Type 021N (belongs to Thiothrix)
Type 021N is a filamentous bacterium possessing the following characteristics:
- not branched;
- immobile;
- straight/bent filaments, usually free in the liquid;
- filament length > 200 µm;
- cell diameter is variable, usually 1.5 to 2.0 µm, but sometimes decreasing towards the tip of a filament;
- no attached growth;
- no sheath;
- septa clearly visible;
- variable cell form (disc shaped, almost square and rod shaped);
- little or no sulphur storage;
- Gram negative;
- Neisser negative.
The ‘name’ Type 021N has been used since 1975 to designate unknown filamentous species which more or less comply with the description above. Nowadays it is known that the designation “Type 021N” stands for a group of filamentous bacteria, all Thiothrix strains (e.g. T. flexilis, T. eikelboomii, T. disciformis), with comparable  morphological features, but differing from each other in phylogenetic location and, to some extent, also in their nutritional requirements. By applying other methods, it is possible to distinguish the various species from each other. However, for plant control this does not make much sense as the growth requirements of the various Type 021N resembling Thiothrix strains do not diverge very much from each other. The characteristic domestic Thiothrix-021N morphotype, usually hardly stores sulpher with the sulphur storage test.

Occurrence in activated sludge
Type 021N is frequently observed in activated sludge. The following process conditions are favourable to this filamentous species:
- a broad spectrum of sludge loading levels (0.05 - 0.4 kg BOD/kg MLSS.day)providing a greater chance of a massive growth at loading levels > 0.1 kg BOD/kg MLSS.day;
- influent containing many easily biodegradable compounds (fatty acids, etc.). These compounds can arise in the transport sewer (stale sewage) as well as originating from industry;
- lack of nutrients, including oxygen;
- complete mixing in the aeration tank.

The substrate that Type 021N needs to grow is removed from the water by floc forming bacteria, under anoxic/anaerobic conditions, in domestic nutrient removal plants. Type 021N, which is strictly aerobic, is only encountered as a secondary filament under these process conditions. Type 021N frequently causes bulking sludge in industrial treatment plants and in domestic plants without nutrient removal.

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 the 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.