132 Environmental Biotechnology
to shearing, which in turn favours the floc-forming bacteria. As these thrive and
produce quantities of sticky, extracellular slime, larger floc particles are formed,
the increasingly irregular shape of which is very apparent on microscopic exam-
ination of the activated sludge. Mucus secretions from rotifers, which become
more numerous as the sludge ages, also contribute to this overall process. Inter-
ruption of this formative succession may occur as a result of high toxicity within
the input effluent, the lack of adequate ciliated protozoan activity, excessive
inter-tank shearing forces or the presence of significant amounts of surfactant.
Process disruption
Toxicity is a particular worry in the operational plant and can often be assessed
by microbiological examination of the sludge. A number of key indicators may
be observed which would indicate the presence of toxic components within the
system, though inevitably this can often only become apparent after the event.
Typically, flagellates will increase in a characteristic ‘bloom’ while higher life
forms, particularly ciliates and the rotifers, die off. The particular sensitivity of
these microbe species to toxic inputs has been suggested as a potential method
of biomonitoring for toxic stress, but the principle has not yet been developed to
a point of practical usefulness.
The floc itself begins to break up as dispersed bacterial growth, characteristic of
an immature sludge, returns, often accompanied by foaming within the bioreactor,
the progressively reducing growth of microbial biomass leading to a lowered
oxygen usage and hence to poor BOD removal. If the toxic event is not so
severe as to poison the entire system, as new effluent input washes through the
tanks, increasingly diluting the concentration of the contaminating substances
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
settings, where the effluent may commonly be deficient in a particular nutrient,
most typically either nitrogen or phosphorus. This results in altered floc formation,
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.