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

Based on our results, the best operating conditions for AD (RT and OLR) can be selected.
But it also depends on the purpose of treatment (Figure 4.14). As we know anaerobic
digestion can be used as a single treatment for waste or in combination with aerobic
process.
In case of dual treatment (anaerobic + aerobic), time spent for anaerobic digestion should
be minimized, so that the saved time could be used for aerobic treatment. Thus RT of 18
days (OLR 8.5 kg VS/m 3 /d) should be preferred for digestion step. But if the purpose is
only a single treatment (i.e. dry anaerobic digestion), then option of higher retention time
(RT 24 d or 30 d) should be chosen to get maximum VS removal and its conversion to
methane.
4.4 Digestate Management and GHG Emissions (Phase III)
Proper management of digestate is needed as it has certain GHG emission potential. So, it
tends to emit methane to the atmosphere, if not stored properly. Moreover, it has certain
amount of nutrients and organic matter, which could be useful if applied on agricultural
soils. Therefore, to protect the environment and to make use of digestate’s economic value,
careful digestate management should be performed. There are several options for digestate
handling and management, for example, digestate dewatering, digestate storage, digestate
application to the land, digestate composting, digestate curing or dumping. But
characteristics of digestate are the key to determine the best and correct option for
management of digestate. Thus, in this section of thesis, characteristics of digestate have
been first described and based on the characteristics, the proper digestate management
options have been suggested and finally their effect studied on its nutrient content and
GHG emission potential.
4.4.1 Characteristics of raw digestate
Digestate was removed from the reactor every day before feeding of fresh waste
throughout the reactor operation period. The freshly withdrawn digestate (raw digestate)
was analyzed for moisture, TS and VS content twice a week. Moreover, digestate was also
characterized for carbon and nitrogen content to calculate its C/N ratio. Characteristics of
digestate have been discussed in this section (Figure 4.15, 4.16 and 4.17; Appendix C,
Table C-3, Appendix D, Table D-3).
With digestion the feed TS decreased, so digestate TS was lower than feed TS. Digestate
TS content was high in the beginning (16 -20%) and started to decrease as the digestion
proceeded. During run 3, it started to increase again almost continuously and reached upto
18.5% at the end of run 4 as shown in Figure 4.15 and given in Appendix C, Table C-3.
This increase in digestate TS corresponds to the increase in feed TS (23 -25%) combined
with increase in OLR. Similar observations were made by Mumme et al., (2010). Also
there was no replenishment of moisture lost through biogas, which is higher in
thermophilic process. Only little changes were observed for the VS content of TS (i.e.
digestate VS/TS), which has been in the range of 0.6-0.7 throughout the study.
The practical consequence of this increase in digestate TS was that it was difficult by the
screw bed pump to feed and re-circulate the feedstock and reactor material respectively.
Thus TS content in the reactor was adjusted by mixing of water with feedstock (making
feed TS 17-20%) starting from run 6 onwards as shown in Figure 4.15 and thereby the
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