Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 129

Figure6.3 Tricklingfilter
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 media 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
applications, but a clinker and slag mix is generally said to give some of the best
results. The ideal filter bed must provide adequate depth to guarantee effluent
retention time, since this is critical in allowing it to become sufficiently aerated
and to ensure adequate contact between the microbes and the wastewater for the
desired level of pollutant removal. It should also have a large surface area for
biomass attachment, with generous void spaces between the particles to allow the
required biomass growth to take place without any risk of this causing clogging.
Finally, it should have the type of surface which encourages splashing on dosing,
to entrap air and facilitate oxygenation of the bed.
The trickling filters in use at sewage works are squat, typically around 8-10m
across and between 1 and 2m deep; though these are the most familiar form,
other filters of comparatively small footprint but 5-20m in height are used to
treat certain kinds of trade effluents, particularly those of a stronger nature and
with a more heavy organic load than domestic wastewater. They are of particular
relevance in an industrial setting since they can achieve a very high throughput
and residence time, while occupying a relatively small base area of land.
To maximise the treatment efficiency, it is, clearly, essential that the trickling
filter is properly sized and matched to the required processing demands. The most