Type 0041
Type 0041 is a filamentous bacterium possessing the following characteristics:
- not branched; - immobile;
- straight/bowed filaments, occurring free in the water as well as attached to the flocs;
- filament length is variable;
- cell diameter is variable (0.6 - 1.5 µm);
- much attached growth always present in domestic plants; "clean" filaments sometimes present in industrial plants;
- sheath present;
- septa visible (if a lot of attached growth is present, only visible at the tip of the filaments);
- square cells;
- no sulphur storage;
- Gram positive;
- Neisser negative.
Type 0041 includes Gram positive, sheath forming bacteria, including the previously described Type 0675, but excluding Type 1851. The latter stains light blue with Gram and consist also of much longer, rectangular cells. Type 0041 comprises a number of "species" which differ from one another in regard to their cell diameters. The somewhat thicker filaments of Type 0041 can be mistaken for those of S. natans (Gram negative and rectangular cells) or for Thiothrix filaments (S granules and Gram negative). Thin Type 0041 filaments resemble those of Type 1851 (light blue with Gram staining and long, rectangular cells).

Occurrence in activated sludge
Type 0041 occurs very commonly in activated sludge. The following process conditions are favourable to this filamentous species:
- sludge loading level < ca. 0.2 kg BOD/kg MLSS.day;
- pre-mixing of raw domestic influent and recycled sludge before these flows enter the aeration tank.
This means that Type 0041 in domestic treatment plants mainly grows on the particulate fraction present in the influent. It is not known if this is also valid for industrial plants. Little is known concerning the factors that determine the growth of this species. The population of Type 0041 is always small in domestic plants, if no pre-mixing is carried out. Pre-mixing stimulates the growth of Type 0041.This practically never causes high SVI values (stabilisation usually at 120 to 130 mg/l), however. A few examples exist of industrial treatment plants in which bulking sludge has been caused by Type 0041.

Control strategies
Examples of controlling symptoms are mainly described in the literature. Only one plant is known of, in which an aerobic selector has been used (successfully) for controlling Type 0041. N.B. If pre-mixing is not carried out in domestic treatment plants, growth of M. parvicella is promoted.