Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 118

sewage sludge produced. Spreading this to land has been one solution which
has been successfully applied in some areas, as a useful fertiliser substitute on
agricultural or amenity land. Anaerobic digestion, which is described more fully
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 sizable quantities of methane gas,
which can be burnt to provide on-site 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 biodegradable material. In
this way, a microcosmic ecological succession is established, with each organ-
ism, or group, in turn providing separate, but integrated, steps within the overall
treatment process. Hence, heterotrophic bacteria metabolise the organic inclu-
sions within the wastewater, carbon dioxide, ammonia and water being the main
by-products 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 aera-
tion, hence the design of typical secondary treatments. Ciliate protozoans feed
on the bacterial biomass produced in this way and nitrifying microbes convert
ammonia first to nitrites and thence to nitrates, which form the nitrogen-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 sub-lethal concentrations,
they may either become chemically bound to either the biomass or the substrate,