Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 128

Table6.4 Illustrativeoxygentransferratesfor
aerationsystemsat20 ◦ C
System

Transfer rate (kg O 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 1.2-2.4

The 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.
The filter is designed to permit good drainage and ventilation and in addition
sedimentation and settling tanks are generally associated with the system. Efflu-
ent, which has been mechanically cleaned to remove the large particles which
might otherwise clog the inter-particulate spaces in the filter bed flows, or is
pumped into the rotating spreader, from which it is uniformly distributed across
the filter bed. This dosing process can take place either continuously or intermit-
tently, depending on the operational requirements of the treatment works. The
wastewater percolates down through the filter, picking up oxygen as it travels
over the surface of the filter medium. The aeration can take place naturally by
diffusion, or may sometimes be enhanced by the use of active ventilation fans.
The combination of the available nutrients in the effluent and its enhanced
oxygenation stimulates microbial growth, and a gelatinous biofilm of micro-
organisms forms on the filter medium. This biological mass feeds on the organic
material in the wastewater converting it to carbon dioxide, water and microbial
biomass. Though the resident organisms are in a state of constant growth, ageing
and occasional oxygen starvation of those nearest the substrate leads to death of