Efficacy of a large-scale integrated constructed wetland for pesticide removal in tail water from a sewage treatment plant

The higher and higher detection frequencies of micro-pollutants such as pesticides in water are nowadays intensifying the investigation for strategies to provide effective engineering methods that could mitigate such substances. Traditional sewage treatment plants (STP) do not design specific processes for micro-pollutants removal in water. As an environmentally-friendly measure, some laboratory-scale wetlands have been proved to be effective in the removal of pesticides in water, but such studies are rarely carried out in large-scale wetlands, especially when they are adopted as a polishing step of STPs. Therefore, the further removals of micro-pollutants in tail water of STPs through the large-scale wetlands and the relevant removal mechanism are still knowledge gaps. In this study, 44 target pesticides were detected in the water of a large-scale integrated constructed wetland (ICW) for four seasons. The ICW was established to further process the tail water from a STP, whose drainage was from domestic sewage of local residents. There were 19, 16, 17, and 19 pesticides detected in spring, summer, autumn, and winter, respectively. The removal values for Σ19 pesticides ranged from 49.99% to 84.96% during the study period, and the removal of these pesticides followed significant seasonal trends, which was likely because the microorganisms responsible for biotic degradation were markedly influenced by seasonal temperature fluctuations. Proteobacteria, Chloroflexi, Acidobacteria, Planctomycetes, and Bacteroidetes were the dominant phyla, and might be associated with the biodegradation of organic pollutants in the ICW. Removal of pesticides by the ICW resulted in overall toxicity reductions in water, but butachlor and chlorpyrifos were still at non-ignorable ecological risks. This study highlights the potential of constructed wetlands for micro-pollutants removal in water as a polishing step in STPs.