Environmental Engineering Reference
In-Depth Information
by co-digestion with mixed-waste feedstocks that are potential sources of
contamination because this reduces the value of the product and limits
disposal options.
6.6.3 Digester type
From a process engineering point of view, optimisation could be regarded as
finding a balance between the energy and raw materials required to operate
the process against the output energy and added value of the digestate. It
may also involve designing the process to meet the particular characteristics
of the feedstock in terms of its energy density, solids content and
biodegradability. Process engineering optimisation can also be interpreted
in economic terms as minimising the cost of the technical equipment and
ongoing operational costs relative to the return on investment. There are
now many different plant design concepts to choose from, which have been
variously catalogued under headings such as wet, dry, single phase, two
phase, leach bed, complete mix, plug flow. Further information on these can
be found in Chapter 5 with detailed accounts in texts such as those by Mata-
Alvarez (2003) for solid wastes and Chernicharo (2007) for high-rate
anaerobic wastewater treatment processes.
Although there are many aspirations in the scientific literature for the
development of faster, smaller and more efficient digestion plants, ultimately
the conversion of carbon substrate to methane gas is a biological function
and the key limitations will therefore remain with the rate of hydrolysis for
solid substrates and the growth rate of methanogens for liquid substrates.
The latter can be overcome to a large extent by high rate systems with
biomass retention and the former, to a lesser extent, by designing reactors to
achieve more effective hydrolysis incorporating mechanical, chemical or
biological pre-treatment systems. At the present time, 90% of reactors for
digestion of solid substrates, sludges and slurries are vertically mounted
CSTR-type digesters operating at mesophilic temperatures, as this type of
reactor suits many of the currently available feedstocks. In cases where the
feedstock has a high proportion of anaerobically non-degradable fibre with
a low moisture content (e.g. municipal solid wastes), high solids 'dry'
digestion systems offer an effective alternative and the low water content
may also allow efficient operation at thermophilic temperatures, fulfilling
the requirement for pasteurisation when used in plug flow mode. CSTR
designs will always result in a proportion of bypass and where this is
undesirable, for example in energy crop digestion, having primary and
secondary digesters in series has been shown to maximise specific methane
yield (Weiland, 2010).
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