Performance of Four Full-Scale Nitrifying Wastewater Treatment Plants Incorporating Selectors

The effect of installing selectors to control filamentous bulking at four full-scale nitrifying wastewater treatment plants was evaluated. The results indicate that a selector can be effective in controlling filamentous sludge bulking. Little difference in effectiveness was observed for aerobic, anaerobic, or anoxic selectors. Long-term operating data indicate that selectors, coupled with an appropriate aeration basin configuration, are capable of producing a nonbulking activated sludge (SVI less than 150 mL/g). Average SVIs in the 60 to 90 mL/g range can be expected, with values as low as 20 to 30 mL/g observed on occasion. An aerobic selector was not effective in controlling bulking in a completemix nitrifying system using surface mechanical aerators.