Actinomycetes (= Nocardioforms, as they are indicated nowadays)

Actinomycetes are filamentous micro-organisms with the following characteristics:
 - real branching;
 - immobile;
 - "twisted" filaments (bunches) in and around the flocs;
 - filament length < 200 µm;
 - cell diameter < 1.0 µm;
 - no attached growth;
 - no sheath;
 - septa not visible;
 - no sulphur storage;
 - Gram positive;
 - sometimes Neisser positive (poly-P granules).
 
 The actinomycetes group is comprised of many different species, that can only be distinguished from each other by further investigation. On account of their characteristic branches, they cannot be mistaken for other filamentous species. M. parvicella is related to the actinomycetes.

Occurrence in activated sludge
Actinomycetes are notorious for scum formation, especially in countries where the average water temperature is higher than that in the Netherlands (5 - 20 °C). Transport of scum into the sludge digestion tank will also cause a scum layer to occur in this tank. The negative effect on the SVI is small. The following process conditions are favourable for the growth of actinomycetes:
- fats or other hydrophobic components in the influent;
- surface active materials in the influent;
- internal recycling of floating material;
- water temperature higher than approximately 15°C;
- ???
The question marks indicate that the available knowledge is still incomplete. Gram positive bacteria often have a hydrophobic cell surface through which fats etc. bind well to their surfaces and thereby can be absorbed from the water. Fats and surface active compounds are always present in domestic waste water. In spite of this, actinomycetes are not always present, even at higher water temperatures. This can be partially explained by the sludge load applied: Actinomycetes are usually found at somewhat higher sludge loading levels ( 0.1 - 0.7 kg BOD/kg MLSS.day). At lower sludge loadings, other Gram positive filamentous bacteria such as M. parvicella are present, which also grow on the fat fraction of the influent. Floc forming bacteria that can use this substrate also exist. It is not yet known which factors are decisive in the competition for this substrate.

Control strategies
1. Systematic skimming, followed by removal/destruction of scum. This is by far the most effective option. Scum must under no circumstances be internally circulated (e.g. to the sewage intake port). In extreme circumstances, a separate flotation unit can also be considered for 'separating' the actinomycetes from the sludge.
2. Drastic reduction of the sludge age is sometimes effective, but is in conflict with the desire to have extensive nitrification.
3. Experiments are being carried out with the dosing of a product which can change the cell surface from hydrophobic to hydrophilic (Al-salts, certain clay types).
 These experiment have not yet resulted in aproven control strategy.
4. Reduction of the concentration of fats, etc. in the influent (in case of industrial discharges).