Aerobes and Effluents 129The general properties of the filter media were discussed earlier. In respect of
sizing the system, the porosity and intergranular spaces govern the interrelation
between relative ease of oxygen ingress, wastewater percolation and nutrient to
biofilm contact. Clearly, the rougher, pitted or irregular materials tend to offer
the greatest surface area per unit volume for microbial attachment and hence,
all other things being equal, it follows that the use of such media allows the
overall filter dimensions to be smaller. In practice, however, this is seldom a
major deciding factor.
In the main, filter systems use rotational dosing systems to ensure a uniform
dispersal of the effluent, though nozzles, sprays and mechanised carts are not
unknown. The feed must be matched to the medium if the surface aeration effect
is to be optimised, but it must also take account of the fluidity, concentration and
quality of the wastewater itself and the character of the resident biofilm.
Since the biological breakdown of effluents within the filter is brought about
by aerobic organisms, the effectiveness of aeration is of considerable impor-
tance. Often adequate oxygenation is brought about naturally by a combination
of the surface effects as the wastewater is delivered to the filter, diffusion from
atmosphere through the filter medium and an in-filter photosynthetic contribution
from algae. Physical air flow due to natural thermal currents may also enhance
the oxygenation as may the use of external fans or pumps which are a feature
on some industrial units.
Activated Sludge Systems
This approach was first developed in Manchester, just prior to the outbreak of the
First World War, to deal with the stronger effluents which were being produced
in increasingly large amount by the newly emerging chemicals industry and were
proving too toxic for the currently available methods of biological processing.
Treatment is again achieved by the action of aerobic microbes, but in this method,
they form a functional community held in suspension within the effluent itself
and are provided with an enhanced supply of oxygen by an integral aeration
system. This is a highly biomass-intensive approach and consequently requires
less space than filter to achieve the same treatment. The main features are shown
in Figure 6.4.
The activated sludge process has a higher efficiency than the previously described
filter system and is better able to adapt to deal with variability in the wastewater
input, both in terms of quantity and concentration. However, very great changes
in effluent character will challenge it, since the resident microbial community is
generally less heterogeneous than commonly found in filters. Additionally, as a
more complex system, initial installation costs are higher and it requires greater
maintenance and more energy than a trickling filter of comparable throughput.
In use, the sludge tanks form the central part of a three-part system, comprising
a settlement tank, the actively aerated sludge vessels themselves and a final