Bulking can be said to have occurred in activated sludge plants when the sludge does not settle easily and has an excessive volume. This can lead to carry over from the final effluent clarifies. A bulking sludge is usually characterised by a sedimentation rate of less than 0.3 m/h, an SSVI or SVI of above 120 and 180 ml/g respectively and a low density structure.
The cause of bulking is normally due to either excessive filamentous growth or micro-organisms producing extracellular such as polysaccharide.
There are a number of short-term control measures which include biocide addition, use of flocculating chemicals or increasing the RAS flow. These measures generally treat the symptoms and not the underlying cause of the problem. The production of extracellular material and excessive filamentous micro-organisms have been related to nutrient deficiency, low dissolved oxygen and configuration of the aeration basin. This article deals with the control of activated sludge bulking based on these causes.

Nutrient Deficiency
A bulking sludge with a nutrient deficiency can be identified via a simple wastewater analysis of the influent wastewater and comparison of the BOD, N and P concentrations. Ideally the ratio of BOD:N:P should be 100:5:1. The limiting nutrients are generally N and P. This ratio however is the maximum requirement of most plant and many plants operate successfully at lower ratios. A bulking sludge with a nutrient deficiency is often viscous due to excessive production of extracellular polysaccharide and may also contain filamentous types 021N and Thiothrix.

Factors within the aeration tank can effect the concentration and requirement of nutrient dosing. Firstly, a decrease in temperature results in more nutrients being needed for carboneous removal as the BOD load is used for cell maintenance. Secondly, high sludge age results in lower nutrient requirements due to cell lysis. Thirdly, nutrient dosing may be required in instances were the BOD is readily available (eg it is present as a simple sugar) but the N and P are organically bound and are therefore not available for utilisation at a sufficiently high rate. Fourthly, when treating a variable carbonaceous loads with a tendency to nutrient deficiency the nutrient should be dosed continually to reduce the impact of shock loads. Yet it is possible to overdose N if only the concentration of ammonia is monitored in the effluent and nitrification occurs.

In general a soluble inorganic N and P concentration of 0.5 to 1 mg/l should be maintained within the aeration tank. However where a highly soluble BOD load is being treated the N and P minimum concentration may have to be increased to 1 to 3 mg/l. Some plants use commercial fertiliser which contains urea, ammonia and nitrate. However in these mixes the nitrate can lead to denitrification, therefore formulas without nitrate should be used.

Low Dissolved Oxygen Concentrations
A high F:M ratio requires a high DO concentration for effective treatment of the BOD load. However at low DO concentrations and at high F:M ratios excessive filamentous growth may occur leading to bulking. The filamentous micro-organism associated with low DO are S. natans, type 1701, M. parvicellaand possibly H. hydrossis.
Bulking due to low DO can occur in a short space of time. A correction mechanism could be to increase the DO in the aeration tank. However this could lead to nitrification due to excess aeration and also for washout of the filamentous micro-organisms three sludge ages are required. Alternatively the F:M ratio could be decreased which would mean an increase in MLSS concentration. An increase in the MLSS concentration may lead to nitrification, due to increased sludge ages, or may exceed the secondary clarifiers design load. There are only really two solutions to this problem to either use a biocide or modify the aeration basin.

Aeration Basin Design
Activated sludge plants, which operate under complete mix system, produce poor settling sludge than systems which incorporate a selector box. In such systems a high substrate concentration zone is artificially created by mixing the RAS with the influent wastewater. This favours the growth of the floc-forming bacteria over filamentous micro-organisms. Floc-forming micro-organisms are favoured due to their high bioadsorption ability which limits the substrate available for other micro-organisms. However if the bioadsorption ability of the RAS has not been regenerated before contact with the influent then the efficiency of this process will be reduced.
Selector boxes can be either anoxic, aerobic or anaerobic. Anoxic zones are primarily used at nitrifying plants to remove nitrate and are effective due to floc-forming micro-organisms ability to denitrify and bioadsorb at high rates. An aerobic selector box should be based on a design F:M of 12 kg COD/kg MLSS, while anoxic selector boxes design load should be 1.2 kg COD/kg MLSS both with a retention time of 20 minutes. Anaerobic selector boxes have a similar F:M load rate to anoxic selector boxes but the retention time is between 45 minutes and two hours.
The above strategies provide starting points for activated sludge bulking control. However to understand the cause of bulking on an activated sludge plant the causative micro-organisms and the operating conditions which give rise to bulking should be first identified.