Microthrix parvicella
Microthrix parvicella is a filamentous bacterium possessing the following characteristics:
- not branched;
- immobile;
- bent/twisted filaments, free in the water or in/around the flocs;
- filament length often < 200 µm;
- cell diameter ca. 0.5 µm;
- usually no attached growth;
- no sheath;
- septa not clearly visible;
- no sulphur storage;
- Gram positive;
- Neisser positive (poly-P granules). These granules become smaller if the growth stagnates.

M. parvicella resembles N limicola I. However, this latter organism forms more robust filaments and stains grey-violet with Neisser staining.

Occurrence in activated sludge
M. parvicella commonly occurs in low loaded domestic treatment plants. This filamentous bacterium is the most important cause of bulking sludge in many countries (including the Netherlands), and is also frequently responsible for scum formation. Transport of surplus sludge containing many M. parvicella filaments to the sludge digestion tank can also cause scum to arise in this tank. The population size of M. parvicella shows a marked seasonal pattern: the population is at its maximum at the end of the winter and at its minimum in summer.
The following process conditions are favourable to the growth of M. parvicella:
- sludge loading level < ca. 0.2 kg BOD/kg MLSS.day;
- waste water containing a substantial amount of higher fatty acids, such as oleic acid. This is always the case with normal domestic waste water;
- circumstances in which the fats/lipids present in the influent are hydrolysed before they reach the aeration tank. This releases the higher fatty acids.Consequently, a long hydraulic retention time in the sewer, the primary sedimentation tank or in the anaerobic zone with Bio-P processes is favourable to M. parvicella;
- a low oxygen level in the aeration tank;
- a large (> 40% of the total volume) anoxic zone in the aeration tank;
- water temperature of < ca. 15°C. M. parvicella grows principally in the late autumn and winter;
- supply of reduced sulphur and nitrogen compounds is also a possible cause. On account of this, recycling of water from the sludge dewatering unit is 'suspect'. This combination means that the process conditions in plants for nutrient removal are extremely favourable to the growth of M. parvicella. It hardly ever occurs in industrial treatment plants.
- nutrient removal conditions favour growth of M. parvicella, particularly when nitrification is incomplete.

Control strategies
1. Dosing with aluminium salts (3.5 g Al/kg MLSS. day). Up to now, this is the only method that has a "guaranteed" effect and which hardly ever negatively influences the desired treatment results. However, a reduction in the average floc size must be taken into account.
2. Strong reduction of the retention time in anoxic zones in the plant. This action is often not possible, depending upon the need for extended denitrification.
3. This is also valid for a major reduction of the sludge age (loss of nitrification).
4. A mixing phase (a few minutes) of raw waste water and returned sludge - before they reach the aeration tank - followed by alternated anoxic and aerobic process conditions (Bio-Denitro). This method is effective for controlling M. parvicella, but stimulates the development of Type 0041, which does not grow quite so massively and does not contribute to scum arising, however.
5. Experience with various types of selectors for controlling M. parvicella have not been consistent up to the present. A non-aerated selector has been introduced between the anaerobic zone (Bio-P) and the anoxic tank in some treatment plants in The Netherlands. Experiences with this configuration have been positive.