Type 1863
Type 1863 is a filamentous bacterium possessing the following characteristics:
- not branched;
- immobile;
- bowed/twisted filaments, free in the liquid;
- filament length < 200 µm;
- cell diameter 0.5 - 0.8 µm;
- no attached growth;
- no sheath;
- septa clearly visible;
- round cells;
- no sulphur storage;
- Gram negative;
- Neisser negative.

The small, short chains of Type 1863 can be mistaken for "Streptococci" (Gram positive) and for Type 0211 (thinner filaments). N.B. It has been put forward recently that Type 1863 comprises a number of morphologically identical but physiologically varying strains.

Occurrence in activated sludge
Type 1863 is principally observed at high sludge loading levels (0.3 - 0.6 kg BOD/kg MLSS.day). It is not yet known if other process conditions also promote the growth of this species. Type 1863 has no effect on the settling properties of the flocs, but can contribute to the formation of scum even if the filaments are Gram negative.

Control strategies
Examples of symptom control only have been mentioned in the literature.