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DRY ANAEROBIC DIGESTION OF MUNICIPAL SOLID

WASTE AND DIGESTATE MANAGEMENT STRATEGIES

by

Zeshan

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

Engineering

Division of Environmental Engineering

Hokkaido University, Japan

Nationality: Pakistani

Previous Degree: Master of Science (Honors) Agriculture

(Soil and Environmental Sciences)

University of Agriculture, Faisalabad

Scholarship donor: Higher Education Commission (HEC)

Pakistan-AIT Fellowship

Asian Institute of Technology

School of Environment, Resources and Development

Thailand

December 2012

i


Acknowledgements

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.

ii


Abstract

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

conditions.

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.

iii


Table of Contents

Chapter Title Page

Title page i

Acknowledgements ii

Abstract iii

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

iv


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

quality

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

system

55

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)

62

4.2.1 Performance of ITDAR during start-up and continuous

operations

63

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)

71

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

v


4.3.3 Effect of organic loading rate on performance parameters of

ITDAR

76

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

86

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

References 95

Appendices 110

Appendix A 110

Appendix B 114

Appendix C 122

Appendix D 127

Appendix E 130

Appendix F 134

vi


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

Systems 36

2.9 Concentration of Organic Pollutants in Digesates and Composts (µg/kg 37

DM)

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

Test 48

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

OFMSW 59

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

vii


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

digester 47

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

64

ITDAR

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

80

experiment

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

viii


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

ix


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

x


1.1 Background

Chapter 1

Introduction

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

waste.

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

al. 2011).

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

leaching.

1


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.

2


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.

3


Chapter 2

Literature Review

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

digester.

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

been reviewed.

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

4


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)

4000

3500

3000

2500

2000

1500

1000

500

0

Figure 2.1 Trend of low solids and high solids anaerobic digestion plants in Europe

(Mattheeuws, 2011)

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.

2.2.1 Hydrolysis

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

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.

5

2000

2001

2002

2003

2004

2005

2006

2007

Year

Wet operation Dry operation

C6H10O4 + 2H2O C6H12O6 + 2H2

2008

2009

Eq. 1

2010


Figure 2.2 Main stages of anaerobic digestion process (Modified from Appels et al.,

2008)

2.2.2 Acidogenesis

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

digestion process.

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.

2.2.3 Acetogenesis

H 2, CO 2

Hydrogenotrophic

Methanogenesis

30% CH 4

Organic Waste

(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

6

Hydrolysis

Soluble Organics

(Simple sugars, Amino acids, Fatty acids)

Acidogenesis

Volatile Fatty Acids

(Propionate, Butyrate, etc), Ethanol

Acetogenesis

CH 4 + CO 2

Acetate

Acetotrophic

Methanogenesis

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

2.2.4 Methanogenesis

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

digestion.

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

prevail.

7


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)

Calcium 2.5-4.5

8 (strongly inhibitory)

Magnesium 1-1.5

3 (strongly inhibitory)

Potassium 2.5-4.5

12 (strongly inhibitory)

Sodium 3.5-5.5

8 (strongly inhibitory)

Heavy metals

Copper (Cu)

Cadmium (Cd)

Iron (Fe)

Chromium (Cr 3+ )

Chromium (Cr 6+ )

Nickel

8

0.0005 (soluble metal)

0.15 a

0.15

1.71 a

0.003

0.5

0.002

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.

2.3.2 Ammonia

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).

9


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

type TS

TAN degree

(%)

(mg/L)

Potato Wet 4.5 Mesophilic 4000-6000 57% loss in Koster

juice

methanogenic and

activity Lettinga,

1988

OFMSW Wet 6 Thermophilic 3500-5500 6-11% loss in Angelidaki

methane et al.,

yield 2006b

Cattle Wet 7 Thermophilic 4000-5000 50% loss in Borja et

manure

specific

growth rate

of

methanogens

al., 1996

Synthetic Wet - Thermophilic 4920-5770 39-64% loss Sung and

wastewater

in methane Liu, 2003

Food Dry 18.4 Thermophilic 3500 50% loss in Gallert and

waste

methane winter,

1997

OFMSW Dry 30 Mesophilic 2800 Process Poggi-

Cease Varaldo et

al., 1997

OFMSW Dry 24.79 Mesophilic 2000 > 50% loss in Jiang et al.,

biogas 2008

OFMSW Dry 30 Thermophilic 1200 - Kayhanian,

1999

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

10


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

2.4.1 pH

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

12


eported, which increased with time, but after some time of continuous feeding, it reduced

gradually.

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

13


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.

2.4.4 Temperature

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

14


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

landfills.

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)

4500

4000

3500

3000

2500

2000

1500

1000

500

0

1990

1991

1992

1993

1994

1995

1996

1997

1998

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

15

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

Year

Mesophilic operation Thermophilic operation

2009

2010


need for heating is also less for dry systems when thermophilic operation is chosen (De

Baere, 2006).

2.4.5 Mixing

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).

Inlet

(a)

Outlet

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

16

Inlet

Mechanical mixing paddle

Biogas

Inlet Outlet

(c)

(b)

Outlet


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.

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

range.

17


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.

OLR =

Q.S

where

S = substrate concentration (kg substrate in terms of TVS)

OLR = organic loading rate (kg substrate/m 3 digester)

V

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,

(2011)

OFMSW - - 6.80 De Baere and

Mattheeuws,

(2011)

SS-OFMSW 20 12.1 5.3 Pavan et al.,

(2000)

OFMSW+Yard 18-40 11 3.70 Hamzawi et al.,

waste

(1999)

MS-OFMSW 18 9.65 5.20 Gallert and

Winter, (1997)

OFMSW+Paper 30 12.6 7.14 Vermeulen et

al., (1993)

OFMSW 23-30 13-15 6 Kayhanian and

Tchobanoglous,

(1993)

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

18

=

S

RT

Eq. 10


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:

Physical pretreatment

Chemical pretreatment

Biological pretreatment (inoculation)

Moreover, co-digestion can also be considered as a good technique to optimize dry

anaerobic digestion.

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

19


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

inoculation.

2.5.4 Co-digestion

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

20


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

researchers.

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.

21


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

different times.

Horizontal

Plug flow

Substrate input mode

Dry Anaerobic Digestion

Single stage continuous Multi stage continuous Single stage batch

Reactor temperature

Thermophilic Mesophilic Thermophilic

Mesophilic

Vertical

Plug flow

Horizontal

Plug flow

Flow pattern

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).

22

Horizontal

Plug flow

No flow

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

23


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

operation.

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)

6000

5000

4000

3000

2000

1000

0

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

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

24


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

25


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.

Feed

Biogas Leachate Recirculation

Leachate

(a)

Biogas

(c)

Partition

wall

Biogas recirculation

Digestate

Fresh

waste

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

26

Fresh

waste

Digestate recirculation

Feed

Digestate

Biogas

(d)

Biogas

Feed

tubes

(b)

Digestate


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.

27


Table 2.5 Performance of Various Kinds of Dry Anaerobic Digesters

Substrate Feed

TS (%)

Food

wastes

SS-

OFMSW

Reactor

Type

Reactor

Size

20 Batch Lab

8L

20 Batch Lab

5L

OFMSW 16 c

Batch Lab

35L

Municipal 35 Batch with Lab

solid

paddle 40L

waste

mixer

Horse 15-30 Batch with Lab

dung with

straw

percolation 57L

Corn 22 Batch Lab

stover

2L

Wheat 22 Batch Lab

straw

from

horse bed

1L

Cow 15-16 Batch Lab

manure +

Sludge

2.5L

SS- 20 Continuous Pilot

OFMSW

CSTR 3m 3 , 1

OFMSW 20 Continuous

CSTR

m 3

Pilot

3m 3

Temperature

(°C)

Retention

Time (d)

28

OLR

(kg

VS/m 3 d)

SMP

LCH4/kg

VS

CH4

(%)

VS

Reduction

Reference

37 33 - 367 90 Cho et al,

(1995)

55 60 - - - 45 Forster-

Carneiro,

2008b

30 32 - 273 64.6 26.1 Dong et al.,

(2010)

37 35 - 200 - - Guendouz et

al., (2010)

35 42 - 170 51-

53

37 40 - 223 e

50-

60

37 30 - 150 55-

60

35 63 - 328 65.1

7

55 11.6 12.1 490 d

44 -49 Kusch et al.,

(2008)

44.4 Zhu et al.,

(2010)

- Cui et al.,

(2011)

54.80 Li et al.,

(2011)

- 59.3 Pavan et al.,

(2000)

55 13.5 9.2 230 68.7 - Bolzonella et

al.,(2003)


Substrate Feed

TS (%)

Reactor

Type

Reactor

Size

Temperature

(°C)

Retention

Time (d)

29

OLR

(kg

VS/m 3 d)

SMP

LCH4/kg

VS

CH4

(%)

VS

Reduction

Reference

MS 18 Continuous Lab 55 19 9.65 342

-OFMSW

4.8L

b

59 65 Gallert and

Winter, (1997)

sim 30 Semi Lab 55 15 11.8 97

OFMSW

continuous 5L

b

- 89 Fdez-Guelfo et

al., (2011)

FW + 30 Contnuous Lab 35 30-100 10

Paper

with lateral 60L

waste

impeller

a

250 - 80 Kim and Oh,

(2011)

OFMSW 25-30 Continuous Lab 55 25-40 4.42-7.5 300

CSTR 4.5L

e

50 80 Montero et al.,

(2008)

Maize - Continuous Lab 55 - 12.7 182 44.1 88.7 Mumme et al.,

silage +

UASS, 26.5L

(2010)

Barley

Magnetic

straw

stirring

Sewage 20 Continuous Lab 35 12 8.5 190 64.9 29 Duan et al.,

sludge

, CSTR 6L

(2012)

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

a 3

The value given is of solid loading rate (i.e. kg TS/m d).

b

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

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

30

30


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

non-hydrolysable lipids).

31


Table 2.6 Effect of Digestion on Properties of Waste

Parameter Feedstock Before After % Change Reference

Digestion Digestion

pH Primary 3.5 7.5 +114 Gomez et al.,

sludge +

OFMSW

2007

Cattle slurry 7.2 8.4 +16.7 Mokry et al.,

2008

Pig slurry 7.0 8.1 +15.7 Mokry et al.,

2008

Cattle 6.9 7.6 +10.1 Gomez et al.,

manure

2007

Total OFMSW 90 g 43.4 g -52 Rao and

Solids

Singh, 2004

Primary 60 g/L 23.6 g/L -60.66 Gomez et al.,

sludge +

OFMSW

2007

Cattle 263 g/L 122.6 g/L -53.38 Gomez et al.,

manure

2007

Cattle slurry 9.9 % 7.1 % -28.28 Mokry et al.,

2008

Pig slurry 7.6 % 4.9 % -35.52 Mokry et al.,

2008

Volatile OFMSW 79.65 33.10 -58.44 Rao and

Solids

Singh, 2004

OFMSW 82.32 % 40.95 % -50.25 Eliyan, 2007

Primary 55.2 g/L 16.5 g/L -70.10 Gomez et al.,

sludge +

OFMSW

2007

Cattle 226.2 g/L 105.4 g/L -53.40 Gomez et al.,

manure

2007

Cattle slurry 86.9 a %TS 75.35 %TS -13.25 Mokry et al.,

2008

Pig slurry 78.5 a %TS 73.26 %TS -6.66 Mokry et al.,

2008

Total N OFMSW 1.4 g 1.06 g -24.28 Rao and

Singh, 2004

Cattle slurry 4.1 kg/t 4.5 kg/t +9.75 Mokry et al.,

2008

Pig slurry 4.1 kg/t 4.5 kg/t +9.75 Mokry et al.,

2008

NH4-N Cattle slurry 1.7 kg/t 2.5 kg/t +47 Mokry et al.,

2008

Pig slurry 2.0 kg/t 3.5 kg/t +75 Mokry et al.,

2008

Manure 70 % of TN 85 % of TN +21.42 Berglund,

2006

a

Calculated by the formula: VS = OM X 1.8/1.72

32


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.

33


Table 2.7 Characteristics of Solid Digestate in Dry Anaerobic Digestion Systems

Substrate pH

MC

(%)

TS

(%)

VS

(% TS)

TN

(% TS)

C/N

NH4-N

(% TS)

P

(%TS)

K

(%TS)

Ca

(%TS)

Mg

(%TS)

OFMSW - 92.77 7.23 76.26

1.06

12.07 - - - - -

Farm manure

(Spring)

Farm manure

(Autumn)

- 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 - -

Digestates

from market

8.5 a

Animal slurry 8.1 a

46.90

53.10

52.64 b

1.53 19.11 c

68.10 31.90 66.30 3.30 11.16 c

34

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

Poultry

manure

Food waste +

8.5 - - - 1.49 - 0.22 2.33 1.19 4.34 0.61

Energy crops

+Animal

manure

7.9 90.40 9.60 77.00 4.40 9.72 c

2.00 - - - -

a

From analysis of 1:2 water suspension

b

Calculated by the formula: VS = OM X 1.8/1.72

c C = VS/1.8

d Characteristics of raw (un-separated) digestate

Reference

Rao and

Singh, 2004 d

Schafer et al.,

2006

Eliyan, 2007

Fuchs et al.,

2008

Jorgensen and

Jensen, 2008

Sanchez et al.,

2008 d

Menardo et

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.

35


Table 2.8 Characteristics of Separated Liquid Digestates from Different Digestion Systems

Substrate

MC

(%)

TS

(%)

VS

(% TS)

TN

(% TS)

C

(%TS)

36

C/N

NH4-N

(% TS)

P

(%TS)

Animal slurry 94.77 5.23 - 7.27 - - - 2.24 -

K

(%TS)

Reference

Moller et al.,

2002

Farm manure

(Spring)

94.33 5.67 70.55 6.53 35.27 5.4 2.12 1.39 6.00

Schafer et al.,

Farm manure

(Autumn)

96.17 3.83 71.00 6.53 23.49 3.6 2.61 1.33 8.36

2006

Manure with

biowaste

99.48 0.52 86.54 30.77 48.10 c

1.6 21.15 2.12 -

Paavola and

Rintala, 2008

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

6.92 b

40.69 c

5.9 3.84 b

2.69 6.92

Mokry et al.,

2008 d

a

Calculated by the formula: VS = OM X 1.8/1.72

b

Feedstock is animal slurry

c

Calculated by the formula: C = VS/1.8

d

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

liquid digestate.

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)

Pollutant Anaerobic

digestion

digestate

Compost Biowaste

compost

37

Digestate of

SS waste

PCB 10 20 33 32 25.6

PAHs 1430 3010 2.659 5925 2680

NP 4770 30 560 - 324

Composted

digestate

(median)

DEHP 29700 30100 1.400 1114 1760

Organic

compounds/

Pesticides

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

2008 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

of PCBs.

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

results.

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

Eliyan, 2007

Cattle manure - 8.05 128 - - - 555

Composted

digestate

Biowaste

digestate

Animal

digestate

0.36 23.00 72 0.097 11.50 26.8 179

38

Sanchez et al.,

2008

Riedel and Marb,

2008

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.

Organic

waste

Biogas

Anaerobic

Digester

Digestate

39

Recycle

Liquid

Digestate

Solid

Digestate

Liquid N

Fertilizer

P Rich

Compost

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

centrifugation.

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

(kg N/ha/y)

Required Storage

Austria 100 6 months

Denmark 170 (cattle)

140 (pig)

30 kg P/ha/y

7 ton DM/ha/y

9

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.

40


Figure 2.11 Changing parameters during aerobic post-treatment (Abdullahi et al.,

2008)

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

methane.

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

storage.

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

41


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

as feedstock.

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

Laholm.

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

42


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

value.

43


Chapter 3

Methodology

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.

Phase I

AD Optimization – C/N ratio

•Testing simulated substrates

with different C/N ratio

•Investigation of AD operational

parameters

•Characterization of digestate

(TS, VS, C, N)

Gas formation

potential test

Phase III

Digestate Management and GHG Emissions

Figure 3.1 Phases of overall research study

44

Phase II

AD Optimization – OLR

•Testing OLRs of feed of selected

C/N ratio

•Investigation of AD operational

parameters

•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

digestate management


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

Composition

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

Characteristics

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

45


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).

3

4

5

2

1

79 cm

4 cm

Figure 3.2 Experimental set-up for gas formation potential test

3.2.2 Experimental set-up for pilot-scale experiments

6

8

7

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

46

9

GC

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

different trials.

Figure 3.3 Pilot-scale experimental setup of inclined thermophilic dry anaerobic

digester

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

47


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

Test

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

solution

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

48


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

Run Duration

(d)

OLR

(kg VS/m 3 .d)

Solid retention

time (d)

Digrr

(Ldig/Lreactor vol.d)

Feedstock 1 (avg. C/N ratio 27)

Start-up 1-14 - 14 1.00

1 15-38 0.65 a

153 0.05

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

a

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-

49


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

subsection (b).

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

following subsections.

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

50


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.

Optimization

of anaerobic

digestion

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

strategy.

51

Dry digestion

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

Liquid 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

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).

52

Storage

Dewatering

Composting

Land Application

Solid Digestate

Rich in OM & P

Curing


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

further analysis.

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.

53


A

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

54

54

A

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.

55


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

origin.

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

manufacture

3 CH4 from storage and N2O from land Fossil CO2 for fertilizer

application

manufacture

4 CH4 from storage and N2O from land Fossil CO2 for fertilizer

application

Scenario 1

Dumping of digestate (baseline)

All the digestate is dumped to a dumpsite

Scenario 2

Direct application of digestate to agricultural land

All the digestate is spread over agricultural land

Scenario 3

Digestate storage and land application

Storage of all the digestate followed by its land application

Scenario 4

Storage, curing and land application of digestate

All the digestate is stored followed by its curing and land

application

Scenario 5

Storage, curing and dumping of digestate

All the digestate is stored and then cured before its dumping

to dumpsite.

5 CH4 from storage and dumping of

digestate

56

manufacture

Nil


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).

where

Methane

emission

g of methane / kg of material

( M TS MCF DOC DOC F ( 16 / 12)

R)

( 1 OX ) 1000

2

F

gCO equivalent/

kg (

g of methane/

kg)

25

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,

2011).

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

IGES, (2011).

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.

58


Table 3.5 Analytical Methods for Various Parameters of Anaerobic Digestion of

OFMSW

Parameters Method/Equipment Interference Reference

Parameters of solid samples (solid waste, inoculum and digestate)

TS (%) Oven drying at 105 ºC Loss of volatile OM

during drying

VS (%TS) Furnace drying to ash at 550 ºC Loss of volatile

inorganic salts like

59

APHA, 2005

APHA, 2005

Organic C (%)

(NH4)2CO3

- Walkley and

Black, 1934

TKN (%) Digestion of sample with H2SO4

and use of Kjeldahl apparatus

for distillation

- APHA, 2005

P (%) HClO4 + HNO3 digestion

- Olsen and

and colorimetric method

Sommer,

1982

Parameters of liquid samples (Liquid digestate or leachate)

pH Glass electrode method Sodium if pH>10 and

temperature

APHA, 2005

ORP Electrode method - APHA, 2005

TAN (mg/L) Distillation method - APHA, 2005

NH3-N (mg/L) Calculation method - Siles et al.,

2010

Alkalinity Titration method - Lahav and

(mg/L as

Morgan,

CaCO3)

2004

VFA (mg/L) Titration method - Lahav and

Morgan,

2004

Parameters of biogas samples

Biogas

composition

SHIMADU-GC14 A Gas

chromatograph with TCD

detector

- APHA, 2005


Chapter 4

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

bottles.

60


Biogas production 100X (NmL)

Specific biogas production (NmL/g VS)

25

20

15

10

5

0

400

350

300

250

200

150

100

50

0

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

Time (days)

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.

61


Biogas production 100X (NmL)

Specific biogas production (NmL/g VS)

25

20

15

10

5

Inoculum 2 Cellulose Feedstock 2

0

250

1 8 15 22 29 36

200

Time Feedstock (days) 2

150

100

50

0

1 8 15 22 29 36

Time (days)

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

Pilot Experiment)

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

62


(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).

63


Dig rr (L dig /L reactor vol .d) pH Methane yield 10x (L/kg VS) VFA/Alk ratio

OLR (kg VS/m 3 .d)

2

1

0

50

40

30

20

10

0

8

7

6

5

4

2

0

20

15

10

5

0

e)

d)

c)

b)

a)

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

Time (days)

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

64


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

65


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

Run

VFA

(mg/L)

VFA/Alk

ratio

pH

Total

Ammonia-N

(mg/L)

66

Free

ammonia

(mg/L)

Specific

methane

production

(L/kgVS)

VS

loss

(%)

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

a

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)

14

12

10

8

6

4

50

40

30

20

10

0

8

7

6

805

60

40

20

0

50

40

30

20

10

0

c)

b)

e)

d)

a)

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

67

Time (days)


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–

68


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 (mg/L)

2800

2300

1800

1300

800

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.

69

1.0

0.8

0.6

0.4

0.2

0.0

TAN/TKN ratio


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

ammonia-N inhibition.

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.

70


Table 4.2 Surplus Energy of ITDAR During Various Runs

Run

Energy

production

(MJ/kg

VS)

Energy Consumption (MJ/kg VS)

Feeding

Heating and

Shredding and Recirculation maintaining

withdrawal

thermophilic

conditions

Surplus

energy

(%)

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

centralized systems.

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

71


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).

pH

9.0

8.5

8.0

7.5

7.0

6.5

6.0

5.5

1 11 21 31 41

Time (days)

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

VFA concentration.

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.

72


VFA 100 X (mg/L)

VFA/Alk ratio

200

180

160

140

120

100

80

60

40

20

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1 11 21 31 41

Time (days)

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.

73

Time (days)

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

70

60

50

40

30

20

10

0

Methane Carbon dioxide GPR

1 11 21 31 41

Time (days)

Figure 4.8 CH4, CO2 and GPR fluctuation during start-up phase

4.3.2 Stability parameters of ITDAR: Effect of organic loading rate

i) pH

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,

74

2.5

2.0

1.5

1.0

0.5

0.0

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.

pH

9.0

8.5

8.0

7.5

7.0

6.5

6.0

5.5

51 71 91 111 131 151

Time (days)

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)

80

70

60

50

40

30

20

OLR 4.55 OLR 6.4 OLR 8.5

OLR 4.55 OLR 6.4 OLR 8.5

51 71 91 111 131 151

Time (days)

Figure 4.10 Concentration of VFA in ITDAR during continuous loading

75


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

system performance.

VFA/Alk ratio

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

OLR 4.55 OLR 6.4 OLR 8.5

51 71 91 111 131 151

Time (days)

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.

76


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)

9

8

7

6

5

4

3

2

OLR 4.55 OLR 6.4 OLR 8.5

51 71 91 111 131 151

Time (days)

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

OLR

(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).

77


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.)

80

60

40

20

0

1 11 21 31 41 51 61

Time (days)

Figure 4.13 Cumulative methane per liter of reactor volume in ITDAR

Dual Treatment

(Anaerobic +

Aerobic)

RT in AD: 18 d

OLR: 8.5 kg VS/m 3 /d

VS removal: 67%

Cumulative methane

yield: 57 L CH4/Lreactor vol

OLR 4.55 OLR 6.4 OLR 8.5

Dry Anaerobic

Digestion

Figure 4.14 Selection of operating conditions based on purpose of waste treatment

78

Single Treatment

(Anaerobic

digestion)

RT in AD: 30 d

OLR: 4.55 kg VS/m 3 /d

VS removal: 78%

Cumulative methane

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

process.

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

methane.

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

79


digestate TS was kept constant at 12-13%.Thus adjustment of TS in the reactor helped in

its smooth operation.

TS (%FM)

Run

Figure 4.15 Comparison of feed and digestate regarding total solids in phase I

experiment

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

35

30

25

20

15

10

5

50

45

40

35

30

25

20

15

10

5

0

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

80

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

3.0

2.5

2.0

1.5

1.0

0.5

0.0

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

(%)

Organic

matter

(%TS)

82

N

(%TS)

C/N

ratio

Reference

Digestate 7.75 88-90 37.28 1.94 18.35 This study

Thai Guidelines* 5.5-8.5 ≤ 35 ≥ 35 ≥ 1 ≤ 20

Rattanaoudom,

2005

Indian Guidelines 5.5-8.5 - > 26 > 1 12-25

Gautam et al.,

2010

*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

this section.

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)

600

500

400

300

200

100

0

568

139 125

OFMSW Digestate Stored Digestate Cured stored

digestate

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.

83

80


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

4.6.

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

84


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

(g CO2-eq/kg

digestate )

GHG saving by fertilizer

substitute (g CO2-eq/kg

digestate)

CH4 N2O N P

85

Net GHG

emission

(g CO2-eq/kg

digestate )

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

digestate.

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)

250

200

150

100

50

0

-50

190

Scenario 1 Scenario 2

-11

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

People

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

community.

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,

86

12 13

Scenario

129


digestion, etc. Moreover, mixing of different kinds of waste will also be done to adjust C/N

ratio.

- 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

Detail

Value/Specification of the reactor

Pilot-scale Real-scale

Amount of waste (kg/d) 22 2000

Reactor size (m 3 ) 0.69 62.5

Dimensions of reactor(m)

-Diameter

0.6

3

-Height

2.4

9

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

Start-up phase

- 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

anaerobic condition.

- 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).

87


- 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

coming week.

- 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

Detail Value/Specification

Dimensions of sand drying beds (m)

-Width

-Length

Thickness of bed layers (cm)

-Sand

-Coarse sand

-Fine gravel

-Medium gravel

-Coarse gravel

Type of sand drying bed Conventional and open

Number of sand drying bed

Under-drainage system

3

-Pipe material

PVC

-Pipe type

Perforated

-Pipe diameter

15 cm

-Pipe spacing

2 m

-Pipe slope

2%

Layer of digestate over bed 20 cm

88

6

16

20

7.5

7.5

7.5

15


Figure 4.20 Layout of conceptual decentralized AD plant for a community

89


- 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

4.4.3 (i).

- 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

amendment.

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

90


Chapter 5

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

section.

5.1 Conclusions

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.

91


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

digestion process.

92


5.2 Recommendations

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

methane.

93


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.

94


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109


Appendix A

Experimental Set-up Pictures

110


Fruit and vegetable

Waste

Closer to real field conditions

Waste material used (5 fractions)

Mechanical shredder

(25 mm size)

Figure A-1 Waste material used in this study

Figure A-2 Set-up for GP21Test (Front view)

111

Shredded waste

Five fractions

Food waste

Vegetables

Fruit waste

Leaf waste

Paper waste


Wet gas

meter

Figure A-3 Set-up for GP21Test (Side view)

Biogas

tube

Withdrawal

port

Hot water

tank

Reactor

Screw

pump

Figure A-4 Pilot-scale inclined thermophilic dry anaerobic reactor

112

Feeding

hopper


Figure A-5 Sand drying bed for dewatering of digestate

113


Appendix B

Calculation of Proposed Decentralized AD Plant

114


Assumptions

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

2000 kg/d

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.

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

Method 1:

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

Method 2:

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

115


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

Composition

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

Characteristics

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

a

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

116


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

Run

Energy

production

(MJ/kg

VS)

Energy Consumption (MJ/kg VS)

Feeding

Heating and

Shredding and Recirculation maintaining

withdrawal

thermophilic

conditions

Surplus

energy

(%)

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

Description Specification

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

http://www.sino-cummins.com

2. http://www.alibaba.com/

Biogas flow rate for 1 kW gas engine: 0.569 m 3 /h (Prakash et al., 2012)

117


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

3

Biogas flow

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

House electricity

1

1000 watt/house

2

0.569 15

1

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

118


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.

Other considerations

-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 %

1400

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

119


(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

used.

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

(L/min)

25 180

Time for 1 cycle of 22 260

recirculation (min)

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

120


= 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

Site/General

-Land area (for all digester set-up and

building)

-Building (store room for equipment,

office, etc)

Feed preparation system

-Shredder or comminuter

-Mixer

-Feed storage tanks 2 m 3 (5 tanks)

Reactor system

-Digester 3m (dia) x 9m (length) steel vessel

Pumping and recirculation system

-Screw bed pump AE1N-1450 (Allweiler)

Heating system

-Steam generator

Biogas conversion to electricity

-Biogas storage tank

-Gas engine

Digestate management

-Sand drying bed

-Digestate storage tanks

Miscellaneous

-Pipes and valves, other spare parts,

tools, etc.

-Bins, containers, etc.

121

100 m 3

14 kW

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

Detail Specification/Remarks

Staff requirement -1 Plant Manager

-1 Process control operator

-2 Maintenance technicians

-2 General labor

Utilities and Fuels -Fuel

-Electricity

-Water

-Natural gas for start-up

Maintenance -Equipment

-Site works

Miscellaneous -Lab analysis cost

-Wastewater treatment cost


Appendix C

Data of Phase I Pilot Experiment

122


Table C-1 Operational Parameters of Anaerobic Digestion During Phase I Pilot

Experiment

Run Time

pH VFA Alkalinity VFA/Alk ratio TAN

(day)

(mg/L) (mg/L)

(mg/L)

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

123

Free Ammonia

(mg/L)


Table C-2 Performance Parameters of Anaerobic Digestion During Phase I Pilot

Experiment

Run Time

GPR % CO2 % CH4 Methane Yield

(L/Lreactor vol./d)

(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

(days)

124

VS Removal

(%)


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

125


Table C-3 Characteristics of Feed and Digestate in Phase I Pilot Experiment

Run

Time

(days)

Feed

TS

(%

FM)

Feed

VS

(%

FM)

Feed

VS/

TS

Digestate

TS

(% FM)

126

Digestate

VS

(% FM)

Digest

ate

VS/TS

Digest

ate

TKN

(%)

Digest

ate

C/N

ratio

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.


Appendix D

Data of Phase II Pilot Experiment

127


Table D-1 Operational Parameters of Anaerobic Digestion During Phase II Pilot

Experiment

Run Time

pH VFA Alkalinity VFA/Alk ratio TAN

(day)

(mg/L) (mg/L)

(mg/L)

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

128

Free Ammonia

(mg/L)

Table D-2 Performance Parameters of Anaerobic Digestion During Phase II Pilot

Experiment

Run Time

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

VS Removal

(%)


Table D-3 Characteristics of Feed and Digestate in Phase II Pilot Experiment

Run

Time

(days)

Feed

TS

(%

FM)

Feed

VS

(%

FM)

Feed

VS/TS

129

Digestate

TS

(% FM)

Digestate

VS

(% FM)

Digestate

VS/TS

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


Appendix E

Methodology for Calculation of Energy Balance

130


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

section.

section.

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:

Feeding

Feeding

Withdrawal

Withdrawal

Recirculation

Recirculation

Shredding

Shredding

Heating

Heating

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

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.

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)

where

where

MP MP = = methane methane production production (L (L CH4)

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)

2010)

HP HP = = Hydrogen Hydrogen production production (L (L H2)

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)

2010)

1.2 1.2 Energy Energy Consumption

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)

(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.

131

131

Biogas


where

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)

where

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)

1000

where

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).

132


Q =

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).

where

Q =

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

133


Appendix F

Sample Calculation of GHG Emission Potential

134


Calculation of GHG Emission Potential of Digestate

The formula for methane emission potential of digestate is given as:

and

where

Methane

emission

g CH 4 / kg digestate

( M TS DOC DOC MCF F (( 16 / 12)

R)

( 1 OX ) 1000

F

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

Thus,

Methane

emission

g of methane / kg digestate

1

0.

1045 0.

356 0.

77 0.

4

0.

5

( 16 / 12)

1000

= 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

135

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