Capturing Stormwater Nitrate and Phosphate with Sorptive Filter Media

Abstract

Soluble phosphate and nitrate are more bioavailable than particulate forms. These nutrients result in eutrophication in both freshwater (typically phosphate-limited) and marine (typically nitrate-limited) systems. In addition, nitrate poses a public health risk at elevated concentrations in drinking water. This research shows that sand filters mixed with 5% iron filings captured, on average, 88% of the influent phosphate in laboratory experiments. Neither incorporation of iron filings into a sand filter nor capture of phosphate had a significant effect on the hydraulic conductivity. Pond-perimeter applications of iron enhanced sand filtration (IESF) with up to 10.7% iron by weight achieved between 29% and 91% phosphate reduction for five events within the first year of operation. After five years, however, a different pond perimeter IESF retained on average 26% of the influent phosphate over three rainy seasons. Retention was best for larger filtered volume events, but negative removal was observed for events with smaller filtered volume and low influent phosphate concentration. Non-routine maintenance improved the hydraulic performance of the pond perimeter IESF and, after a rinsing event, also improved phosphate retention rates to an average of 45%. An IESF was installed to treat agricultural tile drainage and found to reduce total phosphorus loads by 42% to 95% with a flow-weighted mean reduction of 66.3% ± 6.7% (a = 0.05) for 20 events in 2016. The phosphate load reduction varied from 9% to 87% with a flow-weighted mean reduction of 63.9% ± 7.7% (a = 0.05) for 31 events in 2015 and 2016. This research also shows that nitrate is captured abiotically by granular activated carbon (GAC) in laboratory experiments designed to mimic urban and agricultural stormwater runoff. The short contact time and inorganic characteristics of the influent synthetic stormwater suggest that the nitrate was captured by ion exchange, but (bi)carbonate may have competed with nitrate for capture by GAC. Abiotic capture of nitrate requires less stormwater storage volume and less residence time to remove nitrate compared to denitrification, and thus GAC could be used to design smaller treatment practices for nitrate removal.