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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
Page 1 and 2: DRY ANAEROBIC DIGESTION OF MUNICIPA
Page 3 and 4: Abstract Global solid waste generat
Page 5 and 6: 2.9 Characteristics of Digestates 3
Page 7 and 8: List of Tables Table Title Page 2.1
Page 9 and 10: 4.20 Layout of conceptual decentral
Page 11 and 12: 1.1 Background Chapter 1 Introducti
Page 13 and 14: The specific objectives of this res
Page 15 and 16: scale plants of the two processes i
Page 17 and 18: acetogens play their part to run th
Page 19 and 20: In dry anaerobic digestion, recycli
Page 21 and 22: process is inhibited and at that po
Page 23 and 24: eported, which increased with time,
Page 25 and 26: acids and increased downfall of pat
Page 27: performance of a poorly mixed (1 rp
Page 31 and 32: supply of the nutrients missing in
Page 33 and 34: 2.6.2 Single-stage continuous syste
Page 35 and 36: single stage systems are DRANCO, Va
Page 37 and 38: of the solid content around 23% TS
Page 39 and 40: Substrate Feed TS (%) Reactor Type
Page 41 and 42: Second is that it has a high water
Page 43 and 44: 2.9 Characteristics of Digestates D
Page 45 and 46: For instance, Mumme et al., (2010)
Page 47 and 48: Comparison of liquid and solid dige
Page 49 and 50: 2.10 Management Aspects of Anaerobi
Page 51 and 52: Figure 2.11 Changing parameters dur
Page 53 and 54: amount of anaerobic fermentation re
Page 55 and 56: 3.1 Inoculum and Simulations of Was
Page 57 and 58: ed pump, water circulating jacket,
Page 59 and 60: the reactor was increased from 35°
Page 61 and 62: parts (wt/wt bas is) of digestate c
Page 63 and 64: Sand drying bed (SDB) is simple, ea
Page 65 and 66: 3.4.4 Estimation of GHG emissions i
Page 67 and 68: d) Calculation methods i) CH4 emiss
Page 69 and 70: Table 3.5 Analytical Methods for Va
Page 71 and 72: Biogas production 100X (NmL) Specif
Page 73 and 74: (i.e. 5.2 and 3.04 respectively, pl
Page 75 and 76: getting affected with the presence
Page 77 and 78: feedstock 2 is considered as a sudd
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8.0) at most of the above said time
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Table 4.2 Surplus Energy of ITDAR D
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VFA 100 X (mg/L) VFA/Alk ratio 200
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the VFA concentration increased to
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The increase in GPR was almost line
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Based on our results, the best oper
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Based on this property of digestate
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Table 4.5. With curing of digestate
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application (after curing) in CH 4
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digestion, etc. Moreover, mixing of
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Figure 4.20 Layout of conceptual de
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Chapter 5 Conclusions and Recommend
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5.2 Recommendations Following are t
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References Abdullahi, Y. A., Akunna
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Cengel, Y.A. (2003). Heat Transfer
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Forster-Carneiro, T., Pérez, M., R
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Kaparaju, P., Buendia, I., Ellegaar
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Liu, C., Yuan, X., Zeng, G., Li, W.
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Paavola, T. and Rintala, J. (2008).
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Stroot, P. G., McMahon, K. D., Mack
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Zeshan, Karthikeyan, O. P. and Visv
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Fruit and vegetable Waste Closer to
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Figure A-5 Sand drying bed for dewa
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Assumptions Size of the community:
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The flow rate of 0.569 m 3 biogas/h
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Area for 910 kg dry solids = 910 (k
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= 280 m 3 /d x 0.000717 tons/m 3 (D
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Table C-1 Operational Parameters of
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Run Time GPR % CO2 % CH4 Methane Yi
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Appendix D Data of Phase II Pilot E
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Table D-3 Characteristics of Feed a
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Methodology Methodology for for Ene
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Q = Heat transfer rate or heat loss
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Calculation of GHG Emission Potenti
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