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Removal of microorganisms and antibiotic resistance genes during treatment of municipal wastewater treatment by means of pilot scale constructed wetlands

Stavroulakis Konstantinos

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Summary

The present Diploma Thesis deals with the investigation of performance efficiency of three pilot scale constructed wetlands, regarding their ability to treat urban wastewater and to remove pathogenic microorganisms and antibiotic resistance genes. These units are located at the Technical University of Crete and the influent urban wastewater is derived from the output of the primary treatment of the Waste Water Treatment Plant of Chania.The three constructed wetlands are named Control (CW-C) with HDPE filler, Plastic (CW-P) with HDPE filler and Phragmites australis reed vegetation and Gravel (CW-G) with limestone gravel filler and Phragmites australis reed vegetation. Sampling took place within the period November 2021 – May 2022, during which, wastewater samples were taken monthly from the inlet and outlet of the units.Initially, the performance of the constructed wetlands was studied, in terms of the removal of the bacterial indicators Escherichia coli, Enterococcus sp. and Staphylococcus aureus. Generally, the bacterial removals in constructed wetlands CW-C, CW-P, CW-G for Escherichia coli were 90.52%, 99.47% and 70.12%, respectively and for Enterococcus sp. were 95.80%, 99.37% and 90.14% respectively. On the contrary, no substantial removal was recorded for Staphylococcus aureus in any unit. Enterobacteriaceae were removed very satisfactorily in contrast to staphylococci. The combination of high-density polyethylene and reed vegetation of the CW-P constructed wetland makes it the most efficient wetland, as very high removal values of microbial load were observed.Furthermore, within the framework of this thesis, antibiotic resistance profile of the isolated bacteria was studied. Resistance profile was recorded prior to and post treatment within the CWs. The antibiotics tested were Amoxicillin, Ciprofloxacin and Sulfamethoxazole and the applied method was MIC (minimum inhibitory concentration). Generally, the MIC of the tested antibiotics increased after treatment, especially, regarding the bacterium Escherichia coli and the antibiotic Amoxicillin and enterococci and the antibiotic Ciprofloxacin. On the contrary, staphylococci managed to keep their resistance almost stable after treatment, concerning their response to all the three antibiotics.Also, the removal rates of target antibiotic resistance genes (ARGs) were investigated, namely, qnrA, ampC and sul II. The qnrA gene was almost undetectable in the effluents of all CWs, the presence of ampC gene in the effluents varied significantly and sul II gene was detected in high concentrations in all effluent samples.Finally, CWs were studied in terms of their efficiency to remove adenoviruses from wastewater after treatment, Adenoviruses were reduced after treatment, with the removal rate reaching a value up to 99% in some cases. The most effective units were CW-C and CW-P.In conclusion, constructed wetlands are considered as an environmentally friendly technology for the efficient treatment of wastewater, concerning the inactivation of microorganisms and the removal of ARGs and adenoviruses. However, further investigation is required and more studies should be performed, regarding the role of CWs on the effective wastewater treatment and the protection of public health.

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