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Elimination of microbial indicators and antibiotic resistance genes during treatment of landfill leachates with pilot scale constructed wetlands

Meschini Danai

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Summary

The objective of the present diploma thesis is the study of the elimination of microbial indicators, adenoviruses, and antibiotic resistance genes during treatment of landfill leachates with pilot scale constructed wetlands. The leachates derived from Chania waste landfill and were treated in four pilot scale units of horizontal subsurface flow constructed wetlands (CWs). Specifically, the performance efficiency of the units was studied in relation to the different filling materials and operating conditions. The CWs were planted with Phragmites australis and are located in the campus of Technical University of Crete (greenhouse). The first unit (P = Plastic) contained high-density polyethylene (HDPE) as filling material. The second CW (PB = Plastic & Biochar) had HDPE combined with 10% w/v biochar. The filling materials of the third (G = Gravel) and the fourth unit (GB = Gravel & Biochar) were gravel and combined with 10% w/v biochar. Each pilot unit consisted of rectangular iron beds lined internally with HDPE plastic.The experiment was conducted from 28 May to 21 July 2021. During the experiment, consecutive sampling was performed during three periods: May (1st period), June (2nd period), and July (3rd period). Inflow samples were collected from diluted leachates from the landfill site, while outflow samples were collected from the effluents of the four constructed wetlands (P, PB, G, GB).The results of the experiments indicated that regardless of the substrate material used, there was no substantial removal of microbial indicators Escherichia coli, Staphylococcus aureus, and Enterococcus sp. However, adenoviruses were eliminated to a certain extent. Specifically, higher performance occurred during treatment with the wetlands P and PB, as a 2 Log of viral reduction was recorded.Regarding the antibiotic resistance genes (ARGs) studied, namely sul II, ampC, and qnrA, the results were the following:The sul II and ampC genes were not removed during the treatment of the leachates, as they were detected in high concentrations in all effluents. Regarding qnrA, there was no removal in the PB wetland, while in all other effluents there was a significant decrease of its concentration compared to that in the influent. The highest removal occurred during treatment with the GB wetland and was 2 Logs. The results show that there is a correlation between the removal of bacterial indicators and the removal of ARGs. Finally, there is no clear pattern post treatment regarding its effect on the antibiotic resistance profile of the bacteria and the MIC60 values of certain antibiotics, namely ciprofloxacin (CIP), amoxicillin (AMOX) and sulfamethoxazole (SMX). In conclusion, the experiments show that wetlands were not effective in removing microbial markers and ARGs genes but were quite capable of removing adenoviruses. The complex physicochemical and biological mechanisms that take place in wetlands affect the processes of removal of bacteria and viruses and subsequently, their presence in the effluents.

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