Selector applications for bulking control in activated sludge plants have become much more common. Both anoxic and anaerobic selectors prevent the growth of filamentous bacteria in the aerobic zone by removing their food source – namely the readily biodegradable biochemical oxygen demand (BOD) – in the selector ahead of the aerobic zone. As filamentous organisms typically cannot grow successfully without oxygen in the selector, they normally do not compete effectively. The reduced inclination for sludge bulking in plants with selectors has led to more efficient activated sludge system design that enables higher mixed liquor suspended solids levels, allowing rerating of aeration tanks and secondary clarifiers or, in the case of new plants, construction of smaller tanks overall. Operating costs have also been lowered, including reduced sludge chlorination needs for bulking control and reduced energy requirements.