158 Organic waste recycling: technology and management
Table 4.4 Type species of methanogenic bacteria (Ferguson and Mah 1987) (continued)
Bacterium Substrate
Methanothermus fervidus DSM 2088 H 2 -CO 2
Methanothrix concilii GP6 Acetate
Methanothrix soehngenii Opfikon Acetatea May also use mono- and dimethylamine
b Requires the combination of methanol and H 2
4.3 Environmental requirements for anaerobic digestion
Anaerobic reactions in a digester can start quickly with the presence of a good
inoculum or seed, such as digested sludge. During start-up or acclimation, the
seed material should be added to the influent feed material in sufficient quantity,
e.g. at least 50%. The seed volume can then be progressively reduced while
increasing the proportion of the influent feed over a three- or four-week period.
At the end of this period, the influent feed can be fed alone to the digester to
support the growth of anaerobic bacteria. Solid content of the feed material
should be about 5-10%, the remaining being water.
Like any other biological processes, anaerobic digestion is a multi-parameter
controlled process, each individual parameter having overall control over the
process either through their own effect on the system or interaction with other
parameters. These parameters are described below:
4.3.1 Temperature
Temperature and its daily and seasonal variation have a pronounced effect on
the rate of gas production. Generally two ranges of temperature are considered
in methane production. These are mesophilic (25 - 40°C) and thermophilic (50 -
65 °C) similar to those described in Chapter 3. The rate of methane production
increases as the temperature increases, but there is a distinct break in the rise at
around 45°C, as this temperature favours neither the mesophilic nor the
thermophilic bacteria (Figure 4.4). However, no definite relation other than
increasing rate of gas production (within certain limits) can be established.
Below 10°C the gas production reduces drastically; therefore, operation below
this level is not recommended due to limited amount of gas production (among
other technical problems). Above 30 - 35°C, operation of the digester requires
substantial amount of energy input for digester heating and this in turn will
make the operation economically impractical. This suggests that mesophilic