and Winter, 1997). Hartmann
and Ahring, (2005) reported that no inhibition was observed at free ammonia concentration
of 450-620 mg/L under thermophilic digestion. Duan et al., (2012) also reported that with
the concentration of free ammonia lower than 600 mg/L, high-solid anaerobic digestion of
sewage sludge could maintain satisfactory stability. Significant inhibition happened at
TAN concentration of 3000-4000 mg/L and free ammonia concentration of 600-800 mg/L
in their study.
Table 2.2 Change in TAN Inhibitory Concentration with Feed TS and Temperature
Substrate Digestion Feed Temperature Inhibitory Inhibition Reference
Potato Wet 4.5 Mesophilic 4000-6000 57% loss in Koster
OFMSW Wet 6 Thermophilic 3500-5500 6-11% loss in Angelidaki
methane et al.,
Cattle Wet 7 Thermophilic 4000-5000 50% loss in Borja et
Synthetic Wet - Thermophilic 4920-5770 39-64% loss Sung and
in methane Liu, 2003
Food Dry 18.4 Thermophilic 3500 50% loss in Gallert and
OFMSW Dry 30 Mesophilic 2800 Process Poggi-
Cease Varaldo et
OFMSW Dry 24.79 Mesophilic 2000 > 50% loss in Jiang et al.,
OFMSW Dry 30 Thermophilic 1200 - Kayhanian,
Free ammonia concentration is affected mainly by temperature, pH and TAN
concentration. Thus, at 55°C, free ammonia will always be higher than at 35°C (Lin et al.,
2009; De la Rubia et al., 2010) at a given concentration of TAN. Moreover, free ammonia
has been reported to be inhibitory at lower concentration (220 and 215 mg/L) under
mesophilic conditions as compared to thermophilic conditions where its inhibitory
concentration was higher (568 and 468 mg/L) (Gallert and Winter, 1997; El -Hadj et al.,
2009). This also implies that inhibition happens not only by free ammonia, but also by
ionic ammonia, because when threshold concentration of ionic ammonia reaches, the
process is inhibited and at that point the free ammonia value is low and high under
mesophilic and thermophilic conditions respectively.
Two ways to mitigate ammonia inhibition in dry anaerobic digestion systems are: (i)
dilution of digester content and/or removal of leachate and addition of the same amount of
water, and (ii) adjustment of feed C/N ratio where C/N ratio of 27 t o 32 is effective
(Kayhanian, 1999). Mitigation of ammonia accumulation has been further discussed in codigestion
section of the review.
2.4 Optimization of Factors Affecting Dry Anaerobic Digestion
Even though, the pH and VFA are linked to each other but their relation depends on the
waste composition which may differ from the type of waste and the environmental
conditions of anaerobic digestion process. It has been determined that an optimum pH
value for anaerobic digestion lies between 6.5 and 7.5 (Liu et al., 2008). The optimum pH
requirement of the two steps of digestion namely acidogenesis and methanogenesis is
different. High concentrations of acids can result as low pH as
(Ward et al., 2008). Kim and Oh, (2011) found that increase of the substrate concentration
from 40% TS to 50% TS resulted in a drastic decrease in performance. Thus if solid
content of the substrate is not within suitable range, it needs to be adjusted by addition of
water, drying or by mixing with other waste type.
It has been found that organic removal and methane yield decrease with increase in total
solids content. Fernandez et al., (2008) reported that organic removal decreased from 80.69
to 69.05% and methane yield reduced from 110 to 70 L/kg VSremoved, as total solids content
was increased from 20 to 30% under mesophilic conditions. Similarly, increasing the total
solid (TS) content from 20 to 26% reduced biogas yiel d by 35% at S/I ratio of 6.2 and
NaOH loading of 3.5%, where the solid-state anaerobic digestion of fallen leaves was
performed under mesophilic conditions (Liew et al., 2011). Moreover, Duan et al., (2012)
reported that in mesophilic anaerobic digestion of sewage sludge, increasing feeding TS
from 10 to 15% at retention time of 30 days decreased methane yield and volatile solids
removal by 7.4 and 6% respectively. Similar results were obtained by Li et al., (2010)
under similar conditions. Forster-Carneiro et al., (2008) reported similar results for dry
anaerobic digestion of food waste under thermophilic conditions as well. Thus it can be
concluded from above discussion that organic removal and biogas production decrease
with increase in TS content of feed regardless of temperature.
In wastewater treatment, chemical oxygen demand (COD) is usually used as a parameter to
describe organic removal (Rao et al., 2011; Eskicioglu et al., 2011) while in case of dry
anaerobic digestion volatile solids (VS) content is more appropriately used (Dong et al.,
2010; Duan et al., 2012) for the same purpose. Oleszkiewicz and Poggi-Varaldo, (1997)
stated 1.1 kg COD equal to 1 kg VS, and this relation can be used for inter-conversion of
COD and VS. Thus total solids and volatile solids content is used to describe the
concentration of feedstock in dry anaerobic digestion. However, the reactor medium in dry
anaerobic digesters is heterogeneous in structure, composition and size of the solids
(Guendouz et al., 2010). Moreover, the biomass is mixed with the substrate and the liquid
fraction needs to be extracted from the reactor medium for analysis of various parameters.
Thus, it makes in-line measurements very difficult and it is impossible to directly access
the reaction yield.
Volatile solids (VS) in case of dry digester comprise the biodegradable volatile solids
fraction (BVS) and the refractory volatile solids (RVS), which is just similar to soluble
COD and particulate COD in case of wet process. It was reported by Kayhanian and Rich
(1995) that “waste biodegradability, biogas production, C/N ratio and OLR could be
estimated well and correctly with the knowledge of biodegradable volatile solids (BVS)
fraction of solid waste. Lignin is a type of organic material opposite of BVS and is not easy
by anaerobic microbes to degrade it and is called refractory volatile solids or RVS.
For treatment through anaerobic digestion, the most suitable waste is the one with high
volatile solids in which non-biodegradable matter is low. The quality and yield of biogas as
well as quality of compost is affected by waste composition. The waste components
consisting of shrub and tree clippings, straw, bark, sawdust and shavings, which are ligninrich
and woody materials should be avoided to feed into the digester in more than a certain
quantity. The degree of biodegradation can be measured by removal of volatile solids
content, which is an important parameter showing activity status of some groups of
anaerobic microbes. In some study (Elango et al., 2007), 73% of VS reduction was initially
eported, which increased with time, but after some time of continuous feeding, it reduced
2.4.3 C/N ratio
It shows the proportion of quantity of C and N in any organic material. Nitrogen is needed
by microorganisms for their multiplication by making new cells. The proper ratio of
nutrients C:N:P:S for methanation is 600:15:5:3. Therefore, a C/N ratio of 20-30 has been
reported as optimum (Fricke et al., 2007), which can supply sufficient amount of nitrogen
to produce microbial cells and to degrade C of the waste.
In the process of anaerobic digestion, the compounds of reduced N are not removed and
tend to accumulate in the digester and inhibit the digestion process. Therefore, C/N ratio of
the waste material is very crucial parameter. Because if the C/N ratio is high, it means the
waste has low N or high C content, so the low N content will be rapidly consumed by
methanogen, after that there will be lower biogas production due to low N. On the
contrary, if C/N ratio of the material is low, it means it has high N, which will accumulate
inside digester with time that will raise pH to 8 or more, and will become toxic to
methanogens. To minimize ammonia accumulation problem in dry thermophilic anaerobic
digestion, C/N ratio ranging from 27 to 32 has been suggested by Kayhanian, (1999).
Table 2.3 Typical C/N Ratios of Different Materials
Raw material C/N Ratio
Human Excreta 8.0
Duck Dung 8.0
Sewage Sludge 8.6 a -11.3 b
Chicken Dung 10.0
Pig Dung 18.0
Goat Dung 12.0
Sheep Dung 19.0
Food Waste 15.0 c
Cow Dung 24.0
Water Hyacinth 25.0
Fruit and Vegetable Waste 34.0 d
Elephant Dung 43.0
Municipal Solid Waste 40.0
Rice Straw 70.0
Wheat Straw 90.0
Maize Straw 60.0
Saw Dust >200.0
Waste Paper >400.0
Source: a Kymäläinen et al., 2012; b Li et al., 2011; c Liu et al., 2011; d Bouallagui et al.,
2009; RISE-AT, 1998.
Also if the C/N ratio is very high, it slows down degradation and hence VFA production is
very low. Similarly if the C/N ratio is very low, VFA accumulation will happen. Proper
C/N ratio of the feedstock can be obtained by mixing different materials having different
C/N ratios, for example, animal manure or sewage have low C/N ratio which can be mixed
with solid waste having high C/N ratio to get optimum value. Table 2.3 gives C/N ratios of
typical organic materials that can be used for anaerobic digestion after mixing in proper
ratio. This concept has been discussed in detail under the section of co-digestion.
Temperature is very critical parameter of anaerobic digestion process, as the rate of
digestion strongly depends on it. Mainly, methane production through anaerobic digestion
can be performed in two ranges of temperature, which are mesophilic range and
thermophilic range. For mesophilic range, temperature of 35-40°C needs to be maintained
in the digester, whereas temperature range of 50-55°C is the range for thermophilic
operation (De Baere, 2006).
Figure 2.3 Graphical representation of temperature ranges for anaerobic digestion
A thermophilic temperature reduces the required retention time. The microbial growth,
digestion capacity and biogas production could be enhanced by thermophilic digestion,
since the specific growth rate of thermophilic microbes is higher than that of mesophilic
microbes (Kim and Speece, 2002). Figure 2.3 (Mata-Alvarez, 2003) graphically illustrates
the direct relationship between the temperature and the rate of anaerobic digestion.
Thermophilic anaerobic digestion has been reported to generate about 25-50% higher
methane than mesophilic digestion (Khanal, 2008; Yilmaz et al., 2008). The semidry
thermophilic process has a gas production rate 2-3 times the mesophilic process
(Cecchi et al., 1991). These results demonstrate the feasibility of construction of
thermophilic digesters for working at 11-12 days retention time when the OLR is 8 kg
VS/m 3 d or lower providing for the highest volatile solid removal. The elasticity of the
system permits the reduction of retention time down to 8 days by increasing the OLR to 14
kg VS/m 3 d. Li et al., (2002) also reported that thermophilic methane fermentation was
more effective for reducing lipids and had more higher loading capacity compared to
mesophilic condition. Lu et al., (2007) confirmed that thermophilic process was more
feasible for achieving better performance against misbalance, especially during the start-up
period in a dry anaerobic digestion process as compared to mesophilic digestion.
Angelidaki et al., (2006a) also concluded that the best biodegradation results for start-up
could be achieved under thermophilic conditions. Increased destruction rate of organic
acids and increased downfall of pathogen removal is also possible in thermophilic
condition. Besides, thermophilic anaerobic digestion could produce high quality of residue
that could be used further as soil conditioner or fertilizer instead of placing them on
Thermophilic process is, however, more sensitive to toxins and smaller changes in the
environment. Oleszkiewicz and Poggi-Varaldo, (1997) reported that thermophilic process
(at 55°C) was found to be superior to a mesophilic (35°C) one, both in terms of
volatile solid (VS) reduction and specific gas production, but was somewhat less stable at
short mass retention times (MRT). Similarly, Lv et al., (2010) also reported that
thermophilic anaerobic digestion is good as it can perform accelerated hydrolysis by
loosening the structure of polymers and lignocellulose in the substrate. But on the other
hand, it also causes the accumulation of propionate (short chain fatty acid) and hence
decreases methane and increases carbon dioxide in biogas. The reason is that high
temperature decreases microbial diversity. Moreover, at high temperature, solubility of
hydrogen decreases, thus it escapes from the reactor in gaseous form and hence not
available for methane formation. Moreover, solubility of carbon dioxide also reduces with
thermophilic temperature that leads to lesser methane formation (no or less methane
formation through hydrogenotrophic pathway) and removal of carbon dioxide also raises
pH leading to high free ammonia (Gallert and Winter, 1997).
Cumulative capacity (kton/year)
Figure 2.4 Capacity of mesophilic versus thermophilic digestion operation in Europe
The distribution of mesophilic and thermophilic plants is given in Figure 2.4. Mesophilic
plants are more in number than thermophilic plants and these are also mostly for wet
digestion. At the end of 2004, 75% of the capacity was provided by mesophilic plants.
Then a large number of thermophilic plants were constructed in 2005. Almost 96% of
thermophilic plants are provided by dry fermentation systems. The advantages of
thermophilic operation are more important for dry systems than for wet systems, while the
Mesophilic operation Thermophilic operation
need for heating is also less for dry systems when thermophilic operation is chosen (De
Mixing is very important for efficient transfer of organic substrate to the active microbial
biomass and homogenization of reactor medium. Further, mixing enables heat transfer and
thus avoids temperature gradients in both low-solids and high-solids digesters. Also, it
prevents sedimentation of denser particulate material in the digester and helps to release
produced gas from the digester contents (Ward et al., 2008).
There are many methods of mixing including mechanical mixers, recirculation of digestate,
or recirculation of the produced biogas using pumps (Kaparaju et al. 2008) as shown in
Figure 2.5. However, recirculation of digestate has been found the most suitable method of
mixing and compared to mechanical mixer and biogas recirculation, produced higher
biogas volumes from the substrate with TS content more than 10% (Karim et al. 2005b).
Moreover, intermittent mixing has been reported to be suitable for substrate conversion
and higher biogas production (Mills, 1979 and Smith et al., 1979).
Figure 2.5 General methods of mixing in dry anaerobic digestion, a) digestate
recirculation, b) biogas recirculation and c) mechanical mixer
The advantage of dry anaerobic digestion systems is that the reactors do not contain any
moving parts inside, but waste moves through the vessel as new waste is fed and digestate
is removed. However, in dry digestion, a high level of mixing of fresh waste and digested
residues is needed at the time of feeding, where the purpose is to inoculated and moisten
the fresh waste to be fed. Rivard et al., (1990) reported that no significant difference in the
Mechanical mixing paddle
performance of a poorly mixed (1 rpm) and a well mixed (25 rpm) dry digester was
observed. On the other hand, Karim et al., (2005a ) reported that mixed and unmixed
digesters performed quite similar when fed with slurry with 5% TS (wet digestion),
whereas the mixing effect became important when fed with thicker slurry (with 10% and
15% TS), because mixed digesters fed with 10-15% TS feed produced 10-30% more
biogas than unmixed digesters. However Stroot et al., (2001) reported that continuous
mixing in high-solid digestion showed unstable performance at high OLR whereas
minimal mixing showed a better performance at all OLR studied. Moreover, a
continuously mixed unstable reactor became stable (as shown by consumption of
accumulated VFA and simultaneous rise in pH) when the mixing level was reduced. From
above discussion it can be concluded that either intermittent mixing or a low level of
mixing is needed for good performance of dry anaerobic digestion.
Impellers and paddles have been provided in available dry anaerobic digestion
technologies like COMPOGAS and Linde-BRV to facilitate plug-flow and mixing of
reactor medium in the horizontal reactors. Guendouz et al., (2010) stated that one problem
with large-scale reactors is that complete mixing is never achieved. Thus, they designed
and operated a laboratory-scale reactor in which complete mixing was enabled with the use
of a paddle mixer.
2.4.6 Retention time
This is the ratio of volume of reactor to the flow rate of influent substrate (Eq. 9). In other
words, the time required by the waste material to pass through the reactor is called
Retention time (d) = Reactor volume (m 3 )
Flow rate (m 3 /d)
The required retention time for completion of the anaerobic digestion reactions varies with
technologies, process temperature, TS content and waste composition. The retention time
for wastes treated in mesophilic digester is usually higher (up to 40 days) than that of
thermophillic digesters which can be up to 8 days (Cecchi et al., 1991) . Shortening the
retention time decreases the reactor volume and hence saves capital cost. Increase in
retention time, however, increases reactor stability. High retention time increases biogas
due to better stabilization (Poggi-Varaldo and Oleszkiewicz, 1992; Kim and Oh, 2011) on
one hand and decreases it due to lower OLR on the other hand. If the concentration of
organic matter in the substrate is relatively constant, the shorter the RT the higher will be
the value of OLR as can be noted in many reports (Rivard et al., 1990; Gallert and Winter,
1997; Montero et al., 2010; Mumme et al., 2010; Fdez-Guelfo et al., 2011).
One of the disadvantages of dry anaerobic digestion is that it needs high retention time (15-
30 days) as compared to wet anaerobic digestion (where it can be as low as 3 days)
(Nayono, 2010). Thus, feedstock with high TS content needs long RT for digestion. Kim
and Oh, (2011) reported that a t a fixed solid content of 30% total solids, stable
performance was maintained up to an HRT decrease to 40 d. However, the stable
performance was not sustained at 30 d HRT, and hence, HRT was increased to 40 d again.
However, relatively low RT is required in dry digesters operated within thermophilic
2.4.7 Organic loading rate
Organic loading rate is a measure of the biological conversion capacity of the anaerobic
digestion system. Feeding the system above its sustainable OLR results in low biogas
yield, it happens due to accumulation of inhibiting substances such as fatty acids in the
digester slurry. In such a case, the feeding rate to the system must be reduced. Feeding at
below average loading rate was done by Svensson et al., (2006) to build up biomass. In
continuous systems, OLR is an important control parameter. System failures have been
reported by many plants due to overloading (RISE -AT, 1998). The amount of substrate
introduced into the digester is given by Eq. 10.
S = substrate concentration (kg substrate in terms of TVS)
OLR = organic loading rate (kg substrate/m 3 digester)
Table 2.4 High Gas Production Rate in Relation to High Organic Loading Rate in
Dry Anaerobic Digestion
Substrate Feed TS (%) OLR
(kg VS/m 3 d)
GPR (m 3 /m 3 d) Reference
OFMSW 30 2-10 5 Kim and Oh,
OFMSW - - 6.80 De Baere and
SS-OFMSW 20 12.1 5.3 Pavan et al.,
OFMSW+Yard 18-40 11 3.70 Hamzawi et al.,
MS-OFMSW 18 9.65 5.20 Gallert and
OFMSW+Paper 30 12.6 7.14 Vermeulen et
OFMSW 23-30 13-15 6 Kayhanian and
In comparison, wet anaerobic digestion has a gas production rate of 1-2.5 m3/m3/d
(Bouallagui et al., 2004; De La Rubia et al., 2006).
In the dry anaerobic reactor, an increase in TS content results in an equivalent decrease in
required reactor volume, and thus, it enables a higher volumetric organic loading rate, as
described in Guendouz et al., (2010). It has been reported (Vandevivere, 1999) that OLR is
double in high-solids digestion as compared to low-solid digestion. For example, the
sustainable OLR for dry and wet anaerobic digestion is 12 and 6 kg VS/m 3 d respectively
(Hartmann and Ahring, 2006). Duan et al., (2012) reported that high-solid system could
support 4-6 times higher OLR and obtain similar methane yield and VS reduction as
conventional low-solid system at the same SRT, thus reach much higher volumetric gas
production rate as presented in Table 2.4.
2.5 Other Techniques to Optimize Dry Anaerobic Digestion
Most of the digestion systems need pretreatment of waste to get homogeneity in feedstock
or to improve the subsequent digestion process. This section focuses on pre-treatment
process unique to dry anaerobic digestion. Guendouz et al., (2010) stated that less
treatment is required in case of dry anaerobic digestion as compared to wet digestion. The
pretreatment of feedstock for dry anaerobic digestion may involve the removal of the nonbiodegradable
materials, protecting the downstream plant from waste components that may
physically damage it, provision of feedstock of uniform and small particle size for effective
digestion and removal of the materials which may decrease the quality of the digestate.
Hydrolysis is improved and solubilization is increased by the help of chemical and thermal
pre-treatments, which could be helpful to decrease retention time. Similarly, ultrasonic pretreatment
also has been studied in this regard. Major types of pretreatments for dry
anaerobic digestion can be:
Biological pretreatment (inoculation)
Moreover, co-digestion can also be considered as a good technique to optimize dry
2.5.1 Physical pretreatment
The organic waste to be digested is also separated before it is sent to the digester. The
separation method of this waste is either source separation or mechanical separation. The
recyclables like glass, plastics, metals or undesirables like stones can be eliminated by
waste separation. If the waste is not source separated then mechanical separation is done
that involves separation of non-digestible materials with a size larger than 40 mm, the
process is called screening. To reduce particle size to less than 25 mm and 10 mm (for
pilot-scale and lab-scale experiments respectively), shredding is done. Before feeding into
digester, shredding of waste is performed that helps to enhance the availability or surface
area of the substrate to the hydrolyzing enzymes and hence can enhance the digestion rate.
2.5.2 Chemical pretreatment
It mainly consists of alkaline treatment. Alkalis are added to increase the pH to 8-11 during
this process. This is particularly advantageous when using plant or crop material as
feedstock. Chemical pretreatment reduces particulate organic matter of waste into soluble
matter (i.e. carbohydrates, fats, proteins, or even lower molecular weight substances) and
hence changes the composition of waste.
Zhu et al., (2010) tested different NaOH loadings (1%, 2.5%, 5.0% and 7.5% (w/w)) for
solid-state pretreatment of corn stover. Corn stover pretreated with 5% NaOH gave the
highest biogas yield of 372.4 L/kg VS, which was 37% higher than that of the untreated
corn stover. Similarly, Liew et al., (2011) worked on e nhancing the solid-state anaerobic
digestion of fallen leaves through simultaneous alkaline treatment. Three loadings of
NaOH (2, 3.5, and 5%) were tested for th is purpose. About 24-fold enhancement in
methane yield than that of the control (without NaOH addition) was achieved at substrate
to inoculums (S/I) ratio of 6.2 with NaOH loading of 3.5%.
2.5.3 Biological pretreatment (inoculation)
The purpose of this pretreatment is inoculation of feedstock. As the anaerobic digestion is
a complex biological process and its performance is influenced by microbial diversity.
Therefore, balanced active inoculum is essential for the possible degradation to be carried
out. In this view, it is very important to find an appropriate inoculum containing the
necessary microbes for the degradation process to proceed (Lopes, 2004). Another
important factor is the amount of inoculums, which can be described by percentage of
inoculums in reactor medium or more appropriately by substrate to inoculum (S/I) ratio on
volatile solids basis. Digested material from an established reactor, anaerobic sludge or
ruminant manure is often used to seed a new reactor for reducing the start-up time. Many
continuously running reactors inoculate the fresh material with either digested material or
the liquid fraction from the reactor, thus reducing washout of microorganisms. Very short
retention time has been reported to be the cause of microbial washout from the digester.
Forster-Carneiro, (2007) studied the effect of six different types of inoculums (25% w/w)
on thermophilic dry anaerobic digestion (TS 30%) of source-sorted OFMSW performed in
lab-scale digesters. The six inoculums studied were corn silage, restaurant digested waste
mixed with rice hulls, cattle dung, swine dung, digested sludge and 1:1 mixture of swine
dung and digested sludge. Results showed that sludge was the best inoculum with 43% VS
removal and 530 L CH4/kg VS after 60 days. Moreover, swine dung and mixture of swine
dung and digested sludge also performed well. Similarly, microbial pretreatment of corn
stalks was done by Zhou et al., (2012) and it was found that corn straws pretreated with
cow dung and sludge produced 19.6% and 18.9% higher cumulative biogas yield
respectively as compared to untreated straws by solid-state anaerobic digestion performed
for 60 days.
The effect of two different concentrations of inoculums (20% and 30%) was studied on dry
anaerobic digestion of food waste at three different TS percentages, viz., 20%, 25% and
30% (Foster-Carneiro et al., (2008). The percentage of inoculum for good biodegradation
of waste and methane yield was established as 30% w/w at 20% TS in this study.
Optimum S/I ratio for dry anaerobic digestion is 1.0 or lesser. Guendouz et al., (2010) used
S/I ratio ranging from 0.19 to 0.35 in their experiments. The same S/I ratio is used for
BMP test of wastes. Zhou et al., (2011) studied the effect of S/I ratio ( ranging from 0.1 to
3.0) on mesophilic anaerobic digestion of okara at 10% TS (higher TS compared to
conventional digestion) in batch digesters. Results showed maximum methane yield at S/I
values of 0.6-0.9. On the other hand, methane yield decreased when the S/I exceeded 1.0,
which was due to accumulation of volatile fatty acid (VFA) that significantly inhibits
fermentation. Thus VFA accumulation can be avoided by adjusting the S/I ratio and proper
Co-digestion is mostly advantageous to adjust the C/N ratio of waste. By co-digestion in
most cases, biogas yield is improved due to synergism developed in the digester and due to
supply of the nutrients missing in the digestion medium by some of co-substrate.
Kayhanian and Rich (1995) suggested that for dry anaerobic digestion, mixing of two or
three organic wastes could provide a nutrient sufficient feedstock. Moreover, by equipment
sharing during co-digestion, significant economic advantages are also obtained.
Furthermore, adjustment of moisture content or total solids content of feedstock is also
resulted from co-digestion. Common use of access facilities, easier and better management
and handling of mixed wastes are some of the other advantages of co-digestion. Currently,
co-digestion of animal manure and OFMSW is frequently being done in the field. Codigestion
of OFMSW and sewage sludge is also an attractive option being used by many
Co-digestion of waste paper with the waste containing high nitrogen (food waste,
vegetable waste, food processing industry waste and slaughterhouse waste) has been found
beneficial as it can adjust C/N ratio of the medium. Mixing waste paper is useful for
controlled dry digestion (Li et al., 2011) as it can control the accumulation of both the
ammonia and VFA. To get a feedstock with optimum C/N ratio for anaerobic digestion,
many researchers (Wu et al., 2006; Walker et al., 2009) mixed 2-6% paper waste and 5-8%
other slowly degradable waste (e.g. wood chips, straw and fallen leaves) with 86-93% of
OFMSW (foo d, fruit and vegetable waste). This could balance rapid production and
accumulation of VFA. Thus co-digestion could be used as tool to control VFA inhibition.
Li et al., (2011) studied co -digestion effect of undiluted cow manure, wastewater sludge
through dry methane fermentation. Various mixtures of both substrates with different ratios
(1:0, 4:1, 3:2, 2:3, 1:4 and 0:1) were co-digested at 35°C in the laboratory-scale singlestage
batch reactors for 63 days. The results showed that the co-digestion with ratio of 2:3
obtained highest specific biogas generation of 0.503 m 3 /kg VS, specific methane
generation of 0.328 m 3 /kgVS and volatile solids and total organic carbon reductions of
54.80% and 70.71% compared to the other co-digestion ratios and single digestions. It was
also revealed that co-digestion resulted in 3.11- 3.99% higher methane gas yields, due to
synergistic effect. The synergistic effect is mainly attributed to more balanced nutrients
and increased buffering capacity.
Kim and Oh (2011) co -digested food waste and livestock waste in a continuously fed
reactor mixed with impellers, and livestock waste content was gradually increased during
the operation. Until an increase of 40% livestock waste, the reactor exhibited a stable
performance. But, when the livestock waste was increased to 60%, there was a significant
drop in performance, which was attributed to free ammonia inhibition. As the livestock
waste is rich in nitrogen, it caused the accumulation of ammonia in the reactor. Thus care
should be taken to select the proper ratios of co-substrate to avoid deficiency or toxicity of
such nutrients as nitrogen.
2.6 Reactor Design for Dry Anaerobic Digestion
The basic requirements of an anaerobic digester design are to allow for a continuously high
and sustainable organic load rate, a short hydraulic retention time and to get the maximum
methane yield. There are three main types of reactors (Figure 2.6) based on mode of
operation for anaerobic digestion which are batch, one stage continuous and multistage
continuous and are being discussed as under.
2.6.1 Single-stage batch systems
In batch digesters, once the feeding of substrate is done, it is sealed a duration equal to the
retention time and there is no more feeding of substrate during this time until the process
completes. After this, all the digested material is removed and new feed is added into the
digester for next cycle of digestion. In this process, the biogas production is not constant or
continuous. There is low biogas production at the start and in the end, whereas at the
middle time, the rate of gas production is higher. To obtain a constant supply of biogas,
many batch reactors in parallel are operated with feeding or loading of substrate at
Substrate input mode
Dry Anaerobic Digestion
Single stage continuous Multi stage continuous Single stage batch
Thermophilic Mesophilic Thermophilic
Example of technology available
Figure 2.6 Classes of dry anaerobic digestion by operational criteria
Batch operated dry systems are technically simple, less expensive, require less energy and
offer more control over the process than continuous system (Mumme et al., 2010) and
hence are more attractive for developing countries. However, they need heavy inoculum
and mixing for better stabilization of waste plus close observation of safety measures is
also necessary during the opening and emptying of the batches to avoid explosion. Biogas
losses during emptying the reactors and restricted reactor heights are the other drawbacks
(Mumme et al., 2010). To overcome the problems of inoculum addition, mixing and
instability sequential batch system also known as SEBAC was developed.
In batch system, the leachate collected in chambers is sprayed on top of the fermenting
wastes. One technical shortcoming of such a process is that clogging can occur at the
perforated floor. Although the batch systems are well suited to the demands of treating
relatively large quantities of waste yet, biogas production and quality is variable and
somewhat unsteady (Eva ns, 2001). Besides, the batch systems are technically simple but
the land area required by the process is considerably large. Because of these shortcomings,
batch system up to now has not been able to succeed in taking a substantial market
(Bouallagui et al., 2005).
Kompogas DRANCO Valorga Linde - BRV BIOCEL
2.6.2 Single-stage continuous systems
In continuous digestion process, the waste is fed and withdrawn from the reactor
continuously. As the substrate is continuously added, all biochemical reactions involved in
the generation of biogas occur at a reasonably constant rate. The system receives its weight
little by little, spread over time, so that digestion takes place uninterrupted having no end
point. A reasonably constant rate of biogas production is resulted by this. Full-scale single
stage continuous digestion systems in Europe cover over 87% digestion capacity of
biowaste and sewage sludge (De Baere, 2006). Industrialists prefer one-stage systems
because of their simpler designs and lower investment costs. The continuous input
anaerobic digestion requires less land area and its operating cost can be comparable.
Importantly, the higher initial investment cost may be compensated from real state cost
reduction where the land is scarce.
However, a technical difficulty associated with pump has been encountered in loading the
feedstock in continuous manner (Sharma et al. 2000). Mixing is of pivotal importance in
all anaerobic digestion systems, continuous systems rely on pumping for its continuous
operation (Lissens et al , 2001). Further, the continuous system requires high internal
fluidity for the smooth feedstock intake and removal process. Such systems are, therefore,
principally suitable for low solid wastes. For higher solid content, transport and handling
of the waste is carried out with conveyor belts, screws and powerful pumps especially
designed for viscous streams. Such types of equipment are very expensive (Mata-Alvarez
et al., 2003).
There has been a shift of the research focus on semi-continuous mode of operation. Semicontinuous
digesters are fed at continuous intervals of time, as for example on daily basis,
or on more frequent intervals, with simultaneous removal of the digestate (Wang et al.,
2003; Misi and Forster, 2002). These systems are suited to regularly and steadily arising
waste stream. The biogas yield of semi-continuous processes is characteristically higher
and more regular. The higher production rate is attributed to the waste that is kept in their
original state, and not diluted with water (Ole szkiewicz and Poggi-Varaldo, 1997). The
distinction between continuous and semi-continuous system is rather subjective. Most of
the continuous digesters in large scales are not truly continuous. They are operated in semicontinuous
mode (Sharma et al., 2000). The term ‘continuous system’ is used in a broader
sense, which includes truly continuous and semi-continuous digestion systems where the
digesters are fed once or twice a day.
2.6.3 Multi-stage continuous systems
As anaerobic digestion process consists of four steps and each of these requires different
conditions for optimal growth and function of its respective microbial group involved in
the process. For instance, acidogens are more active at relatively low pH (i.e. 5.5–6.0)
while acetogens and methanogens require stable neutral pH and are sensitive to even low
concentrations of inhibitors (e.g., ammonia and VFA). Moreover, acetogens also need
close proximity with methanogens for efficient inter-species hydrogen transfer. However,
most full-scale digestion systems in use are single-stage digesters, in which all of the four
processes have to be carried out simultaneously. In this way, the metabolic activities of
different microbial groups are compromised and the performance of single-stage systems is
often suboptimal with a low reduction of VS. Thus, multi-stage system or more
appropriately, two-stage systems were introduced by the researchers to manage and
examine intermediate steps of digestion process. Hence, continuous anaerobic digestion
process can be grouped into single-stage and multi-stage system on the basis of phase of
Continuous operation of two reactors is generally performed in two stage digestion
systems. In the first reactor, hydrolysis and acetogenesis is done while in the second
reactor, methanogenesis is performed. The limitation of first reactor is hydrolysis rate of
cellulose whereas microbial growth is the limitation for second reactor. By dividing the
digester into two parts like this allows a better control on hydrolysis rate and
methanogenesis. In this type of systems, we can use different techniques to improve the
rate of reaction, for example, hydrolysis rate can be increased by microaerophilic
conditions. One of the advantages of this system is that we can use the digesters as storage
devices. Moreover, for rapidly degradable waste materials (food waste, e tc), a higher
biological stability is achieved.
Cumulative capacity (kton/year)
One-phase process Two-phase process
Figure 2.7 Comparison between one stage and two stage process in Europe
In general, the two-phase system provides rapid and stable treatment increasing the rate of
hydrolysis and methanization (O’Keefe and Chynoweth, 2000). However, the claimed
advantage of the two-phase digestion has not been substantiated in real practice (De Baere,
2000). Contrary to the claim, the added investment cost and operating complexity have
caused this system to be limited in a small market share. Kim and Speece (2002) concluded
that two-phase digestion system showed little benefit over single-phase during start-up
period and no benefit were observed during the long-term period. The high digestion rates
provided by the single phase system makes the system a viable even today.
As can be seen in Figure 2.7, the single-stage anaerobic digestion exhibits the highest trend
of capacity from the period of 1990 up to the present. Some of the examples of high-solid
single stage systems are DRANCO, Valorga and KOMPOGAS processes which have been
discussed in detail in the next section.
2.6.4 Design of available technologies for dry anaerobic digestion
Dry anaerobic digestion technologies used at industrial scale can be divided into three
main categories, which are batch (e.g. BIOCEL), single-stage continuous (e.g. DRANCO
(thermophilic), Valorga (mesophilic) and KOMPOGAS (thermophilic)) (Figure 2.8) and
multi-stage continuous (e.g. Linde -BRV, SUBBOR) systems. In all the dry systems,
because of high solid content, a part of the digested residues is recycled which is mixed
with the feed for inoculation. Due to their high viscosity, waste passes through the vessel
as a slug so that fresh waste is not mixed with the partially digested waste, which is called
as plug flow. This offers the advantage of technical simplicity as no mechanical devices
need to be installed within the reactor (Lissens et al., 2001). The designs of single-stage
dry anaerobic digesters have been discussed in detail below.
BIOCEL: The system is based on a batch-wise dry anaerobic digestion. The total solids
concentration of organic solid wastes as feeding substrate is maintained at 30–40% dry
matter (w/w). The process is accomplished in several rectangular concrete digesters at
mesophilic temperature. The floors of the digesters are perforated and equipped with a
chamber below for leachate collection. Prior to feeding, fresh biowaste substrate and
inocula (digestate from previous feeding) are mixed then loaded to the digester by shovels.
After the loading is finished, the digesters are closed with air tight doors. In order to
control the odor emission; the system is housed in a closed building that is kept at a slight
under-pressure. The temperature is controlled at 35–40ºC by spraying leachate, which is
pre-heated by a heat exchanger, from nozzles on top of the digesters. Typical retention
time in this process is reported to be 15-21 days ( Ten Brummeler, 2000). A full-scale
BIOCEL plant is reported to have successfully treated vegetable, garden and fruit wastes
with the capacity of 35,000 tons/year. Approximately 310 kg of high quality compost, 455
kg of water, 100 kg of sand, 90 kg of biogas with an average methane content of 58% and
45 kg of inert waste are produced from each ton of waste processed (CADDET, 2000).
According to Vandevivere et al., (1999) the BIOCEL plant produces on the average 70 kg
biogas/ton of source-sorted biowaste which is 40 % less than from a single stage low-solids
digester treating similar wastes.
In the DRANCO process, feed is introduced daily into the top of the reactor by pumping
through the feed tubes, and the digested waste is removed from the bottom at the same
time. Part of the digested waste is used as inoculums (one part of fresh waste for six parts
of digested waste) while the rest is dewatered to obtain an organic compost material. There
are no mixing devices in the reactor other than the natural downward movement of the
waste. This process focuses on the conversion of the organic fraction of the municipal solid
wastes to energy and a humus-like final product, called Humotex. The operating
temperature is 55 o C, the total solids concentration is 32% and the residence time is around
18 days. The process produces approximately 100 m 3 biogas/ton input (Chavez-Vazquez
and Bagley, 2002).
The Valorga system is quite different in that the horizontal plug-flow is circular in a
vertical cylindrical reactor, which is partially partitioned (around 2/3 rd of the reactor) by a
central wall or baffle. The partition wall is connected to reactor wall at one end, while the
other end is free allowing the passage of waste. The inlet is on one side of the baffle while
outlet is on the other side. The waste is forced to move around the baffle from inlet to
outlet that creates a plug-flow. Moreover, mixing is done by biogas injection at a high
pressure at the bottom of the reactor. This biogas injection takes place every 15 minutes
through a network of injectors. The residence time is between 18-25 days at 37 o C and
solids content is kept at 30%. The Valorga process is ill suited for relatively wet wastes
because sedimentation of heavy particles inside the reactor takes place when the total
solids content is less than 20% (Lissens et al., 2001). Possible drawbacks of this system are
the clogging of the gas injection ports and the overall maintenance.
Biogas Leachate Recirculation
Figure 2.8 Designs of single-stage dry anaerobic digesters, (a) BIOCEL, (b)
DRANCO, (c) Valorga, (d) KOMPOGAS
The KOMPOGAS process works similarly, except that the plug flow takes place
horizontally in cylindrical reactors. The digested material is removed from the far end of
the reactor after approximately 20 days. The horizontal plug flow is aided by slowly
rotating impellers inside the reactors, which also serve for homogenization, degassing, and
resuspending heavier particles. This process runs at 55 o C and requires careful adjustment
of the solid content around 23% TS inside the reactor. At lower values, heavy particles
such as sand and glass tend to sink and accumulate inside the reactor while higher TS
values cause excessive resistance to the flow. The most significant factor of tubular reactor
is its ability to separate acidogenesis and methanogenesis longitudinally down the reactor,
allowing the reactor to behave as a system of two phases (Bouallagui et al., 2005).
2.7 Research Progress and Research Needs of Dry Anaerobic Digestion
Various research studies on dry anaerobic digestion have been conducted at TS content of
15-30% using different substrates (Table 2.5). Only few research studies conducted on dry
anaerobic digestion have been conducted in pilot-scale digesters (with size of 1-3 m 3 ) but
most of them have been conducted in laboratory-scale digesters (2-60 L). As far as mode
of reactor operation is concerned, both batch and continuous systems are found and almost
all of them are single-stage. Dry anaerobic digestion studies have been performed at both
mesophilic (30-37 o C) and thermophilic (55 o C) temperature ranges. The studies conducted
under continuous mode are up to OLR and RT of up to 7.5-13 kg VS/m 3 d and 12-40 days
respectively. Almost half of the dry AD studies given in this table focused on OFMSW as
substrate. Other than OFMSW, straws and residues of crops, solid livestock waste (e.g.
cow dung, horse dung), food waste and dewatered sewage sludge have been used as feed
for high-solids digestion. Methane yield and VS removal of various studies given here can
not be compared as these vary with the waste type and waste characteristics. However,
process design and experimental conditions can significantly influence the methane yield
and gas production rate.
Research conducted on dry anaerobic digestion so far, as discussed in the above part of
chapter summarizes that proper inoculation, optimum C/N ratio, controlled or minimal
mixing, relatively low TS feed and high or thermophilic temperature can be helpful for
smooth start-up of dry digesters and can control VFA accumulation during start-up of dry
digesters. Moreover, alkaline pre-treatment and thermophilic temperature can enhance
hydrolysis of slowly degradable wastes like straws, and other wastes consisting of crop
residues. In addition, co-digestion or adjustment of C/N ratio to control both VFA and
ammonia accumulation can be used during continuous operation of dry anaerobic
digesters. Thermophilic temperature for dry AD could reduce the retention time and hence
could reduce reactor volume. This together with smaller reactor advantage of dry AD could
sufficiently save the capital cost.
Research on pilot and field scale in the field of dry AD needs to be done. Moreover, the
effect of mixing on performance of dry anaerobic digestion still needs to be studied at
higher range of TS content.
Table 2.5 Performance of Various Kinds of Dry Anaerobic Digesters
20 Batch Lab
20 Batch Lab
OFMSW 16 c
Municipal 35 Batch with Lab
Horse 15-30 Batch with Lab
Corn 22 Batch Lab
Wheat 22 Batch Lab
Cow 15-16 Batch Lab
SS- 20 Continuous Pilot
CSTR 3m 3 , 1
OFMSW 20 Continuous
VS/m 3 d)
37 33 - 367 90 Cho et al,
55 60 - - - 45 Forster-
30 32 - 273 64.6 26.1 Dong et al.,
37 35 - 200 - - Guendouz et
35 42 - 170 51-
37 40 - 223 e
37 30 - 150 55-
35 63 - 328 65.1
55 11.6 12.1 490 d
44 -49 Kusch et al.,
44.4 Zhu et al.,
- Cui et al.,
54.80 Li et al.,
- 59.3 Pavan et al.,
55 13.5 9.2 230 68.7 - Bolzonella et
VS/m 3 d)
MS 18 Continuous Lab 55 19 9.65 342
59 65 Gallert and
sim 30 Semi Lab 55 15 11.8 97
- 89 Fdez-Guelfo et
FW + 30 Contnuous Lab 35 30-100 10
with lateral 60L
250 - 80 Kim and Oh,
OFMSW 25-30 Continuous Lab 55 25-40 4.42-7.5 300
50 80 Montero et al.,
Maize - Continuous Lab 55 - 12.7 182 44.1 88.7 Mumme et al.,
Sewage 20 Continuous Lab 35 12 8.5 190 64.9 29 Duan et al.,
, CSTR 6L
CSTR: Continuously stirred tank reactor
SHW: Slaughter house waste
FW: Food waste,
MS-OFMSW: Mechanically sorted OFMSW
SS-OFMSW: Source sorted OFMSW
UASS: Upflow anaerobic solid-state reactor
The value given is of solid loading rate (i.e. kg TS/m d).
Calculated by dividing methane production rate (L CH4/Lreactor vol..d) by OLR (g VS/Lreactor vol.d).
c TS in reactor medium
d Specific gas production (SGP)
e The unit here is LCH4/kg COD
f Calculated considering 60% methane content
2.8 2.8 Anaerobic Anaerobic Digestion Digestion and and Digestate Digestate Management
Anaerobic Anaerobic digestion digestion treats treats organic organic waste, waste, which which not not only produces biogas, but also other
by-products by-products such such as as liquor/liquid liquor/liquid digestate digestate and and fiber/solid digestate/digestate. The
combination combination of of fiber fiber and and liquor liquor produced produced by by anaerobic anaerobic digestion is termed as digestate. It
has has been been found found that that these these materials materials contain contain a a considerable considerable amount of nutrients and organic
matter matter and and are are useful useful for for agriculture. agriculture. Utilization Utilization of of digestate as a soil amendment or
organic organic fertilizer fertilizer can can improve improve cop cop yield yield and and soil soil properties properties and hence helps promoting
closure closure of of nutrient nutrient cycles. cycles. In In this this way, way, the the need need for for production of synthetic inorganic
fertilizers fertilizers can can decrease decrease and and consequently consequently significant significant amount of energy can be saved and
GHG GHG emission emission can can be be reduced reduced leading leading to to energy energy and and economic benefits.
But But the the problem problem is is that, that, digestate digestate has has potential potential of of methane formation, NH3 NH3 NH3 and N2O
emission emission and and may may also also contain contain pollution pollution causing causing organic organic compounds and other hazardous
material material such such as as heavy heavy metals. metals. Therefore, Therefore, application application of digestate to agricultural land needs
a a sound sound management management and and sometimes sometimes prior prior treatment treatment to improve its quality. Also, after
application, application, careful careful monitoring monitoring of of soil soil properties properties and and plant growth is required.
2.8.1 2.8.1 Need Need of of digestate digestate management management and and digestate digestate utilization
Management Management of of digestate digestate (anaerobically (anaerobically digested digested waste) is needed because of many
reasons. reasons. First First is is to to keep keep the the environment environment safe safe during during and after anaerobic digestion because
its its handling handling and and spreading spreading may may cause cause environmental environmental risk, either due to leakage of nitrate to
recipient recipient waters waters or or due due to to extensive extensive gaseous gaseous losses losses of ammonia and the nitrous oxide as
well well as as because because of of having having certain certain methane methane formation formation potential (Figure 2.9).
Figure Figure 2.9 2.9 Emissions Emissions from from soil soil applied applied digestate digestate to environments
Second is that it has a high water content which makes it expensive to handle, transport
and spread in the field. Thirdly, it is the source of organic matter and nutrients especially
nitrogen and can be used in gardens, forests, recreation and sports ground, and fish pond as
fertilizer or soil conditioner/soil amendment. Moreover, the residue from anaerobic
digestion (digestate) has the potential advantage over untreated slurries that it is consistent
in nutrient content and availability. This makes it easier for farmers to calculate the correct
fertilizer applications to crop requirements (Berglund, 2006). Monetary benefits are also
obtained because the energy consumption for fertilizer manufacturing is decreased if it is
produced from on-farm anaerobic digestion plant. Furthermore, digestate can provide
export revenue, depending on quality. However, Lantz et al., (2007) reported that digestate
contains all the non-biodegradable contamination of the feedstock putting its use as an
organic fertilizer in question.
The post treatment in anaerobic digestion to manage digestate can be liquid digestate
treatment (aeration, nitrogen removal, precipitation of heavy metals) or separation of liquid
and solid fraction of digestate from each other (dewatering, fiber separation, and sand
removal) to solve the problem of its handling and transport (Bauer et al., 2009). If heavy
metal content and other pollutions are within safe limit, the solid digestate can be
composted or spread on agricultural land or used as landfill cover. In such case, it may take
two to four weeks for stabilization of its organic contents before its use for the mentioned
purposes. The liquid fraction is either used directly as a fertilizer in agriculture, recycled
back to the anaerobic digestion process for dilution and inoculation of new waste stream
(especially in dry digesters), treated in a wastewater treatment plant or discharged into
sewage. In case of dry anaerobic digestion, most of the liquid is recycled to the system for
moistening and inoculating the new waste and dewatered digested material is matured to
compost. Other than agricultural use, it can also be utilized for other purpose. For example,
Teater et al., (2011) reported that solid digestate (AD fiber) from a CSTR digesting dairy
manure was a suitable biorefining feedstock (for production of ethanol ) as compared to
switchgrass and corn stover.
2.8.2 Effect of prior digestion on properties of digestate
During digestion, properties of waste change considerably as shown in Table 2.6. Total
solids, organic carbon, volatile organic compounds (odor), GH G emissions potential,
pathogens and weed seeds in the waste decrease because of digestion. pH of the digester
medium increases. Organic N is transformed to NH4-N, so N availability to plants
increases, if the digestate is applied to agricultural land. Moreover, fluidity, homogeneity
and infiltration properties of waste are also improved by digestion, which further increases
the nutrient availability of digestate (Lantz et al., 2007). However, the increased
concentrations of NH4-N and high pH also increase the loss potential of N in NH3 form
through volatilization. There is no much effect of fermentation on P and K availability.
Digestion also improves handling and solids separating characteristics if manure is used as
feedstock, and reduces attractiveness of the manure to rodents and flies. Some of these
effects of digestion on the properties of waste have been presented in Table 2.6.
Tambone et al., 2009 studied the transformation of organic matter during anaerobic
digestion of mixtures of energetic crops, cow slurry, agro-industrial waste and OFMSW.
The anaerobic digestion process proceeded by degradation of more labile fraction (e.g.
carbohydrate-like molecules) and concentration of more recalcitrant molecules (lignin and
Table 2.6 Effect of Digestion on Properties of Waste
Parameter Feedstock Before After % Change Reference
pH Primary 3.5 7.5 +114 Gomez et al.,
Cattle slurry 7.2 8.4 +16.7 Mokry et al.,
Pig slurry 7.0 8.1 +15.7 Mokry et al.,
Cattle 6.9 7.6 +10.1 Gomez et al.,
Total OFMSW 90 g 43.4 g -52 Rao and
Primary 60 g/L 23.6 g/L -60.66 Gomez et al.,
Cattle 263 g/L 122.6 g/L -53.38 Gomez et al.,
Cattle slurry 9.9 % 7.1 % -28.28 Mokry et al.,
Pig slurry 7.6 % 4.9 % -35.52 Mokry et al.,
Volatile OFMSW 79.65 33.10 -58.44 Rao and
OFMSW 82.32 % 40.95 % -50.25 Eliyan, 2007
Primary 55.2 g/L 16.5 g/L -70.10 Gomez et al.,
Cattle 226.2 g/L 105.4 g/L -53.40 Gomez et al.,
Cattle slurry 86.9 a %TS 75.35 %TS -13.25 Mokry et al.,
Pig slurry 78.5 a %TS 73.26 %TS -6.66 Mokry et al.,
Total N OFMSW 1.4 g 1.06 g -24.28 Rao and
Cattle slurry 4.1 kg/t 4.5 kg/t +9.75 Mokry et al.,
Pig slurry 4.1 kg/t 4.5 kg/t +9.75 Mokry et al.,
NH4-N Cattle slurry 1.7 kg/t 2.5 kg/t +47 Mokry et al.,
Pig slurry 2.0 kg/t 3.5 kg/t +75 Mokry et al.,
Manure 70 % of TN 85 % of TN +21.42 Berglund,
Calculated by the formula: VS = OM X 1.8/1.72
2.9 Characteristics of Digestates
Digestate is the liquid-solid suspension that is produced by anaerobic digestion of food
waste, OFMSW, manures and wastewater. It is combination of solid part and liquid phase
respectively called solid digestate or fiber and liquid digestate or liquor. The proportion of
solid and liquid part depends upon the type and nature of feedstock used for digestion as
well as type of digestion process (dry/wet). It has been found to contain certain proportion
of organic matter and plant nutrients like N, P, K, Ca, Mg, etc. that make it suitable to be
used as soil amendment or fertilizer. Liquid part contains high N percentage and solid part
contains high P content. The presence of heavy metals ( Cd, Cr, Pb, Ni, Hg, Cu, Zn) and
organic pollutants on the other hand decreases the possibility of digestate to be used in
agriculture. Some other parameters are also used for characterization of digestates such as
moisture content, total solids and volatile solids content, calorific value of dry fiber, pH,
etc. Physical characteristics of digestates are described by such parameters as bulk density,
cellulose structure/grain size, or presence of plastic, rubber, metals, glass, ceramics, sand
and stone. Biological parameters to describe the digestate are presence of pathogens or
weed seeds and biological stability of the digestate.
2.9.1 Characteristics of solid digestates
In case of dry anaerobic digestion, most of the liquid digestate is recycled to inoculate and
moisten the fresh waste. Thus solid fraction of digestate is the major part of digestate that
needs to be managed in dry anaerobic digestion. Characteristics of solid digestate in dry
anaerobic digestion systems have been shown in Table 2.7. Total solids, volatile solids, salt
content, pH, nutrients (N, P, K, Ca, Mg, etc.) and C/N ratio are assessed to describe the
characteristics of solid digestate. Sometimes, density, pore volume and water holding
capacity are also discussed. The pH of the solid digestate varies between 7.5 and 8.5. Salt
contents vary greatly in composts and digestates.
Through a more consistent choice of the materials of origin, the compost producers can
obtain a more constant salt content in the final product, because this is also influenced by
material of origin. The salinity of biosolids from an animal slurry and cattle manure
digester has been reported as 0.0469 and 2 dS/cm respectively. Organic matter is
principally influenced by the maturity of the products (Fuchs et al., 2008). Similarly, other
characteristics also vary depending on the type of feedstock.
Nutrient contents in the digestates are influenced principally by the materials of origin. The
results of analysis of 100 samples of digestates and composts representative of different
systems and taken from Swiss market show that the nutrient content of the digestates vary
greatly among the digestates. The median values of total N, P, K, Ca, Mg, Fe and Na have
been found to be 1.53%, 0.36%, 1.25%, 4.66%, 0.68%, 0.89% and 0.13% of dry matter
respectively (Fuchs et al., 2008) in this research, while the contents of total N, P and K in
the anaerobic digestion digestate of OFMSW have also been reported as 1.09%, 0.65% and
0.65% respectively (Eliyan, 2007). These differences in nutrient content of different
digestates depend upon the materials of origin.
In a continuously fed dry anaerobic digester, if the moisture content is not maintained by
addition of lost water through digestate and biogas removal, the TS content of the reactor
material (digestate) increases. This increase in TS content of digestate is more rapid, if the
OLR is increased as compared to a constant OLR.
Table 2.7 Characteristics of Solid Digestate in Dry Anaerobic Digestion Systems
OFMSW - 92.77 7.23 76.26
12.07 - - - - -
- 70.55 29.45 89.66 1.46 29.06 0.23 0.28 1.05 - -
- 73.32 26.68 88.95 1.38 30.27 0.17 0.27 1.46 - -
OFMSW - 73.00 27.00 40.95 1.09 20.87 - 0.65 0.65 - -
Animal slurry 8.1 a
1.53 19.11 c
68.10 31.90 66.30 3.30 11.16 c
0.06 0.36 1.25 4.66 0.68
1.05 3.16 - - -
Cattle manure 7.5 - - - 2.38 - 0.36 1.47 1.91 2.61 0.76
Food waste +
8.5 - - - 1.49 - 0.22 2.33 1.19 4.34 0.61
7.9 90.40 9.60 77.00 4.40 9.72 c
2.00 - - - -
From analysis of 1:2 water suspension
Calculated by the formula: VS = OM X 1.8/1.72
c C = VS/1.8
d Characteristics of raw (un-separated) digestate
Singh, 2004 d
Schafer et al.,
Fuchs et al.,
Sanchez et al.,
al., 2011 d
For instance, Mumme et al., (2010) reported an increase in TS concentration from 15.8 to
22.1% continuously since day 39 (during feeding steps with higher OLR of 12.7 and 17 kg
VS/m 3 d). Moreover, compaction of solid-state bed was also observed during the same
time. The increase in TS content of reactor medium has been found to decrease VS
removal of the digester as proved in section 2.4.2. Similarly, increase in TS of digestate
also increases remaining methane formation potential of digestate. Both the TKN and
NH4–N accumulate in the solid residues of a continuously operated digester, which is just
similar to that in liquid digestate.
Fuchs et al., (2008) analyzed one hundred compost and digestate samples representative of
the different composting systems and qualities and concluded that the respiration rate,
enzymatic activities and phytotoxicity varied greatly which depend on maturity and
management process. Mature composts showed less respiration rates and enzymatic
activity. In Europe humic substances are described as parameters for quality of composts in
addition to low contents of pollutants (Binner et al., 2008). Unhurried degradation with
long lasting biological reactivity of the feedstock is important for high compost quality that
leads to the formation of more humic acids. Anaerobic pretreatment seems to be positive
for the development of humic acids during the following rotting period. In contrast,
intensive supply of oxygen mineralizes the metabolic products quickly and completely and
discharges into air. Pure sewage sludge or sewage sludge with low amount of yard wastes
lead to a low development of humic acids.
Tambone et al., (2010) studied a total of 23 samples of digestates, ingestates, composts and
digested sludge to assess their amendment and fertilizing properties. The results shown that
digestates differed from ingestates and also from compost, although the starting organic
mix influenced the digestate final characteristics. In amendment properties, compost and
digestate were better than digested sludge and all of these were better than ingestate.
As to fertilizer properties, digestion produced digestate with very good fertilizing
properties because of the high nutrient (N, P, K) content in available form. Thus, the
digestate appears to be a good candidate to replace inorganic fertilizers, also contributing,
to the short-term soil organic matter turnover.
2.9.2 Characteristics of liquid digestates
It has been found that the liquid digestate resulting from fermentation of farm manures,
agricultural biomass, OFMSW and their combination from biogas plants provide liquid
fertilizer. Typically, the liquid digestates resulting from liquid-solid separation of digested
residues contain low total solids content (3-6 % of farm manure). Moreover, they are rich
in nutrients such as N (> 6% of TS), NH 4-N (> 2% of TS), K (> 4% of TS), Ca and Mg.
Because of high N content of liquid digestates, their C/N ratio is very low (< 6), which
makes them suitable for direct application to the field. The required C/N ratio for an
organic fertilizer (or amendment) to be applied on soil is < 20 (Wood, 2008). However,
high pH (about 8.5) of liquid digestates makes them more prone to be lost to atmosphere
through volatilization, when applied in the field especially at high ambient temperature.
Thus special soil management practices need to be adopted to avoid this loss of N. The
characteristics of liquid digestates from different digesters as investigated by different
environmentalists have been shown in Table 2.8.
Table 2.8 Characteristics of Separated Liquid Digestates from Different Digestion Systems
Animal slurry 94.77 5.23 - 7.27 - - - 2.24 -
Moller et al.,
94.33 5.67 70.55 6.53 35.27 5.4 2.12 1.39 6.00
Schafer et al.,
96.17 3.83 71.00 6.53 23.49 3.6 2.61 1.33 8.36
99.48 0.52 86.54 30.77 48.10 c
1.6 21.15 2.12 -
SS-OFMSW 96.10 3.90 66.40 13.85 36.88 c
2.7 9.85 1.22 3.64 Palm, 2008
Farm manure 93.50 6.50 73.25 a
5.9 3.84 b
Mokry et al.,
Calculated by the formula: VS = OM X 1.8/1.72
Feedstock is animal slurry
Calculated by the formula: C = VS/1.8
Data of raw (un-separated) digestate
Comparison of liquid and solid digestate shows that solid digestates have much higher C/N
ratio (20-30) because of lower N content as compared to liquid digestates and thus need to
be treated and stabilized by composting before they can be applied on agricultural soils as
amendment. Moreover, solid digestates also have higher TS and P content as compared to
2.9.3 Presence of organic pollutants
Digestates are so-called ’secondary fertilizers’ or ’waste fertilizers’ which are used in
agricluture because these can provide a little amount of plant nutrients and organic matter
to soil, these however, contain some pollutants in varying concentrations mostly in the
range of µg/kg, that can contaminate soil and food. Organic pollutants present in the
digestates are PCBs ( polychlorinated biphenyls), PAHs ( polycyclic aromatic
hyrdrocarbons), NPs (nonylphenols), DEHPs (di -2-ethylhexyl phthalates), PBDEs
(polybrominated diphenyl ethers), PCDD/Fs (polychlorinated dibenzo -p-dioxins and
dibenzofurans) and some other organic compounds. The most important pathways of
organic pollutants in the digestate are deposition from air and direct application of
pesticides to the material of origin. Some of these compounds are degraded in soil after
application, so that we can measure concentration in soil to determine their degradation
and persistence. Soil concentrations of pollutants may be 3 to 10 times lower than those in
digestates (Stab et al., 2008).
As shown in the Table 2.9, PCBs have less concentrations in most of digestates or
composts, but other pollutants like PAHs, NP and DEHP are high in concentration.
Usually, these concentrations are low in green waste composts and high in biowaste
composts and composted digestates. This may be because of different levels of
contamination of input materials. Areas having more heating through oil in winter can get
higher concentration of PAHs from atmosphere.
Table 2.9 Concentration of Organic Pollutants in Digesates and Composts (µg/kgDM)
PCB 10 20 33 32 25.6
PAHs 1430 3010 2.659 5925 2680
NP 4770 30 560 - 324
DEHP 29700 30100 1.400 1114 1760
430 130 - 114 -
PBDE - - 13 2.7 26.3
PCDD/Fs - - - 3.2 ng I- 6 ng I-
TEQ/ kg dw TEQ/kg dw
Reference Kordel and Kordel and Stab et al., Kupper, et Riedel and
Herrchen, Herrchen, 2008 al., 2008 Marb, 2008
Note: SS stands for source separated
Estimated flux of organic pollutants (PCB and PAH) to Swiss agricultural soil was
dominated by application of manure and aerial deposition (Brandli et al., 2008). However,
if surface specific loads (loads per hectare) were considered, source-separated compost and
digestate were the most important inputs by more than a factor of 25 and 20 for PCBs and
PAHs. Total PAH loads accounted for 33% of the input from aerial deposition. Loads of
PCBs were low and are even expected to decrease over the next decades due to the banning
2.9.4 Presence of heavy metals
Digestates are so-called ‘recycling fertilizers’ or ‘low price fertilizers’ which are used in
agricluture because these can provide plant nutrients and organic matter to improve soil.
They however, contain some pollutants in the form of heavy metals such as Cd, Cr, Cu,
Hg, Ni, Pb, Zn, etc., that can contaminate soil and food. The most important pathways of
heavy metals in the digestate are deposition from air and direct application of pesticides to
the material of origin. The heavy metal contents in digestates are influenced principally by
the materials of origin (Fuchs et al., 2008). The contents of heavy metals determined
mostly are low except for Cu, Zn as shown in Table 2.10.
2.9.5 GHG emission potential of digestate
The treatment process of anaerobic digestion removes certain amount of carbon from the
waste while the remaining carbon still remains in the digested residues (digestate). Thus
the digestate can act as source of GHGs if not managed properly. Rico et al., ( 2011)
collected four anaerobic effluents from the digester (digesting dairy manure) at different
HRTs and analyzed to measure their residual methane potentials. They reported residual
methane potential of digestates in the range of 12.7 to 102.4 L/g VS. These methane
potentials were highly influenced by the feed quality and HRT of the previous CSTR
anaerobic digestion process. Menardo et al., (2011) also reported that the residual methane
potential of digestate (taken from digesters with feed of animal manure, energy crops and
food industry waste) was very variable (2.88-37.63 NL/kgVS) and depended on the OLR,
HRT and feedstock quality during digestion. Mumme et al., (2010 ) also reported similar
Table 2.10 Heavy Metal Content in Different Types of Digestates (mg/kg DM)
Digestate Type Cd Cr Cu Hg Ni Pb Zn Reference
OFMSW 0.64 13.88 55 0.035 12.16 0 105
Cattle manure - 8.05 128 - - - 555
0.36 23.00 72 0.097 11.50 26.8 179
Sanchez et al.,
Riedel and Marb,
0.28 12.00 40 0.100 10.00 7.0 160 Persson, 2008
0.30 9.30 113 0.080 9.70 4.1 375 Palm, 2008
Similar to the digestion of original waste, specific methanogenic activity (SMA) of
digestate from MSW digester is also linearly linked to moisture content. Hyaric et al.,
(2011) found that low moisture content is detrimental to SMA of digestate. The SMA test
for digestate was performed at mesophilic temperature at four different moisture contents
(65-82%) and it was found that SMA is highest at 82% moisture content.
2.10 Management Aspects of Anaerobic Digestate
2.10.1 Separation of liquid and solid digestate
Moller et al. (2000) suggested that environmental problems may occur on livestock farms
because of presence of huge amount of nutrients in the animal slurry than required by
crops of the locality, so this problem can be mitigated by separation of slurry into nutrient
rich liquid fraction and solid part and the nutrient rich part then can be easily transported to
the agricultural farm having less animals. Solid digestate or fiber rich part of manure that is
rich in organic matter and P makes 10-20 % of total volume of manure or digestate
(Jorgensen and Jensen, 2008; Jensen et al., 2008; Bauer et al., 2009), while the remaining
80-90% proportion consists of liquid fraction.
It has been found that separation of liquid and solid parts also separates N and P
(Tronheim, 2005; Palm, 2008) because solid part contains about 80 % of organic N and P
and liquid fraction contains inorganic N (NH 4-N) and potassium, and this separation also
reduces pollution, odor, and transport cost (Tronheim, 2005). Bauer et al., (2009)
separated fermentation residues (into solid and liquid digestate) of two plants digesting
energy crops and found that solid digestate contained higher dry matter, volatile solids and
carbon, raw ash and phosphate in relation to the mass as compared to liquid digestate,
whereas nitrogen and ammonia nitrogen were slightly enriched in the solid digestate. Only
the potassium content decreased slightly in the solid digestate. Palm (2008) stated that this
liquid part can give a 90 % yield of that given by mineral fertilizer. Figure 2.10 shows the
sequences of separation in anaerobic digestion. Here it can be mentioned that if the type of
anaerobic digestion is dry, most of liquid part will be recycled to digester while the content
of solid part will be high, and we will get a lot of solid digestate that needs to be managed
most probably through composting and this will be the focus of this study.
Figure 2.10 Liquid-solid separation of digestate with production of useful products
Mayer (2008) gave another concept of separation of animal slurry before digestion, 40 %
of liquid part according to him should be digested only to produce biogas and liquid
fertilizer because of having high energy and water content and being odor intensive and 60
% solid structural part with low energy and water content should be composted. Liquidsolid
separation can be done by various methods. Jorgensen and Jensen, (2008) reported
difference in results of various parameters by use of different separation methods like
decantation, mechanical separation and chemical separation of animal slurries and their
digestates. Mechanically, liquid-solid separation can be done by use of screw extractor
separator, rotary screen separator (Bauer et al., 2009; Gioelli et al., 2011), filter press and
2.10.2 Direct land application of liquid digestate
The C/N ratio of digestate should be less than 20 to be fit for land application (Wood,
2008). Palm (2008) stated that this liquid part of digestate, if applied to soild, can give a 90
% yield of that given by mineral fertilizer. The digestate may also be spread directly onto
farmland as slurry. However, its application has been limited to maximum amount of 170
kg N/ha/y by the EU Nitrate Directive in 1999 (Al Seadi et al., 2001).
Load of the nutrients is one of the concerns in recycling anaerobic digestion digestate on
farmland because of danger of nitrate leaching and overloading of phosphorous leading to
surface and ground water pollution. Lack of stability of fresh organic waste is also a
concern. Storage and application of digestate, are therefore, important in this case. Cost of
digestate fertilizer increases because of additional storage and it also increases emission
during storage. To regulate this nutrient loading, limits have been set in different countries
as given in the Table 2.11.
Table 2.11 Regulations of Nutrient Loading on Agricultural Land
Country Nutrient Load
Austria 100 6 months
Denmark 170 (cattle)
30 kg P/ha/y
7 ton DM/ha/y
Italy 170-500 3-6
UK 250-500 4
Modified from Al Seadi et al., 2001
2.10.3 Aerobic post-treatment of solid digestate and its effects on quality
Aerobic post-treatment of the anaerobically digested waste or digestate bring about
changes in pH, solids content and C/N ratio as shown in Figure 2.11. In this study, aeration
was carried out intermittently for 5 h on a daily basis for the treatment. The figure shows
that pH increased between 6.7 and 7.8 during the aeration period.
Figure 2.11 also shows that aeration brings about decrease in VS (%TS) possibly due to
aerobic decomposition of organic matter remaining at the end of anaerobic decomposition.
The decrease in C/N may be an indication of the breakdown of organic matter during this
period. Beyond the 40th day, the C/N ratio becomes considerably less variable due
probably to the depletion of most of the readily biodegradable organic compounds. pH and
C/N therefore seem to be good indicators for assessing compost maturity and stability.
Figure 2.11 also indicates that the compost was fully stabilized on the 40th day.
Thus, an improved aeration frequency can bring about stability in a shorter period. At the
end of the 70 days, the composition of the stabilised compost was found to be pH 7.8; TVS
= 45% and C:N = 12.7 that are recommended values for a compost to use.
Figure 2.11 Changing parameters during aerobic post-treatment (Abdullahi et al.,
2.10.4 Digestate storage and its effects on characteristics
Storage of digestate has a possible influence on its quality. If the digestate is stored for
long, it can undergo decomposition process that can lead to decrease in TS, VS,
transformation of nutrients into their different states especially of N and P and loss of
Paavola and Rintala (2008) demonstrated that during 3-11 months of storage, average
reductions of nitrogen, TS and VS were 0-15%. Soluble chemical oxygen demand (SCOD)
increased slightly from 6.5 to 7.5 g/l after 3 months storage, while after 9-11 months it
decreased from 8.3-11 to 5.6-8.4 g/l. The concentrations of total P and PO4-P in the
separated liquid fractions decreased 40–57% after 3 months storage and 71-91% after 9
months storage compared to the initial concentrations. The methane potential losses during
9-11 months storage corresponded only 0–10% of the total methane potential without
Menardo et al., (2011) reported that generally, digestate storage is in uncovered tanks from
which several gases, such as CO2, NH3, N2O and CH4, are lost to the atmosphere.
Greenhouse gases (GHGs), such as N2O, CO2 and CH4, affect the global environment and
climate while NH3 contributes to general atmospheric pollution. For these reasons, some
European Countries have required that digestate be stored in covered tanks (Palm, 2008).
Gioelli et al., (2011) reported that every day, biogas plants produce huge volumes of
digestate, which can be handled in raw form or after its separation mechanically. Effluents
are usually stored in uncovered tanks, placed aboveground in Italy, which can make them
emitters of biogas into the atmosphere. They estimated the amount of biogas emitted from
non-separated digestate as well as digested liquid fraction to atmosphere during their
storage. The investigation was performed on two biogas plants (1 MWel.) in northwest
Italy. For the residual biogas recovery, a floating system was used, whereas NH3 emission
measurement was done with a set of three wind tunnels. The results showed significant
loss to the atmosphere of each of the gases. About 19.5 and 7.90 m 3 biogas MWhel. -1 were
released to atmosphere everyday from the storage points of non-separated digestate and
digested liquid fraction respectively. It seemed to depend on surface area of the digestate
exposed to temperature and atmosphere. Mixture of different crops and manures was used
2.11 Post Utilization Monitoring Issues of Anaerobic Digestate
2.11.1 Effect of digestate application on soil
Application of digestate to soil may have some positive effects for soil and plant growth
because it contains considerable quantity of plant nutrients and organic matter that can
support soil and plant health. Digestate application has been found to improve physical
characteristics of soil like soil structure, infiltration, and water holding capacity; chemical
characteristics like increased pH in acidic soils, availability of nutrients like N, P and K
and biological characteristics of soil.
Berglund (2006) stated that to improve the poor soil structure by spreading digestate rich in
organic matter on arable land, one of the large-scale biogas plants was built, and ley crops
were introduced intended for anaerobic digestion in cereal-based crop sequences of
Wang et al., 2008 carried out field experiments to study the effect of different treatments of
anaerobic digestion residues on yield and quality of Chinese cabbage ( Brassica rapa L.
var. shanghaisis) and nutrient accumulation in soil. The soil pH was improved, and the
available N, P and K accumulated in the soil were increased by the application of the
anaerobic fermentation residue. Fuchs et al. (2008) performed two field experiments to
evaluate the influence of composts and digestates on soil fertility and plant growth and
concluded that the N-mineralization potential from the most of the digestates applied to
soil was high, in comparison to young composts. Field experiments revealed that digestates
increased the pH-value and the biological activity of soil to the same extent as composts.
According to them, the potential for nitrogen immobilization is affected by maturity,
composition of the composted materials (e.g. digestate) and management of the
composting process. Two parameters predict the risk of nitrogen immobilization in soil:
the NO3-N and the humic acids contents (maturity). Compost should be however
complemented with mineral N and the biogas residues with P (Svensson et al., 2004).
2.11.2 Influence of digestate application on plant growth and health
Digestates affect plant growth positively and negatively. Provision of plant nutrients,
suitable soil characteristics and tolerance against plant diseases, are positive aspects of
digestates for improving plant growth, yield and quality. Svensson et al. (2004) reported
that application of anaerobic digestion residues to soil resulted in less yield than mineral
fertilizer but improved yield and quality than compost. So, digestates should be used in
combination with mineral fertilizer for improved yield and quality of crops. Wang et al.
(2008) concluded from the field experiments that the yield and quality (Vitamin C and
nitrate content) of Chinese cabbage (Brassica rapa L. var. shanghaisis) were significantly
improved. The highest yield of 1364.31 kg/667 m² was achieved at the application of low
amount of anaerobic fermentation residue. It has also been found in another study that low
application rate of digestates with a big time gap between application and planting
enhances their benefits as soil amendment as confirmed in lab by increased seed
germination with dilution of digestate and incubation (Abdullahi et al., 2008).
One of the major negative effects of digestate on plants is its toxicity. Digestates have been
found to be more phytotoxic than composts if applied directly on soil without any
treatment. However, phytotoxic effects can be mitigated by post-treatment (composting) of
digestate (Fuchs et al., 2008; Abdullahi et al., 2008).
2.12 Research Needs for the Dissertation
From this review, it can be noted that ammonia toxicity, VFA accumulation and
incomplete mixing are among the major problems of dry anaerobic digestion. In dry
anaerobic digestion, ammonia inhibition occurs at a lower TAN concentration as compared
to wet digestion. A high C/N ratio feed (having low N) causes a relatively slow microbial
growth and low biodegradation rate due to deficiency of nitrogen (and consequently less
production and accumulation of ammonia and VFA) and hence may alleviate the problem
of both ammonia and VFA accumulation in dry digestion. This can be done by increasing
or adding the fraction of low N (or high C/N ratio) waste. However, there is a maximum
limit of feed C/N ratio (i.e. 30) for the digestion process beyond which digestion is not
feasible. Thus the effect of feed C/N ratio higher than its maximum established limit (i.e.
30) on the performance of dry anaerobic digestion should be investigated in an attempt to
reduce the accumulation of ammonia.
Moreover, most of the previously performed research studies on dry anaerobic digestion
use lab-scale reactor (Table 2.5) with synthetic and well -homogenized feed having the
particle size of around 10 mm. Thus there is a need of research on full-scale or pilot-scale
dry anaerobic digesters operating at closer to the field conditions and optimizing their
continuous operation with practicable organic loading rate.
Furthermore, literature also shows the digested organic waste (digestate) still has certain
residual GHG emission potential, which depends on retention time and loading rate of
previous digestion. Also the stored digestate tends to emit methane to the atmosphere (if
not stored properly) and hence can contribute to the climate change. On the other hand, it
has certain amount of nutrients and organic matter, which could be useful if applied on
agricultural soils. Therefore, it is necessary to carefully analyze various digestate
management options based on their net GHG emission reductions under various scenarios,
so that the best scenario can be selected for digestate management and utilize its economic
The research work of this dissertation consists of three phases as shown below in Figure
3.1. In the beginning, gas formation potential of simulated OFMSW was investigated in
lab-scale set-up. The Phase I and II of this research work, consisting of two separate pilotscale
experiments, mainly focused on optimization of dry anaerobic digestion in terms of
C/N ratio and organic loading rates respectively. Various operational parameters of
digestion (pH, alkalinity, VFA, ammonia) were continuously analyzed to run the digestion
process smoothly and their effect on methane production and VS removal were noticed.
Similarly, characteristics of digestate from the reactor were also determined continuously.
Moreover, in Phase III, various options of digestate management were analyzed from
perspective of GHG emissions.
AD Optimization – C/N ratio
•Testing simulated substrates
with different C/N ratio
•Investigation of AD operational
•Characterization of digestate
(TS, VS, C, N)
Digestate Management and GHG Emissions
Figure 3.1 Phases of overall research study
AD Optimization – OLR
•Testing OLRs of feed of selected
•Investigation of AD operational
•Characterization of digestate
(TS, VS, C)
•GHG emission potential of AD feed, raw digestate,
stored digestate and cured digestate
•GHG emission reduction options for a better
3.1 Inoculum and Simulations of Waste
3.1.1 Inoculum for anaerobic digestion experiments
Inoculum for gas formation potential test was brewery sludge from up-flow anaerobic
sludge blanket reactor (UASB), Singha beer factory, Thailand. Inoculum for start -up of
Phase I experiment was prepared by blending 50, 25 and 25% (w/w) of cow dung,
anaerobically digested food waste and anaerobic brewery sludge from the same UASB as
mentioned above. Thailand. Mixed inoculum was characterized with the TS and VS
content of 13% and 65 %TS, respectively. Inoculum for start-up of Phase II experiment
was prepared by blending 25, 69 and 6% (w/w) of cow dung, anaerobically digested
OFMSW and anaerobic brewery sludge from the same UASB. Mixed inoculums had TS
and VS content of 9.67% and 67.48 %TS respectively.
3.1.2 Simulations of waste
The OFMSW was simulated using food waste, vegetable and fruit waste, leaf waste and
office papers collected from their respective sources. The components were individually
size reduced up to 25mm using mechanical shredder before mixing. Three feedstocks were
prepared with different composition. Feedstock 1 and 2 were used for Phase I experiment
while Feedstock 3 was used for Phase II experiment. Mixing composition was varied to
achieve the C/N ratio of 27 and 32 for the study purpose. The composition and
characteristics of three feedstocks are given in Table 3.1.
Table 3.1 Composition and Characteristics of Simulated Feedstock
Detail Unit Feedstock 1 Feedstock 2 Feedstock 3
Food waste % FW a
42 40 45
Vegetable waste % FW 45 27 33
Fruit waste % FW 5 20 15
Leaf waste % FW 5 8 -
Paper waste % FW 3 5 7
Moisture % 79-84 75-85 81-86
TS (range) % 16-21 15-25 14-19
VS (range) %TS 79-90 80-90 84-88
C (avg.) %TS 51.30 52.10 51.20
TKN (avg.) %TS 1.92 1.63 1.61
C/N (avg.) - 26.72 31.96 31.80
a Percentage based on fresh weight.
In Feedstock 3, composition was set to achieve C/N ratio of 32 but without the use of leaf
waste. Leaf waste was not used in Feedstock 3 to reduce feed TS and to avoid the observed
blockage of pump. To ensure the homogenous feedstock supply for anaerobic digestion
system, it was prepared in bulk quantities and stored at 5 o C. Every day, required amount of
feedstocks were sub-sampled and freeze-thawed under ambient temperature for loading
into the reactor. The simulations were also characterized in different time intervals during
the long time storage. The average total solid (TS), volatile solid (VS) and ca rbon (C)
contents of the Feedstocks 1 & 2, and Feedstock 3 were 20.2%, 85%TS, 51.5%TS and
17%, 86%TS, 51%TS respectively.
3.2 Experimental Set-up
3.2.1 Experimental set-up for gas formation potential test
Gas formation potential test (also called GP21 test) was conducted for Feedstock 1 and 2.
The experimental set-up for it has been shown in Figure 3.2. Laboratory glass bottle of 500
mL with a thick Teflon/silicon (0.01”/0.05”) septum in its arms were used as reaction
vessels for waste. The reaction vessel was connected to a long cylindrical approximately
1L glass tube called eudiometer by plastic tube that was used for measurement of volume
of biogas produced. The eudiometer had barrier solution that was free to move by the
pressure of produced biogas to another 1L plastic bottle (called reservoir tank) connected
to the other end of eudiometer. The reaction vessel was placed in a water bath to attain the
required temperature of 35 ºC (Heerenklage and Stegmann, 2005).
Figure 3.2 Experimental set-up for gas formation potential test
3.2.2 Experimental set-up for pilot-scale experiments
A stainless steel inclined thermophilic dry anaerobic digestion system (ITDAR) was
designed with the total and working volume of 0.69 and 0.55 m 3 , respectively. The internal
diameter of the reactor was 0.6 m and the total height was 2.4 m. It was placed in an
inclination of about 30 o from the ground level to facilitate easy flow of the waste as shown
in Figure 3.3. The ITDAR was externally connected with the waste feeding hopper, screw
1. Sample + inoculum
2. Reaction vessel, 500 mL
3. Water bath
4. Liquid sampling
5. Plastic tube
6. Gas collection tube
(Eudiometer tube ~ 1L)
7. Gas sampling
8. Barrier solution
9. Reservoir tank, 1L
ed pump, water circulating jacket, wet gas meter (to measure flow of biogas) and digested
residue collection opening for continuous operation. Hot water tank (50 L) with immersion
heater was also provided beside and connected with the water circulation jacket to
maintain the temperature of the ITDAR at 55 o C. Temperature sensor inserted in the reactor
was connected with automatic temperature controller to monitor and control the
temperature. Once in a day, simulated feedstock for the given organic loading rate was fed
and almost equal portion of the digested residue was removed from the ITDAR during
Figure 3.3 Pilot-scale experimental setup of inclined thermophilic dry anaerobic
3.3 Experimental Conditions
3.3.1 Experimental conditions for gas formation potential test
The gas formation potential of Feedstock 1 and 2 was determined in the laboratory scale
GP21 test set-up. The basic approach of this test was incubating a small quantity of the
substrate with anaerobic inoculums at 35ºC for 21 days and measure the biogas and
methane generation, usually by simultaneous measurements of gas volume in eudiometer
and gas composition by a Gas Chromatograph (GC). The objective was to find out the
potential of one gram VS of waste material to produce methane gas. The results were
presented according to standard conditions of temperature and pressure, i.e., STP.
Particle size of the waste was reduced to < 10 mm and waste was mixed by blending the
waste sample in a blender or pulverizer in the laboratory. There was no addition of water
during mixing. After getting a homogeneous waste sample, active anaerobic inoculum was
mixed with it. Three replications of this batch test were run with a blank or control run to
accomplish the experiment. A reference set-up was also run using cellulose as substrate to
check the inoculums activity. The sequential procedure of conducting GP21 test has been
described in the Figure 3.4.
Figure 3.4 Method teps for gas formation potential test
Based on this method (Heerenklage and Stegmann, 2005) and the characteristics of
substrates and inoculum (Table 3.2), the substrate to inoculums ratio (S/I ratio) on kg VS
basis achieved in the GP21 reactors was 5.55 and 8.10 for feedstock 1 and 2 respectively.
Table 3.2 Characteristics of Substrate and Inoculum Used in Gas Formation Potential
Characteristics Feedstock 1 Inoculum Feedstock 2 Inoculum
TS (%) 15.84 5.25 25.11 5.36
VS (%TS) 81.94 44.70 79.31 45.79
3.3.2 Experimental conditions for Phase I pilot experiment
In this experiment, the effect of C/N ratio and ammonia-N accumulation was studied on
dry anaerobic digestion of OFMSW. Two simulations of OFMSW with different C/N ratio
(i.e. feedstock 1 and 2) were used in pilot-scale ITDAR for this experiment. The detail of
experimental conditions is given in the following subsections.
a) Start-up operation
Add 50 g of sample (WM) + 50 mL anaerobic
sludge + 200 mL water to all reaction vessels
Flush the reaction vessels with N2 gas after
connecting with eudiometer filled with barrier
Incubate the reaction vessels at 35 ºC for 21
days after lag phase
Shake the reaction vessels occasionally and
take reading of biogas for 21 days regularly
The ITDAR was initially loaded with the 410 kg of feedstock 1 and 180 kg of inoculum
source i.e., 70 and 30 % (w/w), respectively, to its working volume. With this composition,
the S/I ratio (i.e. substrate to inoculums ratio) was 5.2 on kg-VS basis. The temperature of
the reactor was increased from 35°C to 55°C with a gradual increment of 2°C per day to
avoid reactor upset and was maintained at 55°C throughout the study. The system was
operated in a batch mode, without loading any additional feedstock, for first 14 days and
denoted as start-up phase. During the initial start-up phase, the system pH was neutralized
using commercial caustic soda (NaOH) for quick onset of methanogenesis in ITDAR. The
amount of NaOH required for the pH adjustment was calculated based on the simple
laboratory tests using 100 mL of digestate from the ITDAR and pH meter. The reactor
contents were mixed at the rate of about 1 Ldig/Lreactor vol.d (one liter digestate recirculated
per liter reactor volume per day. So Digrr = 1 means that whole contents in reactor is
recirculated for one time in a day) to mix up for homogenous distribution of reactor
contents in these periods.
From day 15 onwards, ITDAR was operated in a continuous mode by loading with the
designed feedstocks under different organic loading rate (OLR) as detailed in below
sections. Table 3.3 provides the details of OLR, solid retention time (SRT) and digestate
recirculation rate (Dig rr) during continuous operation of ITDAR. As the reactor volume
was fixed, the increase in waste loading rate i.e., the OLR, in subsequent runs thus
decreased the SRT. During these periods, the system was continuously monitored for the
fluctuations in process parameters such as biogas, methane, ammonia-N, volatile fatty
acids (VFA), alkalinity and pH as well as other digestate parameters (TS, VS, C and N).
Table 3.3 Operating Conditions of ITDAR for Phase I Pilot Experiment
(kg VS/m 3 .d)
Feedstock 1 (avg. C/N ratio 27)
Start-up 1-14 - 14 1.00
1 15-38 0.65 a
2 39-67 1.60 89 0.10
3 68-99 2.60 54 0.19
Feedstock 2 (avg. C/N ratio 32)
4 100-148 4.00 45 0.34
5 149-170 10.65 13 2.12
6 171-215 4.35 29 2.40
7 216-245 7.70 21 2.90
8 246-280 7.30 19 2.96
Table gives average values of OLR, retention time and recirculation rate for each run.
b) Continuous operation with feedstock 1 (C/N ratio of 27) - Run 1 to 3
As detailed in Table 3.3, the feedstock 1 with the C/N ratio of 27 was loaded into ITDAR
under different OLR of 0.65 to 2.60 kg VS/m 3 .d in three consecutive runs (1 to 3). Each
run was preceded until the biogas yield attained to its steady state, with no further
increment, in ITDAR.
The SRT of 153 days was given during run 1 to avoid the reactor upset with lower Digrr. In
subsequent runs the step-up increase in OLR was studied with decreasing SRT and
increasing Digrr. Every time, one part of the fresh feedstock was mixed up with the two
parts (wt/wt basis) of digestate collected from the ITDAR before loading. Based on mass
balance calculation, specified amount of digestate was removed from the ITDAR. Feeding
and digestate withdrawal was done once a day. To balance the moisture loss due to evapo-
transportation from the ITDAR, water was sprinkled over the feedstock before its feeding
at regular intervals to maintain the TS content for dry digestion.
c) Continuous operation with feedstock 2 (C/N ratio of 32) - Run 4 to 8
From day 100 th onwards, the reactor was loaded with the feedstock 2, which had a higher
C/N ratio of 32. The OLR was varied between 4 and 10.7 kg VS/m 3 .d in different runs (4
to 8). During these periods, the ITDAR was shifted from partial mixing mode to complete
mixing mode by increasing the Digrr to achieve the higher OLR operation and maximize
the biogas yield (Digrr less than 1 Ldig/Lreactor vol.d was considered as partial mixing mode,
for example, Run 1-4, because whole contents in reactor are not recirculated even once in a
day. Digrr more than 1 Ldig/Lreactor vol.d was considered as complete mixing mode, for
example, Run 5-8). Otherwise, the reactor operations followed same as detailed in above
3.3.3 Experimental conditions for Phase II pilot experiment
In this experiment, optimization of thermophilic dry anaerobic digestion by testing the
effect of different OLRs was studied. The simulation of OFMSW, which performed well in
previous experiment and was found optimum in terms of C/N ratio (i.e. C/N ratio 32 or
feedstock 3), was used in pilot-scale ITDAR for this purpose. The study included start-up
operation and continuous operation. The detail of experimental conditions is given in the
a) Start-up operation
The ITDAR was initially loaded with the 329 kg of feedstock 3 and 213 kg of inoculum
source i.e., 60 and 40 % (w/w), respectively, to its working volume. With this composition,
the S/I ratio (i.e. substrate to inoculums ratio) was 3.04 on kg -VS basis. The inoculum
consisted of a mixture of cow dung, anaerobic sludge from a WWTP and anaerobic
digestate of the same reactor. Around 70% (w/w on fresh mass basis) of inoculum
consisted of anaerobic digestate, which was taken from previous run of the same
thermophilic reactor. Therefore, thermophilic temperature (55°C) was maintained
throughout the study, which was achieved by gradual increase of temperature during first 3
days of start-up. The system was operated without loading any additional feedstock, for
first 50 days and it is considered as start-up phase. During the initial start-up phase, the
system pH was neutralized using commercial caustic soda (NaOH) just like Phase I
experiment. The reactor contents were mixed at the average rate of about 1.85 Ldig/Lreactor
vol.d (liter digestate recirculated per liter reactor volume per day) (range: 1 -3 Ldig/Lreactor
vol.d) to mix up for homogenous distribution of reactor contents in these periods.
b) Continuous operation with feedstock 3
From day 51 onwards, ITDAR was operated in a continuous mode by loading the reactor
with designed OLRs. As shown in Figure 3.5, the feedstock 3 was loaded into ITDAR with
different OLRs of 4.5, 6.3 and 8.5 kg VS/m 3 /d in three consecutive runs (1 to 3). Each run
continued until the biogas yield attained to its steady state, just like previous experiment.
The step-up increase in OLR was studied with decreasing SRT from 30 to 18 days. The
Digrr (digestate recirculation rate) was kept constant, at value of 2.8 Ldig/Lreactor vol.d, in this
study. Every time for feeding, one part of the fresh feedstock was mixed up with the two
parts (wt/wt bas is) of digestate collected from the ITDAR. Based on mass balance
calculation, specified amount of digestate was removed from the ITDAR. Feeding and
digestate withdrawal was done once a day. To balance the moisture loss due to evapotransportation
from the ITDAR, water was sprinkled over feedstock before feeding) at
regular intervals to maintain the TS content for dry digestion just like previous experiment.
Run 2 OLR 6.4 kgVS /m 3 .d
Run 1 OLR 4.55 kgVS /m 3 .d
Figure 3.5 Operating conditions of ITDAR for Phase II pilot experiment
During this period, the system was continuously monitored for the fluctuations in process
parameters such as biogas, methane, ammonia-N, volatile fatty acids (VFA), alkalinity and
pH as well as other digestate parameters (TS, VS, C).
3.4 Digestate Management and GHG Emissions Estimation (Phase III)
Management of digestate from anaerobic digestion of municipal solid waste is an
important issue due to wide variation in its characteristics. The digestate characteristics
depend on origin of feedstock, type of feedstock and type of digestion process. Literature
shows the digestate has certain amount of plant nutrients and organic matter and can be
used as organic fertilizer or soil conditioner. However, it is important to indicate that
digestate contains significant amount of methane formation potential, and hence can
contribute to the climate change. Moreover, it can also contain some organic pollutants and
heavy metals. Therefore, it is important to develop a proper digestate management
Feed TS: 14-19%
Feed C/N ratio:32
Run 3 OLR 8.5 kgVS /m 3 .d
Start-Up Constant Dig rr : 2.8 L digestate /L reactor vol. .d
Run 1 RT 30 days
Run 2 RT 24 days
Run 3 RT 18 days
Digestate characteristics analysis, biogas volume and composition
Depending on its characteristics, it could be either applied directly on the agriculture land
or treated by aerobic process of curing or composting. During its handling and
management, the digestate needs to be stored as well as dewatered. Thus, the main purpose
of Phase III is to find environmentally suitable options for digestate management. The
possible unit processes involved in digestate handling and management have been shown
in Figure 3.6. The unit processes shown in dotted boxes are the main sources of GHG
emissions from digestate management system. The processes used for management of
digestate in this study have been detailed in the following sections.
Figure 3.6 Possible unit processes of digestate management system
3.4.1 Storage of digestate
Rich in N & K
The digestate after being withdrawn from the anaerobic digester was temporarily stored in
high density polyethylene (HDPE) plastic drums. The drums were kept covered and were
kept in shade to avoid NH4 volatilization as shown in Figure 3.7. The size of each drum is
150 L. All the withdrawn digestate of each run was collected in these drums and at the end
of the run it was dewatered by the sand drying bed.
3.4.2 Dewatering of digestate
Liquid-solid separation or dewatering of the digestate was done, using simple filtration
system of sand drying bed (Figure 3.8 and 3.9).
Rich in OM & P
Sand drying bed (SDB) is simple, easy to operate and needs low energy for its operation
than mechanical dewatering systems. One circular SDB with a radius and height of 30 and
80 cm respectively was used. The detailed design with top and cross-sectional views of the
bed has been presented in Figures 3.8 and 3.9 respectively. It consists of a 15 cm thick
layer each of coarse, medium and fine gravels. The first layer (15 cm depth) of the bed
from bottom consist of course gravels, followed by other layers of medium and then fine
gravels (15 cm depth each). A perforated central pipe of 2.5 cm diameter with screening
net, placed longitudinally in the center collects the filtrate percolated through the graded
gravel. The slope to the pipe is (6/30*100) 20 percent. Approximately 25 cm layer of
digestate can be placed over the bed, whereas freeboard is 10 cm.
Figure 3.7 Plastic drums for storage of digestate
Digestate dewatering by use of sand bed mostly happens by two mechanisms, i.e., drainage
or downward percolation of water and water evaporation from the surface of digestate.
The dying period (retention time) of digestate was different for different runs. It was in the
range of 2-5 days achieving a TS content of more than 50%. It depended on the
characteristics of digestate and weather conditions. The removal of dewatered digestate
from SDB was done with a shovel and the dewatered digestate was used for curing and
3.4.3 Curing of dewatered digestate
Curing is aerobic treatment of digestate passively (without the use of energy). In this study,
it was done by spreading the digestate with high TS (55%) in thin layer of 5 cm in a tray. It
was done for a period of 30 days under ambient conditions ( at 30-33°C). Dewatering of
digestate was needed to increase its TS before curing.
Figure Figure 3.8 3.8 Sand Sand drying drying bed: bed: Top Top view
Figure Figure 3.9 3.9 Sand Sand bed bed for for digestate digestate dewatering, dewatering, A-A cross-sectional view
60 60 60 cm cm cm
3.4.4 Estimation of GHG emissions in the digestate management system
In this part of study, different integrated digestate management unit processes (called
scenarios) were analyzed for their net GHG emissions. The best scenario with the least
GHG emissions was recommended for proper digestate management. Digestate
management unit processes studied here include digestate dumping, digestate application
on land, digestate storage, aerobic treatment (i.e. curing), etc. The detail of methodology is
given in the following subsections.
a) Goal and functional unit
The goal of this part of study is to evaluate various digestate management systems from
perspective of GHG emissions. A fixed reference point for the GHG emission evaluation is
defined as 1 kg of digestate treated or managed. The digestate used for this estimation is
taken from run 4 of the digestion experiment and its initial moisture content is 89.5%.
b) Process description and comparative scenarios
The digestate is produced continuously throughout the year, but applied to agricultural land
after appropriate treatment. Also, its land application time is not continuous, but only after
harvesting of previous crop (e.g. during spring and autumn). So it needs to be stocked-up
or stored for several months. Thus, digestate can be directly applied to land only if its
nutrient ratio (i.e. C/N ratio < 20) and application season are correct. If those conditions are
not suitable, aerobic treatment and storage are needed respectively.
Several digestate management scenarios have been evaluated for their GHG emissions
which are possible in the field as given in Figure 3.10. In scenario 4 and 5, the process of
curing is done which is passive (without the use of energy) aerobic treatment of digestate.
Dewatering of digestate is needed to increase its TS before curing.
Dumping of digestate is needed, if it has high concentration of heavy metals or other
pollutants and is unfit for application to agricultural land. However, curing of such
digestate is still needed before dumping to minimize its GHG emission potential.
c) System boundaries and sources of GHG emitted or avoided
1. The evaluation includes the GHG emission only from the unit processes of digestate
management system as explained in various scenarios above. Therefore, accounting of
GHG emissions from the use of energy (operation of the facilities and processes as well
as transportation of digestate) is not included in the estimation.
2. Since the digestate was stored for 60 days, thus storage in every scenario means storage
for 60 days in this study.
3. Due to very thin layer, no formation of anaerobic microenvironments is assumed and
hence CH4 emission during dewatering and curing is assumed to be negligible.
4. Nitrous oxide emission from land applied digestate has been considered only, whereas
during digestate storage and curing, it is assumed to be negligible. CH4 emission from
land application of digestate has been reported minimal and has not been considered in
this study. The forms and sources of GHG emitted or avoided are given in Table 3.4.
Figure 3.10 Comparative scenarios of digestate management
5. In each case of digestate management, CO2 produced from biological degradation (e.g.
in digestate land application, storage, dewatering, curing and dumping) has not been
accounted as GHG source. The reason is that the material to be treated is of biogenic
Table 3.4 Forms and Sources of GHG Contributed and GHG Avoided
Scenario GHG contribution GHG avoided
1 CH4 from dumped digestate Nil
2 N2O from land application of digestate Fossil CO2 for fertilizer
3 CH4 from storage and N2O from land Fossil CO2 for fertilizer
4 CH4 from storage and N2O from land Fossil CO2 for fertilizer
Dumping of digestate (baseline)
All the digestate is dumped to a dumpsite
Direct application of digestate to agricultural land
All the digestate is spread over agricultural land
Digestate storage and land application
Storage of all the digestate followed by its land application
Storage, curing and land application of digestate
All the digestate is stored followed by its curing and land
Storage, curing and dumping of digestate
All the digestate is stored and then cured before its dumping
5 CH4 from storage and dumping of
d) Calculation methods
i) CH4 emission potential of dumped digestate (as GHG source)
Estimation of GHG emissions has been done by mass balance method, which calculates
methane formation potential on the basis of weight of carbon in the material. The method
is based on a model given by Bingemer and Crutzen, 1987 (p. 2181) as cited by Kumar et
al. (2004) and used by IGES, (2011). The detail is given in the following steps:
1. The characteristics of waste and digestate material (TS, and C) used for this estimation
were practically found by lab analysis to calculate methane formation potential.
2. Methane potential of material (g/kg waste) was then converted to GHG emission (g
CO2-eq/kg waste) by multiplying with a factor of 25 (IPCC, 2011).
g of methane / kg of material
( M TS MCF DOC DOC F ( 16 / 12)
( 1 OX ) 1000
g of methane/
M = Total mass of material (1 kg)
TS = Total solid content (fraction, e.g. 0.25)
MCF = Methane correction factor (fraction). The frac tion depends upon the
method of disposal and depth available at landfills. The IPCC document
indicated the value 0.4 for open dumps of
ii) Calculation of N2O emission from land applied digestate
The land applied fertilizer, compost or biosolids emits N2O equal to 0.01 of applied N as
stated by IPCC (Brown et al., 2010). However, Moller et al.,(2009) used N 2O emission
factor equal to 0.013-0.017 of applied N to soil. Here we have used N2O emission factor
0.013 of applied N in digestate.
The total amount of N provided by 1 kg of digestate was calculated from the N content of
each type of digestate (Table 4.5). The amount of N2O to be emitted from 1 kg of digestate
was then calculated using the factor 0.013 of applied N. The N2O to be emitted was then
converted to its equivalent amount of CO2 by multiplying with a factor of 298 (IPCC,
iii) Calculation of methane emission during storage
It is equal to the difference of methane emission potential of digestate before and after
storage. The methane emission potential was calculated by the same method as used by
iv) Calculation of avoided GHG by land application of digestate
Land application of raw digestate, stored digestate and stored-cured digestate can somehow
replace the use of synthetic fertilizer and hence can reduce the emission of fossil carbon by
energy use for production of chemical fertilizer. The total amount of N and P provided by
1 kg of digestate was calculated from the N and P content of each type of digestate. The N
and P content of each type of digestate were measured by lab analysis and are given in
Table 4.5. The avoided or saved GHG by land application of digestate was then calculated
by emission factors 8.9 kg CO2-eq/ kg N and 1.8 kg CO2-eq/ kg P (Moller et al., 2009).
3.5 Analytical Methods
Simulated feedstocks, inoculum sources and digested residues (as solid samples) were
characterized for their physico-chemical characteristics such as TS, VS, Carbon (as TOC)
and Nitrogen (as TKN) by standard methods given in Table 3.5. These measurements were
performed once to twice a week to interpret the process performance.
Liquid portion of the digested residues (liquid digestate) was collected by simple
centrifugation (5000 rpm for 20 min). These extracted samples were used for analysis of
pH, oxidation reduction potential (ORP), alkalinity, VFA and total ammonia nitrogen i.e.
TAN (NH3 + NH4 + ). The pH and ORP of digestate were analyzed on daily basis, whereas
all other parameters of liquid digestate were analyzed once to twice a week. Biogas
production was measured using the wet gas flow meter (TG 05, Ritter) connected with
reactor gas outlet port. The biogas composition was analyzed once to twice a week
depending on sensitivity of the reactor conditions.
Table 3.5 Analytical Methods for Various Parameters of Anaerobic Digestion of
Parameters Method/Equipment Interference Reference
Parameters of solid samples (solid waste, inoculum and digestate)
TS (%) Oven drying at 105 ºC Loss of volatile OM
VS (%TS) Furnace drying to ash at 550 ºC Loss of volatile
inorganic salts like
Organic C (%)
- Walkley and
TKN (%) Digestion of sample with H2SO4
and use of Kjeldahl apparatus
- APHA, 2005
P (%) HClO4 + HNO3 digestion
- Olsen and
and colorimetric method
Parameters of liquid samples (Liquid digestate or leachate)
pH Glass electrode method Sodium if pH>10 and
ORP Electrode method - APHA, 2005
TAN (mg/L) Distillation method - APHA, 2005
NH3-N (mg/L) Calculation method - Siles et al.,
Alkalinity Titration method - Lahav and
VFA (mg/L) Titration method - Lahav and
Parameters of biogas samples
SHIMADU-GC14 A Gas
chromatograph with TCD
- APHA, 2005
Results and Discussion
The results and discussion chapter has been divided into four sections. In the first section,
biogas and methane formation potential of waste simulations have been discussed. In the
next section, effect of C/N ratio and ammonia-N accumulation on dry anaerobic digestion
is discussed. In this part of study, effect of two feedstocks with different C/N ratio (27 and
32) and its associated accumulation of ammonia-N have been investigated on the operation
and performance of a pilot-scale thermophilic dry anaerobic digester. In third section, the
effect of different organic loading rates on the stability and performance of the same pilotscale
thermophilic dry digester has been discussed. For this purpose, a third feedstock with
C/N ratio 32 (for detail, please refer to section 3.1.2) was fed to the digester, because feed
with C/N ratio 32 performed well in the earlier experiment. In the final section,
characteristics of digestate have been described and various digestate management options
from perspective of GHG emissions have been discussed.
4.1 Gas Formation Potential of Waste
In this initial part of the thesis, the potential of prepared waste simulations to produce
biogas was measured experimentally. For this purpose, GP21 test set-up was used. Sludge
from anaerobic process was used as a standard inoculum in this study. Similarly, to make
sure that inoculum is active, cellulose was used as a standard substrate, because its biogas
production potential is known in literature. Thus to standardize the test conditions,
experimental runs of cellulose (cellulose + inoculum) were conducted in parallel with the
waste simulations (waste + inoculum). Moreover, blanks (inoculum alone) were also run
for correction. Two separate sets of experiments were conducted for Feedstock 1 and 2.
The results have been discussed below.
The inoculum (alone) used for Feedstock 1 produced lesser biogas (107 NmL) as
compared to that used for Feedstock 2 (125 NmL). The two inoculums used for Feedstock
1 and Feedstock 2 are hereby called as inoculum 1 and inoculum 2 respectively. The same
trend of biogas production was observed from 1 g cellulose by inoculum 1 and 2 (176 and
402 NmL) as shown in Figure 4.1 and 4.2. The inoculum used in both the cases, had TS
and VS content of around 5.5% and 45%TS respectively. The specific biogas production of
inoculum 1 and 2 was 91 and 102 NmL/g VS respectively. These results show that
inoculum 2 was more active as compared to inoculum 1. The results similar to those of
inoculum 2 were obtained from the GP21 test study conducted by Heerenklage and
Stegmann, (2005), where the active inoculum produced around 100 and 400 NmL biogas
from inoculum (alone) and 1 g cellulose respectively.
The cumulative biogas production of Feedstock 1 and 2 (having different TS and VS
characteristics) was almost similar, i.e., 2365 NmL (Figure 4.1 and 4.2). Therefore, the
specific biogas production of Feedstock 1 and 2 was different (i.e. 348 and 225 NmL/g
VSadded respectively) as found in this experiment. The percentage of methane varied
between 65-70 % in biogas samples collected at different time intervals from reaction
Biogas production 100X (NmL)
Specific biogas production (NmL/g VS)
Inoculum 1 Cellulose Feedstock 1
1 6 11 16 21
Time Feedstock (days) 1
26 31 36
1 6 11 16 21 26 31 36
Figure 4.1 Cumulative and specific biogas production by feedstock 1
The test results indicated that the biogas production with maximum percentage of methane
content was recorded between 12 th and 18 th day of incubation at 35 o C. The methane
production potential of the Feedstock 1 and 2 was thus 230±5 and 160±8 mL CH4/g
VSadded, respectively. Guendouz et al. (2010) also reported 187 mL CH 4/g VSadded with
MSW as feedstock for assay which correlates with the present study results.
The expected methane yield for source-sorted OFMSW is more than 350 mL CH4/g VS. In
both tests, the reason for low methane production could be high substrate-to-inoculum (S/I)
ratio (i.e. 5.55 and 8.10 for feedstock 1 and 2 respectively) or in other words overloading.
In literature, the optimum S/I ratio for anaerobic digestion has been described as < 5 and
for BMT test, the usually maintained S/I ratio is 1 or less (Guendouz et al., 2010; Nizami et
al., 2012). Methane production is less in case of Feedstock 2 as compared to 1 as S/I ratio
in case of Feedstock 2 is higher than 1 as stated above. Also, it may be because of higher
percentage of slowly degradable fractions (paper an d leaves) in Feedstock 2 than 1. It is
important to note that, originally, the GP21 test is designed to be run for 21 days. But the
gas production continued beyond 21 days in our experiments, therefore the observations
were made for longer time than its designed duration.
Biogas production 100X (NmL)
Specific biogas production (NmL/g VS)
Inoculum 2 Cellulose Feedstock 2
1 8 15 22 29 36
Time Feedstock (days) 2
1 8 15 22 29 36
Figure 4.2 Cumulative and specific biogas production by feedstock 2
In this set of experiments, we followed GP21 test procedure (Heerenklage and Stegmann,
2005), which considers simple weight of substrate and inoculums. Therefore, the achieved
S/I ratio on kg-VS basis was not in optimum range. Thus research outcomes of GP21 test
attained are that i) Simple weight ratio of substrate and inoculum should not be used to
standardize the experimental conditions of GP21 test, but weight ratio on kg-VS basis
should be used. ii) Low S/I ratio (i.e. 5.5) provided better methane production results as
compared to high S/I ratio (i.e. 8.1).
4.2 Effect of C/N Ratio and Ammonia-N Accumulation on ITDAR (Results of Phase I
The phase I pilot-scale experiments mainly focused on optimization of dry anaerobic
digestion in terms of C/N ratio. Two simulations of OFMSW with different C/N ratio,
namely Feedstock 1 and 2 respectively, were prepared for this purpose. The simulations
were used to operate the pilot-scale reactor under thermophilic conditions. The phase I
pilot experiment consisted of a startup and continuous loading phases. Results of GP21 test
showed that low S/I ratio (5.5) performed better than high S/I ratio (8.1). Therefore, low
S/I ratio was used for start-up of the reactor in both phase I and phase II pilot experiments
(i.e. 5.2 and 3.04 respectively, please refer to section 3.3.2 and 3.3.3). The detailed results
have been discussed in the following sections.
4.2.1 Performance of ITDAR during start-up and continuous operations
The performance of ITDAR for treating the OFMSW with two different C/N ratios (27 and
32) was evaluated under various organic loading rates using the mesophilic inoculum
sources. The overall recital during initial start-up phase, in association with the OLR and
Digrr (digestate recirculation rate) were measured using parameters like pH, alkalinity,
VFA and methane yield as depicted in Figure 4.3(a-e). The results have been discussed in
the following subsections.
a) Monitoring of ITDAR during start-up phase
Initial pH of the substrate contents within the ITDAR was acidic (5.5–5.8) due to
accumulation of very high VFA concentrations of 37,857 mg/L, which was mainly
released from the rapid degradation of readily available biodegradable components in the
simulated feedstock 1 (Table 4.1; Appendix C, Table C-1). The biogas yield was higher
with the higher CO2 and lesser CH4 contents during this period. Therefore, NaOH was
added between 3 rd and 6 th day to attain the pH of near natural range to support with the
methanogenic activity in ITDAR. With the continuous pH maintenance in ITDAR, it
showed the percentage increase in methane content of biogas samples i.e., increased from
3% to 33%, on 10 th day with the alkalinity value of 23,000–27,000 mg/L as CaCO3. Also,
the ammonia-N contents were measured in the range of 1800–2100 mg/L in ITDAR. The
ORP of liquid digestate, which was considered as a good indicator of anaerobic condition,
was measured as -260 mV and -450 mV in ITDAR before and after pH adjustment. Less
than -300 mV of ORP found to be favoring the methanogenesis process in anaerobic
system was reported by Alkaya and Demirer (2011).
Therefore, it was very clear that the ITDAR attained its methanogenesis stage within 14
days of batch operations with the continuous pH adjustment, which can be visualized from
the stable pH, ORP and methane contents.
b) Monitoring of ITDAR during continuous operation phase (Run 1 to 8)
The pH variation in ITDAR during different runs (1–8) has been depicted in Figure 4.3c. It
was observed that, the near neutral pH of 7 and above was maintained throughout the study
period (of 280 days) even with the increasing OLRs (Figure 4.3a) in ITDAR.
The results showed that the system was able to get naturally buffered, which means that the
acid producers and acid consumers were equally dynamic, and produced specific methane
yield of 219 L/kg VSadded on an average from different trials (Figure 4.3d and Appendix C,
Table C-2). In other sense, the ITDAR was continuously acting like an active
methanogenic reactor and the waste fed into the system was comparatively lesser than the
reactor contents, hence the system pH and associated methane yield was not getting much
affected with the increasing OLRs. Generally, the methanogenic system pH was reported
to be in between 6.8 and 7.8 (Lahav and Morgan, 2004).
Dig rr (L dig /L reactor vol .d) pH Methane yield 10x (L/kg VS) VFA/Alk ratio
OLR (kg VS/m 3 .d)
0 30 60 90 120 150 180 210 240 270
1 2 3 4 5 6 7 8
0 30 60 90 120 150 180 210 240 270
Figure 4.3 Time course of dry anaerobic digestion with various parameters in ITDAR
Further, the VFA concentrations were high (5000 –38,000 mg/L) and average alkalinity
was 31,000 mg/L as CaCO3 (range 7700–69,000 mg/L as CaCO3) during different runs
(Table 4.1; Appendix C, Table C-1). Polprasert (2007) reported the VFA concentration of
6000–8000 mg/L as inhibitory. However, the digestion process in ITDAR was not really
getting affected with the presence of VFA concentrations higher than that of inhibitory
levels. The reason is that VFA to alkalinity (VFA/Alk) ratio (a good indicator of digester
failure) during various runs found to be in the range of 0.31–0.56 (Figure 4.3e), except for
run 1 (1.21). Therefore, the methane yield for the run 1 was comparatively lower than the
successive runs and correlated with the previous report by Khanal (2008) . He stated the
VFA/Alk ratio of ≤ 0.4 and 0.8 for successful and faultier reactor functioning, respectively.
The average concentration of 2325 mg/L ammonia-N was recorded in extracted liquid of
digestate in ITDAR. High ammonia-N concentration has also been reported to act as buffer
against the acidification effect of VFA (Lahav and Morgan, 2004). The detailed data of all
these operational parameters has been given in Appendix C, Table C-1.
From overall study, a maximum specific methane yield of 327 L/kg VSadded and minimum
of 121 L/kg VSadded was recorded from runs 3 and 5, respectively (Appendix C, Table C-
2). The possible reasons could be that the lower OLR with higher SRT and higher OLR
with lower SRT, respectively, for run 3 and 5. Further, the higher Digrr and sudden
overloading of reactor and consequent drop in pH could also be the possible reasons for the
lower methane yield during run 5 ( Figure 4.3b). The specific methane yield for the
centralized DRANCO system was reported to be in the range of 210 to 300 L/kgVSadded
(De Gioannis et al., 2008). The present study results, in terms of specific methane yield,
were in line with the performance of centralized units. But, on the other hand ITDAR can
uphold the overall net energy gain by reducing the collection and transportation costs
found to be advantageous for using it in decentralized level.
Further, the understanding of relationship between the pH, ammonia-N and VFA
accumulation with the different feedstock characteristics was considered as important to
improve the reactor performance. Hence, the following sections primarily emphasized the
relationship between feedstock characteristics, ammonia-N accumulation and VFA
interactions in ITDAR (Figure 4.4, Note: VFA data for Day 1-120 has not been included to
clearly show the probable interaction, however, this data is provided in Table C-1 of
Appendix C and in Table 4.1).
4.2.2 Effect of C/N ratio and ammonia-N accumulation in ITDAR
Table 4.1 and Figure 4.4(a–e) depict the important parameters of digestion viz., pH, VFA,
VFA/Alk ratio, ammonia-N, free ammonia, methane yield and VS removal (Appendix C,
Table C-1 and Table C-2), which can directly affect the performance of the ITDAR.
a) Effect of feedstock 1 (C/N ratio of 27) in ITDAR – Run 1 to 3
Feedstock 1 with the C/N ratio of 27 was used in the start-up of ITDAR and in runs 1–3.
The maximum concentration of 3200 mg/L of ammonia-N concentration was recorded
during run 1. Later, the average concentrations subsequently reduced up to 3040 mg/L and
2671 mg/L in run 2 and 3, respectively. The pH was lower during run 1 and increased to
near neutral range during run 2 and 3. Therefore, the escape of ammonia-N as gas during
run 1 was comparatively lesser than the other two runs as noticed from the free ammonia
concentration levels. As stated, the average concentration of free ammonia was 99 mg/L in
run 1, whereas it was around 328 and 284 mg/L during run 2 and 3, respectively. It was
reported that the free ammonia can severely affect the anaerobic system under
concentrations of 200–700 mg/L in thermophilic anaerobic systems by various authors
(Hansen et al., 1998; Straka et al., 2007; Nakakubo et al., 2008; El-Hadj et al., 2009; Yabu
et al., 2011). Therefore, it is very clear that the ITDAR possibly inhibited with the free
ammonia toxicity during the trial runs 1–3 with the feed C/N ratio of 27.
Table 4.1 Digestion Parameters and Methane Yield of ITDAR
Feedstock 1 (avg. C/N ratio 27)
Start-up 37857 a
1.41 6.29 1943 25 9.60 -
1 35920 1.21 6.95 2753 99 176 46.52
2 27567 0.52 7.53 3040 328 286 64.16
3 18025 0.51 7.49 2671 284 327 70.20
Feedstock 2 (avg. C/N ratio 32)
4 11725 0.31 7.75 2360 432 218 54.36
5 11417 0.35 7.35 1895 164 121 35.37
6 9625 0.39 7.50 2161 227 222 53.65
7 7469 0.39 7.29 1791 221 203 52.53
8 6010 0.56 7.34 1758 135 203 54.02
Table gives average values for each run.
In addition, the VFA/Alk ratio was also higher i.e., 1.21 during run 1 and it was around 0.5
for run 2 and 3. Very high concentration of unconsumed VFA from start-up phase carried
to run 1 could be the possible reason for the higher VFA/Alk ratio during run 1. As
discussed earlier, the higher VFA/Alk ratio in run 1 affected over the entire methane
production rate in ITDAR.
In addition to the ammonia-N concentrations, VFA/Alk ratio higher than its optimum value
(i.e. ≤ 0.4) was also found to be affecting the overall methane yield while feeding the
reactor with C/N ratio of 27.
b) Effect of feedstock 2 (C/N ratio of 32) in ITDAR – Run 4 to 8
Feedstock 2 with the C/N ratio of 32 was used for runs 4–8 to reduce the free ammonia
toxicity. Using this range of C/N ratio has been reported (Kayhanian, 1999) as a one of the
best tools to mitigate ammonia-N inhibition in dry thermophilic anaerobic digesters.
During trial 2 with the higher C/N ratio, the ammonia-N concentration further reduced
from 2360 to 1758 mg/L (avg.) in ITDAR during different runs. The sudden change in
feedstock composition (with the C/N ratio of 32) from run 3 (trial 1) with increasing OLR
affected the overall reactor performance during run 4 (trial 2) as shown in Figure 4.4 (b-d).
The reason was that higher C/N ratio helped system to increase alkalinity and pH value
(8.00), which increased free ammonia concentration (657 mg/L) in ITDAR. The detailed
explanation with data of Appendix C, Table C-1 is given here. When feeding of feedstock
2 was started, the change in alkalinity and pH was not very significant (alkalinity changed
from 32000 to 35250 mg/L and pH changed from 7.59 to 7.75) as per data from day 98 to
day 127 in the appendix. This duration can be regarded as acclimatization time for the new
feedstock (i.e. Feedstock 2) for ITDAR being a biological system. After that, the increase
in alkalinity and pH was quite significant (alkalinity changed from 35250 to 68750 mg/L
and pH changed from 7.75 to 8.00) during day 127-141, which increased free ammonia
from 411 to 657 mg/L and directly affected methane yield. This change in parameters by
feedstock 2 is considered as a sudden change, which is just in 14 days (from day 127 to
141), where the retention time of that run was 45 days.
VFA 1000x (mg/L)
Free ammonia 10x (mg/L) pH Methane yield 10x (L/kg VS)
TAN 100x (mg/L)
0 30 60 90 120 150 180 210 240 270
1 2 3 4 5 6 7 8
0 30 60 90 120 150 180 210 240 270
Figure 4.4 Interaction of ammonia and VFA in ITDAR
The higher concentration of free ammonia in ITDAR directly influenced the overall
methane production as can clearly be observed from the Figure 4.4b and d during run 4
(day 135-145). The effect of free ammonia or ammonia-N on the inhibition of biogas
production was instantaneous and the systems succeeded in recovering from the inhibition
only in few cases (Chen et al., 2008). As can be noted, the feed composition variations and
sudden shift in OLR severely affected the performance of ITDAR as evidenced from the
free ammonia accumulation and methane yield. In large scale centralized systems, the
detrimental effect due to the compositional and OLRs variations could be magnified and
may require longer time for the system to get recovered.
Whereas, in a decentralized system like ITDAR, the effect was shortened by altering the
OLR (in run 5), which i n turn lowered the system pH and reduced the free ammonia
concentration (reduced from 432 to 164 mg/L) in ITDAR. Moreover, the lesser SRT and
higher feed C/N ratio reduced the protein solubilization rate and hence produced lesser
ammonia-N concentration within the system, which was found to be advantageous. Straka
et al. (2007) also found that the dilution of waste biomass mixture by addition of activated
sludge in thermophilic anaerobic digestion system reduced the ammonia-N inhibition
effect. On the other hand, the VFA/Alk ratio of the system was also affected and became
0.35 during run 5. The specific methane yield from run 5 was calculated as 121 mg/L
(lowest yield among 8 runs) mainly due to lesser SRT. During run 6 and 7, the average free
ammonia concentration was calculated as 227 and 221 mg/L, respectively. It was even
lower (135 mg/L) during run 8. But, specific methane production rate was almost similar in
all the three runs i.e., around 200 mg/L.
Run 8 performed better among the other runs with higher OLR and lesser SRT. The pH
was near neutral, VFA/Alk ratio was around 0.56 and ammonia-N concentration was
around 1758 mg/L in run 8. Also, long time run stabilized the system performance and
associated parameters in this case.
Although the process control is easy in a decentralized system by judiciously altering the
operating conditions (like OLR, C/N ratio, etc.), but the change should be performed very
carefully and gradually to avoid its effect on performance of biological system. It is
evident in run 4 of this study where sudden change in C/N ratio affected the performance
of the system.
4.2.3 Summary of the effect of ammonia-N accumulation in ITDAR
Total ammonia-N concentration decreased with the increase in C/N ratio of the feedstock
1–2 as depicted in Figure 4.5. The average ammonia-N concentration in the digestate was
2820 and 1990 mg/L for the feedstock 1 (run 1 –3) and 2 (4–8), respectively. Hence, the
decrease of ammonia-N concentration in ITDAR was calculated as 30% with the use of
feedstock 2 (C/N ratio of 32). It can be noted from the Figure 4.4b, that free ammonia
accumulation occurred for short duration (3 –5 days) on days 41, 91 and 187 up to the
levels of 370–425 mg/L (Appendix C, Table C-1), but there was no effect observed in
specific methane yield. Whereas, the accumulation was around 400–660 mg/L and
sustained for a longer duration (10–30 days) especially during run 4 and run 7. It affected
the overall methane yield, which means that the digestion process was inhibited or
moderately inhibited. As the digestion process proceeds, the pH becomes first stable and
then it starts to increase steadily with the increasing alkalinity. As free ammonia is a
function of ammonia-N, pH and temperature, thus it increased with the increasing pH (7.5–
8.0) at most of the above said time periods. It has been reported that a free ammonia
concentration of 700 mg NH3-N/L causes inhibition in methane production under
thermophilic conditions (Yabu et al., 2011). Hartmann and Ahring (2005) also proved that
there was no inhibition at free ammonia concentration of 450–650 mg/L under
thermophilic digestion. In contrast, it has been reported that both gaseous ammonia or free
ammonia (Hansen et al., 1998) and total ammonia-N (El-Hadj et al., 2009; Nakakubo et al.,
2008) cause the inhibition. Chen et al. (2008) highlighted that there was conflicting
information reported in the literature about the sensitivity of acetoclastic and
hydrogenotrophic methanogens. Similarly, there was no clarity in inhibiting concentrations
of ammonia-N or free ammonia in anaerobic digestion system for the particular substrates.
In our case with the ITDAR operations, a drop in methane yield was observed with the
increasing free ammonia concentrations. At free ammonia concentration of 400-660 mg/L,
methane production was found to be inhibited. These results also correlated with the
reported literature. For example, El-Hadj et al. (2009) found that the free ammonia
concentrations of 215 and 468 mg/L reduced 50% of the methane yield under mesophilic
and thermophilic conditions respectively.
TAN TAN/TKN ratio Expected TAN
C/N 27 C/N 32
C/N ratio of feed
Figure 4.5 Variation of total ammonia-N concentration and TAN/TKN ratio with feed
C/N ratio in ITDAR
The subsequent accumulation of VFA after accumulation of ammonia-N in digester has
been reported in other studies as well, but most of them have pointed out that it happened
because of free ammonia (Chen et al., 2008). In our study, accumulation of both ammonia-
N and free ammonia has been found to not directly affect the accumulation of VFA.
Nakakubo et al. (2008) also found that the acetic and propionic acids did not show any
indication with the process imbalance at high ammonia-N concentrations. To support with
this, the pre adapted culture to ammonia-N showed better activity up to 700 mg/L of free
ammonia concentration, while 100–150 mg/L affected the unadapted culture and inhibited
the methane yield as reported by Hansen et al. (1998). They have further stated that the
interaction between free ammonia, volatile fatty acids and pH will lead to an ‘‘inhibited
steady state’’, which is a condition where the process is running stable but with a lower
methane yield. Similar condition was found to be in the case of ITDAR operations under
different OLRs and later recovered with the changing operational sequences.
Total ammonia-N to total nitrogen ratio (TAN/TKN ratio) is considered as a good indicator
for estimating the percentage conversion of total nitrogen into ammonia-N during the
anaerobic digestion. As stated, the calculated percentage of ammonia-N with the feedstock
1 was around 0.77, whereas, it was 0.40 for the feedstock 2. Kayhanian (1999) suggested
that using feedstock C/N ratio from 27 to 32 promotes steady digester operation at
optimum ammonia-N levels. In the present study, feedstock 2 with C/N ratio 32 produced
an overall less concentration of ammonia-N and free ammonia as compared to feedstock 1.
Moreover, there was no accumulation of ammonia-N under commonly used retention time
(20–30 days) for digesters. Thus the feedstock with C/N ratio 32 was found to be better
than that with C/N ratio 27 for dry thermophilic anaerobic digestion to minimize or avoid
In our experiment, Feedstock 2 had low %TKN (1.62%), which produced TAN 1993
mg/L. Using these two values, mathematical calculation for Feedstock 1 having high
%TKN (1.92%) was done, by which it should produce TAN 2350 mg/L (Expected TAN).
But in actual experiment, it produced even higher TAN, i.e., 2821 mg/L. Thus
experimental value of TAN production for waste with high %TKN (or low C/N ratio) was
higher than its calculated value (Expected TAN). The reason may be that the system was
biological but not stoichiometric. The presence of high amount of %TKN in case of
feedstock 1 (C/N 27) as compared to C/N 32 might have resulted a relatively rapid growth
of microbes, which in turn resulted in the more conversion of organic nitrogen into TAN.
Thus experimental TAN was higher than Expected TAN in case of feedstock 1. However,
it needs to be proved experimentally in future studies.
From the results of this experiment, it can be concluded that ammonia nitrogen
accumulation (one main operational problem of dry anaerobic digestion) can be thus
mitigated by use of correct feed mixture (i.e. by adjusting composition or C/N ratio of the
feed). ii) This method to mitigate ammonia accumulation is good for dry anaerobic
digestion as the other methods (dilution of ammonia by addition of water as well as
stripping of ammonia) are not desirable and/or suitable for dry digestion. iii) Moreover,
adjusting the feed composition can be easily managed for a decentralized dry AD system
compared to centralized system.
4.2.4 Energy balance of ITDAR in Phase I pilot experiment
Table 4.2 compiled the net energy gains obtained from two different trials of ITDAR and
most of the runs produced surplus energy. The net energy gain from the reactor is
determined by the quality of produced biogas. The quality of biogas, in turn, depends on
the operational conditions and feedstock composition. Please refer to Appendix E for
methodology of energy balance calculations.
Average energy consumptions during different runs were distributed as 75% for
maintaining the reactor under thermophilic conditions, 12% for the shredding, 2.5% for the
waste loading and withdrawal and 9.6% for the digestate recirculation. From the energy
consumption percentile, it was evident that the continuous temperature maintenance of the
ITDAR upset with the maximum percentage of net energy gained.
Table 4.2 Surplus Energy of ITDAR During Various Runs
Energy Consumption (MJ/kg VS)
Shredding and Recirculation maintaining
Feedstock 1 (avg. C/N ratio 27)
1 14.42 1.33 0.29 0.26 22.34 -68.01
2 28.61 1.27 0.28 0.23 10.15 58.28
3 30.64 1.03 0.23 0.27 6.79 72.86
Feedstock 2 (avg. C/N ratio 32)
4 22.33 0.87 0.19 0.27 6.17 66.38
5 14.71 0.93 0.21 0.69 3.70 62.43
6 15.90 1.49 0.33 1.99 8.89 20.12
7 11.53 1.12 0.25 1.36 6.10 23.42
8 17.97 1.24 0.27 1.45 6.55 47.07
In run 1, however, the net energy production was negative. The reason could be that the
very high retention time of 153 days that increased the energy consumption for
maintaining thermophilic conditions. Similarly, runs 6 and 7 had only a small surplus
energy (i.e. 20% and 23% for run 6 and 7, respectively). The reason could be that the runs
6 and 7 had relatively high retention time compared to run 5 and 8 and thus energy
consumption was higher. Moreover, with the increasing Digrr during runs 4 to 8 of ITDAR
operations resulted with the increasing energy consumption rate compared to that of runs
1–3. But, overall, it can be concluded that the decentralized system (ITDAR) can produce
surplus energy in the range of 50–73% and considered to be economically viable than the
4.3 Optimization of a Pilot-Scale Thermophillic Dry Anaerobic Digester (Results of
Phase II Pilot Experiment)
In phase II pilot experiment, optimization of ITDAR treating OFMSW was performed by
testing different organic loading rates (OLRs). The C/N ratio of OFMSW, which
performed well in the earlier experiment, i.e., 32, was used in this study. The study was
started with a start-up phase (batch mode of operation) followed by continuous operation.
In continuous operation, effect of various organic loading rates on the stability and
performance of ITDAR was evaluated at a constant recirculation rate. The results have
been discussed in the following sections.
4.3.1 Start-up of ITDAR in phase II pilot experiment
For start-up phase, 40% of the reactor’s working volume was filled with inoculum, which
consisted of a mixture of digestate from thermophilic anaerobic reactor, anaerobic sludge
and cow dung. The remaining 60% of working volume was filled with Feedstock 3 (please
see detail of Feedstock 3 in section 3.1.2). The operating temperature in the start-up phase
was in thermophilic range (55°C), which was reached in 3 days by gradual increase. In the
first 50 days (start -up phase), the reactor was not fed and only mixing of the reactor
content was done at the rate of 2.4 Ldig/Lreactor vol.d. Under these conditions, the pH was
initially 7, which started to decrease and reached 6.36. Therefore, small quantities of
NaOH were added to the reactor periodically during days 5-25 to maintain pH at near
neutral range. It can be noted as small peaks during days 11-26 in Figure 4.6. From day 28
onwards, pH did not drop again and started increasing slowly, therefore, NaOH was not
added anymore. It became stable at around 8.2 during days 42-50 (Appendix D, Table D-
1 11 21 31 41
Figure 4.6 pH profile of ITDAR during start-up
At first, the concentration of VFA increased after loading the reactor and reached to its
maximum (19000 mg/L) on day 25. However, once the pH became stable, VFA
concentration started to drop due to its utilization. The concentration of VFA dropped from
19000 to 6300 mg/L only in 20 days (Figure 4.7 and Appendix D, Table D-1). The reason
is that there was no waste feeding throughout the star-up phase (day 1-50). The evolution
of VFA/Alk ratio was different from the VFA concentration. Increase in VFA/Alk ratio
after reactor loading was not observed, rather there was a continuous decrease, which
indicates that alkalinity started to develop and increase just after loading. It may be
because a part of inoculum used for this start-up was taken from the system with the same
conditions (treating OFMSW under thermophilic conditions). After day 30, VFA/Alk ratio
started to decrease in the same way as that of VFA. This may be attributed to decrease in
Methane content in biogas and gas production rate (GPR) was lower in the beginning. The
reason might be unfavorable conditions for methanogenesis, i.e., pH lower than 6.8 and
VFA more than 6000-8000 mg/L (Polprasert, 2007). However, methane content and GPR
started to increase slowly as the system progressed towards stability. On the contrary,
carbon dioxide content was higher at the start (Figure 4.8 and Appendix D, Table D-2),
which is a sign of acidification. But it decreased slowly, as alkalinity and pH increased and
VFA concentration got utilized.
From the above discussion, it can be concluded that pH and VFA concentration of the
reactor were not stable until day 30, thus, methane content was low, carbon dioxide content
was high and GPR was unstable. However, after day 35, the reactor conditions were stable
and, therefore, both the methane content in biogas and GPR increased.
VFA 100 X (mg/L)
1 11 21 31 41
Figure 4.7 Profile of VFA and VFA/Alk ratio during start-up
Overall, this start-up took relatively longer time for attaining stable conditions than a
previous study (Zeshan et al., 2012) in the same reactor. The reason may be th e use of
higher recirculation rate (i.e. 2.4 Ldig/Lreactor vol.d) as compared to previous study (where it
was 1 Ldig/Lreactor vol.d). Suwannoppadol et al., (2011) also recommended that during startup
phase low mixing rate should be used. In the end of start-up phase, GPR decreased,
which may be because most of the accumulated VFA was utilized by microorganisms as
depicted by drop in VFA and rise in pH. This point shows the end of start-up phase in
batch mode where continuous reactor loading should be started.
It was found from the results of this study that the start-up phase ended (at around day 50)
when most of the accumulated VFA was utilized by microorganisms and its concentration
decreased from 19000 mg/L at day 25 to 5700 mg/L at day 54 and further decreased to
4700 mg/L at day 68. With this decrease in VFA concentration, pH increased from 7.2 to
7.8 and the reactor stability conditions reached its maximum by achieving the lowest
VFA/Alk ratio of 0.32. Similarly, volatile solids concentration (VS/TS) of di gestate was
also found at its minimum (i.e. 0.56-0.57) at this point. Thus the end of start-up phase was
marked by the lowest values of VFA concentration, VFA/Alk ratio and VS/TS of digestate.
1 11 21 31 41
Moreover, from the comparison of start-up phases of the two pilot experiments, it was
found that low digestate recirculation rate (i.e. 1 Ldig/Lreactor vol.d) should be used to achieve
stable reactor conditions in less time. The results of continuous loading phase have been
presented in the following sections.
% of Biogas
Methane Carbon dioxide GPR
1 11 21 31 41
Figure 4.8 CH4, CO2 and GPR fluctuation during start-up phase
4.3.2 Stability parameters of ITDAR: Effect of organic loading rate
pH is very basic parameter to describe the stability of anaerobic digestion. With OLR of
4.55 kg VS/m 3 /d, the system stabilized its pH at around 7.75 with a range of 7.5-8 as
shown in Figure 4.9. When the OLR was increased from 4.55 to 6.4 kg VS/m 3 .d, pH fell
down to 7.58 and regulated to an average of 7.67 (7.33 -7.96). As a result of further
increase in OLR to 8.5 kg VS/m 3 /d, a drastic decrease in pH was observed and pH dropped
to the value of 6.89 (Appendix D, Table D-1). Therefore, NaOH was added during days
150-155 to control pH. The decline in pH in the starting days of each of the first two runs
and most of the last run is linked to destabilization of the system as a result of increase in
OLR. The reason is that when organic loading rate is increased, the acidogens also increase
their activity and produce high amount of VFA, as they are fast growing. But, on the other
hand, methanogens owing to their slow specific growth rate can not utilize all the already
produced VFA and need more time to build the required population size. Thus initial and
temporary decrease in pH is due to accumulation of VFA as a result of this imbalance in
the microbial groups, which is recovered until methanogens build their sufficient
population. The decrease of pH is more pronounced while working with higher OLR, i.e.,
8.5 kg VS/m 3 .d. The reason is that the imbalance between acidogenic and methanogenic
activity is more pronounced.
ii) Volatile fatty acids (VFA)
The concentration of volatile fatty acids in the digestate of ITDAR was quite stable at an
average value of 5100 mg/L (range: 4400-5700 mg/L) while operating at OLR of 4.55 kg
VS/m 3 /d (Figure 4.10 and Appendix D, Table D-1). When OLR was increased to 6.40 kg
VS/m 3 /d, VFA concentration started to increase and reached a maximum value of 6500
mg/L with an average value of 5400 mg/L in this run. Finally, at OLR of 8.50 kg VS/m 3 /d,
GPR (L/Lreactor vol./d)
the VFA concentration increased to 7500 mg/L because of increased organic loading rate.
This trend shows the destabilization of the reactor caused by increase in OLR.
51 71 91 111 131 151
Figure 4.9 Evolution of pH in ITDAR during continuous loading
It is important to note that at the start of each OLR, the VFA started to accumulate, which
is related with imbalance of activity of microbial groups and initial temporary
destabilization of reactor as a result of increase in OLR as discussed above in the case of
pH. Similarly, at the end of each of first two OLRs, the concentration of VFA declined,
which is a sign of stability of the system. However, this drop in VFA concentration for
OLR of 8.50 kg VS/m 3 /d was not observed probably because the reactor at this stage was
operated for duration equal to only one cycle of SRT.
VFA 100 X (mg/L)
OLR 4.55 OLR 6.4 OLR 8.5
OLR 4.55 OLR 6.4 OLR 8.5
51 71 91 111 131 151
Figure 4.10 Concentration of VFA in ITDAR during continuous loading
Concentration of VFA and pH (Figure 4.9 and 4.10) are reverse of each other. A low pH
supports the production of VFA (acidogenic activity), whic h in turn suppresses VFA
consumption (methanogenic activity). This could be explained by our results also.
iii) VFA to Alkalinity ratio (VFA/Alk ratio)
VFA/Alk ratio is a good indicator of digester functioning. With OLR of 4.55 kg VS/m 3 /d,
this parameter remained between 0.35-0.45 for most of the time (Figure 4.11). This is a
good range of VFA/Alk ratio for a working digester. But at OLR 6.4 kg VS/m 3 /d, the
average value of VFA/Alk ratio increased to 0.53, which is still acceptable for an operating
digester. The detailed data is provided in Appendix D, Table D-1. However, at OLR of 8.5
kg VS/m 3 /d, VFA/Alk ratio increased to very harmful range (0.67 -0.78), because at
VFA/Alk ratio of 0.8, significant pH reduction and digester failure happen (Khanal, 2008).
The trend of VFA/Alk ratio almost followed the trend of VFA concentration (Figure 4.10),
except at the beginning (day 54), where VFA concentration increased but VFA/Alk ratio
did not follow it. It was because the system had high buffering capacity or alkalinity.
Hence, rise in VFA concentration did not show any adverse effect on this ratio and hence
OLR 4.55 OLR 6.4 OLR 8.5
51 71 91 111 131 151
Figure 4.11 VFA/Alk ratio in ITDAR during continuous loading
4.3.3 Effect of organic loading rate on performance parameters of ITDAR
i) Gas production rate
Gas production rate (GPR) increased in OLR 6.4 and 8.5 kg VS/m 3 /d with an average
value of 6.37 and 7.55 L/Lreactor vol./d respectively as compared to OLR 4.55, where it was
5.01 L/Lreactor vol./d. Figure 4.12 shows biogas production rate of ITDAR as L/Lreactor vol./d
during different stages of continuous loading phase. The biogas contained 47-49% methane
in all the runs (variation is not significant, Appendix D, Table D-2), therefore, the trend of
methane production rate for three OLRs was also similar to GPR, i.e., 2.4, 3.07 and 3.6
L/Lreactor vol./d respectively. It can be noted that at the end of OLR 4.55 and 6.4 kg VS/m 3 /d,
the GPR becomes stable. This is related with stable pH and VFA concentration of the
system at the mentioned time.
The increase in GPR was almost linear with increase in OLR during first two runs. But,
during run 3 (i.e. OLR 8.5 kg VS/m 3 /d), the GPR did not increase with the same rate as
that of OLR. This could be explained by drastic increase in VFA/Alk ratio (o r drop in
alkalinity) during that run. However, this effect of overloading could be alleviated by
further acclimatizing the reactor under those conditions.
ii) VS removal and Specific methane production (SMP)
The VS removal was the highest in OLR 4.55 kg VS/m 3 /d. Thus, in terms of digestate
quality, the best results on kg VS basis were shown at OLR of 4.55 kg VS/m 3 /d as shown
in Table 4.3 and Appendix D, Table D-2. These results are similar to those obtained by
Montero et al., (2008). They obtained VS rem oval of 80% in a thermophilic system
operating at OLR of 4.42-7.50 kg VS/m 3 /d and 25-30% TS. The VS removal decreased as
OLR was increased.
GPR (L/Lreactor vol./d)
OLR 4.55 OLR 6.4 OLR 8.5
51 71 91 111 131 151
Figure 4.12 Gas production rate of ITDAR during different OLRs
The highest methane production per unit weight of volatile solids added (also called
specific methane production, SMP) occurred at OLR of 4.55 kg VS/m 3 /d (330 L CH 4/kg
VS added) whereas it was 20% lower for OLR 8.50 kg VS/m 3 /d. However, methane
production by all the runs of this study (provided in Appendix D, Table D-2) is in line with
the methane yield values found in literature.
Table 4.3 Percentage of VS Removal and Specific Methane Production in ITDAR
(kg VS/m 3 VS Inlet VS Outlet VS Loss SMP
/d) (kg VS) (kg VS) (%) (L CH4/kg VS)
4.55 121.60 26.86 77.90 330
6.40 116.90 28.66 75.48 320
8.50 62.95 20.70 67.00 266
The SMP reported by various authors through dry anaerobic digestion of OFMSW at
thermophilic conditions is in the range of 230-340 L CH4/kg VS added ( Gallert and
Winter, 1997; Pavan et al., 2000; Montero et al., 2008; Bolzonella et al., 2003). Similarly,
SMP reported for a mixture of maize silage and barley straw under thermophilic dry
anaerobic digestion was 182 L CH4/kg VS added (Mumme et al., 2010).
iii) Cumulative methane yield
Cumulative methane yield per liter reactor volume for each OLR tested has been sorted out
(Figure 4.13) for the equal amount of organic waste fed to the system. As shown in the
figure, OLR 4.55 kg VS/m 3 /d produced maximum cumulative methane that is 75 L
CH4/Lreactor vol.. At OLR of 6.4 and 8.5 kg VS/m 3 /d, it is only 57 L CH4/Lreactor vol..
Apparently, the relationship between GPR and OLR shown in Figure 4.12 was almost
linear. But the relationship of cumulative methane yield with each OLR tested (Figur e
4.13) provided better information about the performance of the process.
Cumulative methane (L/Lreactor vol.)
1 11 21 31 41 51 61
Figure 4.13 Cumulative methane per liter of reactor volume in ITDAR
RT in AD: 18 d
OLR: 8.5 kg VS/m 3 /d
VS removal: 67%
yield: 57 L CH4/Lreactor vol
OLR 4.55 OLR 6.4 OLR 8.5
Figure 4.14 Selection of operating conditions based on purpose of waste treatment
RT in AD: 30 d
OLR: 4.55 kg VS/m 3 /d
VS removal: 78%
yield: 75 L CH4/Lreactor vol
Based on our results, the best operating conditions for AD (RT and OLR) can be selected.
But it also depends on the purpose of treatment (Figure 4.14). As we know anaerobic
digestion can be used as a single treatment for waste or in combination with aerobic
In case of dual treatment (anaerobic + aerobic), time spent for anaerobic digestion should
be minimized, so that the saved time could be used for aerobic treatment. Thus RT of 18
days (OLR 8.5 kg VS/m 3 /d) should be preferred for digestion step. But if the purpose is
only a single treatment (i.e. dry anaerobic digestion), then option of higher retention time
(RT 24 d or 30 d) should be chosen to get maximum VS removal and its conversion to
4.4 Digestate Management and GHG Emissions (Phase III)
Proper management of digestate is needed as it has certain GHG emission potential. So, it
tends to emit methane to the atmosphere, if not stored properly. Moreover, it has certain
amount of nutrients and organic matter, which could be useful if applied on agricultural
soils. Therefore, to protect the environment and to make use of digestate’s economic value,
careful digestate management should be performed. There are several options for digestate
handling and management, for example, digestate dewatering, digestate storage, digestate
application to the land, digestate composting, digestate curing or dumping. But
characteristics of digestate are the key to determine the best and correct option for
management of digestate. Thus, in this section of thesis, characteristics of digestate have
been first described and based on the characteristics, the proper digestate management
options have been suggested and finally their effect studied on its nutrient content and
GHG emission potential.
4.4.1 Characteristics of raw digestate
Digestate was removed from the reactor every day before feeding of fresh waste
throughout the reactor operation period. The freshly withdrawn digestate (raw digestate)
was analyzed for moisture, TS and VS content twice a week. Moreover, digestate was also
characterized for carbon and nitrogen content to calculate its C/N ratio. Characteristics of
digestate have been discussed in this section (Figure 4.15, 4.16 and 4.17; Appendix C,
Table C-3, Appendix D, Table D-3).
With digestion the feed TS decreased, so digestate TS was lower than feed TS. Digestate
TS content was high in the beginning (16 -20%) and started to decrease as the digestion
proceeded. During run 3, it started to increase again almost continuously and reached upto
18.5% at the end of run 4 as shown in Figure 4.15 and given in Appendix C, Table C-3.
This increase in digestate TS corresponds to the increase in feed TS (23 -25%) combined
with increase in OLR. Similar observations were made by Mumme et al., (2010). Also
there was no replenishment of moisture lost through biogas, which is higher in
thermophilic process. Only little changes were observed for the VS content of TS (i.e.
digestate VS/TS), which has been in the range of 0.6-0.7 throughout the study.
The practical consequence of this increase in digestate TS was that it was difficult by the
screw bed pump to feed and re-circulate the feedstock and reactor material respectively.
Thus TS content in the reactor was adjusted by mixing of water with feedstock (making
feed TS 17-20%) starting from run 6 onwards as shown in Figure 4.15 and thereby the
digestate TS was kept constant at 12-13%.Thus adjustment of TS in the reactor helped in
its smooth operation.
Figure 4.15 Comparison of feed and digestate regarding total solids in phase I
TKN in digestate dropped at the beginning and was low during day 20-90. After day 90, it
started to increase and became almost constant for rest of the study period in the range of
1.8-2.0%. The significant decrease in TKN at the start of the experiment compared to TKN
in feed suggests that the TKN might have been utilized in the build-up of big size of
microbial biomass sufficient to sustain the reactor. Once sufficient microbial biomass was
developed, consumption of TKN might have been decreased, and thus its concentration
increased in reactor. Conversely, C/N ratio of the digestate was higher in the beginning.
After day 95, C/N ratio was low in the range of 15-20 (Figure 4.16, Appendix C, Table C-
3) and remained in the same range for the rest of study period. This finding is in agreement
with increase in TKN concentration.
C/N ratio of digestate
Feed TS Digestate TS Digestate VS/TS
Start-up 1 2 3 4 5 6 7 8
C/N ratio of digestate Digestate TKN
1 31 61 91 121 151 181 211 241 271
Run Time (days)
Figure 4.16 TKN and C/N ratio of the digestate in phase I experiment
VS/TS of digestate
TKN in digestate (%TS)
Based on this property of digestate (i.e. C/N ratio 15-20), further intensive treatment, for
instance, composting, is not required. The digestate can be directly applied to agricultural
fields as Wood, (2008) stated that C/N ratio of organic material fit for agricultural land
application should be
Table 4.4 Comparison of Digestate Characteristics and Guidelines
Type of digestate pH Moisture
Digestate 7.75 88-90 37.28 1.94 18.35 This study
Thai Guidelines* 5.5-8.5 ≤ 35 ≥ 35 ≥ 1 ≤ 20
Indian Guidelines 5.5-8.5 - > 26 > 1 12-25
Gautam et al.,
*Given by Land Development Department, Thailand.
i) Digestate storage
Digestate is produced continuously throughout the year from digestion unit. However, its
land application time is not continuous, but only after harvesting of previous crop (e.g.
during spring and autumn). So it needs to be stocked-up or stored for several months.
Therefore, in this study, digestate collected from the reactor was stored in the HDPE
storage tanks for a period of 2 months to simulate field conditions and effect of storage on
various characteristics of digestate was observed. The change in concentration of solids
and nutrients with storage has been presented in Table 4.5.
Results show that there was very little change in concentration of solids (TS and VS)
during storage, thus, the removal of organic matter was not significant. But the
concentration of nutrients was significantly affected. The concentration of N and P in
digestate decreased by 32.3 and 33.8% respectively after 2 months storage. This may be
attributed to the prevailing conditions during storage (i.e. high pH, high moisture and high
ambient temperature), which were suitable for the microorganisms responsible for this loss
of nutrients. Similar results were reported by Paavola and Rintala, (2008), who reported
40–57% decrease in the concentrations of total P and PO4-P in the separated liquid fraction
of digestate after 3 months storage.
Table 4.5 Characteristics of Digestate at Different Stages of Management
Type of digestate TS (%) VS (%TS) C (%TS) N (%TS) P (%TS)
Digestate 10.45 64.12 35.60 1.94 0.62
Stored digestate (60 d) 9.34 63.91 35.50 1.47 0.46
Dewatered digestate 54.46 52.80 29.33 1.47 0.46
Stored-cured digestate 54.46 42.00 23.30 1.33 0.41
It can be concluded that storage time of raw digestate should be reduced to avoid nutrient
loss. Moreover, if storage is required, it should be done for dewatered digestate, as
dewatered digestate (low moisture) could lessen microbial activity and hence nutrient loss.
ii) Digestate curing
As concluded before, the digestate in this study did not need any further treatment to be
used as soil amendment because its C/N ratio was
Table 4.5. With curing of digestate, the organic content (C and VS) decreased significantly
(20.55%) and hence, C/N ratio of the digestate decreased to 17.52. This range of C/N ratio
is good for an organic material to be applied on agricultural land as soil amendment.
4.4.3 Digestate management from perspectives of GHG emissions
Data of digestate characteristics (Table 4. 5) was obtained by lab analysis of digestate
samples at each stage. Based on these data and the methods in section 3.4.4-d-(i), GHG
emission potential of digestate was calculated at different stages of digestate management.
Comparison of different digestate management options with regards to GHG emissions
was then made possible. The results regarding this have been presented and discussed in
i) GHG emission potential of digestate
The GHG emission potential of digestate was estimated using equation given in section
3.4.4-d-(i) and compared with that of OFMSW (the original substrate before digestion) as
shown in Figure 4.18. Compared to digestate (139 g CO2-eq/kg waste), the GHG emission
potential of OFMSW is very high (i.e. 568 g CO2-eq/kg waste). Thus, with digestion, GHG
emission potential of OFMSW decreases by about 75%.
GHG emission potential (g CO2-eq/kg)
OFMSW Digestate Stored Digestate Cured stored
Type of waste/digestate
Figure 4.18 GHG emission potential of OFMSW and digestates
Among different types of digestates, the (raw) digestate has maximum GHG emission
potential, whereas it is about 10% less for stored digestate and about 42% less for cured
stored digestate. As the waste treatment process proceeds, the GHG emission potential
decreases. The loss of GHG emission potential is more in case of curing of stored digestate
(i.e. 42%). The reason is that apart from C loss during storage, good aerobic conditions
were provided during the curing process by increasing TS of digestate through dewatering,
which led to sufficient loss of carbon in the form of CO2.
The GHG emission potentials of various types of digestate shown in Figure 4.18
correspond to the GHG emissions, only if they are dumped to the shallow dumpsite.
However, the emissions can be minimized if they are managed well in some better way.
For this purpose, various digestate management options in the form of 5 different scenarios
have been considered and compared with regard to GHG emissions and have been
presented in the following section.
ii) Comparison of GHG emissions from different digestate management scenarios
As digestate has certain residual GHG emission potential, it tends to emit methane to the
atmosphere, if not stored properly and hence can contribute to the climate change.
Therefore, it is necessary to carefully analyze various digestate management options based
on their net GHG emission reductions under various scenarios, so that the best scenario can
be selected for digestate management. In this part of the study, various digestate
management options were considered to assess the reduction in GHG emissions. Results of
GHG emissions from each scenario of digestate management have been presented in Table
Scenario 1: The baseline scenario shows that the net GHG emission is 190 g CO2-eq/kg
digestate. All the emission is from dumpsite or landfill. The emission comes directly from
biodegradation of digestate in CH4 form. Since no flaring, collection and recovery of
methane is considered in scenario 1, thus all the produced methane is released into
atmosphere and contributes to GHG emissions. There is no GHG saving here in this
scenario as well.
Scenario 2: In this scenario, GHG emission is mainly in N2O form, which is equal to 7.85 g
CO2-eq/kg digestate from the land applied digestate. But here the N and P as nutrients
provided by the land applied digestate is the most important factor, which replaces the use
of chemical fertilizer and hence the GHG from fertilizer manufacturing is avoided. Larger
avoidance of GHG emissions from fertilizer production ( -19 g CO2-eq/kg digestate) than
N2O emission (8 g CO 2-eq/kg digestate) from the land applied digestate results in the
negative net GHG emission as shown in Table 4.6.
Scenario 3: In this scenario, GHG emission is in both CH4 and N2O forms. Storage of
digestate for 2 months is the major contributor of CH4 from the stored digestate which is
equal to 20 g CO2-eq/kg digestate. Moreover, N2O from land applied digestate further
contributes to GHG emission. However, the GHG saving from the fertilizer substitution by
application of digestate is lesser than scenario 2. It is because of loss of nutrients during
storage. For instance, GHG saving from fertilizer substitution is mainly through N content
of digestate, but during storage, ammonia is lost through its volatilization from the stored
digestate due to favorable condition of pH. Since the GHG emissions from storage and
land applied digestate (25 CO2-eq/kg digestate) is larger than GHG savings (-13 CO2-eq/kg
digestate), the net GHG emission is positive in this scenario. It can be concluded from the
results of scenario 3 that if land application is the fate of digestate, its storage time should
be minimized as much as possible to reduce the CH4 emission and maximize the GHG
savings from fertilizer substitution by digestate.
Scenario 4: In this scenario, although, curing of digestate has reduced the GHG emission
potential of stored digestate as shown earlier in Figure 4.18. But still the net GHG
emissions here are just like scenario 3, which are during storage (before curing) and land
application (after curing) in CH 4 and N2O forms respectively. Thus, in case, if the stored
digestate is to be applied to soil, curing is less advantageous. However, curing is
recommended if the nutrient ratio of digestate is not suitable for land application (e.g C/N
ratio > 20).
Table 4.6 Net GHG Emissions from All Scenarios of Digestate Management
Scenario GHG emission
GHG saving by fertilizer
substitute (g CO2-eq/kg
CH4 N2O N P
Scenario 1 190 0 0 0 190
Scenario 2 0 7.85 -18.04 -1.17 -11
Scenario 3 20 5.32 -12.22 -0.77 12
Scenario 4 20 4.70 -10.80 -0.67 13
Scenario 5 129 0 0 0 129
Scenario 5: In this scenario, curing has reduced the GHG emission potential of digestate
down to 109 g CO2-eq/kg digestate. However, before curing, there is CH4 emission during
storage equivalent to 20 g CO2-eq/kg digestate. Since dumping of cured digestate is to be
done, there is no N2O emission and also no GHG savings from this scenario. If the
digestate is unfit to be applied on land because of presence of heavy metals or other
pollutants beyond safe levels, then its dumping is needed. In such case, this scenario is
recommended, because before dumping, GHG emission potential of digestate can be
minimized by curing.
iii) Summary of GHG emissions from digestate management options
Net GHG emissions from all scenarios of digestate management were calculated. Scenario
1 produces maximum GHG emissions as there is no management or treatment of digestate,
but only direct landfilling. Scenario 2 performs best out of all other cases in terms of
greenhouse gas emission (Figure 4.19). This is because there is no storage and production
of CH4 and its nutrient content has been fully utilized for GHG savings, so GHG emission
is minimized more than 100% here in scenario 2 as compared to scenario 1.
Scenario 3 and 4 performed almost similar to each other because GHG emission sources
(digestate storage and land application) are common in both scenarios. About 92-93% of
GHG emissions have been however reduced by scenario 3 and 4 as compared to scenario
1. Scenario 5 reduces 32% of GHG emissions as compared to base scenario, which is
however better than direct dumping of digestate. Moreover, GHG emissions in scenario 5
can be further reduced to about 43% as compared to base scenario by avoiding storage of
Some researchers have used the possibility of reduction of N2O emission from land applied
digestate by better agricultural management practices (e.g. digestate application to soil by
placement method or application during peak season of nutrient uptake, etc.), that leads to
different results. Similarly, some studies also include the carbon sequestered into soil as
GHG savings that may lead to difference in results. Sequestered carbon is the carbon
applied to soil in the form of digestate and not released as CO2 from the soil for 100 years.
Also difference in characteristics of digestate (nutrient and carbon content) can produce
different results. From the results of this study, the order of preference to manage the
digestate from perspectives of GHG emissions is that scenario 2 is the best. Scenario 3 and
4 are better than scenario 5, while scenario 1 is the worst.
Net GHG emission (g CO2-eq/kg)
Scenario 1 Scenario 2
Scenario 3 Scenario 4 Scenario 5
Figure 4.19 Net GHG emissions from all scenarios of digestate management
4.5 Decentralized Dry Anaerobic Digestion of OFMSW for a Community of 5000
Based on findings of this thesis work, a decentralized anaerobic digestion system is
designed for treating the OFMSW collected from a community of 5000 people. Such
decentralized AD system will be developed for reducing the burden on central system of
waste management. It is also easy to manage the process at decentralized level. The details
regarding design, feedstock preparation, start-up, continuous operation, methane and
energy generation, digestate management, reduction of GHG emissions and VS balance of
the proposed system have been given in the following sections.
4.5.1 Design of the decentralized AD system
Around 2000 kg/d OFMSW from the mentioned size of community will reach the
decentralized facility (Appendix B). From the results of pilot study given in section 4.3.3,
the selected organic loading rate of 6.5 kgVS/m 3 /d (RT 24 days) will be applied for the
decentralized system. Based on the operating conditions and results of our pilot
experiments, the volume of reactor becomes 62.5 m 3 , which has been designed for the
amount of OFMSW generated by the community. The design data has been summarized in
Table 7.4 (for detailed calculations, please see Appendix B). The AD plant will be
provided with pretreatment facilities to pre-treat the organic solid waste coming from the
4.5.2 Preparation of feedstock for dry AD (Pre-treatment)
- Since the solid waste for the decentralized AD system will be source-separated waste, the
pre-treatment will consist of only shredding, which is required to facilitate pumping,
digestion, etc. Moreover, mixing of different kinds of waste will also be done to adjust C/N
- The OFMSW received from the community will be consisting of a mixture of food waste,
vegetable waste and fruit waste, with C/N ratio in the range of 18-25. Based on the
findings of this thesis, the feedstock C/N ratio 32 is helpful to mitigate ammonia
accumulation problem in dry AD. Thus feedstock C/N ratio should be adjusted by mixing
it with high C/N ratio material (e.g. waste paper, saw dust, etc.).
- In our pilot experiments, various feed mixtures were prepared and used to achieve the
above objective, the details of which are given in Appendix B, Table B-2. The
recommended material from our experimental results is that 5-7% of total waste should
consist of waste paper. However, the composition may be varied depending on the C/N
ratio of feedstock received from the community. Layout of the decentralized digestion
system to be established for treating OFMSW of the community has been shown in Figure
4.20. For this purpose, a shredder-cum -mixer will be used as shown in the Figure.
Table 4.7 Technical Details of Proposed AD Plant and its Comparison to Pilot Plant
Value/Specification of the reactor
Amount of waste (kg/d) 22 2000
Reactor size (m 3 ) 0.69 62.5
Dimensions of reactor(m)
Placement/Orientation Inclined at 30° with ground Inclined at 30° with ground
Material of the reactor Stainless steel Stainless steel
Size of the pump (L/min) 200 1450
Model of the pump Allweiler AE1N-200 Allweiler AE1N-1450
Digestate (Liq + Solid), kg/d 15.4 1400
Biogas production (L/d) 3530 200,000
4.5.3 Operation of decentralized AD system
- The system will be started-up by loading a mixture of waste and inoculum with substrateto-inoculum
(S/I) ratio of ≤ 3. The inoculum should consist of a mixture of variety of
anaerobic materials (e.g. anaerobic sludge, anaerobic digestate from a working digester,
and cow dung).
-At start-up, the reactor will be purged with natural gas to remove oxygen and create
- Low mixing rate (recirculation rate, i.e. 1 Ldig/Lreactor vol./d) will be used during start-up.
- The starting reactor temperature will be same as ambient temperature. It will be then
increased slowly and gradually (i.e. 2°C/day) to reach the designed temperature (i.e. 55°C).
Continuous operation mode
- After start-up, the continuous feeding of reactor should be started with a low OLR. Then
the OLR should be progressively increased based on system’s capacity to reach the
designed OLR (6.5 kg VS/m 3 .d).
- Feeding and digested residue withdrawal will be performed once a day. Based on results
of pilot experiments, the chosen retention time will be 24 days. The feedstock will be
stored in feed storage tank for few days (less than a week) before feeding, because
necessary pretreatment processing (shredding and mixing with waste paper) could be
performed in the preceding week for smooth and un-interrupted running of the plant in the
- Recirculation rate of 2.5 Ldig/Lreactor vol./d will be maintained during continuous operation
of the reactor as used in pilot experiments (please refer to Table 3.3 and Figure 3.5).
- The pH, ammonia, VFA and alkalinity of the liquid extract of digestate and biogas
composition, should be regularly measured to prevent reactor upset and/or to cure it by
methods like decreasing the organic loads or treating with alkali.
4.5.4 Generation of methane and energy
-Methane yield of 96 m 3 CH4/d (or 200 m 3 biogas/d) will be generated by the plant.
-About 33% of the produced biogas from the proposed decentralized system will be used to
provide energy for running of the plant itself, whereas remaining 67% will be surplus
energy (Appendix B) that can be provided to the community.
-Conversion of biogas to electricity is an environmental friendly and clean process. The
system will be provided with immediate conversion facility (Gas engine) of biogas into
electricity. Moreover, biogas storage tank (100 m 3 ) will also be installed for temporary
storage of biogas.
-The amount of power produced from this system will be 14 kW (Appendix B).
4.5.5 Digestate management
Table 4.8 Technical Data of Sand Drying Bed for Digestate Dewatering
Dimensions of sand drying beds (m)
Thickness of bed layers (cm)
Type of sand drying bed Conventional and open
Number of sand drying bed
Layer of digestate over bed 20 cm
Figure 4.20 Layout of conceptual decentralized AD plant for a community
- Storage of digestate will be minimized as much as possible to avoid the loss of carbon
and nutrients as observed from the results of this thesis (please refer to sections 4.4.2(i) and
- Dewatering of digestate will be performed by use of simple sand drying beds, so that its
weight is reduced and it can be transported and managed easily for land application. The
technical details about the design and construction of drying bed for the proposed system
have been summarized in the Table 4.8. For detailed calculations about design of bed, and
weight reduction due to dewatering, please see Appendix B.
-The dewatered solid digestate has coarse cracked surface and dark brown in color. If C/N
ratio of solid digestate is within safe range (< 20), the digestate will be directly applied to
agricultural land, as it stops methane formation and nutrients are utilized by plants. If C/N
ratio of solid digestate is > 20, it should be further treated and then it can be utilized as soil
4.5.6 Reduction of GHG emissions
Managing waste sector is one option for mitigation of global effects of GHGs. By
construction and proper operation of this decentralized AD system, methane emission to
atmosphere will reduce by 280,000 L of CH4 per day which is equal to 5 ton CO2-eq/d
(Appendix B). Thus there is significant effect of introduction of this system in terms of
reduction of GHG emissions.
4.5.7 Material flow (VS balance)
Typical mass balance for volatile solids of the proposed system is given in Figure 4.21. It
is based on the results of run 2 of phase II pilot experiment. It describes that the conversion
efficiency obtained will be up to 80% of VS added to the system. The daily amounts of
feedstock to be fed and residues to be withdrawn are generalized in this figure.
Figure 4.21 Conceptual mass balance for VS of the proposed decentralized system
Conclusions and Recommendations
This study investigated the optimization of thermophilic dry anaerobic digestion of
OFMSW either by testing different feed C/N ratio or by testing different organic loading
rates. Two separate pilot-scale experiments were conducted for this purpose. In the first
experiment, the effect two feed compositions (with C/N ratio 27 and 32) and its associated
ammonia-N accumulation was studied on performance of dry anaerobic digestion. In the
second experiment, effect of different organic loading rates (4.5 -8.5 kg VS/m 3 /d) on
stability and performance parameters of dry anaerobic digestion was studied. In addition,
digestate characteristics were analyzed continuously for both experiments. Various
digestate management options were also analyzed from the perspective of GHG emissions.
The conclusions drawn from three phases of the study have been presented in the following
Conclusions of phase I (Dry digestion optimization by testing different feed C/N ratio)
1. The system accumulated 30% less ammonia-N by the use of feed having C/N ratio 32
as compared to 27. Ammonia nitrogen accumulation in dry anaerobic digestion can be
thus mitigated by use of correct feed mixture (i.e. by adjusting composition or C/N
ratio of the feed).
2. The ITDAR performed well with the various OLRs and produced 200-300 L of
CH4/kg VS. However, it was observed that the free ammonia concentration of 400-660
mg/L adversely inhibited the steady state methane production in ITDAR.
3. Adverse effects of ammonia inhibition reduced with the practice of altering the C/N
ratio, higher OLR and different recirculation rates, which have been proved
advantageous and suggested to overcome the ammonia-N inhibition in ITDAR. The
alterations made during this study are easy to manage in a decentralized system and
thus can help in better process control.
4. Moreover, ITDAR can produce surplus net energy in the range of 50-73%. Therefore,
the system can be effectively implemented at a decentralized level to recover the net
energy from OFMSW.
Conclusions of phase II (Dry digestion optimization by testing different OLRs)
1. Comparison of start-up phases of the two experiments shows that low digestate
recirculation rate (Dig rr) should be used during start-up of the reactor to achieve
reactor stability conditions in less time. Low Digrr (1 Ldig/Lreactor vol./d) in first
experiment achieved stable reactor conditions in shorter time as compared to high
Digrr (2.4 Ldig/Lreactor vol./d) of second experiment.
2. Gas production rate increases linearly with organic loading rate, provided that proper
acclimatization is performed for every increased OLR.
3. The OLR 4.55 kg VS/m 3 /d achieved higher methane yield per kg VS added than other
runs. However, OLR of 6.4 kg VS/m 3 /d also achieved comparable methane yield at a
lesser RT (24 days).
4. Similarly, VS removal was the highest (77.9%) by OLR of 4.55 kg VS/m 3 /d whereas
OLR of 6.4 kg VS/m 3 /d also achieved a good VS removal (i.e. 75.5%).
5. Based on these results, the best conditions with reasonable methane yield and VS
removal for thermophillic dry anaerobic conditions were OLR 6.4 kg VS/m 3 /d with
RT of 24 days. As the methane yield and VS removal decreased with increase in
OLR, therefore, if the objective is partial digestion or dual treatment of waste
(anaerobic + aerobic), then the condition with the lowest RT (i.e. 18 days) and highest
OLR should be preferred.
Conclusions of phase III (Digestate management and GHG emission reduction)
1. Control of total solids content of the reactor helped in smooth operation of the reactor.
Moreover, by comparing the characteristics of digestate from two experiments, it can
be concluded that by increasing TS in feed, the VS/TS ratio of digestate increases, and
hence the removal of solids decreases in dry anaerobic digestion.
2. The digestate didn’t require aerobic treatment (or composting), because the C/N ratio
of raw digestate was optimum for its land application during most of the study period.
3. It can be concluded that storage time of raw digestate from anaerobic reactor should be
reduced to avoid nutrient and carbon loss.
4. Results of the GHG emissions from digestate show that raw digestate has the
maximum GHG emission potential and should not be dumped or landfilled as its
dumping contributes the most to GHG emissions.
5. The study reveals that land application of digestate is very good digestate management
strategy as 93 to more than 100% GHG emissions from digestate can be avoided by its
land application. However, C/N ratio of digestate and time of application to land need
to be considered. Thus the scenarios which include land application are better than the
others whereas base scenario is the worst.
6. Avoid storage of digestate after digestion or otherwise, storage time should be
minimized as much as possible. Results from this study showed that the digestate
emitted 10% of its total GHG potential in 2 months of storage.
7. The results of this study can help the anaerobic digester operators to manage the
digestate efficiently from perspective of GHG emission reduction. It is important to
note that the results are based on digestate characteristics from our pilot-scale
anaerobic reactor. However, these may be different from other studies based on
different digestate characteristics which depend on type of AD substrate and type of
Following are the recommendations for future studies based on the extensive experimental
work of this dissertation:
1. More combinations of wastes consisting of other sources of substrates should be
evaluated for mitigation of ammonia accumulation. For example, feedstock with
high C/N ratio (e.g. straw, crop residues, saw dust, industrial by-products like
distiller grains from cassava ethanol production, waste paper, etc.) should be mixed
with low C/N ratio materials (e.g. kitchen waste, vegetable waste, manures, etc).
2. Combination of only two types of waste (for example, (i) food waste + waste paper,
or (ii) manure + paper ) should also be investigated as it will be helpful in
management of waste supply and easy feed preparation as compared to using 4-5
types of waste.
3. Effect of recirculation rate on both the start-up and continuous operation phases of
dry anaerobic digestion should be further studied. From this study, it was found that
low digestate recirculation rate should be used during start-up of the reactor. It will
help to achieve reactor stability conditions (linked to VFA accumulation and falling
pH) in less time in dry anaerobic digestion. However, more research in this area
should be performed.
4. Reactor design modification in a way to avoid the use of pump for mixing purpose
should also be investigated, because beyond certain limit of TS, the pump gets
blocked and reactor material can not be pumped. Moreover, the modified design to
be studied should provide alternatives means of mixing.
5. Effect of feed total solids percentage on performance of thermophilic dry anaerobic
digestion needs to be studied. In our study, TS for the two pilot experiments was
different that affected the digestion performance. However, this was not a focus
point of our research. Moreover, most of the research already performed on this
issue deals with dry digestion at mesophilic temperature.
6. Based on the findings of this thesis, it is recommended that the feedstock C/N ratio
32 is helpful to mitigate ammonia accumulation problem in dry anaerobic
digestion. Thus the recommended material from our experimental results is that 5-
7% of total waste should consist of waste paper, depending on the C/N ratio of
main portion of feedstock received. This C/N ratio should be used as a base for
future studies dealing with mitigation of ammonia accumulation.
7. Optimum operating conditions for dry anaerobic digestion also depend on the
purpose of treatment. In case of dual treatment (anaerobic + aerobic), time spent for
anaerobic digestion should be minimized, so that the saved time could be used for
aerobic treatment. Thus low retention time (e.g. 15 days) with high OLR (e.g. 8.5-
10 kg VS/m 3 .d) should be used for digestion step. But if the purpose is only a single
treatment (i.e. dry anaerobic digestion), then option of higher retention time (RT 24
d or 30 d) should be chosen to get maximum VS removal and its conversion to
8. Although the process control is easy in a decentralized system by judiciously
altering the operating conditions (like OLR, C/N ratio, etc.), but the change should
be performed very carefully and gradually to avoid its effect on performance of
biological system. Future study should also investigate the difference in reactor
performance by sudden and gradual change in feed C/N ratio.
Abdullahi, Y. A., Akunna, J. C., White, N. A., Hallett, P. D. and Wheatley, R. (2008).
Investigating the effects of anaerobic and aerobic post-treatment on quality and
stability of organic fraction of municipal solid waste as soil amendment. Bioresource
Technology, 99 (18), 8631-8636.
Al Seadi, T., Nielsen, J.B.H., Lindberg, A. and Wheeler, P. (2001). Good practice in
quality management of ad residues from biogas production, IEA Bioenergy,
Retrieved on July 18, 2008 from the web. http://www.ieabiogas.net/Dokumente/managementpaw
Alkaya, E. and Demirer, G.N. (2011). Anaerobic mesophilic co-digestion of sugar beet
processing wastewater and beet pulp in batch reactors. Renewable Energy, 36 (3),
Angeladaki, I. and Sanders, W. (2004). Assessment of the anaerobic biodegradability of
macropollutants. Environmental Science and Bio/Technology, 3, 117-129.
Angelidaki, I., Chen, X., Cui, J., Kaparaju, P. and Ellegaard, L. (2006 a). Thermophilic
anaerobic digestion of source-sorted organic fraction of household municipal solid
waste: Start-up procedure for continuously stirred tank reactor. Water Research, 40
Angelidaki, I., Cui, J., Chen, X., and Kaparaju, P. (2006 b). Operational strategies for
thermophilic anaerobic digestion of organic fraction of municipal solid waste in
continously stirred tank reactors. Environmental Technology, 27 (8), 855-861.
APHA, AWWA, WEF, (2005). Standard methods for the examination of water and
wastewater, 21st Edition, Washington, USA.
Appels, L., Baeyens, J., Degrève, J. and Dewil, R. (2008). Principles and potential of the
anaerobic digestion of waste-activated sludge. Progress in Energy and Combustion
Science, 34 (6), 755-781.
Bauer, A., Mayr, H., Hopfner-Sixt, K. and Amon, T. (2009). Detailed monitoring of two
biogas plants and mechanical solid-liquid separation of fermentation
residues. Journal of Biotechnology, 142 (1), 56-63.
Berglund, M., (2006). Biogas production from a systems analytical perspective. Doctoral
Dissertation, Environmental and Energy System Studies, Lund University, Lund.
Binner, E., Tintner, J., Meissl, K., Smidt, E. and Lechner, P. (2008). Humic acids – A
quality criterion for composts, In Proceedings of the international congress CODIS
2008, Compost and digestate: sustainability, benefits, and impacts for the
environment and for plant production, Solothurn, Switzerland.
Bogner, J., Ahmed, M.A., Diaz, C., Faaij, A., Gao, Q., Hashimoto, S., et al. (2007). Waste
Management, In B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds.),
Climate Change 2007: Mitigation. (pp. 585-618). Contribution of Working Group III
to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change,
Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3 chapter 10.pdf
(Accessed on February 7, 2008).
Bogner, J. and Spokas, K. (1993). Landfill CH4: rates, fates and role in global carbon
cycle. Chemosphere, 26, 369-386.
Bolzonella, D., Innocenti, L., Pavan, P., Traverso, P., and Cecchi, F. (2003). Semi -dry
thermophilic anaerobic digestion of the organic fraction of municipal solid waste:
Focusing on the start-up phase. Bioresource Technology, 86 (2), 123-129.
Borja, R., Sánchez, E., and Weiland, P., (1996). Influence of ammonia concentration on
thermophilic anaerobic digestion of cattle manure in upflow anaerobic sludge blanket
(UASB) reactors. Process Biochemistry, 31 (5), 477–483.
Bouallagui, H., Haouari, O., Touhami, Y., Ben Cheikh, R., Marouani, L. and Hamdi, M.
(2004). Effect of temperature on the performance of an anaerobic tubular reactor
treating fruit and vegetable waste. Process Biochemistry, 39 (12), 2143-2148.
Bouallagui, H., Houhami, Y., Cheikh, R.B., and Hamdi, M. (2005). Biorector performance
in anaerobic digestion of fruit and vegetable waste. Process Biochemistry, 40, 989-
Bouallagui, H., Lahdheb, H., Ben Romdan, E., Rachdi, B. and Hamdi, M. (2009).
Improvement of fruit and vegetable waste anaerobic digestion performance and
stability with co-substrates addition. Journal of Environmental Management, 90 (5),
Brändli, R.C., Bucheli, T.D., Kupper, T., Zennegg, M., Huber, S., Müller, J. et al. (2008).
Input of organic pollutants to soil by compost and digestate application and their
origin, In Proceedings of the international congress CODIS 2008, Compost and
digestate: sustainability, benefits, and impacts for the environment and for plant
production, Solothurn, Switzerland.
Brown, S., Beecher, N. and Carpenter, A. (2010). Calculator tool for determining
greenhouse gas emissions for biosolids processing and end use. Environmental
Science and Technology, 44 (24), 9509-9515.
Bujoczek, G., Oleszkiewicz, J., Sparling, R., and Cenkowski, S. (2000). High solid
anaerobic digestion of chicken manure. Journal of Agricultural Engineering
Research, 76 (1), 51-60.
Buyukkamaci, N. and Filibeli, A. (2004). Volatile fatty acid formation in an anaerobic
hybrid reactor. Process Biochemistry, 39, 11, 1491-1494.
Cecchi, F., Pavan, P., Alvarez, J. M., Bassetti, A., and Cozzolino, C. (1991). Anaerobic
digestion of municipal solid waste: Thermophilic vs. mesophilic performance at high
solids. Waste Management and Research, 9 (4), 305-314.
Cengel, Y.A. (2003). Heat Transfer A Practical Approach. 2 nd Edition. McGraw-Hill
Companies, Inc. ISBN: 0-07-115150-8. (pp. 134-149).
Chavez-Vazquez1, M. and Bagley, D.M. (2002). Evaluation of the performance of
different anaerobic digestion technologies for solid waste treatment. In ASCE
Environmental Engineering Conference, Niagra.
Chen, X., Romano, R. T., and Zhang, R. (2010). Anaerobic digestion of food wastes for
biogas production. International Journal of Agricultural and Biological
Engineering, 3 (4), 61-72.
Chen, Y., Cheng, J. J. and Creamer, K. S. (2008). Inhibition of anaerobic digestion
process: A review. Bioresource Technology, 99 (10), 4044-4064.
Cho, J. K., Park, S. C., and Chang, H. N. (1995). Biochemical methane potential and solid
state anaerobic digestion of korean food wastes. Bioresource Technology, 52 (3),
Cui, Z., Shi, J. and Li, Y. (2011). Solid-state anaerobic digestion of spent wheat straw from
horse stall. Bioresource Technology, 102 (20), 9432-9437.
De Baere, L. (2000). Anaerobic digestion of solid waste: state -of-the-art. Water Science
and Technology, 41 (3), 283-290.
De Baere, L. (2006). Will anaerobic digestion of solid waste survive in the future? Water
Science and Technology, 53 (8), 187-194.
De Gioannis, G., Diaz, L.F., Muntoni, A. and Pisanu, A. (2008). Two -phase anaerobic
digestion within a solid waste/wastewater integrated management system. Waste
Management, 28, 1801-8.
De La Rubia, M. A., Perez, M., Romero, L. I. and Sales, D. (2006). Ef fect of solids
retention time (SRT) on pilot scale anaerobic thermophilic sludge digestion. Process
Biochemistry, 41 (1), 79-86.
De La Rubia, M. T., Walker, M., Heaven, S., Banks, C. J. and Borja, R. (2010).
Preliminary trials of in situ ammonia stripping from source segregated domestic food
waste digestate using biogas: Effect of temperature and flow rate. Bioresource
Technology, 101 (24), 9486-9492.
Dong, L., Zhenhong, Y. and Yongming, S. (2010). Semi -dry mesophilic anaerobic
digestion of water sorted organic fraction of municipal solid waste (WS -
OFMSW). Bioresource Technology, 101 (8), 2722-2728.
Duan, N., Dong, B., Wu, B. and Dai, X. (2012). High-solid anaerobic digestion of sewage
sludge under mesophilic conditions: Feasibility study. Bioresource Technology, 104,
Elango, D., Pulikesi, M., Baskaralingam, P., Ramamurthi, V. and Sivanesan, S. (2007).
Production of biogas from municipal solid waste with domestic sewage. Journal of
Hazardous Materials, 141, 301-304.
Elert, G. (2003). Power consumption of a home. Retrieved September, 11 2012 from
El-Hadj, B., Astals, S., Galí, A., Mace, S. and Mata-Áivarez, J. (2009). Ammonia
influence in anaerobic digestion of OFMSW. Water Science and Technology, 59 (6) ,
Eliyan, C. (2007). Anaerobic digestion of municipal solid waste in thermophilic ontinuous
operation. (Masters Thesis No. EV-07-6, Asian Institute of Technology, 2007).
Pathumthani: Asian Institute of Technology.
Eskicioglu, C., Kennedy, K. J., Marin, J. and Strehler, B. (2011). Anaerobic digestion of
whole stillage from dry-grind corn ethanol plant under mesophilic and thermophilic
conditions. Bioresource Technology,102 (2), 1079-1086.
Evans, G. (2001). Biowaste and biological waste treatment. The Cromwell press. ISBN: 1-
Fdéz.-Güelfo, L. A., Álvarez-Gallego, C., Sales Márquez, D. and Romero García, L. I.
(2010). Start-up of thermophilic-dry anaerobic digestion of OFMSW using adapted
modified SEBAC inoculum. Bioresource Technology, 101 (23), 9031-9039.
Fernández, J., Pérez, M. and Romero, L. I. (2008). Effect of substrate concentration on dry
mesophilic anaerobic digestion of organic fraction of municipal solid waste
(OFMSW). Bioresource Technology, 99 (14), 6075-6080.
Fernández Rodríguez, J., Pérez, M. and Romero, L. I. (2012). Mesophilic anaerobic
digestion of the organic fraction of municipal solid waste: Optimisation of the
semicontinuous process. Chemical Engineering Journal, 193-194, 10-15.
Forster-Carneiro, T. (2009). Anaerobic digestion or biomethanization: dry and
thermophilic conditions. SciTopics. Retrieved February 27, 2012, from
Forster-Carneiro, T., Pérez, M. and Romero, L. I. (2008). Anaerobic digestion of municipal
solid wastes: Dry thermophilic performance. Bioresource Technology, 99 (17), 8180-
Forster-Carneiro, T., Pérez, M. and Romero, L. I. (2008). Influence of total solid and
inoculum contents on performance of anaerobic reactors treating food
waste. Bioresource Technology, 99 (15), 6994-7002.
Forster-Carneiro, T., Pérez, M. and Romero, L.I. (2008). Thermophilic anaerobic digestion
of source-sorted organic fraction of municipal solid waste. Bioresource Technology,
Forster-Carneiro, T., Pérez, M., Romero, L. I. and Sales, D. (2007). Dry -thermophilic
anaerobic digestion of organic fraction of the municipal solid waste: Focusing on the
inoculum sources. Bioresource Technology, 98 (17), 3195-3203.
Fricke, K., Santen, H., Wallmann, R., Huttner, A., and Dichtl, N. (2007). Operating
problems in anaerobic digestion plants from nitrogen in MSW. Waste Management,
Fuchs, J.G., Baier, U., Berner, A., Mayer, J. and Schleiss, K. (2008). Effects of digestate
on the environment and on plant production - results of a research project, In
Proceedings of ECN/ORBIT e.V. Workshop 2008 The future for Anaerobic Digestion
of Organic Waste in Europe, Weimar, Germany.
Gallert, C. and Winter, J., (1997). Mesophilic and thermophilic anaerobic digestion of
source-sorted organic wastes: effect of ammonia on glucose degradation and methane
production. Applied Microbiology and Biotechnology, 48, 405–410.
Gautam, S.P., Bundela, P.S., Pandey, A.K. Awasthi, M.K. and Sarsaiya, S. (2010).
Composting of municipal solid waste of Jabalpur city. Global Journal of
Environmental Research, 4 (1), 43-46.
Gioelli, F., Dinuccio, E. and Balsari, P. (2011). Residual biogas potential from the storage
tanks of non-separated digestate and digested liquid fraction, Bioresource
Technology, 102 (22), 10248-10251.
Gomez, X., Cuetos, M.J., Garcıa, A.I. and Moran, A. (2007). An evaluation of stability by
thermogravimetric analysis of digestate obtained from different biowastes, Journal of
Hazardous Materials, 149, 97–105.
Guendouz, J., Buffière, P., Cacho, J., Carrère, M. and Delgenes, J. (2010). Dry anaerobic
digestion in batch mode: Design and operation of a laboratory-scale, completely
mixed reactor. Waste Management, 30 (10), 1768-1771.
Hamzawi, N., Kennedy, K.J. and McLean D.D. (1999). Review of applications of highsolids
anaerobic digestion to solid waste management, Journal of Solid Waste
Technology and Management, 26 (3), 119–132.
Hansen, K.H., Angelidaki, I. and Ahring, B.K. (1998). Anaerobic digestion of swine
manure inhibition by ammonia. Water Research, 32 (1), 5-12.
Hartmann, H. and Ahring, B.K. (2005). A novel process configuration for anaerobic
digestion of source-sorted household waste using hyper-thermophilic posttreatment.
Biotechnology and Bioenergy, 90, 830–837.
Hartmann, H. and Ahring, B.K. (2006). Strategies for the anaerobic digestion of the
organic fraction of municipal solid waste: an overview. Water Science and
Technology 53, 7–22.
Heerenklage, J. and Stegmann, R. (2005). Analytical methods for the determination of the
biological stability of waste samples, In Proceedings Sardinia 2005, Tenth
International Waste Management and Landfill Symposium S. Margherita di Pula,
Cagliari, Italy; 3-7 October, 2005.
Hoornweg, D. and Bhada-Tata, P. (2012). What a waste: a global review of solid waste
management. Urban development series, knowledge papers no. 15. The Worldbank,
Washington D.C. United States. Retrieved August 16, 2012 from
Hu Hu, Z. and Yu, H.Q. (2006). Anaerobic digestion of cattail by rumen cultures. Waste
Management, 26, 1222-1228.
Hyaric, R., Benbelkacem, H., Bollon, J., Bayard, R., Escudié, R. and Buffière, P. (2011).
Influence of moisture content on the specific methanogenic activity of dry mesophilic
municipal solid waste digestate.Journal of Chemical Technology and Biotechnology,
In Press, DOI: 10.1002/jctb.2722.
IGES (Institute for Global Environmental Strategies), (2011). Practical guide for improved
organic waste management: Climate benefits through the 3Rs in developing Asian
countries, IGES, Kanagawa, Japan, ISBN: 978-4-88788-077-1.
IPCC ( Intergovernmental Panel on Climate Change ), (2011). Climate Change 2007:
Working Group I: The Physical Science Basis, Last Retrieved May 7, 2012 from
Jensen, L.S., Luxhøi, J., Magid, J., Birkmose, T. and Jørgensen, K. (2008). Utilization of
separated animal manure bio-solids – opportunities for improved nutrient cycling and
reduced environmental impact, with Denmark as an example, In Proceedings of the
international congress CODIS 2008, Compost and digestate: sustainability, benefits,
and impacts for the environment and for plant production, Solothurn, Switzerland.
Jiang, J., Wu, S., Sui, J. and Wang, Y. (2008). Research on single phase high solid
anaerobic digestion of organic fraction of municipal solid wastes. Huanjing
Kexue/Environmental Science, 29 (4), 1104-1108.
Jiunn-Jyi, L., Yu-You, L. and Noike, T. (1997). Influences of pH and moisture content on
the methane production in high-solids sludge digestion. Water Research, 31 (6),
Jokela, J.P.Y., Rintala, J.A (2003). Anaerobic solubilisation of nitrogen from municipal
solid waste (MSW). Reviews in Environmental Science and Bio/Technology, 2, 67-
Jørgensen, K. and Jensen, L.S. (2008). Quality characterization of separated animal
manure bio-solids - key parameters important for composting and nutrient
availability, In Proceedings of the international congress CODIS 2008, Compost and
digestate: sustainability, benefits, and impacts for the environment and for plant
production, Solothurn, Switzerland.
Kaparaju, P., Buendia, I., Ellegaard, L. and Angelidaki, I. (2008). Effects of mixing on
methane production during thermophilic anaerobic digestion of manure: Lab-scale
and pilot-scale studies. Bioresource Technology, 99 (11), 4919-4928.
Karim, K., Hoffmann, R., Klasson, K. T. and Al-Dahhan, M. H. (2005a ). Anaerobic
digestion of animal waste: Effect of mode of mixing. Water Research, 39 (15), 3597-
Karim, K., Hoffmann, R., Klasson, K. T. and Al-Dahhan, M. H. (2005b ). Anaerobic
digestion of animal waste: Waste strength versus impact of mixing. Bioresource
Technology, 96 (16), 1771-1781.
Kayhanian, M. (1999). Ammonia inhibition in high-solids biogasification: an overview and
practical solutions. Environmental Technology, 20, 355–365.
Kayhanian, M. and Rich, D. (1995). Pilot -scale high solids thermophilic anaerobic
digestion of municipal solid waste with an emphasis on nutrient
requirements. Biomass and Bioenergy, 8 (6), 433-444.
Kayhanian, M. and Tchobanoglous, G. (1993). Innovative two-stage process for the
recovery of energy and compost from the organic fraction of municipal solid waste
(MSW). Water Science and Technology, 27 (2), 133-143.
Khanal, S.K. (2008). Anaerobic biotechnology for bioenergy production: principles and
applications. Blackwell Pb. Co. ISBN: 978-0-813-82346-1.
Kim, D. and Oh, S. (2011). Continuous high-solids anaerobic co-digestion of organic solid
wastes under mesophilic conditions. Waste Management, 31 (9-10), 1943-1948.
Kim, M. and Speece, R.E. (2002). Reactor configuration -Part II comparative process
stability and efficiency of thermophilic anaerobic digestion. Environmental
Technology, 23, 643-654.
Kördel, W. and Herrchen, M. (2008). Organic pollutants in secondary fertilizers, In
Proceedings of the international congress CODIS 2008, Compost and digestate:
sustainability, benefits, and impacts for the environment and for plant production,
Koster, I. W. and Lettinga, G. (1988). Anaerobic digestion at extreme ammonia
concentrations. Biological Wastes, 25 (1), 51-59.
Kumar, S., Gaikwad, S.A., Shekdar, A.V., Kshirsagar, P.S. and Singh, R.N (2004).
Estimation method for national methane emission from solid waste landfills,
Atmospheric Environment, 38 (21), 3481-3487.
Kupper, T., Brändli, R.C., Bucheli, T.D., Stämpfli, C., Zennegg, M., Berger, U., et al.
(2008). Organic pollutants in compost and digestate: occurrence, fate and impacts, In
Proceedings of the international congress CODIS 2008, Compost and digestate:
sustainability, benefits, and impacts for the environment and for plant production,
Kusch, S., Oechsner, H. and Jungbluth, T. (2008). Biogas production with horse dung in
solid-phase digestion systems. Bioresource Technology, 99 (5), 1280-1292.
Kymäläinen, M., Lähde, K., Arnold, M., Kurola, J. M., Romantschuk, M. and Kautola, H.
(2012). Biogasification of biowaste and sewage sludge - measurement of biogas
quality. Journal of Environmental Management, 95(SUPPL.), S122-S127.
Lahav, O. and Morgan, B. (2004). Titration methodologies for monitoring of anaerobic
digestion in developing countries - A review. Journal of Chemical Technology and
Biotechnology, 79, 1331-1341.
Lantz, M., Svensson, M., Björnsson, L. and Börjesson, P. (2007). The prospects for an
expansion of biogas systems in sweden - incentives, barriers and potentials. Energy
Policy, 35 (3), 1819-1829.
Li, D., Sun, Y., Guo, Y., Yuan, Z., Ma, L. and Kong, X. (2010). Methane production for
three total solids anaerobic digestion of water sorted organic fraction of municipal
solid waste. Taiyangneng Xuebao/Acta Energiae Solaris Sinica, 31 (11), 1391-1396.
Li, H. and Wang, Y. (2011). Influence of total solid and stirring frequency on performance
of dry anaerobic digestion treating cattle manure, Applied Mechanics and Materials,
Li, J., Jha, A. K., He, J., Ban, Q., Chang, S. and Wang, P. (2011). Assessment of the effects
of dry anaerobic codigestion of cow dung with waste water sludge on biogas yield
and biodegradability.International Journal of Physical Sciences, 6 (15), 3679-3688.
Li, Y., Park, S. Y. and Zhu, J. (2011). Solid -state anaerobic digestion for methane
production from organic waste. Renewable and Sustainable Energy Reviews, 15 (1),
Li, Y. Y., Sasaki, H., Yamashita, K., Saki, K. and Kamigochi, I. (2002). High-rate methane
fermentation of lipid-rich food wastes by a high-solids co-digestion process. Water
Science and Technology, 45 (12), 143-150.
Liew, L. N., Shi, J. and Li, Y. (2011). Enhancing the solid -state anaerobic digestion of
fallen leaves through simultaneous alkaline treatment. Bioresource Technology, 102
Lin, L., Yuan, S., Chen, J., Xu, Z. and Lu, X. (2009). Removal of ammonia nitrogen in
wastewater by microwave radiation. Journal of Hazardous Materials, 161, 1063–
Lissens, G., Vandevivere, P., De Baere, L., Biey, E.M. and Verstraete, W. (2001). Solid
waste digesters: process performance and practice for municipal solid waste
digestion. Water Science and Technology, 44 (8), 91-102.
Liu, C., Yuan, X., Zeng, G., Li, W. and Li, J. (2008). Prediction of methane yield at
optimum pH for anaerobic digestion of organic fraction of municipal solid waste.
Bioresource Technology, 99 (4), 882-888.
Liu, G., Zhang, R., El-Mashad, H. M., Dong, R. and Liu, X. (2011). Biogasification of
green and food wastes using anaerobic-phased solids digester system. Applied
Biochemistry and Biotechnology, 1-13.
Lopes, W.S., Leite, V. D. and Prasad, S. (2004). Influence of inoculum on performance of
anaerobic reactors for treating municipal solid waste. Bioresource Technology, 94,
Lu, S., Imai, T., Ukita, M. and Sekine, M. (2007). Start-up performances of dry anaerobic
mesophilic and thermophilic digestions of organic solid wastes. Journal of
Environmental Sciences, 19 (4), 416-420.
Luning, L., Van Zundert, E.H.M. and Brinkmann, A.J.F. (2003). Comparison of dry and
wet digestion for solid waste. Water science and technology, 48 (4), 15-20.
Lv, W., Schanbacher, F. L. and Yu, Z. (2010). Putting microbes to work in sequence:
Recent advances in temperature-phased anaerobic digestion processes. Bioresource
Technology, 101 (24), 9409-9414.
Mata-Alvarez, J. (2003). Biomethanization of the Organic Fraction of Municipal Solid
Waste. IWA Publishing. ISBN: 1-900222-14-0.
Mattheeuws, B. (2011). State of the art of anaerobic digestion of municipal solid waste in
Europe, In International Conference on Solid Waste 2011 Moving Toward
Sustainable Resource Management, 2-6 May 2011, Hong Kong.
Mayer, M. (2008). Cost effective solutions with partial stream digestion, In Proceedings of
ECN/ORBIT e.V. Workshop 2008, The future for Anaerobic Digestion of Organic
Waste in Europe, Weimar, Germany.
Menardo, S., Gioelli, F. and Balsari, P. (2011). The methane yield of digestate: Effect of
organic loading rate, hydraulic retention time, and plant feeding, Bioresource
Technology, 102 (3), 2348-2351.
Mills, P. J. (1979). Minimisation of energy input requirements of an anaerobic
digester. Agricultural Wastes, 1 (1), 57-66.
Misi, S.N. and Forster, C.F. (2002). Semi-continuous anaerobic co-digestion of agro
wastes. Environmental Technology, 23, 445-451.
Mokry, M., Bolduan, R., Michels, K. and. Wagner, W. (2008). Fertilization of arable crops
with digestates of agricultural biogas plants, In Proceedings of the international
congress CODIS 2008, Solothurn, Switzerland.
Møller, H.B. Sommer, S.G. and Ahring, B.K. (2002). Separation efficiency and particle
size distribution in relation to manure type and storage conditions, Bioresource
Technology, 85, 189-196.
Moller, H.B., Lund, I. and Sommer, S.G. (2000). Solid -liquid separation of livestock
slurry: efficiency and cost, Bioresource Technology, 74, 223-229.
Møller, J., Boldrin, A. and Christensen, T. H. (2009). Anaerobic digestion and digestate
use: Accounting of greenhouse gases and global warming contribution. Waste
Management and Research, 27 (8), 813-824.
Montero, B., Garcia-Morales, J. L., Sales, D. and Solera, R. (2008). Evolution of
microorganisms in thermophilic-dry anaerobic digestion. Bioresource
Technology, 99 (8), 3233-3243.
Montero, B., Garcia-Morales, J. L., Sales, D. and Solera, R. (2010). Evolution of butyric
acid and the methanogenic microbial population in a thermophilic dry anaerobic
reactor. Waste Management, 30 (10), 1790-1797.
Mumme, J., Linke, B. and Tölle, R. (2010). Novel upflow anaerobic solid -state (UASS)
reactor. Bioresource Technology, 101 (2), 592-599.
Nakakubo, R., Møller, H.B., Nielsen, A.M. and Matsuda, J. (2008). Ammonia inhibition of
methanogenesis and identification of process indicators during anaerobic digestion.
Environmental Engineering Science, 25, 1487-1496.
Nayono, S. E. (2010). Anaerobic digestion of organic solid waste for energy production.
PhD Thesis. KIT Sci. Pb. Co. ISBN: 978-3-86644-464-5.
Nizami, A. S., Orozco, A., Groom, E., Dieterich, B. and Murphy, J. D. (2012). How much
gas can we get from grass? Applied Energy, 92, 783-790.
O’Keefe, D.M., and Chynoweth, D.P. (2000). Influence of phase separation, leachate
recycle and aeration o treatment of municipal solid waste in simulated landfill cells.
Bioresource Technology, 72, 55-66.
Oleszkiewicz, J.A. and Poggi-Varaldo, H.M. (1997). High -solids anaerobic digestion of
mixed municipal and industrial waste. Journal of Environmental Engineering, 123
Olsen, S.R. and Sommer, L.E. (1982). Phosphorus, In: A.L. Page, R.H. Miller, D.R.
Keeney (Eds.), Methods of Soil Analysis Part 2: Chemical and microbiological
properties, second ed., American Society of Agronomy & Soil Science Society of
America, Wisconsin, US, (pp. 403-430). ISBN 0-89118-072-9.
Ostrem, K. (2004). Greening waste: anaerobic digestion for treating the organic fraction of
municipal solid wastes. Retrieved November 1, 2008 from http://www.seas.col
Paavola, T. and Rintala, J. (2008). Effects of storage on characteristics and hygienic quality
of digestates from four co-digestion concepts of manure and biowaste Bioresource
Technology, 99, 7041-7050.
Palm, O. (2008). The quality of liquid and solid digestate from biogas plants and its
application in agriculture, In Proceedings of ECN/ORBIT e.V. Workshop 2008, The
future for Anaerobic Digestion of Organic Waste in Europe, Weimar, Germany.
Pavan, P., Battistoni, P., Mata-Alvarez, J. and Cecchi, F. (2000). Performance of
thermophilic semi-dry anaerobic digestion process changing the feed
biodegradability, Water Science and Technology, 41 (3), 75-81.
Persson, P. (2008). The practice of integrated anaerobic digestion concepts including waste
management, agriculture and energy production, In Proceedings of ECN/ORBIT e.V.
Workshop 2008, The future for Anaerobic Digestion of Organic Waste in Europe,
Poggi-Varaldo, H. M. and Oleszkiewicz, J. (1992). Anaerobic co-composting of municipal
solid waste and waste sludge at high total solids levels. Environmental Technology,
Poggi-Varaldo, H.M., Rodríguez-Vázquez, R., Fernández-Villagóme, G. and Esparza-
García, F. (1997). Inhibition of mesophilic solid-substrate anaerobic digestion by
ammonia nitrogen. Applied Microbiology and Biotechnology, 47, 284–291.
Polprasert, C. (2007). Organic waste recycling technology and management. 3 rd Ed. IWA
Pb. Co. ISBN: 184339121X.
Prakash, G., Bounthavy, S. and Bastiaan, T. (2012). Reader. In Design and Implementation
of Domestic Biogas Programmes, 6 th to 10 th August, Energy Field of Study, AIT,
Rao, A. G., Surya Prakash, S., Joseph, J., Rajashekhara Reddy, A. and Sarma, P. N.
(2011). Multi stage high rate biomethanation of poultry litter with self mixed
anaerobic digester. Bioresource Technology, 102 (2), 729-735.
Rao, M. S. and Singh, S. P. (2004). Bioenergy conversion studies of organic fraction of
MSW: Kinetic studies and gas yield-organic loading relationships for process
optimisation. Bioresource Technology, 95 (2), 173-185.
Rico, C., Rico, J.L., Tejero, I., Muñoz, N. and Gómez, B. (2011). Anaerobic digestion of
the liquid fraction of dairy manure in pilot plant for biogas production: Residual
methane yield of digestate, Waste Management, 31 (9-10), 2167-2173.
Riedel, H. and Marb, C. (2008). Heavy me tals and organic contaminants in Bavarian
composts – an overview, In Proceedings of the international congress CODIS 2008,
Compost and digestate: sustainability, benefits, and impacts for the environment and
for plant production, Solothurn, Switzerland.
RISE-AT. (1998). Review of current status of anaerobic digestion technology for treatment
of municipal solid waste. Retrieved October 25, 2008. Chiang Mai University,
Institute of Science and Technology Research and Development. http://www.ist.cmu
Rivard, C. J., Himmel, M. E., Vinzant, T. B., Adney, W. S., Wyman, C. E. and Grohmann,
K. (1990). Anaerobic digestion of processed municipal solid waste using a novel high
solids reactor: Maximum solids levels and mixing requirements. Biotechnology
Letters, 12 (3), 235-240.
Ruggeri, B., Tommasi, T. and Sassi, G. (2010). Energy balance of dark anaerob ic
fermentation as a tool for sustainability analysis. International Journal of Hydrogen
Energy, 35, 10202-10211.
Sanchez, M., Gomez, X., Barriocanal, G., Cuetos, M.J. and Moran, A. (2008). Assessment
of the stability of livestock farm wastes treated by anaerobic digestion, International
Biodeterioration & Biodegradation, In Press, doi:10.1016/j.ibiod.2008.04.0
Schafer, W., Lehto, M. and Teye, F. (2006). Dry anaerobic digestion of organic residues
on-farm- a feasibility study, Agrifood Research Reports, 77, 98. Retrieved July 16,
2008 from www.mtt.fi/met/pdf/met77.pdf.
Sharma, V.K., Testa, C., Lastella, G., Cornacchia, G. and Comparto, M.P. (2000).
Inclined-plug-flow reactor for anaerobic digestion of semi-solid waste. Applied
Energy, 65, 173-185.
Siebert, S. (2008). Quality requirements and quality assurance of digestion residuals in
Germany, In Proceedings of ECN/ORBIT e.V. Workshop 2008, The future for
Anaerobic Digestion of Organic Waste in Europe, Weimar, Germany.
Siles, J.A, Brekelmans, J., Martín, M.A, Chica, A.F. and Martín, A. (2010). Impact of
ammonia and sulphate concentration on thermophilic anaerobic digestion.
Bioresource Technology, 101, 9040-9048.
Smith, R. J., Hein, M. E. and Greiner, T. H. (1979). Experimental methane production
from animal excreta in pilot-scale and farm-size units. Journal of Animal Science, 48
Stäb, J., Kuch, B., Rupp, S., Fischer, K., Kranert, M. and Metzger, J. W. (2008).
Determination of organic contaminants in compost and digestates in Baden-
Württemberg, South-West Germany, In Proceedings of the international congress
CODIS 2008, Compost and digestate: sustainability, benefits, and impacts for the
environment and for plant production, Solothurn, Switzerland.
Straka , F., Jenicek, P., Zabranska, J., Dohanyos, M. and Kuncarova, M. (2007). Anaerobic
fermentation of biomass and wastes with respect to sulfur and nitrogen contents in
treated materials. In Proceeding of 11 th International waste management and landfill
symposium, Cagliari, Italy, 1-5 October, 2007.
Stroot, P. G., McMahon, K. D., Mackie, R. I. and Raskin, L. (2001). Anaerobic codigestion
of municipal solid waste and biosolids under various mixing conditions-I. digester
performance. Water Research, 35 (7), 1804-1816.
Sung, S. and Liu, T. (2003). Ammonia inhibition on thermophilic anaerobic
digestion. Chemosphere, 53 (1), 43-52.
Surroop, D. and Mohee, R. (2012). Technical and economic assessment of power
generation from biogas. In 2012 International Conference on Environmental Science
and Technology, Singapore.
Suwannoppadol, S., Ho, G., and Cord-Ruwisch, R. (2011). Rapid start-up of thermophilic
anaerobic digestion with the turf fraction of MSW as inoculum. Bioresource
Technology, 102 (17), 7762-7767.
Svensson, K., Odlare, M. and Pell, M. (2004). The fertilizing effect of compost and biogas
residues from source separated household waste, The Journal of Agricultural
Science, 142, 461-467.
Svensson, L. M., Björnsson, L. and Mattiasson, B. (2006). Straw bed priming enhances the
methane yield and speeds up the start-up of single-stage, high-solids anaerobic
reactors treating plant biomass. Journal of Chemical Technology and
Biotechnology, 81 (11), 1729-1735.
Tambone, F., Genevini, P., D'Imporzano, G. and Adani, F. (2009). Assessing amendment
properties of digestate by studying the organic matter composition and the degree of
biological stability during the anaerobic digestion of the organic fraction of
MSW. Bioresource Technology, 100 (12), 3140-3142.
Tambone, F., Scaglia, B., D'Imporzano, G., Schievano, A., Orzi, V., Salati, S. and Adani,
F. (2010). Assessing amendment and fertilizing properties of digestates from
anaerobic digestion through a comparative study with digested sludge and
compost. Chemosphere, 81 (5), 577-583.
Tchobanoglous, G., Burton F.L. and Stensel H.D. (2003). Wastewater Engineering
Treatment and Reuse. Fourth edition, Tata McGraw-Hill, New Delhi. ISBN: 0-07-
Teater, C., Yue, Z., MacLellan, J., Liu, Y. and Liao, W. (2011). Assessing solid digestate
from anaerobic digestion as feedstock for ethanol production. Bioresource
Technology, 102 (2), 1856-1862.
Ten Brummeler, E. (2000). Full scale experience with the BIOCEL process. (2000). Water
Science and Technology, 41 (3), 299-304.
Tronheim (2005). Separation -optimal utilization of digestate, Nordic Bioenergy Project,
Retrieved July 20, 2008 from http://www.bioenergy2005.no/downloads/Present
asjoner / 26. %20oktober/1C/Lars%20Baadstorp.pdf.
Vandevivere, A., Baere, LD. and Verstraete, W. (1999). Types of anaerobic digesters for
solid wastes. Retrieved October 30, 2008 from http://www.ees.adelaide.edu.au/
Vermeulen, J., Huysmans, A., Crespo, M., Van Lierde, A., De Rycke, A. and Verstraete,
W. (1993). P rocessing of biowaste by anaerobic composting to plant growth
substrates, Water Science and Technology, 27 (2), 109-119.
Walker, L., Charles, W. and Cord-Ruwisch, R. (2009). Comparison of static, in-vessel
composting of MSW with thermophilic anaerobic digestion and combinations of the
two processes. Bioresource Technology, 100 (16), 3799-3807.
Walkley, A. and Black, I.A. (1934). An examination of Degtjareff method for determining
organic carbon in soils: effect of variations in digestion conditions and of inorganic
soil constituents. Soil Science, 63, 251-263.
Wang, J.Y., Xu, H-L., Zhang, H. and Tay, J-H. (2003). Semi -continuous anaerobic
digestion of food waste using a hybrid anaerobic solid-liquid bioreactor. Water
Science and Technology, 48 (4), 169-174.
Wang, Y., Shen, F., Liu, R. and Wu, L. (2008). Effects of anaerobic fermentation residue
of biogas production on the yield and quality of Chinese cabbage and nutrient
accumulations in soil, International Journal of Global Energy Issues, 29 (3), 284-
Ward, A. J., Hobbs, P. J., Holliman, P. J. and Jones, D. L. (2008). Optimisation of the
anaerobic digestion of agricultural resources. Bioresource Technology, 99 (17),
Weiske, A. (2005). Survey of technical and management based mitigation measures in
agriculture. Retrieved July 21, 2008 from http://www.ieep.eu/publications /pd
Wood, C.W. (2008). Fertilizers, Organic. In W. Chesworth, (ed.) Encyclopedia of Soil
Science (pp. 263-270). Springer, Berlin, ISBN 978-1-4020-3994-2.
Wu, M. C., Sun, K. W. and Zhang, Y. (2006). Influence of temperature fluctuation on
thermophilic anaerobic digestion of municipal organic solid waste. Journal of
Zhejiang University.Science.B., 7 (3), 180-185.
Yabu, H., Sakai, C., Fujiwara, T., Nishio, N. and Nakashimada, Y., (2011). Thermophilic
two-stage dry anaerobic digestion of model garbage with ammonia stripping. Journal
of Bioscience and Bioengineering, 111, 312–319.
Yilmaz, T., Yuceer, A. and Basibuyuk, M. (2008). A comparison of the performance of
mesophilic and thermophilic anaerobic filters treating papermill
wastewater. Bioresource Technology, 99 (1), 156-163.
Zeshan, Karthikeyan, O. P. and Visvanathan, C. (2012). Effect of C/N ratio and ammonia-
N accumulation in a pilot-scale thermophilic dry anaerobic digester. Bioresource
Technology, 113, 294-302.
Zhou, S. X., Dong, Y. P. and Zhang, Y. L. (2012). Solid-state anaerobic digestion for
methane production from corn stalks with stack-pretreated. Materials Science
Forum, 697, 326-330.
Zhou, Y., Zhang, Z., Nakamoto, T., Li, Y., Yang, Y., Utsumi, M. and Sugiura, N. (2011).
Influence of substrate-to-inoculum ratio on the batch anaerobic digestion of bean
curd refuse-okara under mesophilic conditions. Biomass and Bioenergy, 35 (7),
Zhu, J., Wan, C. and Li, Y. (2010). Enhanced solid-state anaerobic digestion of corn stover
by alkaline pretreatment. Bioresource Technology, 101 (19), 7523-7528.
Experimental Set-up Pictures
Fruit and vegetable
Closer to real field conditions
Waste material used (5 fractions)
(25 mm size)
Figure A-1 Waste material used in this study
Figure A-2 Set-up for GP21Test (Front view)
Figure A-3 Set-up for GP21Test (Side view)
Figure A-4 Pilot-scale inclined thermophilic dry anaerobic reactor
Figure A-5 Sand drying bed for dewatering of digestate
Calculation of Proposed Decentralized AD Plant
Size of the community: 5,000
Total waste generated by the community: 5 t/d (Waste generation in low-income countries
is 1.0 kg/p/d (Bogner et al., 2007)
Weight of organic waste: 60 % of total waste = 0.60 x 5 = 3 t/d,
By applying the average waste collection efficiency i.e., 65-70% (Hoornweg and Bhada -
Tata, 2012) of South Asian and East Asian regions, the resulted waste quantity: 2.0 t/d or
TS of waste: 18 % (please refer to Table B-2)
VS of waste: 84 %TS (please refer to Table B-2)
Density of pretreated waste: 1000 kg/m 3 (please refer to Table B-2 of Appendix B)
Reactor working volume: 75 % of total volume
Operating conditions (loading rate and retention) and methane yield used for design
calculations have been taken from results of run 2 of phase II pilot experiment as given in
Table B-1 Operating Conditions Versus Methane Yield of Phase II Pilot Experiment
Detail Run 1 Run 2 Run 3
OLR (kg VS/m 3 /d) 4.55 6.40 8.50
RT (d) 30 24 18
Methane yield (L/kg VS) 330 320 266
1. Calculations for volume of reactor
Working volume of reactor = VS (kg) to be added per day
VS load (kg VS/m 3 .d)
VS to be added per day = 2000 kg/d x 18/100 (TS) x 84/100 (VS) = 300 kg VS/d
Working volume = 300 (kgVS/d) / 6.4 (kgVS/m 3 /d) = 46.88 m 3
Reactor total volume: 46.88 x 100/75 = 62.5 m 3
VS to be added per day = 2000 kg/d x 18/100 (TS) x 84/100 (VS) = 300 kg VS/d
Volume of organic waste = weight/density = 2000 (kg/d) / 1000 kg/m 3 = 2.0 m 3 /d
Working volume of reactor (L ) = Flow rate (L/d) * RT (d)
= 2000 x 24 = 48,000 L = 48 m 3
OLR at this volume = 300 (kg VS/d)/ 48 m 3 = 6.25 kg VS/ m 3 .d
Working volume of reactor at OLR of 6.4 kg VS/m 3 .d = 48 x 6.25/6.4 = 46.88 m 3
Total volume: 46.88 x 100/75 = 62.5 m 3
The dimensions of the digester: 3 m (dia) x 9 m (height)
Daily amount of waste to feed: 2000 kg/d
2. Preparation of correct feed mixture for dry anaerobic digestion
The main purpose of this research was to prepare a correct feed mixture (in terms of C/N
ratio and TS) for dry anaerobic digestion, which can reduce the problem of ammonia
inhibition. To achieve that purpose, various feed mixtures were prepared, which consisted
of different percentage of different types of waste (i.e. food waste, fruit and vegetable
waste, waste paper and leaves). The detail of feed mixtures and their characteristics is
given in Table B-2.
Table B-2 Composition and Characteristics of Various Feed Mixtures
Detail Unit Feedstock 1 Feedstock 2 Feedstock 3
Food waste % FW a
42 40 45
Vegetable waste % FW 45 27 33
Fruit waste % FW 5 20 15
Leaf waste % FW 5 8 -
Paper waste % FW 3 5 7
Moisture % 79-84 75-85 81-86
TS (range) % 16-21 15-25 14-19
VS (range) %TS 79-90 80-90 84-88
C (avg.) %TS 51.30 52.10 51.20
TKN (avg.) %TS 1.92 1.63 1.61
C/N (avg.) - 26.72 31.96 31.80
Density (avg.) kg/m 3
1045 1000 1020
Percentage based on fresh weight.
Feedstock with C/N ratio 32 performed well and had 30% lesser ammonia accumulation
than C/N ratio 27. Thus, feedstock with C/N ratio 32 is recommended for the proposed
decentralized AD system, which can be prepared by mixing kitchen waste with 5-7% of
waste paper as shown in Table B-2.
3. Calculation for methane or biogas yield
Specific methane production = 320 L/kg VS (From pilot experiment, please see Table B-1)
Methane production = volatile solids x specific methane production
= 300 (kgVS/d) x 320 (L/kg VS)
= 96000 L CH4/d or 96 m 3 CH4/d
= 96 m 3 (CH4/d) x 100/48.12 (biogas/methane) = 200 m 3 biogas/d
= 8.33 m 3 biogas/h
The flow rate of 0.569 m 3 biogas/h is required to convert the biogas into electricity for a
gas engine of 1 kW (Prakash et al., 2012). Thus the flow rate of biogas produced from the
proposed decentralized AD plant is sufficient to generate electricity.
4. Energy and electricity generation
Energy balance of the pilot system has been calculated (Table B -3) based on the
methodology given in Appendix E. Results of run 2 have been considered to calculate the
electricity generation for real scale proposed decentralized AD plant.
Table B-3 Surplus Energy of ITDAR During Various Runs of Experiment 2
Energy Consumption (MJ/kg VS)
Shredding and Recirculation maintaining
1 20.60 1.29 0.28 2.09 4.149 62.02
2 18.21 1.16 0.26 1.68 2.76 67.77
3 15.98 1.21 0.27 1.18 2.50 67.75
Energy production: 18.21 MJ/kg VS (from pilot experiment 2, run 2, Table B-3)
Energy available: 67% of produced (33% spent on operation of the plant)
= 18.21 (MJ/kg VS) x 0.67 = 12.2 MJ/kg VS
Input of proposed plant = 300 kg VS/d
Energy produced = 12.2 (MJ/kg VS) x 300 (kg VS/d) = 3660 MJ/d = 3.66GJ/d
We know that 1 GJ/d = 11.57 kW (1 J/s = 1 W)
3.66 GJ/d = 3.66 x 11.57 = 42.35 kW
If efficiency of gas engine is 33% (Surroop and Mohee, 2012), then
Electricity Generation = 42.35 x 33% = 13.97 kW
Thus electricity of 14 kW will be generated by use of a 14 kW gas engine.
Table B-4 Technical Detail and Availability of Gas Engine
Ranked power 14 kW (continuous operation)
Voltage 400 V
Current 27 A
Approximate weight 800 kg
Electrical efficiency 34.80%
Biogas consumption < 0.5 m 3 /kWh
Suppliers Address 1. EMAC International
Biogas flow rate for 1 kW gas engine: 0.569 m 3 /h (Prakash et al., 2012)
Biogas flow rate for 14 kW gas engine: 0.569 x 14 = 7.96 m 3 /h. This flow rate can be
supplied by the proposed system, as it has flow rate of 8.33 m 3 /h. The detail of the
commercially available gas engine is given in the Table B-4.
5. Use of the produced biogas
The produced biogas can be used for various purposes like cooking, lighting and electricity
production. The extent of use of the produced biogas for various purposes has been given
in the Table B-5.
Table B-5 Use of Produced Biogas (8.33 M 3 /H) for Various Purposes
Purpose of use Specifications
required (m 3 Number of uses at
/h) a time
Stove burners 10 cm 0.28 30
Mantle Lamps 60 watts equivalent 0.195 42
Electricity consumption of a house in US and Australia is 900-1100 watt (Elert, 2003)
2 For 1000 watt power production
3 Prakash et al., (2012)
6. Technical details of digestate management
Digestate withdrawn from the reactor will not be stored because storage causes emission of
GHGs as found from the results of this study (please refer to sections 4.4.2(i) and 4.4.3 (i).
As the C/N ratio of digestate is within safe range to be applied on agricultural land (please
refer to Figure 4.16 and Table 4.4), therefore, it will be sent to use for agricultural purposes.
But before sending, its weight will be reduced by dewatering for easy management.
Technical details of dewatering have been given below.
i) Design of sand drying bed for dewatering digestate from proposed system
Sand drying bed will be used to dewater digestate. The advantages of sand drying bed are
low cost and simple.
Quantity of digestate: 1400 kg/d
Solid content of raw digestate: 6.5%
Dewatering time: 10 days
Depth of digestate layer on bed: 20 cm
Calculation for area of sand drying bed
Area of sand drying bed can be calculated from the loading of dry solids.
Typical dry solids loading: 120 kg dry solids/m 2 -yr (Tchobanoglous et al., 2003)
For a drying time of 10 d: 120/365 x 10 = 3.28 kg dry solids/m 2
Thus area needed for 3.28 kg dry solids = 1 m 2
Digestate produced as dry solids from proposed system
= 1400 (kg/d) x 10 d x 6.5% dry solids
= 910 kg dry solids
Area for 910 kg dry solids = 910 (kg dry solids)/3.28 (kg dry solids/m 2 )
= 277 m 2
Standard dimensions of drying beds are 6 m (wide) x 6 to 30 m (long)
Therefore, 3 drying beds with dimensions of 6 m (wide) x 16 m (long) will be
constructed to fulfill the digestate dewatering requirement of proposed system.
-Starting from the bottom to top, the bed will consist of 15 cm coarse gravel layer and
7.5 cm thick layer each of medium gravel, fine gravel and coarse sand. At the top,
there will be a 15-25 cm layer of sand.
-Digestate will be placed on bed in 20 cm layer.
-The bed will be equipped with lateral drainage lines under it, which will consist of
perforated 15 cm plastic pipes spaced 2 m apart and sloped at 2 percent.
ii) Calculation for reduction of weight (%) by digestate dewatering
Total amount of raw digestate: 1400 kg/d
Solid content of raw digestate: 6.5%
Solid content of dewatered digestate: 50%
Leachate solid content: 2.0%
TS in raw plant residues or digestate = 1400 * 0.065
= 91 kg
Total weight of dewatered residues = X
Total weight of leachate = Y
We know that, X + Y = 1400 kg or X = 1400 – Y Eq. (i)
Weight of TS added = Weight of TS removed
or 91 = 0.50 *X + 0.02 * Y
or 91 – 0.02Y = 0.5X
or X = (91 – 0.02Y)/0.5 = X Eq. (ii)
Comparing Eq. (i) and (ii), we have
X = 131 kg
Y = 1269 kg
Weight reduction = (1400 - 131) * 100 = 90.6 %
7. Size of the pump
The size of the pump used in pilot-scale ITDAR is AE1N-200. The number 200 means
theoretical delivery of the pump is 200 L/min. But in practice, its delivery (flow rate) was
observed as 25 L/min probably due to viscosity of the reactor material. The pump head
(vertical height to deliver digestate to the reactor) for the real scale reactor is just 3 times
that of pilot reactor as given in Table B-6. But, to meet the high delivery (L/min) due to
huge size of real plant, much higher size (not just 3 times like head) of pump should be
Calculations for selecting the size of pump
As the same operating conditions and feed composition will be replicated, almost the same
viscosity of the reactor material will achieve in real scale reactor. Also the practical flow
rate of real scale pump can be calculated from pilot observation as follows:
The practical flow rate of pilot pump (200 L/min) = 25 L/min
If we select a pump size of 1450 L/min for the proposed real-scale reactor,
Its practical flow rate will be: = 25/200 x 1450 = 181 L/min
Time taken to recirculate whole reactor material for 1 time
= 47000 (L)/181 (L/min)
= 260 min
Recirculation time required to achieve the recirculation rate of 2.5 Ldig/Lreactor vol./d
= 2.5 x 260 min = 650 min
Recirculation frequency: 4 times/d (in every 6 h) = 650/4 = 160 min ≈ 3 h
Table B-6 Comparison of Pump System Between Pilot and Real Reactor
Detail Pilot-scale plant Real proposed plant
Size (m 3 ) 0.7 (0.6 m dia x 2.4m height) 62.5 (3 m dia x 2.4m height)
Pump head (m) 1.7 5
Pump size (L/min) 200 1450
Observed flow rate
Time for 1 cycle of 22 260
Full Model Number AE1N-200 AE1N-1450
Company Allweiler Allweiler
The pump (size 1450 L/min) needs to run for 3 hours in every 6 hours to achieve the
required recirculation rate (or recirculation every alternate hour i.e. 1h on and 1h off).
Thus, this is the minimum pump size (1450 L/min) for the proposed reactor (working vol.
47 m 3 ). Because if pump size smaller than this is used, it needs to be turned-on longer time
than turned-off, which is not good. However, if we want to reduce turned-on time than
turned-off time, we need to select a pump size bigger than size 1450 L/min.
This pump size (1450) will also allow p umping of 3 times grain size and 2 times fiber
length of the material as compared to pump size 200.
8. Calculation of GHG emission reduction
Landfill methane production = 140 L/kg waste
(Bogner and Spokas, 1993)
Total methane emission, if the community landfills all the waste,
= 2000 kg waste /d 140 *L CH4/kg waste = 280000 L CH4 = 280 m 3 CH4/d
= 280 m 3 /d x 0.000717 tons/m 3 (Density of methane)
= 0.2 tons CH4/d = 0.2 (ton CH4/d) x 25 (tons CO2-eq/ton CH4)
= 5 ton CO2-eq/d
9. Plant capital and operating and maintenance cost (Things to consider)
Considerations for plant (2000 kg/d OFMSW (Dry thermophilic anaerobic digestion
plant) capital as well as O&M costs are given in Tables B-7 and B-8.
Table B-7 Considerations for Calculating Capital Cost of the Proposed AD Plant
Detail Specification/Characteristics/ Remarks
-Land area (for all digester set-up and
-Building (store room for equipment,
Feed preparation system
-Shredder or comminuter
-Feed storage tanks 2 m 3 (5 tanks)
-Digester 3m (dia) x 9m (length) steel vessel
Pumping and recirculation system
-Screw bed pump AE1N-1450 (Allweiler)
Biogas conversion to electricity
-Biogas storage tank
-Sand drying bed
-Digestate storage tanks
-Pipes and valves, other spare parts,
-Bins, containers, etc.
100 m 3
6 m (wide) x 16 m (long) (3 beds)
2 m 3 (5 tanks)
Table B-8 Considerations to Calculate Operating and Maintenance Cost of AD Plant
Staff requirement -1 Plant Manager
-1 Process control operator
-2 Maintenance technicians
-2 General labor
Utilities and Fuels -Fuel
-Natural gas for start-up
Miscellaneous -Lab analysis cost
-Wastewater treatment cost
Data of Phase I Pilot Experiment
Table C-1 Operational Parameters of Anaerobic Digestion During Phase I Pilot
pH VFA Alkalinity VFA/Alk ratio TAN
1 5.43 49130 23450 2.10 1766 2
6 5.98 35640 27360 1.30 1928 7
13 6.92 28800 35000 0.82 2134 65
18 6.68 37930 28860 1.31 2600 46
24 7.14 34250 28800 1.19 2480 123
29 6.9 37750 28860 1.31 2990 87
35 7.12 33750 32850 1.03 2940 139
41 7.61 30900 65500 0.47 3206 427
47 7.48 28500 52700 0.54 3115 318
53 7.39 26500 48450 0.55 2944 249
56 7.53 25250 49050 0.51 2947 334
63 7.49 24000 46350 0.52 2986 312
70 7.41 19125 38950 0.49 2811 248
77 7.40 20000 37400 0.53 2779 240
83 7.36 20750 36000 0.58 2912 232
91 7.70 15500 31350 0.49 2618 416
98 7.59 14750 32000 0.46 2233 286
105 7.45 13750 28500 0.48 2205 212
119 7.71 12750 33750 0.38 2566 416
127 7.75 10750 35250 0.30 2352 411
133 7.82 11100 44400 0.25 2436 486
141 8.00 10750 68750 0.16 2400 657
147 7.78 11250 36250 0.31 2200 407
154 7.45 11250 35100 0.32 1862 179
160 7.44 10500 33600 0.31 1883 177
168 7.30 12500 30450 0.41 1939 136
175 7.51 12000 30250 0.40 2114 230
182 7.35 10750 30250 0.36 2163 168
187 7.72 10000 27025 0.37 2254 372
196 7.47 8750 22425 0.39 2310 231
203 7.31 8375 19425 0.43 2086 149
210 7.49 7875 19800 0.40 2037 213
217 7.33 8375 19000 0.44 1897 141
224 7.41 7800 19250 0.41 1820 161
233 7.84 6700 22650 0.30 1850 383
240 7.55 7000 16500 0.42 1680 198
246 7.50 5150 15062 0.34 1708 182
253 7.25 8150 11700 0.70 1512 95
261 7.00 5750 7700 0.75 1897 69
264 7.37 5500 9100 0.60 2135 173
271 7.47 5500 14000 0.39 1540 154
Table C-2 Performance Parameters of Anaerobic Digestion During Phase I Pilot
GPR % CO2 % CH4 Methane Yield
(L CH4/kg VS)
15 0.16 58.55 34.31 76 33.38
16 0.35 59.93 31.87 70 33.38
18 0.27 59.72 32.71 72 33.38
19 0.28 56.22 33.23 107 49.53
20 0.24 51.31 36.50 112 49.00
22 0.26 37.26 46.35 131 49.00
23 0.12 39.33 48.10 200 49.53
24 0.31 44.74 46.22 197 49.54
25 0.46 35.06 42.68 175 49.53
26 0.39 39.77 49.74 197 49.53
27 0.44 37.69 50.08 160 39.55
28 0.54 37.69 50.08 167 39.55
29 0.51 37.51 51.51 167 39.54
30 0.60 37.58 49.96 170 39.55
31 0.83 34.3 54.53 167 39.55
32 0.77 34.89 50.80 264 55.79
33 1.00 32.66 56.58 250 55.79
34 0.85 35.32 54.46 278 55.79
35 1.09 32.53 56.48 259 55.76
36 0.94 35.32 54.52 279 55.79
37 1.01 32.39 54.27 259 55.79
38 1.06 32.38 54.23 273 55.79
39 1.00 33.00 51.17 275 56.22
40 1.16 34.00 54.60 262 56.22
41 0.99 36.96 52.94 268 56.21
42 1.22 32.76 56.48 249 56.22
43 1.07 35.17 54.43 280 56.22
44 1.16 36.71 53.08 262 56.22
45 1.34 32.52 56.63 274 61.43
46 1.14 32.00 50.43 308 61.43
47 1.38 36.62 52.97 289 61.43
48 1.67 36.14 51.01 274 61.43
49 1.83 35.58 54.72 269 61.43
50 1.67 38.58 52.32 285 61.43
51 1.96 36.72 53.78 280 65.46
53 2.18 38.64 52.07 294 65.48
54 2.32 38.04 52.22 278 65.46
56 2.84 35.79 54.02 282 65.46
58 3.27 35.45 54.42 300 65.46
62 3.19 40.04 50.50 297 64.25
64 3.50 40.41 50.02 261 64.25
Run Time GPR % CO2 % CH4 Methane Yield VS Removal
(days) (L/Lreactor vol./d)
(L CH4/kg VS) (%)
69 3.00 36.95 52.78 307 69.56
76 3.18 34.92 55.26 457 96.73
78 3.27 38.40 52.33 457 96.73
82 3.61 41.28 49.91 257 65.07
85 3.73 43.78 47.34 265 67.18
88 3.84 35.33 51.80 237 65.00
90 4.01 32.61 57.27 292 65.00
93 5.32 36.79 53.71 335 66.59
95 6.05 34.11 56.08 309 68.83
100 5.76 36.83 53.64 312 69.81
105 4.47 42.00 49.32 277 71.57
107 3.94 41.90 49.42 277 71.57
112 3.79 40.40 49.88 285 71.06
117 3.09 36.69 50.71 255 58.98
125 4.84 40.45 50.00 215 53.52
130 4.73 38.75 51.54 226 53.52
135 6.11 38.83 51.43 214 50.79
140 5.75 42.50 48.84 124 32.53
149 7.53 40.76 47.99 134 34.49
155 10.28 43.66 47.85 104 28.09
163 8.64 50.60 42.70 97 31.77
169 5.20 49.22 42.40 151 49.00
172 4.40 44.50 47.21 177 49.00
175 5.69 39.75 50.53 216 52.72
181 2.80 40.33 50.72 221 54.37
185 2.31 32.66 56.70 270 54.00
191 1.62 34.42 54.83 257 54.22
205 3.22 41.15 47.72 211 55.00
218 3.21 42.00 48.40 218 57.01
227 4.79 40.82 49.51 217 54.79
232 3.71 42.00 41.50 188 55.00
239 6.30 35.29 55.00 191 48.14
245 6.86 41.17 48.78 138 34.79
251 7.41 40.08 49.44 250 63.04
260 6.29 42.61 45.00 221 61.54
264 6.82 45.12 46.00 122 34.94
267 7.64 42.31 47.88 132 34.92
272 7.58 43.66 46.69 202 55.69
278 4.09 42.91 46.72 292 79.40
Table C-3 Characteristics of Feed and Digestate in Phase I Pilot Experiment
1 - - - 18.62 14.15 0.76 1.59 26.55
6 - - - 18.68 13.45 0.72 1.30 30.81
15 15.80 13.13 0.83 12.06 7.79 0.65 1.71 20.98
24 15.80 13.13 0.83 10.17 5.90 0.58 0.71 45.41
29 15.80 13.13 0.83 11.48 7.07 0.62 0.82 41.71
35 15.80 13.13 0.83 9.00 5.17 0.57 0.87 36.68
41 15.80 13.13 0.83 8.84 5.26 0.60 0.89 37.14
47 17.26 13.80 0.80 8.48 4.98 0.59 0.96 33.95
53 17.26 13.80 0.80 7.88 4.46 0.57 0.90 34.90
63 17.26 13.80 0.80 8.12 4.61 0.57 0.91 34.68
70 20.75 16.47 0.79 7.81 4.69 0.60 0.89 37.45
77 20.75 16.47 0.79 7.94 4.92 0.62 0.90 38.22
83 20.75 16.47 0.79 7.80 4.87 0.62 0.92 37.71
91 20.75 16.47 0.79 8.13 4.96 0.61 1.32 25.67
98 22.09 18.65 0.84 8.18 5.17 0.63 1.80 19.52
105 25.11 19.91 0.79 8.07 4.87 0.60 2.01 16.69
119 25.11 19.91 0.79 10.09 6.62 0.66 1.96 18.58
127 25.11 19.91 0.79 11.52 7.50 0.65 2.17 16.66
133 25.11 19.91 0.79 12.12 7.94 0.65 1.93 18.85
141 25.11 19.91 0.79 16.87 10.88 0.65 1.88 19.06
147 25.11 19.91 0.79 16.70 11.38 0.68 1.86 20.36
151 22.83 18.64 0.82 18.47 12.49 0.68 ND ND
158 22.83 18.64 0.82 17.40 12.18 0.70 ND ND
165 22.83 18.64 0.82 13.10 9.10 0.70 ND ND
173 22.83 18.64 0.82 11.61 8.09 0.70 ND ND
180 22.83 18.64 0.82 11.63 7.80 0.67 ND ND
185 22.83 18.64 0.82 11.37 7.50 0.66 ND ND
193 22.83 18.64 0.82 11.39 7.64 0.67 ND ND
200 22.83 18.64 0.82 11.77 7.66 0.65 ND ND
207 22.83 18.64 0.82 11.95 7.83 0.66 ND ND
215 22.83 18.64 0.82 11.03 7.32 0.66 ND ND
224 22.83 18.64 0.82 12.16 7.69 0.63 1.74 20.18
233 18.10 14.84 0.82 11.52 7.73 0.67 1.94 19.22
240 18.10 14.84 0.82 12.57 8.91 0.71 1.99 19.79
246 18.10 14.84 0.82 18.63 11.20 0.60 2.03 16.46
253 18.10 14.84 0.82 9.62 6.35 0.66 2.68 13.69
261 18.10 14.84 0.82 10.16 6.61 0.65 1.80 20.07
264 19.22 15.15 0.79 16.17 9.71 0.60 2.02 16.52
271 19.49 15.46 0.79 10.74 7.17 0.67 2.36 15.72
280 19.49 15.46 0.79 17.73 8.85 0.50 1.84 15.07
%FM: Percentage of fresh matter, ND: Not determined.
Data of Phase II Pilot Experiment
Table D-1 Operational Parameters of Anaerobic Digestion During Phase II Pilot
pH VFA Alkalinity VFA/Alk ratio TAN
11 7.14 15300 13300 1.15 1540 76
19 7.16 17775 17000 1.05 1652 85
25 7.20 19000 18000 1.06 1785 101
32 7.43 17050 17450 0.98 1660 153
39 8.10 11100 17700 0.63 1442 464
47 8.23 6300 19500 0.32 1491 582
54 7.82 5700 15450 0.37 1862 371
61 7.91 5325 13750 0.39 2016 473
68 7.76 4700 11750 0.40 1890 337
75 7.71 4975 11600 0.43 1880 305
81 7.70 5100 11550 0.44 2058 327
89 7.70 5000 11000 0.45 2065 328
96 7.56 5350 10250 0.52 1897 228
103 7.85 4400 10500 0.42 1974 416
112 7.75 3575 10500 0.34 2198 384
119 7.54 4450 9400 0.47 2107 244
127 7.82 6450 11400 0.57 2100 419
135 7.75 5900 10750 0.55 2114 370
141 7.55 6525 8800 0.74 2030 240
146 7.23 5700 8500 0.67 2030 122
154 7.19 7350 9650 0.76 3066 169
160 7.00 7500 9500 0.79 2100 76
Table D-2 Performance Parameters of Anaerobic Digestion During Phase II Pilot
GPR % CO2 % CH4 Methane Yield
(days) (L/Lreactor vol./d)
(L CH4/kg VS)
55 4.54 41.69 48.79 317 82.16
63 5.30 41.60 48.02 308 80.46
74 5.65 40.32 48.58 331 83.96
84 5.53 40.89 48.42 309 79.32
89 4.73 40.83 49.18 321 81.49
102 5.70 39.98 49.37 353 88.06
114 5.75 38.73 50.64 361 86.67
119 6.77 42.00 48.00 310 81.56
126 5.64 41.90 47.69 305 80.43
135 6.64 36.90 47.07 326 79.59
145 6.07 43.15 46.82 283 77.13
146 6.04 42.76 47.28 161 43.50
159 7.79 43.53 46.76 278 74.86
Table D-3 Characteristics of Feed and Digestate in Phase II Pilot Experiment
11 - - - 5.68 3.40 0.60
19 - - - 11.91 7.27 0.61
25 - - - 8.22 4.86 0.59
32 - - - 7.42 4.76 0.64
39 - - - 8.28 5.05 0.61
47 - - - 9.77 5.73 0.59
51 16.07 13.97 0.87 5.21 2.97 0.57
58 15.89 13.70 0.86 5.83 3.32 0.57
65 14.66 12.61 0.86 5.17 2.91 0.56
71 16.38 14.39 0.88 5.17 2.91 0.56
78 15.08 13.17 0.87 5.06 2.87 0.57
79 13.44 11.38 0.85 5.64 3.32 0.59
86 13.88 11.93 0.86 5.63 3.34 0.59
93 13.88 11.93 0.86 5.20 2.92 0.56
100 17.85 15.28 0.86 4.82 2.66 0.55
108 17.85 15.28 0.86 4.38 2.35 0.54
110 17.85 15.28 0.86 5.09 2.83 0.56
116 17.85 15.28 0.86 6.80 3.74 0.55
124 17.85 15.28 0.86 7.06 3.95 0.56
132 17.85 15.28 0.86 6.64 3.64 0.55
139 16.83 14.45 0.86 6.86 3.99 0.58
146 17.59 15.18 0.86 6.43 3.57 0.55
150 17.59 15.18 0.86 4.99 3.05 0.61
160 16.26 14.01 0.86 4.99 3.05 0.61
%FM: Percentage of fresh matter
Methodology for Calculation of Energy Balance
Methodology Methodology for for Energy Energy Balance Balance Calculation Calculation of of Inclined Inclined Reactor
The The energy energy balance balance of of inclined inclined reactor reactor is is based based on on its energy consumption and energy
production. production. Energy Energy is is required required for for shredding shredding the the waste waste (ES), waste feeding and withdrawal
(EFW), (EFW), re-circulation re-circulation of of reactor reactor content content (ER) (ER) and and heating heating (EH) which includes energy for
heating heating the the substrate substrate (EHS) (EHS) as as well well as as energy energy for for maintaining constant thermophilic
temperature temperature (EMT). (EMT). The The production production of of energy energy (EP) (EP) is only from the methane produced.
Method Method for for calculation calculation of of each each type type of of these these energies energies has been given in the following
The The net net energy energy production production NEP NEP (MJ) (MJ) is is the the difference difference between the energy produced and
consumed consumed by by the the process process that that is is shown shown in in Figure Figure 1 1 and and given by the following equation:
NEP NEP = = EP EP - - ES ES – – EFW EFW – – ER – EH
Figure Figure E-1 E-1 Energy Energy inputs inputs and and output output of of inclined anaerobic digester
1.1 1.1 Energy Energy Production
The The energy energy production production EP EP (MJ) (MJ) equivalent equivalent to to methane methane and hydrogen content of the
produced produced biogas biogas can can be be calculated calculated by by the the equation equation below.
EP EP = = (MP (MP X X L.H.V. L.H.V. of of CH4) CH4) + + (HP (HP X L.H.V. of H2)
MP MP = = methane methane production production (L (L CH4)
L.H.V. L.H.V. of of CH4= CH4= 0.03618 0.03618 MJ/L MJ/L CH4 CH4 CH4 (Ruggeri (Ruggeri et et al., al., 2010)
HP HP = = Hydrogen Hydrogen production production (L (L H2)
L.H.V. L.H.V. of of H2 H2 H2 = = 0.0108 0.0108 MJ/L MJ/L H2 H2 H2 (Ruggeri (Ruggeri et et al., al., 2010)
1.2 1.2 Energy Energy Consumption
Energy Energy consumption consumption has has been been described described under under sub-categories sub-categories as follows:
1.2.1 1.2.1 Energy Energy required required for for shredding shredding (ES)
The The assumption assumption here here is is that that about about 100 100 kg kg of of waste waste is is shredded by the use of 0.5 L gasoline.
34.8111 MJ = 1L Petrol
ES (MJ) = 0.5 L (Petrol) x weight of waste (kg) x 34.8111
100 kg (waste)
1.2.2 Energy required for feeding and withdrawal (EFW)
P = Motor power (kW)
T = Duration of operation (h)
3.6 MJ = 1 kWh
EFW (MJ) = P x T x 3.6
1.2.3 Energy required for recirculation (ER)
It is calculated using the same formula as EFW.
1.2.4 Energy required for heating (EH)
a. Energy required for heating the substrate (EHS)
This is the energy required to heat influent substrate. The energy consumed for the heating
of substrate for each organic loading rate has been measured. The total heat energy (EH,
MJ) required to raise the temperature of waste from its ambient value (To i.e. 28 °C,
average ambient temperature of Pathumthani province) to the reactor’s temperature Ti (50
°C) can be calculated by equation below.
EH (MJ) = M x Cp x (Ti - To)
M = Weight of waste in killograms
Cp = Specific heat of feed (kJ/kg.°C)
1000 = Factor to change kJ to MJ
The influent had TS and moisture content of about 15% and 85% respectively. Therefore,
for calculation of EH, Cp of water (i.e. 4.186 kJ/kg.°C) has been used for 85% moisture
and Cp of peat (i.e. 1.88 kJ/kg.°C) has been used for 15% solids. The reason for using Cp
of peat is that the solids in my reactor feed have 80% volatile solids or organic matter and
are similar in nature to peat which is the soil formed of partially decayed plants.
b. Energy for maintaining thermophilic temperature (EMT)
This is energy required to maintain the reactor temperature more than ambient temperature
(i.e. at 50 °C). Since the temperature of the reactor was kept constant, therefore EMT is
equal to the heat energy lost through the walls of the reactor. Heat transfer rate or heat loss
(Q in Joule/second) from the cylindrical surface of the reactor (a composite wall consisting
of three layers namely stainless steel, cotton and Al/PE foam insulation) was calculated
using the following formula (Cengel, 2003 p.149).
Heat transfer rate or heat loss (Q) from the top and bottom bases of the reactor was
calculated using the following formula (Cengel, 2003 p.134).
Ti - T0
1/hi(2πr1L) + ln (r2/r1)/K1(2πL) + ln (r3/r2)/K2(2πL) + ln (r4/r3)/K3(2πL) + 1/h0(2π4L)
Ti - T0
1/hiA + L1/K1A + L2/K2A + L3/K3A + 1/h0 A
Ti = Temperature inside the reactor (°C)
T0 = Temperature outside the reactor (°C)
r1 = Radius of cylinder w.r.t first layer (stainless steel) of composite wall
r2 = Radius of cylinder w.r.t second layer (cotton) of composite wall
r3 = Radius of cylinder w.r.t third layer (Al/PE foam insulation) of composite wall
r4 = Radius of cylinder w.r.t outer surface of third layer of composite wall
L = Length of cylinder
K1, K2 and K3 = thermal conductivities of layers 1, 2 and 3
hi = Heat transfer coefficient from inside reactor to first layer
ho = Heat transfer coefficient from last layer to air
L1, L2 and L3 are thicknesses of layers 1, 2 and 3
A = Surface area of the base of cylinder
Sample Calculation of GHG Emission Potential
Calculation of GHG Emission Potential of Digestate
The formula for methane emission potential of digestate is given as:
g CH 4 / kg digestate
( M TS DOC DOC MCF F (( 16 / 12)
( 1 OX ) 1000
GHG emission (gCO2-eq/kg digstate) = Methane emission (g CH4/kg digestate) x 25
M = Mass of digestate = 1 kg
TS = Total solid content of digestate = 0.1045
DOC = Carbon content of digestate in TS = 0.356
DOCF = Fraction of DOC dissimilated. The model is described as 0.014T+0.28, at
35C, the value is computed as 0.77.
MCF = Methane correction factor for open dumpsite of < 5 m depth = 0.4
F = Fraction of methane in landfill gas = 0.5
R = Recovered methane (kg) = 0
OX = Oxidation factor = 0
g of methane / kg digestate
( 16 / 12)
= 7.6 g CH4/kg digestate
GHG emission of digestate = 7.6 (g CH4/kg digestate) x 25 = 190 gCO2-eq/kg digstate
Since, 0.7335 kg digestate was produced by digester for 1 kg waste fed, therefore, the
GHG emission potential of digestate per unit weight of waste fed will be:
GHG emission potential of digestate obtained from 1 kg waste fed = 190 x (0.7335/1)
= 139 gCO2-eq/kg waste
The weights of material decreased at every step of its management, which were used for
GHG emission potential calculation and are given as follows:
Waste fed 1 kg
Digestate 0.7335 kg
Stored digestate 0.7335 kg
Dewatered-stored digestate 0.153 kg
Stored-cured digestate 0.123 kg