Performance Assessment of an Iron-Enhanced Sand Filtration Trench for Capturing Dissolved Phosphorus

Abstract

Nutrients (phosphorus and nitrogen) in excess can cause nuisance algae blooms that generate negative aesthetic and eutrophic conditions in receiving lakes and rivers (U.S. EPA., 1999). In temperate fresh water, dissolved phosphorus is the limiting nutrient (Aldridge and Ganf, 2003; Schindler, 1977) and exists in the form of phosphates (HXPO4, (Stumm and Morgan, 1981)) contributed to urban stormwater from sources such as lawn fertilizers, leaf litter, grass clippings, unfertilized soils, detergents, and rainfall, among others (American Public Health Association, 1998; U.S. EPA., 1999). A recent study of nationwide monitoring data (Pitt et al., 2005) reports that the median values of total and dissolved phosphorus (phosphates) are 0.27 and 0.12 mg P/L, respectively and therefore the fraction of dissolved phosphorus to total phosphorus is approximately 44%. Removing dissolved phosphorus from stormwater at a substantial rate requires capture of both the particulate and dissolved fractions of total phosphorus. While most stormwater treatment practices can capture particulate phosphorus through settling or filtration, very few practices have a mechanism to consistently capture dissolved phosphorus over the life-cycle of a treatment practice. Wet detention basins, in particular, are typically designed to capture greater than 80% total suspended solids and, on average, achieve a ~50% total phosphorus load reduction but do little to remove dissolved contaminants from stormwater. Because dissolved phosphorus has a higher bioavailability factor than particulate forms (Sharpley et al., 1992), removing only particulate fractions from stormwater only minimally reduces phosphorus bioavailability. To capture dissolved phosphorus, a chemical adsorption or precipitation process must be added to stormwater treatment practices. Adding steel wool or elemental iron to a sand filter has been shown to capture a significant amount of dissolved phosphorus (Erickson et al., 2007). As the elemental iron forms iron oxides (rust), dissolved phosphorus binds to these iron oxides by surface adsorption.