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

performance of a poorly mixed (1 rpm) and a well mixed (25 rpm) dry digester was
observed. On the other hand, Karim et al., (2005a ) reported that mixed and unmixed
digesters performed quite similar when fed with slurry with 5% TS (wet digestion),
whereas the mixing effect became important when fed with thicker slurry (with 10% and
15% TS), because mixed digesters fed with 10-15% TS feed produced 10-30% more
biogas than unmixed digesters. However Stroot et al., (2001) reported that continuous
mixing in high-solid digestion showed unstable performance at high OLR whereas
minimal mixing showed a better performance at all OLR studied. Moreover, a
continuously mixed unstable reactor became stable (as shown by consumption of
accumulated VFA and simultaneous rise in pH) when the mixing level was reduced. From
above discussion it can be concluded that either intermittent mixing or a low level of
mixing is needed for good performance of dry anaerobic digestion.
Impellers and paddles have been provided in available dry anaerobic digestion
technologies like COMPOGAS and Linde-BRV to facilitate plug-flow and mixing of
reactor medium in the horizontal reactors. Guendouz et al., (2010) stated that one problem
with large-scale reactors is that complete mixing is never achieved. Thus, they designed
and operated a laboratory-scale reactor in which complete mixing was enabled with the use
of a paddle mixer.
2.4.6 Retention time
This is the ratio of volume of reactor to the flow rate of influent substrate (Eq. 9). In other
words, the time required by the waste material to pass through the reactor is called
retention time.
Retention time (d) = Reactor volume (m 3 )
Flow rate (m 3 /d)
The required retention time for completion of the anaerobic digestion reactions varies with
technologies, process temperature, TS content and waste composition. The retention time
for wastes treated in mesophilic digester is usually higher (up to 40 days) than that of
thermophillic digesters which can be up to 8 days (Cecchi et al., 1991) . Shortening the
retention time decreases the reactor volume and hence saves capital cost. Increase in
retention time, however, increases reactor stability. High retention time increases biogas
due to better stabilization (Poggi-Varaldo and Oleszkiewicz, 1992; Kim and Oh, 2011) on
one hand and decreases it due to lower OLR on the other hand. If the concentration of
organic matter in the substrate is relatively constant, the shorter the RT the higher will be
the value of OLR as can be noted in many reports (Rivard et al., 1990; Gallert and Winter,
1997; Montero et al., 2010; Mumme et al., 2010; Fdez-Guelfo et al., 2011).
One of the disadvantages of dry anaerobic digestion is that it needs high retention time (15-
30 days) as compared to wet anaerobic digestion (where it can be as low as 3 days)
(Nayono, 2010). Thus, feedstock with high TS content needs long RT for digestion. Kim
and Oh, (2011) reported that a t a fixed solid content of 30% total solids, stable
performance was maintained up to an HRT decrease to 40 d. However, the stable
performance was not sustained at 30 d HRT, and hence, HRT was increased to 40 d again.
However, relatively low RT is required in dry digesters operated within thermophilic
range.
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