Biomedical Engineering Reference
In-Depth Information
since they thrive in a relatively narrow pH threshold of 6.6-7.0, becoming pro-
gressively more impaired as the pH falls below 6.4. In this event, the persistence
of unmodified VFAs can have potentially serious implications for the final use
or disposal of the material derived.
There are four main groups of bacteria involved in AD, as shown below, with
some typical examples of each:
•
Hydrolytic fermentative bacteria -
Clostridium
and
Peptococcus
.
•
Acetogenic bacteria -
Syntrophobacter
and
Syntrophomonas
.
•
Acetoclastic methanogens -
Methanosarcina
and
Methanothrix
.
•
Hydrogenotrophic methanogens -
Methanobacterium
and
Methanobrevibac-
terium
.
In reality, these are not the only species present in a digester and, though the
stages previously described represent the main desired biochemical reactions, a
number of additional bacterial types and biochemical pathways play a role in
the overall breakdown process. As with composting, there is much interaction
between these various organisms.
Applying AD to Waste Management
The nature of anaerobic digestion inevitably means that its applications to
waste management are relatively large-scale operations, there being no effective
equivalent of home composting. Hence, whether the application is as an onsite
treatment for process effluent or as part of a centralised municipal waste initiative,
the approach relies heavily on engineering, a schematic plant being shown in
Figure 8.2. This is in clear contrast with composting and, together with the
attendant additional costs, probably goes further to explain the overall lower
Figure8.2
ADplantschematicflowchart
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