Environmental Engineering Reference
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compared to the actual data. For the input feed characteristics, it may not
be necessary to measure the content of all components if the feedstock is
dominated by composite material. For example, in monodigestion of grass
silage, the model input may be assumed as a composite material because
grass silage comprises approximately only 10% of readily soluble
components (such as soluble sugars and VFAs); 90% of the feedstock is
comprised of particulate components (Thamsiriroj and Murphy, 2011). The
particulate feedstock is converted into soluble components during the
anaerobic process, which narrows the gap between simulation and actual
observation of such components. However, if the input feed is low in solids
content but high in soluble COD such as industrial wastewater, then a more
detailed analysis of feed characteristics is required to enable an accurate
simulation.
5.8.5 Optimisation of anaerobic digestion using computer
models
The behaviour of an anaerobic digester is complex and highly dynamic and
involves a series of processes that are interrelated. Failure of an individual
process can result in failure of the entire system. The stability of an
anaerobic digester depends on many factors such as feedstock character-
istics, organic loading rate, retention time, reactor operating conditions and
reactor configuration (dry or wet, batch or continuous, number of
connected digesters, separation of phases, recirculation of digestate).
Modelling allows variation of one factor while the others are fixed and
thus the effect of an individual factor on the digester performance can be
simulated. For example, a model created by Lai et al. (2009) was used to
predict the optimal concentrations of bicarbonate initially added to produce
high methane yields in a batch reactor digesting OFMSW; Thamsiriroj and
Murphy (2011) applied the ADM1 to simulate the behaviour of a two-stage
CSTR system digesting grass silage in which they found that recirculation of
digestate can limit the build-up of total VFAs, resulting in long-term stable
operation. The simulation of many individual factors can help to illustrate
the optimal conditions in which the design and operation will be most
effective. Modelling can also be used to identify a process that becomes rate
limiting and eventually leads to digester failure. A modelling study by
Thamsiriroj et al. (2011) on grass monodigestion suggested that acetogenesis
is a critical step that can become rate limiting in long-term operation due to
the scarcity of micronutrients. This would concur with findings by Banks
(2011) that selenium and cobalt are essential micronutrients for food waste
digestion.
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