Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 125

in greater detail in Chapter 8, 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.

Nitrogenous Wastes

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 (NH 3 ) can be represented as:

2NH 3 + 3O 2 −→ 2NO 2 − + 2H + + 2H 2 O

This reaction releases energy which is subsequently used by Nitrosomonas
to reduce carbon dioxide. A secondary oxidation of the nitrate produced
by Nitrobacter forms nitrate ions, with energy again being released for use by
this bacterium.

2NO 2 − + O 2 −→ 2NO 3 −

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 mentioned in Chapter 2. 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 oxi-
dation. As has been mentioned earlier, the former is covered elsewhere in this
topic; the rest of this chapter will largely address the latter.