Fungi are filamentous micro-organisms possessing the following properties: 
- real branching;
- immobile;
- straight/bowed filaments;
- filament length > 200 µm;
- cell diameter is usually > 2.5 µm;
- no attached growth;
- no sheath;
- septa clearly visible;
- rectangular cells;
- no sulphur storage;
- Gram "negative": fungal filaments often do not stain evenly;
- Neisser negative.

Fungi form robust, branched filaments (hyphae), which can be easily distinguished from other filamentous organisms, even at a low magnification.

Occurrence in activated sludge
Fungi are only seldom observed in domestic treatment plants, and they are never dominating the filamentous population. Fungi also almost never cause bulking sludge in industrial plants. If fungi do come to dominate the sludge population the reason is usually a very low pH level.

Control strategy

 Increasing the pH level to a neutral level

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