Removal efficiency of indicator organisms and tetM prevalence in enterococci in a constructed wetland for wastewater treatment

Sammanfattning: Indicator organisms like coliforms, E. coli and enterococci are normally associated with the intestinal tract of warm-blooded animals, and can thus be used for indicating fecal contamination of water. This study investigated the removal efficiency of indicator organisms in a free water surface constructed wetland used for additional treatment of municipal wastewater. The species composition of enterococci and the prevalence of the tetracycline resistance gene tetM were also investigated. Samplings were done at five different sites: at the inlet, within and at the outlet of the wetland, as well as upstream and downstream of the wetland outlet into the adjoining river. A total of 86 enterococci isolates were characterized as E. faecalis, E. faecium or other Enterococcus spp., and the two former were also screened for the tetM gene. The investigated wetland showed a high removal rate of indicator organisms, with a removal of approximately 98%. E. faecium was identified as the predominant species in the wetland at a mean of 55.8%. In comparison, only 9.3% were E. faecalis, while other Enterococcus spp. were found to comprise 34.9%. Of the 56 isolates identified as E. feacalis or E. faecium 10.7% were positive for the tetM gene. The high removal of bacteria shown by this investigation provides further evidence of the benefits of constructed wetlands on wastewater treatment. It also offers a first indication of the Enterococcus spp. composition in Ekeby wetland, Eskilstuna, Sweden, and shows that the prevalence of the tetM gene is relatively low at this point in time.