Prokaryotic viruses (phages) existing in activated sludge (AS), a biological treatment process widely used in wastewater treatment plants (WWTPs), act to regulate the composition of microbial community in the activated sludge. Phages are major bacterial predators, through virus-host interactions with key bacterial populations in AS systems, they can influence the removal efficiency of pollutants. Phages of high specificity could be used to curb undesired bacteria, e.g., the undesired foaming-associated filamentous bacteria that could disrupt the removal efficiency of AS system.