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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
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: acetogens play their part to run th
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
Page 69 and 70:
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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