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

Moreover, from the comparison of start-up phases of the two pilot experiments, it was
found that low digestate recirculation rate (i.e. 1 Ldig/Lreactor vol.d) should be used to achieve
stable reactor conditions in less time. The results of continuous loading phase have been
presented in the following sections.
% of Biogas
70
60
50
40
30
20
10
0
Methane Carbon dioxide GPR
1 11 21 31 41
Time (days)
Figure 4.8 CH4, CO2 and GPR fluctuation during start-up phase
4.3.2 Stability parameters of ITDAR: Effect of organic loading rate
i) pH
pH is very basic parameter to describe the stability of anaerobic digestion. With OLR of
4.55 kg VS/m 3 /d, the system stabilized its pH at around 7.75 with a range of 7.5-8 as
shown in Figure 4.9. When the OLR was increased from 4.55 to 6.4 kg VS/m 3 .d, pH fell
down to 7.58 and regulated to an average of 7.67 (7.33 -7.96). As a result of further
increase in OLR to 8.5 kg VS/m 3 /d, a drastic decrease in pH was observed and pH dropped
to the value of 6.89 (Appendix D, Table D-1). Therefore, NaOH was added during days
150-155 to control pH. The decline in pH in the starting days of each of the first two runs
and most of the last run is linked to destabilization of the system as a result of increase in
OLR. The reason is that when organic loading rate is increased, the acidogens also increase
their activity and produce high amount of VFA, as they are fast growing. But, on the other
hand, methanogens owing to their slow specific growth rate can not utilize all the already
produced VFA and need more time to build the required population size. Thus initial and
temporary decrease in pH is due to accumulation of VFA as a result of this imbalance in
the microbial groups, which is recovered until methanogens build their sufficient
population. The decrease of pH is more pronounced while working with higher OLR, i.e.,
8.5 kg VS/m 3 .d. The reason is that the imbalance between acidogenic and methanogenic
activity is more pronounced.
ii) Volatile fatty acids (VFA)
The concentration of volatile fatty acids in the digestate of ITDAR was quite stable at an
average value of 5100 mg/L (range: 4400-5700 mg/L) while operating at OLR of 4.55 kg
VS/m 3 /d (Figure 4.10 and Appendix D, Table D-1). When OLR was increased to 6.40 kg
VS/m 3 /d, VFA concentration started to increase and reached a maximum value of 6500
mg/L with an average value of 5400 mg/L in this run. Finally, at OLR of 8.50 kg VS/m 3 /d,
74
2.5
2.0
1.5
1.0
0.5
0.0
GPR (L/Lreactor vol./d)