Effects of bioretention media compost volume fraction on toxic metals removal, hydraulic conductivity, and phosphorous release

Batch and column experiments were conducted to investigate the effects of compost volume fraction (CVF; 0-50%) on bioretention media hydraulic conductivity, toxic metal (i.e., Cd, Zn, and Cu) capture, and phosphorus (P) release. Three sources of compost were tested and all exhibited high Cd and Zn sorption capacities. Thus, metal breakthrough times in the columns increased with increasing CVF. Typical breakthrough behavior was not observed for Cu, because little to no Cu was observed in the column effluents after more than 1,000 pore volumes. The unique behavior of Cu was attributable to its precipitation in the synthetic storm water with removal primarily attributed to filtration rather than sorption. Although beneficial for metal removal from storm water, increasing the CVF of the media resulted in reduced hydraulic conductivity and significant release of P. The release of nutrients from compost is a significant concern, and addition of a second layer of media beneath the compost-amended sand to promote P retention, such as iron-amended sand, should be considered.