Thiothrix
The name Thiothrix encompasses some filamentous bacteria possessing the following characteristics:
- rarely branched;
- immobile;
- straight/bowed filaments, which mostly protrude from the flocs;
- filament length: highly variable;
- cell diameter also variable (0.5 to 2.0 µm), sometimes narrowing within the filament towards the tip (= tapering);
- no attached growth;
- sheath ???, possibly present around thick filaments;
- septa clearly visible if the cells are not filled with sulphur globules;
- square to rectangular cells;
- sulphur granules present (in vivo or after the S test);
- usually Gram negative;
- Neisser negative.

Thiothrix filaments are usually attached to the flocs. The sulphur globules are very characteristic. If only a little S is present in the thicker filaments, Thiothrix can easily be confused with Type 021N. Besides low molecular compounds such as fatty acids, Thiothrix also uses reduced sulphur compounds. The sulphides are oxidised and elementary sulphur is temporarily stored in the cell as an intermediary product. These are the bright globules that can be microscopically observed. It is known nowadays that morphotype Thiothrix includes several species.

Occurrence in activated sludge
The following process conditions are favourable to Thiothrix:
- reduced sulphur compounds in the influent;
- many low molecular carbon compounds in the influent;
- sludge load > ca. 0.1 kg BOD/kg MLSS.day;
- lack of O2, N or P;
- complete mixing in the aeration tank.
Thiothrix hardly ever occurs in modern nutrient plants, unless unusually high sulphide levels are present in the influent. Large Thiothrix populations, that cause bulking sludge, can be present at high sludge loading levels and in industrial treatment plants.

Control strategies
1. Remove sulphides from the influent.
2. Remove deficiencies: O2 > 2 mg/l and BOD : N : P = 100 : 5 : 1.
3. Aerobic selector.
4. Anoxic zone if sufficient nitrite/nitrate is available for the removal of the dissolved fraction from the influent through denitrification.
5. Controlling symptoms
 
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