Sphaerotilus natans, more commonly known as S. natans, is a filamentous bacterium that can be found in wastewater treatment plants. It is often responsible for bulking and foaming in activated sludge. Sphaerotilus bacteria grow well under low oxygen concentrations, when there are high levels of hydrogen sulfide either coming into a wastewater treatment plant or being added by chemicals like Ferric sulfate.

Below is a sample from a municipal activated sludge plant that gets periodic loadings from a local cheese producer. It was wet mount stained 1:1 with 1:100 crystal violet viewed under bright field:


Bulking Filaments in Wastewater

Sample above viewed at 100X magnification. Floc structure has many long filaments extending into the bulk solution. Some filaments appear to exhibit branching.

 Sphaerotilus Natan Filaments in Wastewater

Sample above viewed at 450X magnification. The filaments in this sample were long, thin, without epiphytic growths and had evidence of branching.

Occurrence of S. Natans

S. Natans In Wastewater - FilamentsSphaerotilus natans hardly ever occur in modern, low-loaded nutrient-removal plants. They regularly cause sludge bulking  in industrial treatment plants containing many easily biodegradable compounds in the influent. Growth in mass of S. natans results in an extremely high SVI.

The following process conditions are favorable to S. natans’ growth in activated sludge:

  • Low levels of Oxygen, Nitrogen, or Phosphorous
  • In the agriculture industry, several low molecular compounds in the influent
  • Sludge load > ca. 0.2 kg BOD/kg MLSS.day
  • Complete mixing in the aeration tank
  • Advantage in high current areas where sheaths allow a means of attachment
  • Optimal growth at 86oF Optimal growth at pH 6.4-8.1


Sample above viewed at 1000X magnification. Filament appears to display false branching; meaning two trichomes are stuck together and grow outward without contiguous cytoplasm between the trichomes.

Below is the sample from the activated sludge plant displaying slow settling of solids due to S. natans. From left to right: initially mixed, after 1 hour, and after 2 hours.

Do you think you might have S. natans in your wastewater treatment process? Following are some of S. natans’ most recognizable characteristics which may help you identify the bacteria. Or, send us a sample and we will identify the filament for you. Please give us a call or visit our Lab Services for more information about this service.

  • Often false branching (this is the only filament that exhibits 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 when organism is growing slowly
  • 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
  • Spherical PHB granules (about 3 per cell) often observed
  • Filaments radiate outward from floc structure into bulk solution

Note: 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 Type 0961 (cells clearly longer and distinctly transparent).


  1. Glymph, Tony. Wastewater Microbiology: A Handbook for Operators. Denver, CO: American Water Works Association, 2005. pp.76-78, 95-97
  2. asissludge.com. 2000. Activated sludge information systems.

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