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

et al., 2011). Therefore, it is very clear that the ITDAR possibly inhibited with the free
ammonia toxicity during the trial runs 1–3 with the feed C/N ratio of 27.
Table 4.1 Digestion Parameters and Methane Yield of ITDAR
Run
VFA
(mg/L)
VFA/Alk
ratio
pH
Total
Ammonia-N
(mg/L)
66
Free
ammonia
(mg/L)
Specific
methane
production
(L/kgVS)
VS
loss
(%)
Feedstock 1 (avg. C/N ratio 27)
Start-up 37857 a
1.41 6.29 1943 25 9.60 -
1 35920 1.21 6.95 2753 99 176 46.52
2 27567 0.52 7.53 3040 328 286 64.16
3 18025 0.51 7.49 2671 284 327 70.20
Feedstock 2 (avg. C/N ratio 32)
4 11725 0.31 7.75 2360 432 218 54.36
5 11417 0.35 7.35 1895 164 121 35.37
6 9625 0.39 7.50 2161 227 222 53.65
7 7469 0.39 7.29 1791 221 203 52.53
8 6010 0.56 7.34 1758 135 203 54.02
a
Table gives average values for each run.
In addition, the VFA/Alk ratio was also higher i.e., 1.21 during run 1 and it was around 0.5
for run 2 and 3. Very high concentration of unconsumed VFA from start-up phase carried
to run 1 could be the possible reason for the higher VFA/Alk ratio during run 1. As
discussed earlier, the higher VFA/Alk ratio in run 1 affected over the entire methane
production rate in ITDAR.
In addition to the ammonia-N concentrations, VFA/Alk ratio higher than its optimum value
(i.e. ≤ 0.4) was also found to be affecting the overall methane yield while feeding the
reactor with C/N ratio of 27.
b) Effect of feedstock 2 (C/N ratio of 32) in ITDAR – Run 4 to 8
Feedstock 2 with the C/N ratio of 32 was used for runs 4–8 to reduce the free ammonia
toxicity. Using this range of C/N ratio has been reported (Kayhanian, 1999) as a one of the
best tools to mitigate ammonia-N inhibition in dry thermophilic anaerobic digesters.
During trial 2 with the higher C/N ratio, the ammonia-N concentration further reduced
from 2360 to 1758 mg/L (avg.) in ITDAR during different runs. The sudden change in
feedstock composition (with the C/N ratio of 32) from run 3 (trial 1) with increasing OLR
affected the overall reactor performance during run 4 (trial 2) as shown in Figure 4.4 (b-d).
The reason was that higher C/N ratio helped system to increase alkalinity and pH value
(8.00), which increased free ammonia concentration (657 mg/L) in ITDAR. The detailed
explanation with data of Appendix C, Table C-1 is given here. When feeding of feedstock
2 was started, the change in alkalinity and pH was not very significant (alkalinity changed
from 32000 to 35250 mg/L and pH changed from 7.59 to 7.75) as per data from day 98 to
day 127 in the appendix. This duration can be regarded as acclimatization time for the new
feedstock (i.e. Feedstock 2) for ITDAR being a biological system. After that, the increase
in alkalinity and pH was quite significant (alkalinity changed from 35250 to 68750 mg/L
and pH changed from 7.75 to 8.00) during day 127-141, which increased free ammonia
from 411 to 657 mg/L and directly affected methane yield. This change in parameters by