Enhanced Sand Filtration for Storm Water Phosphorus Removal

Abstract

Batch studies with an initial phosphorus concentration typical of storm water were conducted at the University of Minnesota on C 33 sand, calcareous sand, limestone, three blast oxygen furnace (BOF) by-products, aluminum oxide, and chopped granular steel wool for the removal of dissolved phosphorus from synthetic storm water runoff. Based on the findings of these batch studies, sand filtration enhanced with steel wool, calcareous sand, or limestone has the potential to be a practical and cost-effective method of removing dissolved phosphorus from storm water runoff. Column studies are then performed on four enhancements with C 33 sand filtration: calcareous sand, limestone, chopped granular steel wool, and steel wool fabric. Synthetic storm water runoff with a variable dissolved phosphorus concentration passed through the columns while the flow rate was measured and effluent samples were taken and analyzed for total and dissolved phosphorus concentration and pH. As found in the batch studies, C 33 sand retained dissolved phosphorus but the capacity was quickly exhausted. Combinations of C 33 sand with limestone or calcareous sand clogged the columns and prevented them from draining completely. Steel wool, however, significantly increased the duration and level of phosphorus retention as compared to C 33 sand alone and did not clog the columns. Between 34 and 81% of the dissolved phosphorus was retained by the six steel-enhanced columns. Fine oxidized iron particles observed in the effluent are too small to be completely captured by typical geotextile fabric and may compromise phosphorus removal performance, but phosphorus adsorbed to iron oxide will be of limited bioavailability. Steel-enhanced sand filtration is modeled with contact time, total mass of phosphorus retained, and influent concentration as variables. Enhancing sand filtration systems with steel wool fabric would minimally increase installation costs and would increase the material cost by 3–5%. Based on these findings, steel-enhanced sand filtration is a potentially cost-effective treatment for removing dissolved phosphorus from storm water runoff.