Investigating the fundamental basis for selectors to improve activated sludge settling.

Aerobic, anoxic, and anaerobic selectors have become popular for controlling filamentous bulking in activated sludge systems; however, selectors are not always successful. Regression analyses of data collected from 48 full-scale wastewater treatment plants, with operating selectors, provided a method for ranking the importance of selector operating and design parameters (with respect to their influence on activated sludge settleability) and suggests optimum numerical ranges for these parameters for best selector performance. Selectors do not appear to control filamentous bulking in long mean cell residence time (MCRT) plants.