Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 113

6
Aerobes and Effluents
The easiest thing that a sewage works is required to treat is sewage; a large
number of industrial or commercial activities produce wastewaters or effluents
which contain biodegradable contaminants and typically these are discharged to
sewers. The character of these effluents varies greatly, dependent on the nature
of the specific industry involved, both in terms of the likely BOD loading of
any organic components and the type of additional contaminants which may
also be present. Accordingly, the chemical industry may offer wastewaters with
high COD and rich in various toxic compounds, while tannery water provides
high BOD with a chromium component and the textile sector is another high
BOD effluent producer, with the addition of surfactants, pesticides and dyes.
Table 6.1 shows illustrative examples of typical effluent components for various
industry sectors.
Table 6.2 shows the illustrative effluent BOD by industry sector, from which
it should be apparent that the biodegradable component of any given wastewater
is itself highly variable, both in terms of typical values between industries, but
also in overall range. Thus, while paper pulping may present an effluent with
a BOD of 25 000 g/m 3 , sewage returns the lowest of the BODs quoted, clearly
underlining the point of this chapter's opening statement.
As might reasonably be expected from the foregoing discussion, the direct
human biological contribution to wastewater loading is relatively light. A 65 kg
person produces something in the region of 0.1-0.5 kg of faeces per day on
a wet weight basis, or between 30-60 g of dry solids. The same person also
produces around 1-1.5 kg of urine per day, with a total mass of dry solids
amounting to some 60-80 g. Of course, the actual effluent arriving at a sewage
works for treatment contains the nitrogen, phosphorus and other components
originally excreted in the urine or faeces, but in a higher dilution due to flush-
ing water and, often, storm drainage also. Local conditions, climate, details
of the sewer system and water availability are, clearly, all potential factors
affecting this, though a 49:1 ratio of water to solids is fairly typical for devel-
oped nations.
Sewage Treatment
Looking at sewage works in the strictly literal term, the aims of treatment can
be summarised as the reduction of the total biodegradable material present, the