Recycled crushed concrete as a filter medium for phosphorus and heavy metal removal from semi-synthetic stormwater.

Facing escalating urban drainage issues, cities are increasingly adopting nature-based solutions, which have proven effective for flood prevention and filtration of particulate contaminants. However, retention of dissolved nutrients and heavy metals has remained a challenge. This column study investigates the long-term implications of using crushed concrete as a filter medium for retention of phosphorus (P), copper (Cu), and zinc (Zn). Results show that crushed concrete achieved significantly higher P retention (98.1 ± 1.6 %) than filter sand and gravel (7.13 ± 9.5 %). Both materials demonstrated high metal removal capabilities (>95 % for Cu and >99 % for Zn). While the filter sand and gravel exhibited leaching of P and Zn during stress tests with demineralised water, the pollutants retained in the crushed concrete column were not prone to remobilisation. A follow-up mini column experiment confirmed that the crushed concrete can sustain its high removal efficiencies for P, Cu, and Zn over extended periods. Freshly crushed concrete with a low degree of carbonation was critical for efficient P removal, whereas grain size was the more influential factor for heavy metal retention. Nevertheless, high P removal coincided with an elevated effluent pH (>11) and pronounced chromium (Cr) leaching in the initial phases of both column experiments. While crushed concrete has strong potential to serve as a long-lasting, sustainable, and cost-effective medium for removal of P, Cu, and Zn from urban runoff, these drawbacks could be a barrier to wider adoption. Therefore, further research should explore optimised sourcing, tailored pretreatment, and application possibilities.