Enhancement of Phosphorus Removal in Bioretention Cells by Soil Amendment

Abstract

Phosphorus (P) removal in bioretention cells has been highly variable. The objective ofthis study was to find inexpensive filter media with high P sorption and adequate hydraulicconductivity. Batch sorption experiments were conducted to screen filter media. Incorporation of flyash increased P sorption of two Oklahoma soils, Teller loam and Dougherty sand significantly. Flyash addition decreased the hydraulic conductivity of the sand exponentially. Maximum sorptioncapacity predicted by Langmuir isotherms was only 23.8 mg/kg for Dougherty sand, but 385 mg/kgfor Dougherty sand with 5% by weight fly ash, and 82.0 mg/kg for expanded shale. Dougherty sandreleased most sorbed P while P released by the sand/fly ash mixture was negligible. A linearequilibrium convection-dispersion transport model was applied to estimate retardation factors byfitting observed breakthrough curves (BTCs) obtained from column flow-through experiments.Phosphorus BTC in Dougherty sand suggested its retardation factor to be close to one, whileretardation factor of Dougherty sand with 2.5% and 5% fly ash and expanded shale was 199, 470,and 15.7, respectively. Incorporation of a sand/fly ash layer in bioretention cells could improve Premoval dramatically.