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

Total ammonia-N to total nitrogen ratio (TAN/TKN ratio) is considered as a good indicator
for estimating the percentage conversion of total nitrogen into ammonia-N during the
anaerobic digestion. As stated, the calculated percentage of ammonia-N with the feedstock
1 was around 0.77, whereas, it was 0.40 for the feedstock 2. Kayhanian (1999) suggested
that using feedstock C/N ratio from 27 to 32 promotes steady digester operation at
optimum ammonia-N levels. In the present study, feedstock 2 with C/N ratio 32 produced
an overall less concentration of ammonia-N and free ammonia as compared to feedstock 1.
Moreover, there was no accumulation of ammonia-N under commonly used retention time
(20–30 days) for digesters. Thus the feedstock with C/N ratio 32 was found to be better
than that with C/N ratio 27 for dry thermophilic anaerobic digestion to minimize or avoid
ammonia-N inhibition.
In our experiment, Feedstock 2 had low %TKN (1.62%), which produced TAN 1993
mg/L. Using these two values, mathematical calculation for Feedstock 1 having high
%TKN (1.92%) was done, by which it should produce TAN 2350 mg/L (Expected TAN).
But in actual experiment, it produced even higher TAN, i.e., 2821 mg/L. Thus
experimental value of TAN production for waste with high %TKN (or low C/N ratio) was
higher than its calculated value (Expected TAN). The reason may be that the system was
biological but not stoichiometric. The presence of high amount of %TKN in case of
feedstock 1 (C/N 27) as compared to C/N 32 might have resulted a relatively rapid growth
of microbes, which in turn resulted in the more conversion of organic nitrogen into TAN.
Thus experimental TAN was higher than Expected TAN in case of feedstock 1. However,
it needs to be proved experimentally in future studies.
From the results of this experiment, it can be concluded that ammonia nitrogen
accumulation (one main operational problem of dry anaerobic digestion) can be thus
mitigated by use of correct feed mixture (i.e. by adjusting composition or C/N ratio of the
feed). ii) This method to mitigate ammonia accumulation is good for dry anaerobic
digestion as the other methods (dilution of ammonia by addition of water as well as
stripping of ammonia) are not desirable and/or suitable for dry digestion. iii) Moreover,
adjusting the feed composition can be easily managed for a decentralized dry AD system
compared to centralized system.
4.2.4 Energy balance of ITDAR in Phase I pilot experiment
Table 4.2 compiled the net energy gains obtained from two different trials of ITDAR and
most of the runs produced surplus energy. The net energy gain from the reactor is
determined by the quality of produced biogas. The quality of biogas, in turn, depends on
the operational conditions and feedstock composition. Please refer to Appendix E for
methodology of energy balance calculations.
Average energy consumptions during different runs were distributed as 75% for
maintaining the reactor under thermophilic conditions, 12% for the shredding, 2.5% for the
waste loading and withdrawal and 9.6% for the digestate recirculation. From the energy
consumption percentile, it was evident that the continuous temperature maintenance of the
ITDAR upset with the maximum percentage of net energy gained.
70