Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 130

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 non-linear nature of the effect of temperature on contaminant
removal. Although the speed of chemical reactions is well known to double for
every 10 ◦ C rise, 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
increasingly inhibited.
The general properties of the filter media were discussed earlier. In respect of
sizing the system, the porosity and inter-granular 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 both 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.