Aerobes and Effluents 127Figure 6.3 Trickling filter
The filter is designed to permit good drainage and ventilation and in addition
sedimentation and settling tanks are generally associated with the system. Efflu-
ent, which has been mechanically cleaned to remove the large particles which
might otherwise clog the interparticulate spaces in the filter bed, flows, or is
pumped, into the rotating spreader, from which it is uniformly distributed across
the filter bed. This dosing process can take place either continuously or intermit-
tently, depending on the operational requirements of the treatment works. The
wastewater percolates down through the filter, picking up oxygen as it travels
over the surface of the filter medium. The aeration can take place naturally by
diffusion, or may sometimes be enhanced by the use of active ventilation fans.
The combination of the available nutrients in the effluent and its enhanced
oxygenation stimulates microbial growth, and a gelatinous biofilm of micro-
organisms forms on the filter medium. This biological mass feeds on the organic
material in the wastewater converting it to carbon dioxide, water and microbial
biomass. Though the resident organisms are in a state of constant growth, ageing
and occasional oxygen starvation of those nearest the substrate leads to death of
some of the attached growth, which loosens and eventually sloughs, passing out
of the filter bed as a biological sludge in the water flow and thence on to the
next phase of treatment.
The filter medium itself is of great importance to the success of these systems
and in general the requirements of a good material are that it should be durable
and long lasting, resistant to compaction or crushing in use and resistant to frost
damage. A number of substances have been used for this purpose including
clinker, blast-furnace slag, gravel and crushed rock. A wholly artificial plastic
lattice material has also been developed which has proved successful in some