A novel stormwater treatment media has been developed using expanded shale aggregate, Al-based water treatment residual (WTR), and psyllium-based binder. The media (HPMM) has sufficient structural capacity and hydraulic conductivity to serve as a permeable pavement base material and demonstrated effective phosphorus (P) retention in lab- and field-scale studies. Long-term adsorption capacity is projected to exceed 600 years of useful life before P saturation under conditions typical of urban stormwater in Maryland (i.e., 0.20 mg/L dissolved P (DP) influent and 100 cm of direct rainfall per year). A dynamic model was developed to describe DP adsorption onto the media based on lab testing and verified under field monitoring. The model predicted 62% DP concentration reduction and 65% DP mass load reduction. Actual reductions from 17 months of monitoring in a field pilot study were 67% for DP concentration and 69% DP mass load.

Total Cu and Zn were also removed from stormwater in lab and field studies. Percent concentration reductions of 59-69% for Cu and 78-90% for Zn were shown in lab studies using synthetic stormwater. Mass load was reduced in field monitoring by 32 and 21% for Cu and Zn, respectively.

WTR in the media was shown to be a potential source of nitrogen (N). An internal water storage (IWS) zone was established in a 5-cm permeable pavement base layer to mitigate N export by promoting denitrification. The IWS was shown to effectively lower N concentrations in simulated stormwater when carbon (C) was available in excess (~10 mg/L total C as C). Elevated Al concentrations were found in some filtrate samples from the field study, resulting from washout of fines from the media. Improved HPMM mix preparation methods have been developed and are critical to prevent Al washout and export.

This research resulted in development of the first known enhanced stormwater treatment media to retain DP in a permeable pavement base layer. With appropriate N and Al control, the novel media can be an effective tool and can enhance permeable pavements to improve urban stormwater quality.