Full‐scale evaluation of factors affecting the performance of anoxic selectors

Five full‐scale nitrifying activated‐sludge systems incorporating anoxic selectors were evaluated. Operating records were reviewed to determine selector performance (filament control) and to identify operating conditions that affected selector performance. Key operating parameters found to influence anoxic selector performance were selector loading, expressed in terms of the selector food‐to‐microorganism (F:M) ratio, wastewater temperature, and activated‐sludge system aerobic solids retention time (SRT). A target selector F:M ratio of 0.7 to 1.2 kg CBOD5 (carbonaceous biochemical oxygen demand)/kg MLSS (mixed liquor suspended solids) · d (lower values at cooler temperatures, higher values at warmer temperatures) and an aerobic SRT of 8 to 12 days were found to produce excellent selector performance. Increasing selector F:M or system aerobic SRT to greater than these ranges was found to result in persistent filamentous bulking.