Iron Enhanced Sand Filter Performance for Reducing Phosphorus from a Regional Stormwater Pond

摘要

William Street Pond is a stormwater basin located in Roseville, MN. The pond receives stormwater from the surrounding residential neighborhood and discharges directly to Lake McCarrons, a 75-acre deep lake that supports recreational opportunities. During the past 15 years, several projects have been implemented in Lake McCarrons and its watershed to reduce the phosphorus load in the lake. Currently, Lake McCarrons is not listed as impaired for nutrients. Capitol Region Watershed District (CRWD) regards stormwater inputs, including particulate and dissolved phosphorus loading, as a high priority for improvement efforts. In 2011, William Street Pond underwent significant improvements to enhance its performance, including a retrofit of the pond outlet that incorporates two iron-enhanced sand filter benches (IESF's). The IESF's remove dissolved phosphorus via surface sorption to oxidized iron. To assess the efficacy of the IESF's for phosphorus removal, CRWD collected influent and effluent samples from 2013 to 2016 to be analyzed for multiple phosphorus species. Samples were collected within 24 hours of a significant storm event to capture runoff. Ninety-seven percent of samples taken during the sample period showed a reduction of total phosphorus. Similar reductions were observed for dissolved phosphorus and ortho-phosphate with nearly half of the effluent samples being analyzed under the reporting limit for both species. Further, effluent samples from both filters showed very consistent, low concentrations for all phosphorus species. These results suggest that the amount of iron in the sand and the contact time with the iron is sufficient to achieve near-maximum reductions in phosphorus concentrations.