Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 126

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.
The UFB system is the most expensive, both to install in the first place and
subsequently to run, as it requires comparatively high maintenance and needs a
filtered air supply to avoid air-borne particulates blocking the narrow diffuser
pores. Illustrative UFB aeration results, based on operational data, obtained from
the amelioration of post anaerobic digestion liquor from a horticultural waste
processing plant, are shown in Table 6.3.
Though the comparatively simple approaches which produce large to medium
sized bubbles are the least efficient, they are commonly encountered in use since
they offer a relatively inexpensive solution.
Mechanical aeration systems
In this method, a partly submerged mechanically driven paddle mounted on floats
or attached to a gantry vigorously agitates the liquid, drawing air in from the
surface and the effluent is aerated as the bubbles swirl in the vortex created.