116 Environmental Biotechnology
in the context of waste management in Chapter 8, has also been used as a means
of sludge treatment. The use of this biotechnology has brought important benefits
to the energy balance of many sewage treatment works, since sludge is readily
biodegradable under this regime and generates sizeable quantities of methane
gas, which can be burnt to provide onsite electricity.
At the same time, water resources are coming under increasing pressure, either
through natural climatic scarcity in many of the hotter countries of the world, or
through increasing industrialisation and consumer demand, or both. This clearly
makes the efficient recycling of water from municipal works of considerable
importance to both business and domestic users.
Though in many respects the technology base of treatment has moved on,
the underlying microbiology has remained fundamentally unchanged and this
has major implications, in this context. In essence, the biological players and
processes involved are little modified from what would be found in nature in
any aquatic system which had become effectively overloaded with biodegrad-
able material. In this way, a microcosmic ecological succession is established,
with each organism, or group, in turn providing separate, but integrated, steps
within the overall treatment process. Hence, heterotrophic bacteria metabolise the
organic inclusions within the wastewater; carbon dioxide, ammonia and water
being the main byproducts of this activity. Inevitably, increased demand leads
to an operational decrease in dissolved oxygen availability, which would lead to
the establishment of functionally anaerobic conditions in the absence of external
artificial aeration, hence the design of typical secondary treatments. Ciliate proto-
zoans feed on the bacterial biomass produced in this way and nitrifying microbes
convert ammonia first to nitrites and thence to nitrates, which form the nitro-
gen source for algal growth. Though the role of algae in specifically engineered,
plant-based monoculture systems set up to reduce the nitrogen component of
wastewaters is discussed more fully in the next chapter, it is interesting to note,
in passing, their relevance to a ‘traditional’ effluent treatment system.
One of the inevitable consequences of the functional ecosystem basis underly-
ing sewage treatment plants is their relative inability to cope with toxic chemicals
which may often feature in certain kinds of industrial wastewaters. In particu-
lar, metabolic poisons, xenobiotics and bactericidal disinfectants may arrive as
components of incoming effluents and can prove of considerable challenge to
the resident microbes, if arriving in sufficient concentration. This is a fact often
borne out in practice. In 2001, considerable disruption was reported as a result
of large quantities of agricultural disinfectant entering certain sewage works as a
consequence of the UK’s foot and mouth disease outbreak. A number of poten-
tial consequences arise from such events. The most obvious is that they kill off
all or part of the biological systems in the treatment facility. However, depen-
dent on the nature of the substances, in microbially sublethal concentrations,
they may either become chemically bound to either the biomass or the substrate,
or be subject to incomplete biodegradation. The effective outcome of this is