118 Environmental Biotechnology
component. The viability of land treatment depends, however, on the prevention
of groundwater quality degradation being afforded a high priority. In the early
days of centralised sewage treatment, the effluent was discharged onto land and
permitted to flow away, becoming treated over time by the natural microbial
inhabitants of the soil. This gave rise to the term a ‘sewage farm’ which persists
today, despite many changes in the intervening years. Clearly, these systems are
far less energy intensive than the highly engineered facilities common in areas
of greater developed urbanisation.
The most common forms of effluent to be treated by land spread, or the related
soil injection approach, are agricultural slurries. According to the European Com-
mission’s Directorate General for Environment, farm wastes account for more
than 90% of the waste spread on land in Europe and this is predominantly
animal manure, while wastes from the food and beverage production industry
form the next most important category (European Union 2001a). Removal of the
constituent nutrients by soil treatment can be very effective, with major reduc-
tions being routinely achieved for suspended solids and BOD. Nitrogen removal
generally averages around 50% under normal conditions, though this can be
significantly increased if specific denitrifying procedures are employed, while a
reduction in excess of 75% may be expected for phosphorus. Leaving aside the
contribution of plants by nutrient assimilation, which features in the next chapter,
the primary mechanisms for pollution abatement are physical filtration, chemical
precipitation and microbiological metabolism. The latter forms the focus of this
discussion, though it should be clearly understood that the underlying principles
discussed in the preceding chapter remain relevant in this context also and will
not therefore be lengthily reiterated here.
The activity is typically concentrated in the upper few centimetres of soil,
where the individual numbers of indigenous bacteria and other micro-organisms
are huge and the microbial biodiversity is also enormous. This natural species
variety within the resident community is fundamental to the soil’s ability to biode-
grade a wide range of the components in the wastewaters applied to it. However,
it must be remembered that the addition of exogenous organic material is itself
a potential selective pressure which shapes the subsequent microbial comple-
ment, often bringing about significant alterations as a result. The introduction of
biodegradable matter has an effect on the heterotrophic micro-organism popula-
tion in both qualitative and quantitative terms, since initially there will tend to
be a characteristic dying off of sensitive species. However, the additional nutri-
ents made available, stimulate growth in those organisms competent to utilise
them and, though between influxes, the numerical population will again reduce
to a level which can be supported by the food sources naturally available in the
environment, over time these microbes will come to dominate the community.
In this way, the land spreading of wastewater represents a selective pressure, the
ultimate effect of which can be to reduce local species diversity. Soil experiments
have shown that,in extremis, this can produce a ten-fold drop in fungal species