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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
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: scale plants of the two processes i
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 and 28: performance of a poorly mixed (1 rp
Page 29 and 30: conventional low-solid system at th
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
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Table 3.5 Analytical Methods for Va
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Biogas production 100X (NmL) Specif
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(i.e. 5.2 and 3.04 respectively, pl
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getting affected with the presence
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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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