Relatively large, non-motile filaments (100-500 µm). Straight, smoothly curved or bent filaments with no branching. Cells are square to rectangular (1.2-1.6 x 1.5-2.5 µm) and contained in a tight fitting sheath. There are no indentations at the cell septa. Filaments are found within the floc structure causing open, lacy floc or radiating outwards causing inter-floc bridging. The filament staining is Gram positive or Gram variable when in the floc structure and Neisser negative with Neisser positive granules observed infrequently. Neisser positive slime coat frequently observed in industrial wastes. Usually easy to identify due to its flat, ribbon like structure with twists that make it difficult to focus on more than a few cells at a time under phase contrast. Intracellular granules rarely observed. No sulfur granules. Can be heavy attached growth when observed inside the floc structure. No attached growth when extending into the bulk solution (usually when in a rapid growth phase, watch out for this, it usually means significant trouble). Sheath is difficult to detect except when missing cells.

Type 0675 is similar although smaller in diameter and is usually more Gram positive. Type 021N is similar when in the bulk solution and with no attached growth, although Type 021N has no sheath.

This filament is usually found in environments where there low F/M with a long MCRT. It can also be found in low nutrient environments. Very rarely is it a problem filament. Usually it causes a nice backbone for the floc structure.

F/M can be changed by increased sludge wasting, changing from complete mix to plug flow or use of a selector. RAS chlorination should be definitely used if in the rapid growth phase. After chlorination, sheath may still be present and interfere with SVI. Nutrient deficient wastes can be checked by effluent values of residual NH³ and o-PO⁴ and should be supplemented if necessary.

0041 ranks 4th in number of predominance. Common in over 50% of sludge in low abundance. Can be beneficial at low abundance as a backbone structure for the floc.