For those effluents, however, which are consigned to land treatment regimes, the fate of nitrogen is of considerable importance. In aerobic conditions, the biological nitrification processes within the soil produce nitrate from ammonia and organic nitrogen, principally by the chemotrophic bacteria, Nitrosomonas and Nitrobacter, which respectively derive first nitrites and then finally nitrates. The oxidation of ammonia (NH3) can be represented as:
2NH3+3O2−−−> 2NO2−+ 2H++ 2H2O
This reaction releases energy which is subsequently used by Nitrosomonas to reduce carbon dioxide. A secondary oxidation of the nitrate produced by Nitrobac-ter forms nitrate ions, with energy again being released for use by this bacterium.
However, in anoxic conditions nitrate compounds can be reduced to nitrogen gas as a result of the activities of various species of facultative and anaerobic soil bacteria, in which the nitrate ion acts as an alternative electron acceptor to oxygen in respiration. As a result, it becomes possible to view the interlinked processes of nitrogen losses via volatisation, denitrification and plant uptake as control mechanisms for the nitrogenous component in wastewaters in land applications. Approximately 20 – 30% of the applied nitrogen is lost in this way, a figure which may rise to as much as 50% under some circumstances, as factors such as high organic content, fine soil particles and water-logging all provide favourable conditions for denitrification within a soil.
Though amelioration processes involving land spreading or injection clearly have beneficial uses for some kinds of wastewaters, in general effluents, particu-larly those of industrial origin, require more intensive and engineered solutions. In this respect, whether the liquors are treated on-site by the producers themselves, or are tankered to external works is of little significance, since the techniques involved will be much the same irrespective of where they are applied. The con-tribution of environmental biotechnologies to the safe management of effluents principally centres on microbial action, either in anaerobic digestion where the carbon element is fully reduced, or in aerobic processes which lead to its oxidation.
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