A survey to investigate the proliferation of filamentous micro-organisms, which cause filamentous bulking in activated sludge wastewater treatment processes, was conducted for the first time in the tropical zone. Activated sludge samples were collected from six wastewater treatment processes in Bangkok City, Thailand. The activated sludge plants investigated include those for 1) an industrial wastewater from an industrial estate, 2) a domestic wastewater from apartment houses, 3) a brewery wastewater, 4) a hotel wastewater, 5) a textile industry wastewater and 6) a milk processing wastewater. The identification of proliferating filamentous micro-organisms in the samples was attempted. Type 021N was exclusively dominant in the milk processing wastewater treatment plant. The characteristics of the 021N present were different from “typical” 021N. The most proliferating filament in the textile wastewater treatment plant was suspected to be Nostocoida limicola. There was a predominant filament in the brewery wastewater treatment plant, which is suspected to be a new type. Other major filaments proliferating include Eikelboom Type 021N, 1701, 0041 and 0092. Nocardia was also found in a few plants.

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