Type 1851
Type 1851 is a filamentous bacterium possessing the following characteristics:
- not branched;
- immobile;
- straight/slightly bent filaments which sometimes form bundles;
- filament length variable, but usually > 200 µm;
- cell diameter 0.5 - 0.7 µm;
- usually much attached growth, but sometimes clean filaments in industrial treatment plants;
- sheath present;
- septa often not clearly visible;
- rectangular cells;
- no sulphur storage;
- Gram positive (light blue);
- Neisser negative.

Type 1851 can be confused with thin Type 0041 filaments (dark blue with Gram staining and shorter cells).
It is known nowadays that morphotype 1851 includes several species

Occurrence in activated sludge
Type 1851 is regularly observed in low loaded plants, but practically never predominates in domestic treatment systems. Large populations of Type 1851 can be present in industrial treatment plants (agro-industry).
The following process conditions are favourable to Type 1851:
- sludge loading levels < ca. 0.15 kg BOD/kg MLSS.day;
- low molecular compounds (agro-industry);
- ???.

Control strategies
Well documented references concerning the control of Type 1851 are not available. Depending upon the nature of the substrate used, it may be expected that aerobic, anoxic or anaerobic selectors and two step configurations are suitable for controlling Type 1851.