Relatively short, non-motile filaments (20-100 µm). Straight or bent filaments with no branching. Cell shape is concealed (0.5 µm). Filaments are found within the floc structure causing elevated SVI. Free in the bulk solution in severe cases. The filament staining is Gram negative and Neisser negative. Usually easy to identify due to its needle like appearance in a pin cushion. Intracellular granules not observed. No sulfur granules. Bundles of trichomes and attached growth are common. Sheath is difficult to detect and no cell septa.

None.

This filament is usually found in environments where there low F/M, nutrient deficiency and low DO. Usually overlooked due to its size at low magnification.

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. Increase DO and add nutrients if this is the situation. Nutrient deficient wastes can be checked by effluent values of residual NH³ and o-PO⁴ and should be supplemented if necessary.

H. hydrosis ranks 9th in number of predominance.