An additional use of tertiary clarifiers: potential for ecology-based control of bacterial and genetic pollution in wastewater

Despite advancements in urban wastewater treatment, final disinfection remains the primary, yet often inefficient or costly, measure to control fecal bacteria and antibiotic resistance genes (ARGs). A solution to enhance performance while reducing the impact of chemical disinfection could be found in a different management of the often-overlooked tertiary clarifiers integrated into the coagulation–flocculation stage. By analyzing three different wastewater treatment plants (WWTPs) combining amplicon and shotgun metagenomics, flow cytometry, and water chemistry, we demonstrated that tertiary clarifiers shape microbial communities, reducing bacterial abundance while maintaining community richness. Whereas fecal/opportunistic pathogens decline, environmental bacteria either persist or increase, suggesting selective pressures at play. Furthermore, while ARG abundance decreased, their relative prevalence remained constant or even increased, raising concerns about potential horizontal gene transfer. Biofilms within clarifiers act as reservoirs, stabilizing diversity despite environmental fluctuations. These findings challenge the traditional perception of tertiary clarifiers as mere sedimentation tanks and highlight their potential role for microbial control. Through optimized retention times and ecological management, tertiary clarifiers could complement disinfection by reducing environmental footprints and operational costs, lowering the overall intensity required to mitigate bacterial and genetic pollution. This study provides a foundation for an additional use of tertiary clarifiers in WWTP design, integrating eco-engineered solutions to enhance treatment efficacy.