Oxidation of sulfide and thiosulfate and storage of sulfur granules in Thiothrix from activated sludge

Thiothrix from activated sludge is, in the presence of oxygen, able to oxidize sulfide and thiosulfate and store many clear sulfur granules at sulfide concentrations less than about 1 mM and thiosulfate concentrations of 0.1-20 mM. On this basis, a sulfur storage test for the identification of Thiothrix in activated sludge was developed. It is shown that the filamentous bacteria, Type 021N (commonly causing bulking in sewage plants), is able to store sulfur granules and thus is a Thiothrix. In enrichment cultures with thiosulfate typical Thiothrix rosettes and motile gonidia are developed, and the presence of a sheath is demonstrated. Filamental diameter varies between 0.8-2.4 μm, the youngest filaments being thinnest. Attempts to isolate Thiothrix have been unsuccesful, but Thiothrix filaments with only a few contaminants are able to grow on solid media with acetate as the only carbon and energy source, indicating the presence of heterotrophic strains. The filamentous organisms of 14 Danish sewage treatment plants are identified, Thiothrix being the most common in 8, and responsible for bulking in 4 plants.