Influent pathogenic bacteria may go straight into effluent in full scale wastewater treatment plants

摘要

Incoming microorganisms to wastewater treatment plants (WWTPs) are usually considered to be adsorbed onto the activated sludge flocs, consumed by protozoan or to just die off. Analyses of the effluent generally show a very high degree of reduction of pathogens supporting this assumption. Thus, it is assumed that the bacteria present in the effluent comprise primarily of those bacteria that thrive/grow in the plants. However, standard techniques for detecting bacteria in the effluent, particularly pathogens, are based on culture-dependent methods, which may give erroneous results by underestimating the number of bacteria escaping WWTPs. The aim of this study was to determine if bacteria from the incoming wastewater are being incorporated into the activated sludge community, whether they die off, or whether they stay dispersed in the water phase and thus are discharged from WWTP together with the effluent. Culture-independent 16S rRNA gene amplicon sequencing was applied for the identification and quantification of the microorganisms. Over a sampling period of three months, the microbial community composition was investigated by 16S rRNA gene amplicon sequencing (V1 to V3 region) and MiDAS curated taxonomy (McIlroy et al. 2015). In total, 304 samples from 14 full-scale Danish WWTPs were investigated. Net growth rates and cell numbers of specific OTU’s were calculated using bacterial mass balances (Saunders et al. 2015) and operational data from the WWTPs.Analyses of the microbial composition in influent, process tank and effluent in the 14 WWTPs showed that the microbial communities in incoming wastewater were very similar across the plants. The same was observed for communities in the activated sludge in the process tanks. In contrast, the effluent community was in some WWTPs very similar to the process tank community, in other plants influent and effluent communities were very similar to each other while in some plants it was a mixture. This indicates that in some plants many microorganisms went straight through the plant from influent to effluent without being incorporated into activated sludge flocs. Some of these are known as pathogens. One of these was from the genus Arcobacter (Campylobacteraceae) and it included one particularly abundant OTU found in both influent and effluent in all 14 investigated WWTPs. This single Arcobacter OTU accounted for up to 14% of all bacteria found in the effluent. This indicates that Arcobacter, and perhaps other pathogenic genera, are shortcutting in full-scale plants without being removed from the plant by the activated sludge flocs. Calculation of net growth rates and establishing mass balances for all OTUs confirmed that Arcobacter did not grow in the plants, indicating that Arcobacter cells found in the effluent originated from the influent wastewater. This poses a potential serious health safety problem. Further investigations are needed including perhaps implementing methods to prevent “short-cutters” from escaping the WWTPs into the effluent.