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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
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DRY ANAEROBIC DIGESTION OF MUNICIPA
Page 3 and 4:
Abstract Global solid waste generat
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2.9 Characteristics of Digestates 3
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List of Tables Table Title Page 2.1
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4.20 Layout of conceptual decentral
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1.1 Background Chapter 1 Introducti
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The specific objectives of this res
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scale plants of the two processes i
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acetogens play their part to run th
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In dry anaerobic digestion, recycli
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process is inhibited and at that po
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eported, which increased with time,
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acids and increased downfall of pat
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performance of a poorly mixed (1 rp
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conventional low-solid system at th
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supply of the nutrients missing in
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2.6.2 Single-stage continuous syste
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single stage systems are DRANCO, Va
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of the solid content around 23% TS
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Substrate Feed TS (%) Reactor Type
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Second is that it has a high water
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2.9 Characteristics of Digestates D
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For instance, Mumme et al., (2010)
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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
Page 79 and 80: 8.0) at most of the above said time
Page 81 and 82: Table 4.2 Surplus Energy of ITDAR D
Page 83 and 84: VFA 100 X (mg/L) VFA/Alk ratio 200
Page 85 and 86: the VFA concentration increased to
Page 87 and 88: The increase in GPR was almost line
Page 89 and 90: Based on our results, the best oper
Page 91 and 92: Based on this property of digestate
Page 93 and 94: Table 4.5. With curing of digestate
Page 95 and 96: application (after curing) in CH 4
Page 97 and 98: digestion, etc. Moreover, mixing of
Page 99 and 100: Figure 4.20 Layout of conceptual de
Page 101: Chapter 5 Conclusions and Recommend
Page 105 and 106: References Abdullahi, Y. A., Akunna
Page 107 and 108: Cengel, Y.A. (2003). Heat Transfer
Page 109 and 110: Forster-Carneiro, T., Pérez, M., R
Page 111 and 112: Kaparaju, P., Buendia, I., Ellegaar
Page 113 and 114: Liu, C., Yuan, X., Zeng, G., Li, W.
Page 115 and 116: Paavola, T. and Rintala, J. (2008).
Page 117 and 118: Stroot, P. G., McMahon, K. D., Mack
Page 119 and 120: Zeshan, Karthikeyan, O. P. and Visv
Page 121 and 122: Fruit and vegetable Waste Closer to
Page 123 and 124: Figure A-5 Sand drying bed for dewa
Page 125 and 126: Assumptions Size of the community:
Page 127 and 128: The flow rate of 0.569 m 3 biogas/h
Page 129 and 130: Area for 910 kg dry solids = 910 (k
Page 131 and 132: = 280 m 3 /d x 0.000717 tons/m 3 (D
Page 133 and 134: Table C-1 Operational Parameters of
Page 135 and 136: Run Time GPR % CO2 % CH4 Methane Yi
Page 137 and 138: Appendix D Data of Phase II Pilot E
Page 139 and 140: Table D-3 Characteristics of Feed a
Page 141 and 142: Methodology Methodology for for Ene
Page 143 and 144: Q = Heat transfer rate or heat loss
Page 145: Calculation of GHG Emission Potenti
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