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

The higher concentration of free ammonia in ITDAR directly influenced the overall
methane production as can clearly be observed from the Figure 4.4b and d during run 4
(day 135-145). The effect of free ammonia or ammonia-N on the inhibition of biogas
production was instantaneous and the systems succeeded in recovering from the inhibition
only in few cases (Chen et al., 2008). As can be noted, the feed composition variations and
sudden shift in OLR severely affected the performance of ITDAR as evidenced from the
free ammonia accumulation and methane yield. In large scale centralized systems, the
detrimental effect due to the compositional and OLRs variations could be magnified and
may require longer time for the system to get recovered.
Whereas, in a decentralized system like ITDAR, the effect was shortened by altering the
OLR (in run 5), which i n turn lowered the system pH and reduced the free ammonia
concentration (reduced from 432 to 164 mg/L) in ITDAR. Moreover, the lesser SRT and
higher feed C/N ratio reduced the protein solubilization rate and hence produced lesser
ammonia-N concentration within the system, which was found to be advantageous. Straka
et al. (2007) also found that the dilution of waste biomass mixture by addition of activated
sludge in thermophilic anaerobic digestion system reduced the ammonia-N inhibition
effect. On the other hand, the VFA/Alk ratio of the system was also affected and became
0.35 during run 5. The specific methane yield from run 5 was calculated as 121 mg/L
(lowest yield among 8 runs) mainly due to lesser SRT. During run 6 and 7, the average free
ammonia concentration was calculated as 227 and 221 mg/L, respectively. It was even
lower (135 mg/L) during run 8. But, specific methane production rate was almost similar in
all the three runs i.e., around 200 mg/L.
Run 8 performed better among the other runs with higher OLR and lesser SRT. The pH
was near neutral, VFA/Alk ratio was around 0.56 and ammonia-N concentration was
around 1758 mg/L in run 8. Also, long time run stabilized the system performance and
associated parameters in this case.
Although the process control is easy in a decentralized system by judiciously altering the
operating conditions (like OLR, C/N ratio, etc.), but the change should be performed very
carefully and gradually to avoid its effect on performance of biological system. It is
evident in run 4 of this study where sudden change in C/N ratio affected the performance
of the system.
4.2.3 Summary of the effect of ammonia-N accumulation in ITDAR
Total ammonia-N concentration decreased with the increase in C/N ratio of the feedstock
1–2 as depicted in Figure 4.5. The average ammonia-N concentration in the digestate was
2820 and 1990 mg/L for the feedstock 1 (run 1 –3) and 2 (4–8), respectively. Hence, the
decrease of ammonia-N concentration in ITDAR was calculated as 30% with the use of
feedstock 2 (C/N ratio of 32). It can be noted from the Figure 4.4b, that free ammonia
accumulation occurred for short duration (3 –5 days) on days 41, 91 and 187 up to the
levels of 370–425 mg/L (Appendix C, Table C-1), but there was no effect observed in
specific methane yield. Whereas, the accumulation was around 400–660 mg/L and
sustained for a longer duration (10–30 days) especially during run 4 and run 7. It affected
the overall methane yield, which means that the digestion process was inhibited or
moderately inhibited. As the digestion process proceeds, the pH becomes first stable and
then it starts to increase steadily with the increasing alkalinity. As free ammonia is a
function of ammonia-N, pH and temperature, thus it increased with the increasing pH (7.5–
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