Sphaerotilus natans is a filamentous bacterium possessing the following
- often false branching;
straight to slightly bowed filaments;
- filament length >200
- filament diameter: 1.0 µm to 2.0 µm;
growth sometimes present;
- sheath present ( often better visible after
- 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).
The following process conditions are favourable to
- lack of O2, N or P;
- many low molecular compounds
in the influent (agro industry);
- sludge load > ca. 0.2 kg BOD/kg
- 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.
Remove deficiencies: O2 > 2 mg/l and BOD : N : P = 100 : 5 :
2. Aerobic selector.
3. Anoxic zone if sufficient nitrite/nitrate is
available for removal of the dissolved fraction from the influent
4. Anaerobic zone if a combination with a
Bio-P process is an option.
5. Two step configuration (aerobic/aerobic
6. Controlling symptoms