134 Environmental Biotechnology
the total active biomass content in an activated sludge system, which is termed
the mixed liquor suspended solids (MLSS), is an important factor in process
efficacy. Accordingly, it is routinely measured at sewage works being important
in the calculation of the F/M ratio, which can be more properly defined as:
F/M=
flowrate(m^3 /d)×BOD(kg/m^3 )
volumeofsludgetank(m^3 )×MLSS(kg/m^3 )
Although the preceding systems are the most common forms likely to be seen
in use, a number of other systems exist which may sometimes be encountered,
some of which will be briefly outlined for the sake of completeness.
Deep Shaft Process
In many respects this is an activated sludge derivative, which was borne out of
ICI’s work on the production of proteins from methanol in the 1970s. Figure 6.5
shows the main features of the system, which is based around a shaft 50–
100 metres deep.
Figure 6.5 ICI deep shaft process