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Municipal wastewater treatment by means of pilot scale constructed wetlands: study of the removal of microbial indicators and antibiotic resistance genes

Aretaki Maria-Antonia

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Summary

In the context of this specific thesis, the performance of three pilot scale constructed wetlands was studied for the treatment of municipal wastewater from the wastewater treatment plant of Chania. In particular, samples from their outflow were examined in terms of the removal of bacterial indicators of fecal contamination, adenoviruses (AdVs), resistance genes (ARGs), as well as the possible differentiation of the resistance profile of the bacterial strains to different antibiotics was investigated.The experiment concerns three vertical sub-surface flow constructed wetlands which were placed in the outdoor area of the greenhouse of the Technical University of Crete and were studied for an operational period of four months (July-October). The two pilot scale units were planted with the common reed Phragmites Australis and had as filling material gravel and the other recycled plastic HDPE while the third unit was the control, without vegetation and also with HDPE as filling material. The three constructed wetlands are labeled as CWG (Gravel), CWP (Plastic) and CWC(Control).Regarding the results of the removal of the bacterial load, three bacteria were specifically examined: Escherichia coli, Staphylococcus sp. and Enterococcus sp. The results of the average removal efficiency of the above bacteria for the units are as follows: In CWC there was a reduction of 78.70% of E. coli, 31.19% of Staphylococcus sp. and 84.43% of Enterococcus sp. In the CWG for E. coli there was a removal of 93.43%, for Staphylococcus sp. 40.48% and for Enterococcus sp. 99.69%. Finally, regarding the CWP performance, E. coli was reduced by 94.12%, Staphylococcus sp. 32.20% and Enterococcus sp. 85.18%. Overall, Enterococcus sp.showed better removal rates due to the high sedimentation speeds compared to E. coli, while it is also considered that, in general, the basic mechanism for the removal of microorganisms in constructed wetlands is the filtration of the sewage on the substrate. Regarding the removal of adenoviruses (AdVs), the best removal values seem to have been shown by CWC as there were effluent samples in which no concentration of adenovirus genes was detected. CWG and CWP also showed a noticeable decrease reaching up to more than 2Logs.HDPE as a filling material, although it does not appear so often in literature references, is a light material and a reliable alternative that provides quite satisfactory results and has advantages over other substrate materials (gravel or sand) as shown in this specific study.The potential differentiation of the resistance profile of the three bacteria mentioned above was examined for three antibiotics: Ciprofloxacin (CIP), Amoxicillin (AMOX) and Sulfamethoxazole (SMX). This test was based on the Minimum Inhibitory Concentration method, MIC60 and the results showed that thebacterial strains from the CWC effluent had the greatest increase in this value for almost all antibiotics. In the other two wetlands CWG and CWP with vegetation, theresults showed that perhaps there is an indirect contribution of plants to the reduction of antibiotic resistance. However, there is no clear pattern on how the resistance profile of bacteria changes.The resistance genes that were studied are sul ll, ampC, qnrA which are associated with the antibiotics under study and the 16s rRNA ribosomal bacterial genetic material. From the results obtained, the significant increase of resistance genes and their distribution in the outflow of the three wetlands can be concluded.However, there is also a reduction of the genes in some cases comparing their concentrations with the inflow sewage.To sum up, constructed wetlands can be used as alternative, environmentally friendly technologies for wastewater treatment as they can achieve good removal rates for the microbial load and also for ARGs, which are considered to be a threat to the environment and to the human health as well.

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