136 Environmental Biotechnology
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 a sometimes used for sewage treatment and is characterised by a con-
structed 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 biological contactors
are typically used for small installations, and are particularly useful for applications
with high seasonal variations like caravan site sewage systems.
Figure 6.7 Rotating biological contactor