Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 135

The shaft contains the wastewater to be treated, compressed air being blown in
at the base, which travels up the central section, setting up an opposing counter
flow in the outer part of the shaft. Screened secondary effluent is allowed to settle
and a portion of the sludge produced is returned to the input zone, just as in a
traditional activated sludge tank, though degassing is required to remove nitrogen
and carbon dioxide bubbles from the floc to allow for proper sedimentation.
The high pressures at the base force far more oxygen into solution than nor-
mal, which aids aeration enormously and allows the process to achieve an oxygen
utilisation of around 90%, which is some 4.5 times better than conventional acti-
vated sludge systems. The bubble contact time produced, averaging 90 seconds
or more, is over 6 times longer than in standard diffused air systems. It has a
low footprint, making it ideal for use in restricted areas.
Pure Oxygen Systems
With process efficacy so closely dependent on aeration and the ability to support
a high microbial biomass, the use of pure oxygen to enhance the effective levels
of the gas dissolved in the effluent has an obvious appeal. The UNOX process,
which was developed by the Union Carbide Corporation is probably amongst the
best known of the pure oxygen activated sludge systems and Figure 6.6 shows
the general layout of the bioreactor vessels.
Pure oxygen obviously gives a better oxygen transfer rate per unit volume of
the bioreactor than can be achieved using conventional aeration methods. In turn,
this allows a heavier organic loading per unit volume to be treated compared
with ordinary air-fed systems, which enables this system to be used to deal
with stronger effluents and permits a high throughput where space is restricted.
Typically these systems are fed using liquid oxygen tanks.
Despite their clear advantages, pure oxygen systems suffer with some major
drawbacks. For one thing, the capital costs involved in installing them in the first
place are considerable, as are their running costs and maintenance requirement.
The pure oxygen itself represents an explosion risk, thus necessitating intrinsically
Figure6.6 TheUNOX pureoxygensystem