Strategies for Controlling Filamentous Bulking in Activated Sludge Wastewater Treatment Plants: The Old and the New

Filamentous bulking and foaming are the most common settling problems experienced in activated sludge (AS) wastewater treatment plants (WWTPs). The quality of the final effluent is poor during episodes of bulking and foaming, which is an environmental, human health and economic burden. Remedial measures are often ineffective, and traditional non-specific methods such as chlorination may also negatively impact important functional bacterial species such as nitrifiers. Modifications to older methods as well as new strategies are required for controlling filamentous bulking. Laboratory testing needs to be followed by testing at scale in WWTPs. This review describes the filamentous bacteria responsible for filamentous bulking, with a focus on their global distribution and known factors which are selective for the growth of specific filaments. Traditional and new non-specific and biological control strategies are reviewed and discussed. Research gaps are identified with the aim of promoting continued efforts to establish effective control strategies for filamentous sludge bulking.