Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 137

Figure6.6 TheUNOX ™ pureoxygensystem

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 intrinsi-
cally safe operational procedures and, in addition, leads to accelerated corrosion
of the equipment used. However, for some applications and for certain kinds of
effluents, they can prove particularly appropriate.

The Oxidation Ditch

This is sometimes used for sewage treatment and is characterised by a constructed
ellipsoidal ditch, in which the effluent is forced to circulate around the channel
by brush aerators. The ditch itself is trapezoidal in cross section to maintain
uniform effluent velocity throughout the channel. Effluent is fed into the system
without any prior primary sedimentation and typically gives rise to only 50% of
the surplus sludge produced by a typical activated sludge process.

The Rotating Biological Contactor

This system, shown in Figure 6.7, is a derivative of the biological filter. It
effectively combines the advantages of this previously described approach, like
the absence of a complicated settlement system for sludge return and a low
maintenance requirement with the smaller footprint and long microbial exposure
characteristic of the active sludge process.
They have submerged internal disc baffles which act as sites for the attached
growth of biomass, which are slowly turned by electric motor causing the
microbes to be alternately aerated and immersed in the effluent. Rotating biologi-
cal contactors are typically used for small installations, and are particularly useful
for applications with high seasonal variations like caravan site sewage systems.