Environmental Engineering Reference
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digestate high in solids content that may be suitable as a feedstock for a
biorefinery concept, has a higher methane content in the biogas, but
produces 24% less methane (Nizami et al., 2011b).
5.8 Modelling and optimisation of anaerobic digestion
5.8.1 Modelling of anaerobic digesters
Mathematical modelling allows use of data from experiments or actual
operating digesters to simulate operating conditions under different loading
conditions. The model takes into account the specific configuration of the
digesters.
.
A vertical dry continuous reactor with mixing of feedstock and digestate
before pumping to the top of the reactor as described by De Baere
(2010) (and as indicated in Fig. 5.4(a)) is modelled as a series of CSTRs
with the effluent from one reactor acting as the influent for the next
reactor.
.
A horizontal dry continuous reactor (Fig. 5.4(b)) with an axial paddle
stirrer is modelled in a similar fashion to the vertical reactor but allows
for bifurcation and recycling of part of the bacterial biomass (Zaher and
Chen, 2006).
.
A wet continuous reactor (Fig. 5.3(a)) with biogas recirculation under
pressure to provide mixing can be modelled as a single CSTR (Zaher
and Chen, 2006).
This section examines Anaerobic Digestion Model No 1 (ADM1)
(developed by Batstone et al. (2002) under the International Water
Association (IWA) Task Group for Mathematical Modelling of
Anaerobic Digestion Processes) as applied to the CSTR (Fig. 5.3(a)). The
model has been widely used by researchers to investigate anaerobic digestion
of various feedstocks and digester configurations.
5.8.2 Description of ADM1
ADM1 is a mathematical model that simulates the behaviour of different
particulate (solid) and soluble (liquid) components within an anaerobic
digester. The model deals with anaerobic digestion in three dimensions:
processes, components and time. Input feedstock is assumed to be a
composite particulate material that can be broken down into different
substrate components through processes taking place within the digester
(Fig. 5.1). Conservation of mass is applied. The processes are categorised
into two main processes - biochemical and physico-chemical processes
(Batstone et al., 2002).
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