Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 199

pH and volatile fatty acids concentration
These are interdependent factors which need to be considered together. Adequate
process control and digester optimisation requires suitable pH monitoring, since
many of the bacteria involved are pH sensitive. In particular, acidogens, having
better tolerance to acidity, may produce acids faster than the increasingly inhibited
methanogens can use it, in conditions of low pH, leading to spiralling acidity and
the potential for process collapse. A number of acid-base reactions exist within
the typical AD process, which lead to a measure of natural, inbuilt resistance to
major pH swings. However, under certain circumstances, the need for external
interference may arise and the amount of such intervention necessary to maintain
proper equilibrium will depend on the nature of the material. For some wastes,
pH control may only be necessary during start-up or in overload conditions; for
others where acidity is habitually shown to be a problem, continuous control may
be necessary.
Volatile fatty acid concentration is one of the most important process indica-
tors. Elevated VFAs are characteristic of AD instability and thus they may be
the first indication of a developing problem, though the actual cause may be
less immediately obvious. Inadequate mixing, excessive loading, poor temper-
ature control or bacterial inhibition can all lead to an increase in VFAs and a
decrease in pH. Considering the inconvenience and cost of being forced to empty
a sick reactor, commercial AD operations rely greatly on routine monitoring of
this kind.
Biogas
The production of methane-rich biogas, which is an essential characteristic of
anaerobic digestion, has been used as an important marketing tool, since the
generation of energy from waste by biological means has much attraction, not
least amongst those countries with a popular dislike of incineration. A cohesive
discussion of bioenergy appears in a later chapter and accordingly, a consideration
of the wider implications of biogas fuel will be deferred until then. However, it is
worth pointing out in the waste-specific context that, though much has been made
of the apparent dual benefit of biowaste treatment and energy production, in an
actual application, it is a practical impossibility to optimise both simultaneously.
Other Biotechnologies
Although composting and anaerobic digestion between them account for by far
the bulk of biological waste treatment globally and each has a well-established
track record, as is so often the case with practical applications of environmental
biotechnology, neither is a clear winner. Dependent on the specifics of the situ-
ation, the particular waste composition, local factors and so on, either can have
clear benefits to offer and, as we have seen, both can form adequate responses