Successful phosphorus removal by a nutrient removal activated sludge plant, depends ultimately on the ability of the biomass to store carbon in the form of polyhydroxybutyrate and phosphate as polyphosphate. This study has shown that floc-forming cells in the aerobic zone of an efficient phosphorus removing plant contain large polyphosphate inclusions, and are usually larger in size than anaerobic zone cells. Monitoring the development of biomass in a new activated sludge plant by seeding with mixed liquor from the Johannesburg Goudkoppies Works, demonstrated the rapid formation of a typical activated sludge floc. The distinct differences between polyhydroxybutyrate and polyphosphate storage in the anaerobic, anoxic and aerobic zones observed in Goudkoppies sludge were however, not evident in the new plant until two weeks after start-up and the large clusters of polyphosphate containing bacteria were not observed until even later. The formation of large clusters of bacteria observed in phosphorus removing plants was considered contingent on the production of extracellular slime. This was borne out by the examination of various plants for the presence of extracellular slime, which revealed that the presence of this material correlates well with satisfactory phosphorus removal.

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