dry anaerobic digestion of municipal solid waste and digestate ...

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
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