Enhancement of Phosphorus Removal in Bioretention Cells by Soil Amendment

摘要

Phosphorus (P) removal in bioretention cells has been highly variable. The objective of this study was to find inexpensive filter media with high P sorption and adequate hydraulic conductivity. Batch sorption experiments were conducted to screen filter media. Incorporation of fly ash increased P sorption of two Oklahoma soils, Teller loam and Dougherty sand significantly. Fly ash addition decreased the hydraulic conductivity of the sand exponentially. Maximum sorption capacity predicted by Langmuir isotherms was only 23.8 mg/kg for Dougherty sand, but 385 mg/kg for Dougherty sand with 5% by weight fly ash, and 82.0 mg/kg for expanded shale. Dougherty sand released most sorbed P while P released by the sand/fly ash mixture was negligible. A linear equilibrium convection-dispersion transport model was applied to estimate retardation factors by fitting observed breakthrough curves (BTCs) obtained from column flow-through experiments. Phosphorus BTC in Dougherty sand suggested its retardation factor tobe close to one, while retardation 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 P removal dramatically.