Type 1701
Type 1701 is a filamentous bacterium possessing the following characteristics:
- occasional false branching;
- immobile;
- straight/bowed filaments, often protruding from the flocs;
- filament length < 200 µm;
- cell diameter 0.7 - 0.9 µm;
- attached growth often present;
- sheath present;
- septa visible (if heavy attached growth is present, then only at the clean tip of a filament);
- rod shaped cells;
- no sulphur storage;
- Gram negative;
- Neisser negative.
If branching is present, the identification key can indicate S. natans (markedly thicker filaments). If the septa are not observed, Type1701 can be confused with Type 1851 (Gram positive), H. hydrossis (thinner filaments without attached growth) or Type 0092 (Neisser positive).

Occurrence in activated sludge
Type 1701 is not very often observed in activated sludge. The following process conditions are favourable to this filamentous bacterium:
- sludge loading levels > ca. 0.2 kg BOD/kg MLSS.day;
- waste water containing a high level of carbohydrates, especially starch;
- low oxygen levels in the aeration tank; - relatively high water temperatures (> ca. 15°C);
- ???.

This means that Type 1701 practically never occurs in domestic treatment plants where nutrient removal is implemented. Type 1701 grows mainly in plants in which waste water originating from certain agro-industries is treated and where it can cause high SVI values.

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 Bio-P processes is an option.
5. Two step configurations (aerobic/aerobic or anaerobic/aerobic).
6. Controlling symptoms.