FIELD EVALUATION OF ENHANCED PHOSPHORUS AND NITROGEN REMOVAL IN STORMWATER CONTROL MEASURES

Abstract

This project evaluates two innovative stormwater control measures (SCMs) installed on the University of Maryland campus in College Park, Maryland. One project retrofitted an existing bioretention cell with 5% (by mass) aluminum-based water treatment residual (Al-WTR) to enhance phosphorus removal (P1 site). The other combined a porous parking area with underground anoxic vaults to promote nitrogen removal (N1 site).

At the P1 site, the net reduction of the total runoff was 40% and the volume reduction ratios ( ) were lower than before the retrofit. The total suspended solids (TSS), total phosphorus (TP), and particulate phosphorus (PP) concentrations were significantly reduced by the bioretention cell, due to the filtration of the particulate matter, while TP export occurred before WTR retrofit. Soluble reactive phosphorus (SRP) and dissolved organic phosphorus (DOP) concentrations in the stormwater runoff were not obviously changed compared to the system effluent. The near constant outflow of SRP and DOP concentrations suggest an equilibrium adsorption treatment mechanism. Mass loads were reduced for TSS and all P species. WTR incorporation decreased the bioretention media phosphorus saturation index (PSI) from approximately 0.075 to approximately 0.041, which stayed relatively constant during the two year study period, even with the media P continually increasing, indicating a significant increase in media P sorption capacity.

At the N1 site, the flow management achieved nearly zero runoff discharge due to infiltration from the vaults during dry weather. With regard to the porous pavement, approximately 34% of the total nitrogen (TN) (4.7 kg/ha-yr) was mitigated by filtration processes; 6.5 kg/ha-yr particulate organic nitrogen (PON) and 1.8 kg/ha-yr ammonium nitrogen ( ) were removed, and 4.0 kg/ha-yr oxidized nitrogen (NOx) were created. In the denitrification vaults, approximately 26% of the TN (3.7 kg/ha-yr) was decreased by system reaction, mostly due to the decrease in NOx (3.8 kg/ha-yr). PON was reduced slightly, by 0.2 kg/ha-yr. The small amount of DON and produced likely resulted from leaching from the wood logs. As a result, the N mass reduction that occurred was not only due to volume reduction, but also to system reactions (60%).