Dynamics and interrelationships between antibiotic resistance, organic micropollutants and bacterial communities in full-scale rural constructed wetlands

Abstract

Constructed wetlands systems (CWs) are increasingly regarded as promising alternatives or complements to conventional wastewater treatment processes. However, the fate of chemical and biological contaminants in real-world treatment processes is understudied in this type of systems. This study aimed to fill this gap by evaluating the response of three horizontal subsurface flow CWs, in Northern Portugal, planted with Phragmites australis, in operation for >7 years, to reducing the load of fecal contamination, antibiotic resistance genes and organic micropollutants (OMPs).

Influent, effluent and sediments samples (n = 36) were examined for abundance of cultivable Escherichia coli and total coliforms, total bacteria (16S rRNA gene), 10 genetic biomarkers associated with anthropogenic contamination (uidA, crAssphage, intI1, sul1, ermB, ermF, mefC, qacEΔ1, tetX and aph(3″)-Ib) by quantitative PCR, non-target LC-MS of OMPs and 16S rRNA gene-based bacterial community analysis.

The three CWs showed reduction values (log-units/mL) up to 4.8 of E. coli and 3.6 of biomarkers, with the highest values observed in warmer periods. No evidence of for the accumulation microbiological contaminants in the sediments was observed. Among the 59 OMPs detected, reduction rates varied, and the concentration of the most abundant pharmaceutical compounds in the final effluent varied –reaching ng/L concentrations of ∼36 000 for fenofibric acid, ∼14 000 for acetaminophen, ∼3000 for oxazepam and ∼2000 for irbesartan, which can be considered high to discharge in the receiving environment. The bacterial community was dominated by members of the class Gammaproteobacteria, with treatment contributing to significant reduction of the relative abundance of members of the classes Clostridia, Bacilli and Actinomycetes. Compared with wastewater, sediments had significantly higher relative abundance of Alphaproteobacteria.

The study confirms that CWs are an adequate alternative for the treatment of domestic wastewater in small communities, although it warns of the need for regular monitoring and adjustment of treatment conditions, especially during cooler periods.