Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 134

and the process recovers, excessive filament formation may occur leading to
a condition known as 'filamentous bulking'. As a result, it is sometimes said
that toxic inputs favour filamentous bacteria but, with the exception of hydrogen
sulphide contamination, this is not strictly true. It is, however, fair to say that the
disruption caused by a toxic influx permits their burgeoning growth, particularly
since they are generally the fastest group to recover.
By contrast, 'slime bulking' can often occur in industrial activated sludge set-
tings, where the effluent may commonly be deficient in a particular nutrient,
most typically either nitrogen or phosphorus. This results in altered floc forma-
tion, reduced settling properties and, in some cases, the production of the slimy,
greyish foam at the surface of the aeration vessel, which gives this event its name.
This greasy, extracellular polymer interferes with the normal settling processes,
altering the sludge buoyancy by entrapping air and encouraging foaming. The
situation can generally be managed simply by adding appropriate quantities of
the missing nutrient, though where relatively easily biodegradable soluble BOD
is readily available, it may be necessary to deliberately create higher levels of
nitrogen and phosphorus within the system than a straightforward analysis might
otherwise indicate.
Foaming 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
typically blossom after the floc has been formed and when most of the efflu-
ent'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 never-the-less perform an important function. Their prin-
cipal 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