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

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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.

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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.

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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
pH Kd, mL/g
Teller loam 6.2 0.41
Dougherty sand 6.3 2.08
Fly ash 11.5 2180
Limestone 9.0 12.1
Peat moss 2.9 -5.79
M-shale (KS) 6.4 280
N-shale (MO) 8.6 1.21
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Heavy Metal Sorption, Kd (ml/g)
Material Cu Pb Zn
Dougherty sand 11.6 335 8
Teller Loam 1650 557 351
Slaughterville Loam 4680 646 113
Fly Ash 8410 3050 4010
Dougherty Sand 155 gt1220 21
D 2.5 F 266 gt1220 618
D 5 F 239 gt1220 843
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Amending soils with fly ash

• The addition of fly ash increased P sorption of
  both soils significantly, especially Dougherty
  sand.

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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
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P Sorption Isotherms
Langmuir Langmuir Langmuir Freundlich Freundlich Freundlich
Sm, mg/kg b, L/mg r2 Kf, L/kg n r2
Dougherty sand 23.8 0.278 0.948 4.93 0.622 0.914
M-shale 82.0 3.30 0.997 52.9 0.254 0.996
D5F 385 2.89 0.998 203 0.295 0.985
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Desorption

• Dougherty sand desorbed average 42 of initially
  sorbed P, expanded shale 7, and D5F negligible
  amounts.
• Possible irreversible sorption in D5 and shale.

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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.

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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).

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P Column Results
Observed and fitted P breakthrough curves
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P Column Results
Dougherty sand M-shale D2.5F D5F
Retardation (R) 1 16 199 470
Kd calc. from R 0 10 38 80
Kd from batch sorp. 2.1 280 307 398
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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.

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Metal Column Results
Medium Metal R Kd (ml/g)
Dougherty sand Cu 1100 264
Pb 2,350 564
Zn 490 117
D 2.5 F Cu 6,700 1,320
Pb 7,100 1,400
Zn 2,000 394
D 5 F Cu 175,000 29,000
Pb gt2,950 gt48,900
Zn 145,000 24,000
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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.

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Transport Modeling
Medium Element Lifetime (years) Lifetime (years)
Medium Element Pavements Lawns
Dougherty sand Cu 22 62
Dougherty sand Pb 48 133
Dougherty sand Zn 10 27
D2.5 F Cu 96 264
D2.5 F Pb 102 280
D2.5 F Zn 28 79
D5 F Cu 1111 3050
D5 F Pb gt1861 gt5107
D5 F Zn 925 2539
P 4 11
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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.

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SMI - Nitrate tests

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

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SMI Nitrate Results
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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.