Testing the effect of selectors in the control of bulking and foaming in full scale activated-sludge plants.

Selectors were operated at four full-scale activated sludge plants to control bulking and foaming problems due to filamentous microorganisms. Selector effectiveness was not related to reduction of biodegradable organic matter in the contact zone, but was related to soluble COD levels in selector effluent. Significant reductions in the numbers of filamentous m icroorganisms were reported.