Medium length, non-motile filaments (100-400 µm). Irregularly coiled filaments with no branching and no cell septa. Cells are square (0.6 x 0.8 µm). Filaments are found in spaghetti-like tangles within the floc structure causing open, lacy, diffuse floc or in loose patches free in the bulk solution. The filament staining is strongly Gram positive or Gram variable when in the floc structure and Neisser negative with Neisser positive granules observed frequently. Short clear spaces may occur in the filament. Usually easy to identify due to its staining characteristics. Intracellular PHB granules may give beaded effect. No sulfur granules. No attached growth or sheath present. It is a common cause of foaming.

Nocardia is similar although Microthrix parvicella lacks the branching that Nocardia has.

This filament is usually found in environments where there low F/M (0.05-0.2 lbs BOD) and with a long MCRT (10-40 days). High wastewater grease and fat content may also be a cause. Found more commonly in the colder climates in the north.

Municipalities that have high grease in lift stations can have high problems with this filament as well as Nocardia.

F/M can be changed by increased sludge wasting, changing from complete mix to plug flow or use of a selector. Extensive chlorination or RAS chlorination should definitely be used. Reducing sludge age helps. If possible increase grease and fat control on the influent if applicable.

Microthrix parvicella ranks 7th in number of predominance.