Final report. (29.606.2013). study of the temporal variability in the load of resistant bacteria at the WWTP lausanne, Switzerland
Rationale: Wastewater is increasingly recognized as a source of antibiotic resistant bacteria to receiving aquatic systems like rivers and lakes. The spread of resistance factors into the environment has been recognized as an environmental pollutant. In the context of the global increase of antibiotic resistance among pathogens the release of resistance factors may increase the risk of further dissemination and evolutionary spread of resistance factors. For the WWTP of Lausanne previous research demonstrated that the current treatment allows a considerable number of resistant bacteria to reach Lake Geneva with the treated wastewater, and that elevated resistance factor concentrations are present in the sediment of Vidy Bay around the point of wastewater discharge. The city of Lausanne currently plans the renovation of the WWTP, including additional advanced treatment stages and collaborated with Eawag to obtain data on the effectiveness of the planned facility with regards to the removal of resistance factors. Purpose: The WWTP of Lausanne tested a pilot scale system for activated charcoal powder (CAP) adsorption and ultrafiltration (UF). In addition to further substantiating results regarding the efficiency of resistance factor removal obtained during a first study conducted in 2012, the purpose of this study was to obtain additional data on the degree of temporal variability in resistance factors in the inflow, biologically treated wastewater, and CAP-UF treated wastewater. Approach: During operation of the CAP-UF pilot, samples were taken at the WWTP’s sewage inflow, the outflow of the biological treatment stage (= inflow for the pilot), and from the CAP-UF treated water. Two sampling campaigns were carried out in March and April 2013. Each treatment stage was sampled three times per day. Total microbial cell counts, total counts of cultivable bacteria, counts of cultivable bacteria resistant to three combinations of 2-3 antibiotics, and the abundance of three antibiotic resistance genes were determined. Results: The results of the 2013 campaign confirm that CAP-UF is effective in strongly reducing the number of bacteria in the discharged water, and with it the number of resistant bacteria and resistance genes. Following the CAP-UF treatment, no cultivable multiresistant bacteria were detected. Resistance genes are partly still detectable but are reduced by 3 to 4 orders of magnitude compared to the biological treatment stage, resulting in concentrations that are as low or lower than what is typically found in the environment. The most likely explanation for the remaining bacteria is regrowth of bacteria in the water after passage of the ultrafiltration membrane. The temporal variability was overall limited. A system failure (clogged membrane) on the second day of sampling resulted in an increased load of cells and resistance genes later that day. Burning of sludge from the WWTP was found to remove all resistance factors, however the wet part of the process produces material that is colonized by bacteria and contains considerable amounts of resistance factors. Interpretation: The CAP-UF stage represents a very substantial improvement over the current treatment method with regards to the amount of resistant bacteria and resistance genes that are released to the environment. Even under the worst conditions observed (post membrane clogging) the loads of resistant bacteria and resistance genes were near or below natural background resistance in Lake Geneva. Integration of membrane ultrafiltration into the treatment procedure can therefore be recommended in the context of retaining antibiotics resistance factors. Since the treatment also removes micropollutants including antibiotics it further reduces potential selection pressure for antibiotic resistance in the natural environment. However, we currently cannot compare it to other treatment alternatives (ozonation, UV, chemical disinfection, etc.). The full-scale ultrafiltration unit should be monitored for its effectiveness in retaining bacteria.