122 Environmental Biotechnology
Limits to land application
There are, then, limits to the potential for harnessing the processes of natural
attenuation for effluent treatment. While centuries of use across the world tes-
tify to the efficacy of the approach for human sewage and animal manures, its
application to other effluents is less well indicated and the only truly ‘industrial’
wastewaters routinely applied to the land in any significant proportion tend to be
those arising from food and beverage production. This industry is a consumer
of water on a major scale. Dairy production uses between 2–6 m^3 of water per
1m^3 of milk arriving at the plant, the manufacture of preserves requires any-
thing between 10–50 m^3 of water per tonne of primary materials consumed and
the brewing industry takes 4–15 m^3 of water per tonne of finished beer pro-
duced (European Union 2001b). A significant proportion of the water is used
for washing purposes and thus the industry as a whole produces relatively large
volumes of effluent, which though not generally dangerous to human health or
the environment, is heavily loaded with organic matter.
The alternative options to land spreading involve either dedicated on-site treat-
ment or export to an existing local sewage treatment works for coprocessing with
domestic wastewater. The choice between them is, of course, largely dictated by
commercial concerns though the decision to install an on-site facility, tanker
away to another plant or land spread, is often not solely based on economic
factors. Regional agricultural practice also plays an important part, in terms of
fertiliser and irrigation requirements as well as with respect to environmental
and hydrological considerations. It is, of course, a fundamental necessity that the
approach selected can adequately cope with both the physical volume of the max-
imum effluent output on a daily or weekly basis, and the ‘strongest’ wastewater
quality, since each is likely to vary over the year.
Although it is convenient to consider the food and beverage industry as a single
group, the effluent produced is extremely variable in composition, depending on
the specific nature of the business and the time of the year. However, there are
some consistent factors in these effluents, one of which being their typically heavy
potassium load. Much of their nutrient component is relatively readily available
both for microbial metabolism and plant uptake, which obviously lends itself to
rapid utilisation and in addition, the majority of effluents from this sector are
comparatively low in heavy metals. Inevitably, these effluents typically contain
high levels of organic matter and nitrogen and, consequently, a low C/N ratio,
which ensures that they are broken down very rapidly by soil bacteria under even
moderately optimised conditions. However, though this is an obvious advantage
in terms of their treatability, the concomitant effect of this additional loading
on the local microbiota has already been mentioned. In addition, these effluents
may frequently contain heavy sodium and chloride loadings originating from the
types of cleaning agents commonly used.