Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 195

Figure8.3 Classifyinganaerobicdigestionsystems
figure. Alternatively, the temperature range at which they are operated can be
used, thus leading to defining AD systems as either mesophilic or thermophilic.
The loading regime adopted can also be a useful means of distinguishing
digester types for some purposes, allowing a distinction to be drawn between
'batch' and 'continuous' systems. The former are filled in a single go, then
permitted to digest the contents before being emptied and recharged, while the
latter have a continuous cycle of new biowaste being added and processed mate-
rial being drawn off. Figure 8.3 shows the inter-relationship of these various
branches of the AD family tree.
However, these are, in effect, operational criteria and as such, though useful
in themselves, they can tend to unite dissimilar technologies within essentially
artificial groupings, giving little clue as to which best suits what type of biowaste.
For this, an examination of aspects of the digester design and engineering prin-
ciples can often provide a better insight, as in the following descriptions of some
of the major examples.
Anaerobic baffled reactor (ABR)
These generally feature a horizontal flow of biowaste through the digester vessel,
and are suitable for a wide range of materials. The Valorga process, with its
patented gas recirculation and mixing system, is based on this approach.
Anaerobic fixed film reactor (AFFR)
These digesters have a fixed growth plate on which a bacterial biofilm is estab-
lished, digestion taking place on this surface. They are ideal for relatively weak
biowastes with low solids content, but are of less use in other applications.
Completely mixed contact reactor (CMCR)
In this design, the biomass derived during processing is recycled after dewatering
to increase the retention time for the solids. This approach is typically used to
treat high strength, industrial biowastes.