196 Environmental Biotechnology
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.
Continuously stirred tank reactor (CSTR)
Intended to treat slurries and liquid biowastes, this system is essentially the same
as the preceding CMCR design, but without the need for the solids’ recycle.
Fluidised bed reactor (FBR)
Sometimes termed the expanded bed reactor, this system relies on an inter-
nal microbial growth medium, which is fluidised by the waste liquid circulat-
ing within it. Accordingly they are only suitable for liquid biowastes or very
dilute slurries.
Multi-phasic processes (MPP)
Physically separating the stages of AD into different reactors, these systems
are principally of use as experimental tools for increasing understanding of the
pathways and mechanisms of the anaerobic digestion process. They are featured
here for completeness, but they do not generally have any commercial application.
Upflow anaerobic sludge blanket (UASB)
These systems hold relatively high numbers of active bacteria, which makes them
suitable for treating biowastes of low solids content and they are commonly used
to process high-strength industrial waste liquids or light suspensions.