Biofuels production 1614.3.4 Loadings
This term can be expressed as organic loading {kg COD or volatile solids
(VS)/m^3 -day} and hydraulic loading or retention time (HRT). A too-high
organic loading will normally result in excessive volatile fatty acid production
in the digester (sour condition) with a consequent decrease in pH, and adversely
affecting the methanogenic bacteria. A too-low organic loading will not provide
sufficient quantity of biogas for other uses and will make the digester size
unnecessarily too large. Because organic materials feeding to anaerobic
digesters are in semi-solid form, organic loading to a digester can be
conveniently interpreted in terms of VS.
Section 4.4 describes the two main types of anaerobic digesters i.e. the
dispersed-growth digesters (those employing dispersed-growth bacteria) and the
attached-growth digesters (those employing attached-growth bacteria).
Optimum organic loadings to dispersed-growth digesters have been reported to
be 1-4 kg VS/(m^3 -day) and 1-6 kg COD/(m^3 -day); for attached-growth
digesters, they are 1-15 and 5-30 kg COD/(m^3 -day) for anaerobic filters and up-
flow sludge blanket digesters, respectively (Barnet et al. 1978 and Brown and
Tata 1985).
HRT has an equally significant effect on digester performance. Too short an
HRT will not allow sufficient time for anaerobic bacteria, especially the
methane-forming bacteria, to metabolize the wastes. Too long an HRT could
result in excessive accumulation of digested materials in the digester, and
construction of the digester too large in size. Similar to the organic loadings, an
optimum HRT depends on the characteristics of influent feed materials and
environmental conditions in the digesters. For dispersed-growth digesters, the
optimum HRT falls within the range of 10-60 days; while for the attached-
growth digesters, they are 1-10 and 0.5-6 days for anaerobic filters and up-flow
sludge blanket digesters, respectively (Brown and Tata 1985).
It appears from the above information that the attached-growth digesters can
be operated at higher organic loadings or shorter HRT than the dispersed-growth
digesters. This advantage is attributed to the nature of attached-growth bacteria
that attach to the media and/or stay in the digester for a long period of time.
They are thus present in the digesters in high concentrations, not easily washed
out/overflowed in the digester slurry, and are well acclimated to the incoming
wastes. To increase the process performance or achieve higher loading rates, the
dispersed growth digesters can have part of their slurry recycled back to the
digesters in order to retain more active biomass and increase the solids retention
time.