Some immigrating Pathogenic Bacteria Goes Straight Through Full-Scale Wastewater Treatment Plants

Bacteria immigrating to wastewater treatment plants (WWTPs) are usually considered to be absorbed to the activated sludge flocs or die off. Therefore it is assumed that bacteria highly abundant in the effluent comprise primarily of those that have active growth in the plant. However standard methods for detecting bacteria in effluent are highly based on culture dependent methods, which may give erroneous results by underestimating the bacteria that are not removed. The aim of this study was to determine if immigrating bacteria are incorporated in the activated sludge microbial community, die off, or stay dispersed in the water phase and thus are discharged from the WWTPs with the effluent. Culture independent 16S rRNA gene amplicon sequencing (V1 – V3 region) was applied for the identification of bacteria using MiDAS curated taxonomy (McIlroy et al. 2015). In total 304 samples were investigated from influent, process tanks and effluent at 14 Danish full–scale WWTPs. Net growth rates and cell numbers were calculated for specific OTU’s using bacterial mass balances (Saunders et al., 2916). The microbial community composition in influent wastewater was very similar across plants. The same was observed for the process tank communities. The most abundant bacteria from the influent had 3-8% relative abundance in the process tanks, but no active net growth. Some genera were found in high relative abundance in the influent and effluent but not in process tanks, which indicates that specific genera stayed in the water phase and were discharged. One of these was the genus Arcobacter, where some species are known pathogens. Mass balances confirmed that Arcobacter did not grow in the plants indicating that cells of Arcobacter originated from the influent. This poses a potential health safety problem and further investigations are needed including perhaps implementing new methods to remove bacteria like Arcobacter before they are discharged with the effluent.