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

of the solid content around 23% TS inside the reactor. At lower values, heavy particles
such as sand and glass tend to sink and accumulate inside the reactor while higher TS
values cause excessive resistance to the flow. The most significant factor of tubular reactor
is its ability to separate acidogenesis and methanogenesis longitudinally down the reactor,
allowing the reactor to behave as a system of two phases (Bouallagui et al., 2005).
2.7 Research Progress and Research Needs of Dry Anaerobic Digestion
Various research studies on dry anaerobic digestion have been conducted at TS content of
15-30% using different substrates (Table 2.5). Only few research studies conducted on dry
anaerobic digestion have been conducted in pilot-scale digesters (with size of 1-3 m 3 ) but
most of them have been conducted in laboratory-scale digesters (2-60 L). As far as mode
of reactor operation is concerned, both batch and continuous systems are found and almost
all of them are single-stage. Dry anaerobic digestion studies have been performed at both
mesophilic (30-37 o C) and thermophilic (55 o C) temperature ranges. The studies conducted
under continuous mode are up to OLR and RT of up to 7.5-13 kg VS/m 3 d and 12-40 days
respectively. Almost half of the dry AD studies given in this table focused on OFMSW as
substrate. Other than OFMSW, straws and residues of crops, solid livestock waste (e.g.
cow dung, horse dung), food waste and dewatered sewage sludge have been used as feed
for high-solids digestion. Methane yield and VS removal of various studies given here can
not be compared as these vary with the waste type and waste characteristics. However,
process design and experimental conditions can significantly influence the methane yield
and gas production rate.
Research conducted on dry anaerobic digestion so far, as discussed in the above part of
chapter summarizes that proper inoculation, optimum C/N ratio, controlled or minimal
mixing, relatively low TS feed and high or thermophilic temperature can be helpful for
smooth start-up of dry digesters and can control VFA accumulation during start-up of dry
digesters. Moreover, alkaline pre-treatment and thermophilic temperature can enhance
hydrolysis of slowly degradable wastes like straws, and other wastes consisting of crop
residues. In addition, co-digestion or adjustment of C/N ratio to control both VFA and
ammonia accumulation can be used during continuous operation of dry anaerobic
digesters. Thermophilic temperature for dry AD could reduce the retention time and hence
could reduce reactor volume. This together with smaller reactor advantage of dry AD could
sufficiently save the capital cost.
Research on pilot and field scale in the field of dry AD needs to be done. Moreover, the
effect of mixing on performance of dry anaerobic digestion still needs to be studied at
higher range of TS content.
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