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

NaOH were added to the reactor periodically during days 5-25 to maintain pH at near
neutral range. It can be noted as small peaks during days 11-26 in Figure 4.6. From day 28
onwards, pH did not drop again and started increasing slowly, therefore, NaOH was not
added anymore. It became stable at around 8.2 during days 42-50 (Appendix D, Table D-
1).
pH
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
1 11 21 31 41
Time (days)
Figure 4.6 pH profile of ITDAR during start-up
At first, the concentration of VFA increased after loading the reactor and reached to its
maximum (19000 mg/L) on day 25. However, once the pH became stable, VFA
concentration started to drop due to its utilization. The concentration of VFA dropped from
19000 to 6300 mg/L only in 20 days (Figure 4.7 and Appendix D, Table D-1). The reason
is that there was no waste feeding throughout the star-up phase (day 1-50). The evolution
of VFA/Alk ratio was different from the VFA concentration. Increase in VFA/Alk ratio
after reactor loading was not observed, rather there was a continuous decrease, which
indicates that alkalinity started to develop and increase just after loading. It may be
because a part of inoculum used for this start-up was taken from the system with the same
conditions (treating OFMSW under thermophilic conditions). After day 30, VFA/Alk ratio
started to decrease in the same way as that of VFA. This may be attributed to decrease in
VFA concentration.
Methane content in biogas and gas production rate (GPR) was lower in the beginning. The
reason might be unfavorable conditions for methanogenesis, i.e., pH lower than 6.8 and
VFA more than 6000-8000 mg/L (Polprasert, 2007). However, methane content and GPR
started to increase slowly as the system progressed towards stability. On the contrary,
carbon dioxide content was higher at the start (Figure 4.8 and Appendix D, Table D-2),
which is a sign of acidification. But it decreased slowly, as alkalinity and pH increased and
VFA concentration got utilized.
From the above discussion, it can be concluded that pH and VFA concentration of the
reactor were not stable until day 30, thus, methane content was low, carbon dioxide content
was high and GPR was unstable. However, after day 35, the reactor conditions were stable
and, therefore, both the methane content in biogas and GPR increased.
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