Some pathogenic bacteria may go directly to effluent in WWTPs

Urban sewers and wastewater treatment systems are critical for human health protection. Pathogenic bacteria are generally considered removed efficiently in biological wastewater treatment plants but this understanding is almost solely based on culture-based controls measures. Here we revisit the removal efficiency and effluent quality by culture-independent methods. We investigated the microbial composition in influent and effluent of 14 municipal Danish WWTPs over a 3 months period. The general microbial composition in the influent was very similar in all plants with the genus Arcobacter as one of the most abundant genera, reaching up to 35% of all bacteria. This genus contains several pathogenic species including Arcobacter butzleri. This genus was poorly removed by the activated sludge treatment plants, compared to most other genera found in the influent. Detailed studies of bulk water phase and the shear sensitive fraction of the activated sludge flocs showed that Arcobacter flocculated poorly, a large fraction stayed in bulk water phase and went straight through the plants into the effluent. The known human pathogenic species A. butzleri were isolated from influent and effluent. Genome sequencing confirmed the presence of virulence genes typical for the pathogenic strains. Metagenomic analysis of wastewater, activated sludge, and effluent confirmed the presence of known pathogenic Arcobacter and their virulence genes. Visualization by fluorescence in situ hybridization suggested that Arcobacter were alive in the effluent and thus posed a potential threat for humans and livestock in the environment. There is a need for new standardized non-culture based measurements of pathogens in effluent wastewaters, and a need for simultaneous removal of pathogens that are not caught and removed with the activated sludge.