Identification and significance of filamentous microorganisms in activated sludge

Based on observations of 226 samples from 78 plants throughout the U.S., the 10 most frequently encountered filamentous types were in order of decreasing frequency: Type 1701, Nocardia spp., Type 0041, Type 021N, Type 0092, Haliscomenobacter hydrossis, Sphaerotilus natans, Microthrix parvicella, Type 0581, and Type 1851. Identification was possible for 96% of the filamentous types present in large numbers, and detailed analysis of a sample could be completed in 1 to 2 hours. Types 1701, 021N, S. natans, and 1863 were associated with bulking at low mixed liquor DO concentrations. Types 0041, 0092, M. parvicella, and 0581 were associated with bulking occurring at low organic loadings. The conditions associated with the presence of excessive amounts of Nocardia spp., H. hydrossis, 1851, and several other types were not clear. Identification of filamentous micro-organisms was highly useful in determining the factors contributing to episodes of activated sludge bulking.