Aerobes and Effluents 117that the degree of contaminant removal achievable becomes uncertain and less
easily controlled. Partial mineralisation of toxic substances is a particular con-
cern, often leading to the accumulation of intermediate metabolites in the treated
wastewater, which may represent the production of a greater biological threat.
The incomplete metabolism of these chemicals under aerobic conditions typi-
cally results in oxidised intermediary forms which, though less intrinsically toxic
than their parent molecules, are often more mobile within the environment. In
addition, when the treatment efficiency is subject to monitoring, as intermedi-
ate metabolites, these substances may not be picked up by standard analytical
techniques, which may result in an unfairly high measure of pollutant removal
being obtained.
Moreover, the extension of sewage treatment facilities to ameliorate trade efflu-
ents also has implications for the management of true sewage sludge. It is not
economically viable to develop processing regimes which do not lead to the
concentration of toxic contaminants within the derived sludge. This was shown
to be a particular problem for plants using the activated sludge process, which
relies on a high aeration rate for pollutant removal, which proceeds by making
use of biotransformation, air stripping and adsorption onto the biomass. Adsorp-
tion of toxic inorganic substances like heavy metals, or structurally complex
organic ones, onto the resident biomass, poses a problem when the microbial
excess is removed from the bioreactor, particularly since dewatering activities
applied to the extracted sludge can, in addition, catalyse a variety of chemical
transformations. Accordingly, sewage sludge disposal will always require careful
consideration if the significant levels of these chemicals are not subsequently to
cause environmental pollution themselves.
Land Spread
The previous chapter discussed the inherent abilities of certain kinds of soil
microbes to remediate a wide range of contaminants, either in an unmodified
form, or benefiting from some form of external intervention like optimisation,
enhancement or bioaugmentation. Unsurprisingly, some approaches to sewage
treatment over the years have sought to make use of this large intrinsic capacity as
an unengineered, low-cost response to the management of domestic wastewaters.
Thus, treatment by land spread may be defined as the controlled application of
sewage to the ground to bring about the required level of processing through
the physico-chemical and biological mechanisms within the soil matrix. In most
applications of this kind, green plants also play a significant role in the overall
treatment process and their contribution to the wider scope of pollutant removal
is discussed more fully in the next chapter.
Although it was originally simply intended as a disposal option, in a classic
case of moving a problem from one place to another, the modern emphasis is
firmly on environmental protection and, ideally, the recycling of the nutrient