Biomedical Engineering Reference
In-Depth Information
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 ani-
mal manure, while wastes from the food and beverage production industry form
the next most important category (European Commission's Directorate General
for Environment, 2001). Removal of the constituent nutrients by soil treatment
can be very effective, with major reductions being routinely achieved for sus-
pended solids and BOD. Nitrogen removal generally averages around 50% under
normal conditions, though this can be significantly increased if specific deni-
trifying 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 microbio-
logical 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
biodegrade a wide range of the components in the wastewaters applied to it.
However, it must be remembered that the addition of exogenous organic mate-
rial is itself a potential selective pressure which shapes the subsequent microbial
compliment, often bringing about significant alterations as a result. The introduc-
tion of biodegradable matter has an effect on the heterotrophic micro-organism
population in both qualitative and quantitative terms, since initially there will
tend to be a characteristic dying off of sensitive species. However, the additional
nutrients 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 10-fold drop in fungal species
and that Pseudomonas species become predominant in the bacterial population
(Hardman, McEldowney and Waite, 1994).
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