Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 124

between 10 and 50m 3 of water per tonne of primary materials consumed and
the brewing industry takes 4-15m 3 of water per tonne of finished beer pro-
duced (European Commission's Directorate General for Environment, 2001). 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 co-processing 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 micro-biota 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.
The land application of such liquors requires care since too heavy a dose may
lead to damage to the soil structure and an alteration of the osmotic balance.
Long term accumulation of these salts within the soil produces a gradual
reduction of fertility and ultimately may prove toxic to plants if left to proceed
unchecked. Moreover, the characteristically high levels of unstabilised organic
material present and the resultant low carbon to nitrogen ratio tends to make
these effluents extremely malodorous, which may present its own constraints
on available options for its treatment. It is inevitable that issues of social
acceptability make land spread impossible in some areas and, accordingly, a
number of food and drink manufacturers have opted for anaerobic digestion as an
on-site treatment for their process liquors. This biotechnology, which is described