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 testify 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 m3 of water per 1 m3 of milk arriving at the plant, the manufacture of preserves requires any-thing between 10 – 50 m3 of water per tonne of primary materials consumed and the brewing industry takes 4 – 15 m3 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.
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 bal-ance. 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 accept-ability 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 is extremely effective at transforming the organic matter into a methane-rich biogas, with a high calorific value which can be of direct benefit to the operation to offset the heating and electrical energy costs. Under this method, the organic content of the effluent is rapidly and significantly reduced, and a minimum of sludge produced for subsequent disposal.