Laboratory fill-and-draw activated sludge systems were started with mixed liquor from a domestic treatment plant and fed glucose mineral salts medium. Two units each were maintained at pHs of 4, 5, 6, and 7. Effluent biochemical oxygen demand, total organic carbon, and suspended solids quickly dropped to low levels in all units. The sludge volume index was low at all four pHs until day 30 then increased dramatically (bulking) for the units at pHs of 4 and 5. Fungal filaments were noted earlier at pHs of 4 and 5 and became abundant by day 30. At pHs of 6 and 7, fungal filaments remained at low concentrations. These results provide support for the hypothesis that fungi may be responsible for activated sludge bulking at a low pH.
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