198 Environmental Biotechnology
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.
Wetness
Anaerobic digestion 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 dominate the commercial world, but the relatively thicker contents inevitably
demand more energy to mix effectively, off-setting much of the advantage. Com-
parisons of ‘wet’ or ‘dry’ approaches, like those of mesophilic or thermophilic
processes, generally yield no clear winner. Each system has particular advantages
and applications for certain kinds of biowaste, and selecting the right one for any
given use is almost always best done on this basis.
Feedstock
As with composting, the particle size and nature of the material to be treated play
an important role. The ease of breakdown is largely defined by the characteristics
of the biowaste material to be treated, but generally finer particles allow for better
processing and a homogeneous slurry or suspension is the ideal feedstock for AD.
It must be stressed, however, that some biowaste types, particularly the likes of
lignin-rich, woody material, are relatively resistant to this process.
Loading rate
Loading depends on the characteristics of the waste, its degree of wetness,
digester volume, the expected retention period and similar system design param-
eters. It is typically expressed as the chemical oxygen demand per cubic metre
of digester void-space (COD/m^3 ) or, for continuous or semi-continuous process,
per unit time (COD/day, COD/hr).