Aerobes and Effluents 133Foaming can be a significant and unsightly nuisance in operational facilities
and, as has been discussed, may occur as a result of either nutrient deficiency
or the growth of specific foam-generating filamentous organisms. Microscopic
examination of the fresh foam is often the best way to determine which, and thus
what remedial action is necessary.
Typical protozoans present in the sludge include amoebae, ciliates and flagel-
lates and, together with rotifers, they play secondary roles in the activated sludge
treatment of wastewaters. The presence or absence of particular types can be
used as valuable biological indicators of effluent quality or plant performance.
In this way, the incidence of large numbers of amoeba often suggests that a
shock loading has taken place, making large quantities of food available within
the system, or that the dissolved oxygen levels in the tanks have fallen, since
they are better able to tolerate conditions of low aeration. A large flagellate pop-
ulation, particularly in mature sludges, suggests the persistence of appreciable
quantities of available organic nutrients, since their numbers are usually limited
by competition with bacteria for the same dissolved foodstuff. Since ciliates, like
rotifers, feed on bacteria, their presence indicates a healthy sludge, as they typ-
ically blossom after the floc has been formed and when most of the effluent’s
soluble nutrients have been removed. As protozoa are more sensitive to pH than
floc-forming bacteria, with a typical optimum range of 7.0–7.4 and tolerating
6.0–8.0, they can also provide a broad measure of this parameter in the system.
The population of rotifers seldom approaches large numbers in activated sludge
processes, though they nevertheless perform an important function. Their princi-
pal role is the removal of dispersed bacteria, thus contributing to both the proper
development of floc and the reduction of wastewater turbidity. Taking the longest
time of all members of the microbial community to become established in the
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:
F/M=
mass of BOD applied to the biological phase each day
total microbial biomass in the biological phaseThe 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
suggests little available nutrients and consequently slow growth results. Clearly,