Environmental Engineering Reference
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5.2 One-stage dry batch digester with sprinkling of liquor in a closed
loop (adapted from Nizami and Murphy, 2010).
(Nizami and Murphy, 2010). Reactors may be operated under mesophilic or
thermophilic temperature conditions.
5.5.2 Dry batch reactors
Reactors may take the form of a simple batch process (Fig. 5.2); this is
commonly used for feedstocks with solids content of 30-40% (Hobson,
1990). The feedstock is fed to the digester, and gas production initiates,
increases, peaks, decreases and then ceases. At this stage the batch is opened
and approximately half of the feedstock is removed. The remainder is left as
innoculum for the next batch (Nizami and Murphy, 2010). The effective
retention time is therefore of the order of 50% greater than the period
between feeding frequency. Typically, liquor drained from the digester is
showered on the feedstock and fed in a closed loop, repeatedly showering
the process. The advantage of the batch digester is its simplicity. The
parasitic energy demand is low. The solids content is high so a relatively
small amount of thermal energy input is required. The processing required is
also low, leading to a low electrical parasitic demand. The benefit of the
system is exemplified when treating waste such as OFMSW. Contaminants
are not of issue for the system; if, for example, a piece of metal is in the
feedstock this will not encroach on any moving part. The system typically
employs existing agricultural or construction equipment to insert and
remove the feedstock. Vertical garage door systems are often employed in
these systems. The main disadvantage of the system is that it may not
maximise the methane production per unit of feedstock. For commercial
developments, numerous batches are employed that are fed sequentially to
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