Recycled Concrete Aggregates in Bioretention Cells: Pollutant Retention, Start-Up and Stabilization Dynamics, and Resource Conservation

This study evaluated recycled concrete aggregates (RCAs) as a filter medium and secondary mineral resource in bioretention cells treating synthetic urban runoff, with emphasis on pollutant retention, start-up, and stabilization dynamics. Four laboratory-scale columns containing RCAs were operated for 12 weekly cycles under unplanted and vetiver-planted configurations, with and without a 2 cm granular activated carbon layer. Ammonium, nitrate, total nitrogen, phosphate, Pb, Zn, pH, and electrical conductivity (EC) were monitored throughout the experiment. Ammonium was consistently retained, reaching removal efficiencies above 95% in all configurations by the final cycles, while Pb and Zn remained below detection in nearly all effluents. Nitrate, total nitrogen, and phosphate showed initial export and greater variability during start-up, followed by more consistent retention, particularly in the vegetated configurations. The GAC layer did not provide a consistent improvement in nutrient retention. EC generally decreased during operation, indicating progressive conditioning of the RCA media, whereas effluent pH remained persistently alkaline. Altogether, RCAs provided effective pollutant retention while exhibiting distinct conditioning and stabilization trajectories. Their use as the main mineral component of bioretention filter beds also offers a pathway for substituting virgin aggregates and valorizing construction and demolition waste, although long-term performance and life-cycle impacts require further evaluation.