Large wastewater treatment plants (WWTP) are increasingly supplemented with quaternary treatment. Classical monitoring hereby relies mostly on the measurement of oxygen demand, micropollutants and the nutrients phosphorus and nitrogen. From a microbiological perspective, relevant parameters to assess treatment performance include the removal efficacies of the fecal gene load as a proxy of pathogenic risk, antibiotic resistance genes and the bacterial regrowth potential. For this purpose, a combination of flow cytometry and quantitative PCR, together with a viability assessment, was applied to characterize a full-scale pilot plant. The pilot plant comprised conventional treatment and MBR and ozonation for advanced treatment. The assessment of fecal gene load was based on the quantification of Bacteroidales of human origin, as these obligate anaerobic bacteria cannot replicate within wastewater treatment plants. Whereas conventional treatment resulted in only moderate removal of these parameters, quaternary treatment typically led to a much stronger decrease. MBR treatment contributed most strongly to the removal with an appr. 6 log reduction compared to the primary clarification effluent, corroborating its microbiological merit for wastewater treatment. In addition to removing microorganisms and their genetic content, data also suggested a 95% reduction in extracellular DNA. Ozonation further enhanced microbiological removal. From an analytical perspective, the study shows the added value of using a long amplicon qPCR approach together with sample treatment with a viability dye to minimize false-positive signals and to avoid underestimation of treatment performance. The chosen diagnostic approach shows promise in assessing the microbiological treatment efficacy of WWTPs and as a basis to decide on the microbiological necessity of treatment upgrades.
Nocker et al. (Wed,) studied this question.