Haliscomenobacter hydrossis

Haliscomenobacter hydrossis is a filamentous bacterium possessing the following characteristics:
- not branched;
- immobile;
- straight filaments, protruding from the flocs;
- filament length < 200 µm;
- cell diameter 0.3 - 0.4 µm;
- usually no attached growth;
- sheath present;
- invisible septa;
- no sulphur storage;
- Gram negative;
- Neisser negative.

The thin dead straight filaments of H. hydrossis are so characteristic that they are not easily confused with other filaments. The tiny stems of Hyphomicrobium cells can indeed be mistaken for H. hydrossis filaments.
It is known nowadays that morphotype H. hydrossis includes several species.

Occurrence in activated sludge
H. hydrossis commonly occurs in activated sludge plants. The following process conditions are favourable to the growth of this filamentous species:
- sludge loading levels (domestic waste water) of > ca. 0.2 kg BOD/kg MLSS .day;
- H. hydrossis can develop en masse in industrial plants, also at lower loading levels;
- many low molecular compounds in the influent (agro industry);
- an influent with a high concentration of nitrogen compounds;
- complete mixing in the aeration tank;
- a low oxygen concentration in the aeration tank;
- probably also a deficiency of phosphate.
 The H. hydrossis population is always small in domestic treatment plants. This filamentous bacterium can indeed develop en masse in industrial plants where many easily biodegradable compounds are present in the influent. A large population results in a high SVI.

Control strategies
1. Remove deficiencies: O2 > 2 mg/l and BOD : N : P = 100 : 5 : 1.
2. An aerobic selector.
3. An anoxic zone, if sufficient nitrite/nitrate is available for the removal of the dissolved fraction from the influent through denitrification.
4. An anaerobic zone, if a combination with a Bio-P process is an option.
5. A two step configuration (aerobic/aerobic or anaerobic/aerobic).
6. Controlling symptoms.