Biofuels production 159range provides the optimal operational range of temperature, although pathogen
inactivation will be less than those to be achieved in the thermophilic range
(Figure 3.1).
During winter period, heating of biogas digesters may be necessary so that
growth of anaerobic bacteria, especially the methanogens, will be possible. The
heating of a digester can be accomplished by heating the influent feeding
materials (e.g. with the biogas produced) and feeding it to the digester, or by the
recirculation of hot water through coils of pipe installed inside the digester.
Other means to heat a digester include (Brown and Tata 1985):
- housing the digester in an enclosure lined with a thick transparent
plastic film; the heat within the enclosure can be 5 - 10°C higher
than the ambient temperature. - designing the digester in such a way that water can be held on the
roof of the digester and heated by solar radiation. - insulating the digester with suitable materials available locally or by
placing compostable material like leaves in an annular space built
around the digester.
Figure 4.4 Effect of temperature on gas production (Price and Chermisinoff 1981)
4.3.2 pH and alkalinity
Operational range of pH in anaerobic digesters should be between 6.6 and 7.6,
with the optimum range being 7 to 7.2. Although acid-forming bacteria can
tolerate a pH as low as 5.5, the methanogenic bacteria are inhibited at such low
pH values. The pH of a digester may drop to below 6.6 if there is an excessive
Gas production (volume / time)Temperature (°C)