Enhancement of Pollutant Removal in Bioretention Cells by Soil Amendment

1. Enhancement of Pollutant Removal in Bioretention Cells by Soil Amendment Glenn O. Brown, Professor, PE, Ph.D., D.WRE Biosystems and Agricultural Engineering Oklahoma State University August 20, 2009

2. Background Phosphorus and Nitrate removal in bioretention cells has been reported to be highly variable, and in some cases, cells have been a P and NO3- source. Our long term objective hasbeen to find an inexpensivefilter media with highpollutant sorption andadequate hydraulic conductivity.

3. Materials and Methods • Soils: Dougherty sand, Teller loam • Sorbent media: fly ash, peat moss, limestone, expanded shales and sulfur modified iron. • Batch sorption experiments conducted to screen media. • Hydraulic conductivity tests performed to determine infiltration capacity of media. • Column study and transport modeling carried out to determine transport parameters and predict long-term cell performance.

4. Media Screening • Distribution coefficients were measured to screen media. Fly ash and an expanded shale from KS displayed the largest P sorption. P Kd ml/g Batch Sorption

5. Heavy Metal Sorption, Kd (ml/g)

6. 5% fly ash Amending soils with fly ash • The addition of fly ash increased P sorption of both soils significantly, especially Dougherty sand.

7. 5% fly ash Ks= 3.59 cm/hr Hydraulic Conductivity • Teller loam: 0.29 cm/hr; Dougherty sand: 40 cm/hr; Expanded shale: 39 cm/hr. The addition of fly ash decreased Ks of Dougherty sand markedly. Falling head permeameter Ks of sand/fly ash mixture

8. P Sorption Isotherms

9. Desorption • Dougherty sand desorbed average 42% of initially sorbed P, expanded shale 7%, and D+5%F negligible amounts. • Possible irreversible sorption in D+5% and shale.

10. Column Experiments • Column: 14.4 cm I.D., 14.3 cm long. Loading rate: 3 cm/hr. • Influent concentration: 1 mg/L P. • Samples analyzed by ICP. • Evaluate sorption in a dynamic condition.

11. Transport Modeling • One dimensional linear equilibrium adsorption convection-dispersion transport model in CXTFIT 2.1 in the STANMOD software package developed by the U.S. Salinity Laboratory. • No decay, no production, third-type inlet boundary and step input. • Fit observed breakthrough curves by the model to estimate hydrodynamic dispersion coefficient (D) and retardation factor (R).

12. P Column Results Observed and fitted P breakthrough curves

13. P Column Results

14. Metald Column Results • Only Zn was observed in the effluent after 250 to 350 pore volumes. • Retardation estimated by destructive sampling of the columns and fitting using CXTFIT 2.1.

15. Metal Column Results

16. Estimating Lifetime • Hypothetical Scenario • Filter media depth: 1 m • Influent concentrations: P, Cu, Zn & Pb 1 mg/L • Effluent limits: P 0.037 mg/L; Cu, Zn & Pb 0.01 mg/l. • Fifty years of Grove OK precipitation data were used to estimate the runoff loading. • Used fitted parameters from column tests. • Conservative assumption of reversible adsorption.

17. Transport Modeling

18. Sulfur Modified Iron • A variation of “zero valance iron” • Shown to reduce • Nitrate • Arsenic • Chromium • Chlorinated Solvents • Other Metals • Screening tests conducted in Spring of 2009.

19. SMI - Nitrate tests • Batch reactor. • Two types of SMI. • Pure SMI, and mixed with sand and flyash. • Solution concentrations of0 to 300 mg/l.

20. SMI – Nitrate Results

21. Conclusions • The addition of fly ash increased P sorption of all soils significantly. • Phosphorous sorption is at least partially irreversible. • Soils tested have significant heavy metal sorption, but fly ash will make them effectively infinite sinks. • Amended with 5% fly ash, Dougherty sand exhibited high P sorption and adequate hydraulic conductivity. • Sulfur Modified Iron has potential to remove nitrate. We can assume it will also remove organic compounds.