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.
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 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.
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