Environmental Engineering Reference
In-Depth Information
6.5.4 Mixing and mass transfer
Much early work in the 1950s on improving the AD process focused on
heating and mixing the digester, increasing feedstock solids content and
reducing the HRT. Laboratory and field experience at that time suggested
that continuous mixing of the digester contents was required for successful
operation. Even today, there is considerable debate in the literature as to
whether mixing should be continuous or intermittent, by mechanical means
or gas recirculation (Karim et al., 2005); and there are several reports where
mixing has been shown to slow down or reduce biogas production (Stroot
et al., 2001; Kaparaju et al., 2008). There is still general agreement, however,
that some mixing is necessary for a high rate of reaction.
Mixing helps to prevent stratification, temperature gradients, deposition
of solids and scum, and crust formation. In doing this it maintains physical,
chemical and biological uniformity in the digester and minimises short
circuiting in CSTR designs. From a biological perspective, mixing systems
provide active contact between the biomass and the substrate, distribute the
feedstock effectively and, in a CSTR, dilute inhibitory substances.
In large-scale commercial systems possibly one of the biggest impacts
associated with mixing, or the lack of it, is the build-up of inert high-density
mineral materials (grit) in the base of the digester. This effectively reduces
the size of the digestion tank, increases the organic loading and reduces the
retention time - all factors that could adversely affect both the specific
methane yield and the volumetric methane production if the digester is
working close to its metabolic or hydraulic limits. Mixing is an energy-
intensive process and consumes a proportion of the energy produced: an
efficient mixing system for a CSTR should therefore aim to maintain all
solids in suspension with the minimum energy input. The types of mixing
systems and their relative merits are discussed in Chapter 8.
In dry high-solids digestion where reactors often have plug flow
characteristics or may be operated in batch mode, mixing within the reactor
itself is not normally applied but there is a high degree of mixing of fresh
feedstock with inoculum at the start of the process to ensure good contact
with the substrate. These reactors tend to be used with feedstocks that have
a high fibre content, where a close association between substrate and
degrading organisms through the formation of cellulosomes (Shoham et al.,
1999; Schwarz, 2001) may be important for species such as Clostridium,
which are common fermentative agents in anaerobic systems.
Close association between different microorganisms is also important
where syntrophy is an essential part of the metabolic pathway (Speece et al.,
2006). In these circumstances it has been suggested that mixing may
adversely affect productivity and alternatives such as low or intermittent
mixing and biomass retention systems may be preferable.
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