Solids retention time (SRT), biological scum trapping and recycle, and the dynamic equilibrium between Nocardioform populations in the foam and the mixed liquor are the controlling factors in activated sludge foaming events caused by Nocardioform bacteria. For the operating modes described
in this paper, a cured mixed liquor foaming condition (filament counts of approximately 105 intersections/g volatile suspended solids) was only achieved when SRT control, selective wasting, and polymer addition were in effect. Solids retention time control, with the SRT remaining
below 1.5 days, and selective wasting will cure a severely foaming mixed liquor, but effects will only be observed after 3 or 4 months after implementation. The combined wastage of Nocardioform bacteria from selective wasting and SRT control can ensure long-term foam control to the operation
of a pure-oxygen activated sludge system with foam-trapping features. An SRT of 0.3 days will result in the complete washout of Nocardioform bacteria from the activated sludge system, which can then operate at an SRT of 3 days free of Nocardioform. Polymer addition to mixed liquor is only
effective for foam control when a large portion of the system biomass exists as a heavy layer of foam above the mixed liquor.
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