From waste to resource. Stormwater treatment using crushed concrete and iron-coated granulate in pilot-scale bioretention cells

Bioretention systems are increasingly implemented for urban stormwater treatment. However, removal of dissolved nutrients and metals remains challenging, contributing to eutrophication and toxicity in receiving waters. Granular amendments can enhance dissolved pollutant retention, yet concerns regarding material sustainability and potential contaminant release necessitate evaluation of alternative recycled materials. This study evaluates the performance of locally sourced recycled filter materials used as amendments in three pilot-scale bioretention cells - a control, one amended with iron-coated granulate (FeG), and one amended with crushed concrete (CC), explicitly assessing the trade-offs between pollutant removal and trace-metal leaching. The cells were irrigated with semi-synthetic stormwater over two wet periods separated by a dry phase. Both amended cells markedly improved pollutant removal compared to the control. During the second wet period, average P removal exceeded 90% in the CC and FeG cells compared to an average of 64% in the control cell. The FeG cell exhibited rapid stabilization and consistent P removal already in the top layer of the cell, whereas the CC amended cell achieved higher polishing at depth. Cu and Zn removal exceeded 90% in all systems, although transient declines occurred following rewetting. Trace metal leaching analyses showed substantial Cr and moderate As leaching from the CC cell, particularly during initial operation and immediately after the dry period, whereas leaching from the control and FeG cells were negligible. Overall, the study demonstrates that recycled amendments can significantly enhance bioretention performance, but careful material selection and monitoring are essential to mitigate potential metal leaching risks. • Recycled filter granulates enhanced P removal in bioretention cells. • Repurposed drinking water filter granulate showed low leaching and high stability. • Elevated pH and trace metal leaching limit crushed concrete use in bioretention. • Organic-rich top layers dominated Cu and Zn retention across all systems. • Drying caused transient decline in treatment without long-term performance loss.