126 Environmental Biotechnology
Figure 6.2 Turbine sparger aeration system
Table 6.4 Illustrative oxygen transfer rates
for aeration systems at 20◦C
System Transfer rate
(kgO 2 /kWh)
Diffused air
Coarse bubble 0.6–1.2
Medium bubble 1.0–1.6
Fine bubble 1.2–2.0
Brush aerator 1.2–2.4
Turbine sparger Aerator 1.2–2.4The design of the system and the processing vessel is crucial to avoid problems
of oxygen transfer, liquid stratification and foaming, all of which can be major
problems in operation. The time taken to effect treatment depends on the regime
used and the nature of the effluent. In this context, Table 6.4 shows typical oxygen
transfer rates for aeration systems at 20◦C.
The value of aeration in the treatment process is not restricted to promoting
the biological degradation of organic matter, since the addition of oxygen also
plays an important role in removing a number of substances by promoting direct
chemical oxidation. This latter route can often help eliminate organic compounds
which are resistant to straightforward biological treatments.
Trickling Filters
The trickling or biological filter system involves a bed, which is formed by a
layer of filter medium held within a containing tank or vessel, often cast from
concrete, and equipped with a rotating dosing device, as shown in a stylised form
in Figure 6.3.