Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 123

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, which is described
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:

2NO 2 − + 2H + +

2NH 3 + 3O 2 −−−→

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 Nitrobac-
ter forms nitrate ions, with energy again being released for use by this bacterium.

2NO 2 − + 2NO 3 −

O 2 −−−→

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,