124 Environmental Biotechnology
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
book; the rest of this chapter will largely address the latter.
Aeration
Introducing air into liquid wastes is a well-established technique to reduce pol-
lutant potential and is often employed as an on-site method to achieve discharge
consent levels, or reduce treatment costs, in a variety of industrial settings. It
works by stimulating resident biomass with an adequate supply of oxygen, while
keeping suspended solids in suspension and helping to mix the effluent to opti-
mise treatment conditions, which also assists in removing the carbon dioxide
produced by microbial activity. In addition, aeration can have a flocculant effect,
the extent of which depends on the nature of the effluent. The systems used fall
into one of two broad categories, on the basis of their operating criteria:
- Diffused air systems.
- Mechanical aeration.
This classification is a useful way to consider the methods in common use,
though it takes account of neither the rate of oxygen transfer, nor the total dis-
solved oxygen content, which is occasionally used as an alternative way to define
aeration approaches.
Diffused air systems
The liquid is contained within a vessel of suitable volume, with air being intro-
duced at the bottom, oxygen diffusing out from the bubbles as they rise, thus
aerating the effluent.
These systems can be categorised on the basis of their bubble size, with the
crudest being coarse open-ended pipes and the most sophisticated being spe-
cialised fine diffusers. Ultra-fine bubble (UFB) systems maximise the oxygen
transfer effect, producing a dense curtain of very small bubbles, which conse-
quently have a large surface area to volume ratio to maximise the diffusion.