128 Environmental Biotechnology
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
8–10 metres across and between 1–2 metres deep; though these are the most
familiar form, other filters of comparatively small footprint but 5 to 20 metres 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
important factors in arriving at this are the quality of the effluent itself, its input
temperature, the composition of the filter medium, detail of the surface-dosing
arrangements and the aeration. The wastewater quality has an obvious signifi-
cance in this respect, since it is this, combined with the eventual clean-up level
required, which effectively defines the performance parameters of the system.
Although in an ideal world, the filter would be designed around input character,
in cases where industrial effluents are co-treated with domestic wastewater in
sewage works, it is the feed rate which is adjusted to provide a dilute liquor
of given average strength, since the filters themselves are already in existence.
Hence, in practice, the load is often adjusted to the facility, rather than the other
way about.
The input temperature has a profound influence on the thermal relations within
the filter bed, not least because of the high specific heat capacity of water at
4200 J/kg/◦C. This can be of particular relevance in industrial reed bed systems,
which are discussed in the following chapter, since a warm liquor can help to
overcome the problems of cold weather in temperate climes. By contrast the
external air temperature appears to have less importance in this respect. The
situation within the reaction space is somewhat complicated by virtue of the
nonlinear nature of the effect of temperature on contaminant removal. Although
the speed of chemical reactions is well known to double for every 10◦Crise,
at 20◦C, in-filter biodegradation only represents an increase of 38% over the
rate at 10◦C. Below 10◦C, the risk of clogging rises significantly, since the
activity of certain key members of the microbial community becomes increas-
ingly inhibited.