Biotechnology and Waste 197From all of this discussion, it should be obvious that, however proprietary AD
technologies are classified, no one type is universally ideal or superior, each
having certain characteristics which make it appropriate for particular wastes and
less suitable for others. This means, of course, that although comparisons of the
various approaches are of great interest to potential users, in practice they are
difficult to make in any meaningful way. Even certified data from an operating
plant can only be taken as broadly indicative of how a similar one might perform
elsewhere, especially in respect of breakdown efficiency and biogas generation
or quality.
Process parameters
Efficient AD requires the development and maintenance of an optimised internal
environment to facilitate biological activity. This is of particular importance in
the commercial setting and a number of both physical and chemical factors must
be taken into account to achieve it, of which the most important are:
- temperature;
- retention period;
- agitation;
- wetness;
- feedstock;
- loading rate;
- pH and volatile fatty acids concentration.
Temperature
As mentioned previously, in commercial systems, digesters are operated at around
35 ◦C (mesophilic) or 55◦C (thermophilic). Irrespective of which approach is
adopted for any particular application, a relatively constant temperature is essen-
tial for the process to run at its greatest efficiency.
Retention period
Although the amount of biowaste degraded depends on its character, the avail-
ability of bacteria and the time allowed for processing, temperature governs both
the rate of breakdown itself and the particular bacterial species present in the
digester. Hence, there is a direct relationship between temperature and the reten-
tion period. Some AD technologies have attempted to shorten the retention period
by separating the stages of the process within the digester. The separation of the
acidogenic and methanogenic stages permits each to be optimised and this has
been well demonstrated at laboratory scale using a completely mixed digester,
with phase-isolation being achieved by pH manipulation. Despite the greater effi-
ciency, higher biogas yield and enhanced process stability claimed, it has seen