Removal of microplastics, organic pollutants and metals from stormwater in bioretention filters with added sorbent material during simulated extreme rainfall events under winter conditions with dormant plants

This study investigates the impacts of extreme rainfall events during winter and plant dormancy on bioretention filters' removal efficiencies of microplastics (MP), metals, and organic pollutants (OP) in stormwater. The studied filters had sorption materials such as waste-to-energy bottom ash, biochar, and Sphagnum peat mixed with sandy loam. Besides influent and effluent sampling, pore water sampling was conducted. The filters were subjected to two cycles of simulated rainfall, each corresponding to an approximate 50-year rainfall event. Results showed that all the filters effectively reduced the quantity of MP > 10 µm, OP, and most metals, even under high flows. The biochar and ash filters had higher removal efficiencies (RE) for metals, while ash and peat showed slightly higher RE for OP. A decrease in removal efficiencies was observed towards the end of the second rain for some metals, especially for Cr. The pore-water sampling showed that dissolved metal concentrations varied with filter depth, indicating multiple sorption and/or desorption processes. Additionally, pollutant removal differed depending on the characteristics of the sorption materials. This study highlights the resilience of bioretention filters with added sorption materials exposed to extreme conditions, providing valuable insights into their performance and potential optimisation for robust urban stormwater management.