Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 133

makes some understanding of the microbiology of activated sludge essential.
Bacteria account for around 95% of the microbial mass in activated sludge and
most of the dispersed growth suspended in the effluent is bacterial, though ide-
ally there should not be much of this present in a properly operating activated
sludge process. Generally speaking this tends only to feature in young sludges,
typically less than three or four days old, and only before proper flocculation
has begun. Ciliates are responsible for much of the removal of dispersed growth
and adsorption onto the surface of the floc particles themselves also plays a part
in its reduction. Significant amounts of dispersed growth characterises the start-
up phase, when high nutrient levels are present and the bacterial population is
actively growing. However, the presence of excessive dispersed growth in an
older sludge can often indicate that the process of proper floc formation has been
interrupted in some way. When floc particles first develop they tend to be small
and spherical, largely since young sludges do not contain significant numbers of
filamentous organisms and those which are present are not sufficiently elongated
to aid in the formation process. Thus, the floc-forming bacteria can only flocculate
with each other in order to withstand shearing action, hence the typical globular
shape. As the sludge ages, the filamentous microbes begin to elongate, their num-
bers rise and bacterial flocculation occurs along their length, providing greater
resistance to shearing, which in turn favours the floc-forming bacteria. As these
thrive and produce quantities of sticky, extra-cellular slime, larger floc particles
are formed, the increasingly irregular shape of which is very apparent on micro-
scopic examination of the activated sludge. Mucus secretions from rotifers, which
become more numerous as the sludge ages, also contribute to this overall process.
Interruption of this formative succession may occur as a result of high toxicity
within the input effluent, the lack of adequate ciliated protozoan activity, exces-
sive 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 bio-monitoring 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