What is the purpose of a selector in an activated sludge system and how does it contribute to filamentous bacteria control?

A selector in an activated sludge system is a small reactor or zone placed at the head of the aeration basin, where influent wastewater is mixed with return activated sludge (RAS). The RAS is the settled biomass recycled from the secondary clarifier. The purpose of a selector is to create specific environmental conditions that favor the growth of floc-forming bacteria over filamentous bacteria. Floc-forming bacteria are desirable because they form dense, compact flocs that settle well in the clarifier, leading to good solids separation. Filamentous bacteria, on the other hand, are undesirable because they can cause bulking sludge, which has poor settling characteristics and can lead to solids carryover in the effluent. Selectors contribute to filamentous bacteria control by creating a competitive advantage for floc-formers. There are several types of selectors, including aerobic, anoxic, and anaerobic selectors. Each type creates different conditions to suppress filamentous growth. Anaerobic selectors are commonly used. In an anaerobic selector, the mixed liquor is subjected to conditions without oxygen but with a high concentration of readily biodegradable organic carbon, typically in the form of volatile fatty acids (VFAs). Under these conditions, floc-forming bacteria, particularly polyphosphate-accumulating organisms (PAOs) if phosphorus removal is also a goal, are able to rapidly uptake the VFAs and store them as polyhydroxyalkanoates (PHAs). Filamentous bacteria are generally less efficient at rapidly utilizing VFAs under anaerobic conditions. Therefore, the floc-formers gain a competitive advantage. Aerobic selectors provide high dissolved oxygen (DO) concentrations and a high initial food-to-microorganism (F/M) ratio. This favors bacteria that can rapidly consume readily biodegradable organic matter under aerobic conditions, again outcompeting many filamentous types. Anoxic selectors utilize nitrate as an electron acceptor in the absence of free oxygen. This selects for bacteria capable of denitrification, which can also help control certain types of filamentous bacteria. By creating these selective environments, selectors help to maintain a healthy and well-settling sludge, preventing bulking and improving overall treatment performance. For example, an anaerobic selector can be particularly effective in controlling filamentous bulking caused by nutrient deficiency, as the PAOs can store carbon under anaerobic conditions and then utilize it for growth under aerobic conditions, outcompeting filamentous organisms that are limited by the lack of stored carbon.