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.