Innovative Methods to Treat Polluted Materials from Used Bioretention Filters for Purification of Polluted Stormwater

Abstract

Urbanization reduces surface areas for natural infiltration and increases the transport of pollutants such as metals, microplastics, and organic pollutants in stormwater runoff. Bioretention filters are widely implemented as green infrastructure for stormwater treatment and have demonstrated high pollutant removal efficiency. Limited knowledge exists regarding the accumulation, distribution, and long-term fate of pollutants within filters, as well as the management of used filter materials after operation. The aim of this thesis was to investigate the distribution and characteristics of metals and microplastics in used bioretention filters and to evaluate potential treatment strategies for microplastics removal and metal recovery. Two pilot-scale bioretention filters with different sorption materials as a main layer were studied: one containing municipal incineration bottom ash, and another containing biochar. Metals and microplastics contents were analyzed at different depths and filter material layers. Sieving was conducted to evaluate metal distribution in different particle size fractions, while sequential extraction was used to assess metal mobility in polluted materials. Density separation was investigated for removal of microplastics, and acid leaching was evaluated for recovery of metals. Different treatment sequences combining these methods were assessed. The results showed that metals originating from stormwater accumulated in the surface layer of the biochar-based filter, while elevated metal contents were observed throughout the filter profile in the ash-based filter due to the contribution of metals from the ash materials themselves. Although the finest particle fraction contained the highest metal contents, most of the total metal load was associated with larger particles. Sequential extraction indicated relatively high mobility of Cd and Zn, while Cr and Ni were stable. Microplastics and rubber related polymers were quantified in both filters, with rubber particles largely retained in the surface layer. No consistent depth-dependent trend was observed for microplastics. Density separation could partially remove certain polymers from the filter materials but showed limited ability to remove rubber particles. Acid leaching combined with density separation achieved the highest metal recovery performance and showed potential as a combined treatment strategy. The results suggest that different management strategies may be required for organic and inorganic materials. Further research is needed to assess the technical, environmental, and economic feasibility of the proposed treatment pathways.