Biomedical Engineering Reference
In-Depth Information
sludge, their presence indicates increasing stabilisation of the organic components
of the effluent.
Organic loadings
Calculating the organic loadings for a given activated sludge system is an impor-
tant aspect of process control. Measuring the BOD of the incoming wastewater
gives a value for the amount of biodegradable matter available for microbial use,
which can be used together with an estimate of the resident biomass to derive a
relationship termed the food to micro-organism (F/M) ratio. This, which is also
sometimes known as the organic loading rate, is given as follows:
mass of BOD applied to the biological phase each day
total microbial biomass in the biological phase
F/M
=
The F/M ratio is a useful indication of anticipated micro-organism growth and
condition, a high F/M value yielding rapid biomass increase, while a low one sug-
gests little available nutrients and consequently slow growth results. Clearly, the
total active biomass content in an activated sludge system, which is termed
the Mixed Liquor Suspended Solids (MLSSs), 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:
BOD
(
kg/m
3
)
volume of sludge tank
(
m
3
)
×
MLSS
(
kg/m
3
)
flow rate
(
m
3
/
d
)
×
F/M
=
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 born 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
50-100m deep.
The shaft contains the wastewater to be treated, compressed air being blown in
at the base, which travels up the central section, setting up an opposing counter
flow in the outer part of the shaft. Screened secondary effluent is allowed to settle
and a portion of the sludge produced is returned to the input zone, just as in a
traditional activated sludge tank, though degassing is required to remove nitrogen
and carbon dioxide bubbles from the floc to allow for proper sedimentation.
The high pressures at the base forces far more oxygen into the solution than
normal, which aids aeration enormously and allows the process to achieve an
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