The role of dispersed nocardioform filaments in activated sludge foaming

The role of dispersed nocardioform filaments in activated sludge foaming was studied in bench-scale activated sludge systems fed with primary effluent from a full-scale wastewater treatment plant. Additional batch experiments were also conducted using activated sludges from several wastewater treatment plants. Dispersed nocardioforms showed a greater propensity for foaming than floc-bound nocardioforms.; The mode of effluent withdrawal from the aeration basin was shown to have a major impact on the relative proportion of dispersed and floc-bound nocardioforms in the activated sludge. Reactors with "trapping" features (sub-surface effluent withdrawal) had significantly higher dispersed nocardioform populations than reactors with "non-trapping" features (surface effluent withdrawal). The total nocardioform population was also higher in reactors with "trapping" features than in non-trapping reactors.; High dispersed nocardioform filament concentrations were correlated with a high propensity for foaming. A threshold dispersed nocardioform filament concentration was required to initiate foaming. Below this threshold level, no foaming occurred, whereas, above this level, foaming increased with increases in the dispersed nocardioform organism counts. No such correlation was observed between the floc-bound nocardioform population and the propensity for foaming.; Cationic polymer and poly-aluminum chloride both reduced foaming by flocculating dispersed nocardioforms into the activated sludge flocs, thereby converting them to floc-bound nocardioforms. A non-ionic surfactant (alkyl phenol ethoxylate) deflocculated floc-bound nocardioform organism filaments, converting them to the foam-causing dispersed growth form. This could act as a trigger for initiating the rapid-onset nocardioform foaming events observed at activated sludge plants.