Environmental Engineering Reference
In-Depth Information
6.5
Increasing the metabolic capacity of a digester
Where the maximum load on a digester is defined by its metabolic capacity
then improvement in the volumetric methane productivity could be gained
by increasing that metabolic capacity. This concept applies mainly to
digesters that are receiving substrate at a relatively high concentration,
although there are cases where the performance of slurry digesters can also
be improved when the limitation is shown not to be the HRT. Factors that
might be considered and which could contribute to increasing the metabolic
capacity of a digester are operating temperature, availability of macro-
nutrients, micronutrient status and aspects of digester design.
6.5.1 Operating temperature
Temperature has a strong effect on a range of kinetic parameters, including
the maximum specific growth rate of microorganisms, the half-saturation
constant, the growth yield and the decay rate. The maximum rates at which
microorganisms can utilise substrates are thus strongly temperature
dependent, with methanogens more sensitive than hydrolytic or acidogenic
bacteria. Methanogenic organisms can grow over a very wide temperature
range (Madigan et al., 2010) but well-established optima exist at around 35-
40
C (thermophilic) and these temperature
ranges are therefore the most common ones for full-scale operation. Some
methanogens are known that have higher optimal growth temperatures (e.g.
Zeikus and Wolee, 1972; Lauerer et al., 1986; Miller et al., 1988; Kurr et al.,
1991) but these extreme thermophiles have not so far been exploited on a
commercial scale. Depending on the substrate, the rate of methane
generation in thermophilic systems may be 25-50% higher than in
mesophilic systems, allowing shorter retention times. At the same time,
thermophilic systems are more susceptible to variations in operating and
environmental conditions, and are more likely to accumulate volatile fatty
acids (VFA). When optimising an AD system for net energy production, the
additional energy required to raise and maintain feedstock and digester
contents at this temperature must also be taken into account. Figure 6.4
shows an example of the calculated surplus energy available from the
digestion of a fixed daily volume (100m
3
) of feedstocks with different COD
concentrations entering a mesophilic (35
8
C (mesophilic) and around 55
8
C) digester
with a minimum 12-day retention time at an influent and ambient
temperature of 15
8
C) or thermophilic (55
8
C, assuming that all of the COD is converted to methane
and taking into account heat transfer losses from a typical insulated digester
(Salter and Banks, 2009). In terms of optimising the process, the increase in
volumetric production that could be obtained by operating at thermophilic
temperature needs to be compared with the additional energy consumed in
8
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