Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 197

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
little large-scale use, probably as a result of the higher cost implications of such
asystem.
Agitation
The agitation of the digester contents has a number of benefits, one of the most
obvious being that it helps to mix up material, evening out any localised con-
centrations, thus also helping to stop the formation of 'dead zones' or scum.
In addition, it increases the waste's availability to the bacteria, helps remove
and disperse metabolic products and also acts to ensure a more uniform tem-
perature within the digester. There have been some suggestions that efficient
mixing enhances methane production, but the evidence is inconclusive, so it
seems likely that this may only be of noticeable benefit for some systems or
operational regimes.
We t ne s s
AD is a wet process and any biowaste which is too dry in its natural state will
require the addition of a suitable liquid, typically water, recycled AD process
liquor or slurries, either sewage or agricultural, before processing can begin. In
order to minimise digester size, so-called 'dry' systems have tended to domi-
nate the commercial world, but the relatively thicker contents inevitably demand
more energy to mix effectively, off-setting much of the advantage. Comparisons
of 'wet' or 'dry' approaches, like those of mesophilic or thermophilic processes,
generally yield no clear winner. Each system has particular advantages and appli-
cations for certain kinds of biowaste, and selecting the right one for any given
use is almost always best done on this basis.