Sphaerotilus natans
Sphaerotilus natans is a filamentous bacterium possessing the following charactristics:

 - often false branching;
 - immobile;
 - straight to slightly bowed filaments;
 - filament length >200 µm;
 - filament diameter: 1.0 µm to 2.0 µm;
 - attached growth sometimes present;
 - sheath present ( often better visible after Gram staining);
 - septa clearly visible;
 - rectangular to rod shaped (at the tip of a filament) cells;
 - no sulphur storage;
 - Gram negative ;
 - Neisser negative.

 If branching is absent, this species can be confused with Type 0041 (Gram positive, shorter cells), Type 021N (shorter cells, no attached growth, no sheath) or with Type 0961 (cells clearly longer and distinctly transparent).

Occurrence in activated sludge
 The following process conditions are favourable to S. natans:
- lack of O2, N or P;
- many low molecular compounds in the influent (agro industry);
- sludge load > ca. 0.2 kg BOD/kg MLSS.day;
- complete mixing in the aeration tank.

S. natans hardly ever occurs in modern low loaded nutrient removal plants. It regularly causes bulking sludge in industrial treatment plants containing many easily biodegradable compounds in the influent. Growth en masse of S. natans results in an extremely high SVI.
 
Control strategies
 1. Remove deficiencies: O2 > 2 mg/l and BOD : N : P = 100 : 5 : 1.
 2. Aerobic selector.
 3. Anoxic zone if sufficient nitrite/nitrate is available for removal of the dissolved fraction from the influent through denitrification.
 4. Anaerobic zone if a combination with a Bio-P process is an option.
 5. Two step configuration (aerobic/aerobic or anaerobic/aerobic).
 6. Controlling symptoms
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