7/29/2018 1 Development of Stormwater Treatment Methods Robert Pitt, P.E., Ph.D., BCEE Emeritus Cudworth Professor of Urban Water Systems Department of Civil, Construction, and Environmental Engineering University of Alabama, Tuscaloosa, AL Treatability of Stormwater Toxicants and Bench‐Scale Tests • Particle size distributions • Pollutant strengths of different sized particulates • Sequential digestions and extractions to determine forms of metals and organics • Bench‐scale treatability tests (settling columns, aeration, photodegradation by different wavelengths, precipitation, sorption, ion exchange, etc.)
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7/29/2018
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Development of Stormwater Treatment Methods
Robert Pitt, P.E., Ph.D., BCEEEmeritus Cudworth Professor of Urban Water SystemsDepartment of Civil, Construction, and Environmental EngineeringUniversity of Alabama, Tuscaloosa, AL
Treatability of Stormwater Toxicants and Bench‐Scale Tests
• Particle size distributions
• Pollutant strengths of different sized particulates
• Sequential digestions and extractions to determine forms of metals and organics
• Bench‐scale treatability tests (settling columns, aeration, photodegradation by different wavelengths, precipitation, sorption, ion exchange, etc.)
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Measured Particle Sizes, Including Bed Load Component, at Monroe St. Detention Pond, Madison, WI
Need to remove very small particlesfor high levels of stormwater control
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High levels of pollutant reduction require the capture of very fine particulates, and likely further capture of filtered pollutant fractions.
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Analytical scheme used to determine pollutant associations with particle size, colloids, and organic complexes (samples always split using USGS/Decaportcone splitter)
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Filterable metal
percentage in
ionic forms
Filterable metal
percentage bound in
organic complexes
Zinc 15 85
Copper 70 30
Cadmium 10 90
Lead 12 88
Filterable forms of the metals determine their ability to be removed using ion exchange or sorption methods (higher valence ionic forms easiest to remove, large organic‐metal complexes are difficult to remove)
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PAH% Association
Water Particulate Matter
Naphthalene 22 78
Fluorene 3 97
Phenanthrene 2 99
Anthracene 8 92
Fluoranthene 29 71
Pyrene 19 81
Benzo(a)anthracene 3 99
Chrysene 1 99
Benzo(b)fluoranthene 1 99
Benzo(k)fluoranthene 2 98
Benzo(ghi)perylene 1 99
Benzo(a) pyrene 1 99
Observed PAH Associations with Stormwater Particulates (MCTT Treatability Tests)
Fugacity modeling generally under‐predicted the particulate bound fractions, but was very useful in identifying significant factors affecting the partitioning.
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Settling and Scour of Stormwater Particulates
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Settling velocity of discrete particulates as a function of size and specific gravity (Reynolds and Richards 1996)
Erodibility of previously settled material based on size and shear stresses (Chow 1959)
Traditional methods can be used to calculate settleability of stormwater particulates and scour of previously settled material.
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86420-2
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Specific Gravity (3-250 um) (g/cc)
Percent
3.167 1.674 30 0.799 0.034
1.533 0.6348 30 0.268 0.661
Mean StDev N AD P
Influent S.G. (3-250 um)
Effluent S.G. (3-250 um)
Variable
Normal - 95% CI
Changes in Specific Gravity with Sedimentation Treatment at an Industrial SiteInfluent: 5th to 95th percentile, 1.3 to 6 g/cc (median: 3.2 g/cc)Effluent: 5th to 95th percentile, 0.5 to 2.3 g/cc (median: 1.5 g/cc)
Preferential removal of higher specific gravity materials results in a shift to lower overall specific gravity of particulates in effluent water (and greater migration distance in receiving water after discharge).
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Sieve size range (μm)Average Specific Gravity (g/cc)
Specific Gravity and Volatile Solids of Sediment Collected from Stormwater Treatment Device
Specific gravity decreases as the volatile solids content increases; larger particle sizes have lower specific gravity and greater volatile solids as they contain larger amounts of light‐weight organic debris for these industrial area stormwater sediment samples. Their settling rates are still large due to their large sizes.
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Three flow rates: 10, 5, and 2.5 LPS (160, 80, and 40 GPM)Velocity measurements (Vx, Vy, and Vz) Five overlying water depths above the sediment: 16, 36, 56, 76,
and 96 cmG 12 19 20
F 5 11 18 21 27
E 4 10 17 22 28
D 3 9 16 23 29
C 2 8 15 24 30
B 1 7 14 25 31
A 6 13 26
y
x
Total points per test: 15530 instantaneous velocity measurements at each point
1636
5676
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Scour of Captured Sediment in Storm Drain Catchbasin Inlets
CFD Modeling to Calculate Scour/Design Variations
Used CFD (Fluent 6.2 and Flow 3D) to determine scour from stormwater controls; results verified with full‐scale physical model
This is an example of the effects of the way that water enters a sump on the depth of the water jet and resulting scour
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Buffered Aluminum Sulfate (mg/L)
% r
edu
ctio
n
Turbidity
Copper
Lead
Toxicity
Chemical Coagulation and Precipitation Stormwater Treatment for Turbidity, Lead and Copper
Alum usually had adverse toxicity effect, while ferric chloride with microsand gave best overall reductions.
Bench‐scale to Full‐scale Treatment Schemes
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Design Modifications to Enhance Control of Toxicants in Stormwater Controls and Pilot‐
Scale Tests
• Capture of fine particulates
• Photo‐degradation (enhanced vertical circulation, but not complete mixing that can scour sediments)
• Aeration
• Floatation (subsurface discharges) to increase trapping of floating litter
Development of Stormwater Control Devices
• Multiple treatment processes can be incorporated into stormwater treatment units sized for various applications.
– Gross solids and floatables control (screening)
– Capture of fine solids (settling or filtration)
– Control of targeted “dissolved” pollutants (sorption/ion exchange)
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Laboratory Media Studies • Rate and Extent of Metals Capture
– Capacities (partitioning)
– Kinetics (rate of uptake)
• Effect of pH & pH changes due to media, particle size, interfering ions, etc
• Packed bed filter studies
• Physical properties and surface area determinations
Some laboratory and field pilot‐scale test setups (EPA and WERF‐supported research at Univ. of Alabama). Critical that tests use actual stormwater, not artificial mixtures.
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Development and Testing of Treatment Methods
Multi Chamber Treatment Tank (MCTT)
• Developed under support of the US EPA to provide treatment before stormwater infiltration
• In the public domain, not commercialized
• Targets organic and metallic toxicants
• Very high levels of control through multiple treatment unit processes
• Relatively slow treatment flow rate
• An underground treatment device
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MCTT Cross‐Section
Milwaukee, WI, Ruby Garage Maintenance Yard Drainage Area
Effluent concentrations possible with treatment train using sedimentation along with sorption/ion exchange
Particulate solids (mg/L)
10 to 45 <5 to 10
Phosphorus (mg/L) 0.2 to 0.3 0.02 to 0.1
TKN (mg/L) 0.9 to 1.3 0.8
Cadmium (g/L) 3 0.1
Copper (g/L) 15 3 to 15
Lead (g/L) 12 3 to 15
Zinc (g/L) 37 <20
PAHs (g/L) 10 to 100 <1 to 5
Stormwater Treatability Findings• Biological receiving water problems due to stormwater in
heavily urbanized areas associated with contaminated sediment and habitat destruction (along with safety, trash, secondary contact); water column concentrations rarely have direct effects.
• Need to reduce sediment‐associated pollutants in discharges; try to remove particles down to about 5 μm. Very low NELs may require filtered forms also to be partially removed, usually by active treatment media (sorption and ion exchange). Filtered forms may mostly be as colloids and organic complexes and not amenable to ion exchange.
• Allowing stormwater to flow through captured debris (as in some netting and filter systems) can cause degradation of debris with increased effluent concentrations.
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Stormwater Treatability Findings (cont.)
• Treatment media may be source of pollutants (such as phosphorus from compost, or release of previously captured material if anaerobic, or snowmelt affecting ion exchange)
• Clogging and related maintenance issues are very critical. Flows decrease with use; chemical capacity is reduced with use; sediment and trash storage capacity is reduced with use.
• Snowmelt hinders performance due to increased viscosity of water (slower settling rates), SAR detrimental effects on clays and organics in treatment media, groundwater contamination by chlorides, etc.
Stormwater Treatability Findings (cont.)
• Evaluation of stormwater controls requires both controlled and long‐term tests using actual stormwater (“synthetic” stormwater does not have correct mixture of particulate bound pollutants and interfering major ions, and variability of concentrations).
• Tests must represent the range of expected concentrations and flows.
• Long‐term evaluations are needed to indicate decreased performance with use and associated maintenance issues.