The occasional occurrence of a biological scum of problematic proportions in activated sludge plants, is widespread. The causitive organisms have in Southern African plants been identified as members of the Nocardia and Microthrix species. When causing scum problems, these organisms have the property to be selectively separated from the settleable activated sludge floc by a process of selective flotation. By applying the process of selective flotation a quantitative method for the determination of biological scum has been developed. This method was used to compare the scum intensity of different activated sludge plants and to evaluate the efficiency of scum removal upon treatment. Pilot scale studies were undertaken to evaluate the process of selective flotation as a simple practical method for the control of scum forming micro-organisms in activated sludge plants.

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