Biotechnology and Waste 195take-up of this technology than any other factor. It should also be apparent that
more resources, and primarily a more skilled workforce, are essential prerequisites
for success. However, for wastes which are particularly suited to this form of
biotechnology, a number of cost-benefit analyses over the years have shown that
these drawbacks may often be outweighed by the advantages inherent in the
system. As with so many practical applications of environmental biotechnology,
there is seldom one catch-all solution and the most appropriate approach can
only really be judged on the specifics of the problem. There will always be cases
when either composting or AD is self-evidently the most suitable route; when
the matter is less clear-cut, however, the technology decision is often much more
difficult to make.
There are many ways in which anaerobic digestion systems may be cate-
gorised as will be briefly discussed below. However, it is important to realise
that, irrespective of their individual construction, they all fundamentally consist
of isolated vessels of some kind, designed to exclude air and maintain internal
conditions at the optimum for bacterial action. It is possible to describe systems
treating slurries of 15% total dry solids (TDS) or less as ‘wet’, or ‘dry’ if their
TDS exceeds this 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 per-
mitted to digest the contents before being emptied and recharged, while the latter
have a continuous cycle of new biowaste being added and processed material
being drawn off. Figure 8.3 shows the interrelationship 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
Figure 8.3 Classifying anaerobic digestion systems