Difference between revisions of "BMPs for stormwater filtration"
BMPs for stormwater filtration

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Green Infrastructure: Filtration practices can be an important tool for detention of stormwater runoff and treatment of pollutants in stormwater runoff. If the practice utilizes vegetation, additional benefits may include cleaner air, carbon sequestration, improved biological habitat, and aesthetic value.

Filtration practices treat stormwater by filtering pollutants. They are effective at attenuating suspended sediment and pollutants associated with that sediment. Most filtration practices utilize an underdrain and engineered soil media to attenuate pollutants and can be sized, in appropriate conditions, to treat the water quality volume. Some infiltration and/or loss of water through evapotranspiration occurs in most filtration practices. Filtration Best Management Practices include, but are not limited, to

biofiltration (bioretention with an underdrain),
permeable pavement with an underdrain,
tree trenches and tree boxes with an underdrain,
dry swale with an underdrain (with or without check dams),
wet swales, and
green roofs.

Biofiltration basin (bioretention with an underdrain)

Bioretention basin (Source: CDM Smith)

Applications and treatment capabilities for biofiltration basins
Applications		Treatment capabilities^{2, 3}
Residential	Yes	TSS	High⁴
Commercial	Yes	TN	Low/Medium
Ultra-urban	Limited⁶	TP	Medium/high⁵
Industrial	Yes¹	Chloride	Low
Highway/road	Yes	Metals	High
Recreational	Yes	Oils and grease	High
		Pathogens	High
¹ Unless the filtration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. ² This is only for the portion of flow that enters the filtration basin; by-passed runoff does not receive treatment; ³ Low = < 30%; Medium = 30-65%; High = 65 -100%); ⁴ Assumes adequate pretreatment; ⁵ Certain soil mixes can leach P; ⁶ Due to a size restriction Sources: EPA Factsheet, 1999; Davis et al., 2001, 2003, 2006; Hsieh and Davis, 2005; Hong et al., 2006; Hunt et al., 2006; NPRPD, 2007; Li and Davis, 2009; Diblasi et al., 2009; Passeport et al., 2009; Brown et at., 2011a, b; Komlos et al., 2012; Denich et al., 2013; Li and Davis, 2013; California Stormwater BMP

Biofiltration basins, often called rain gardens, use soil (typically engineered media or mixed soil) and native vegetation to capture runoff and remove pollutants. The media has a high infiltration rate but the underlying soil typically has a low infiltration rate (C or D soils). Therefore an underdrain is incorporated into the design. Water captured by the BMP is filtered through the engineered media and most of the treated water is captured by the underdrain and returned to the storm sewer system. Some water infiltrates below the underdrain and some water is taken up by plants. The amount of water that infiltrates can be maximized by raising the underdrain above the native soil or using an upturned elbow in the underdrain. Water captured by the BMP must drawdown within 48 hours. For more information, see the following pages in this Manual.

Permeable pavement (with an underdrain)

Permeable pavement (Source: CDM Smith)

Applications and treatment capabilities for permeable pavement
Applications		Treatment capabilities^{2, 3}
Residential	Yes	TSS	High⁴
Commercial	Yes	TN	Medium/High
Ultra-urban	Yes	Nitrate	Low/Medium
Industrial	Yes¹	TP	Medium/High
Retrofit	Yes	Chloride	Low
Highway/road	Yes	Metals	High
Recreational	Yes	Oils and grease	High
		Pathogens	⁵
¹ Unless the filtration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. ² This is only for the portion of flow that enters the filtration basin; by-passed runoff does not receive treatment; ³ Low = < 30%; Medium = 30-65%; High = 65 -100%); ⁴ Assumes adequate pretreatment; ⁵ Insufficient information Source: Schueler, 1987; Pratt et al, 1999; Adams, 2003; Brattebo and Booth, 2003; Adams, 2003; Bean et al, 2007; SEMCOG, 2008; International Stormwater Database, 2012

Permeable pavements are paving surfaces that allow stormwater runoff to filter through surface voids into an underlying stone reservoir for filtration and/or storage. They are suitable for driveways, trails, parking lots, and roadways with lighter traffic. The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt, and permeable interlocking concrete pavers (PICP). All permeable pavements have a similar design layering system, consisting of a surface pavement layer, an underlying stone aggregate reservoir layer, optional underdrains for filtration and geotextile over non-compacted soil subgrade. Discharge of this stored runoff occurs primarily to underdrains, though some infiltration occurs into the underlying soil. The amount of water that infiltrates can be maximized by raising the underdrain above the native soil or using an upturned elbow in the underdrain. Water captured by the BMP must drawdown within 48 hours. The drainage area leading to permeable pavements should not exceed twice the surface area of the final pavement surface.

For more information, see the following pages in this Manual.

Tree box/Tree trench (with an underdrain)

Tree box (Source: CDM Smith)

Applications and treatment capabilities for tree box/tree trench
Applications		Treatment capabilities^{2, 3}
Residential	Yes	TSS	High⁴
Commercial	Yes	TN	Low/Medium
Ultra-urban	Yes	TP	Medium/High⁵
Industrial	Yes¹	Chloride	Low
Highway/road	No	Metals	High
Recreational	Yes	Oils and grease	High
		Pathogens	High
¹ Unless the filtration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. ² This is only for the portion of flow that enters the filtration basin; by-passed runoff does not receive treatment; ³ Low = < 30%; Medium = 30-65%; High = 65 -100%); ⁴ Assumes adequate pretreatment; ⁵ Certain soil mixes can leach P. Source: see [1]

Tree trenches are a system of trees that are connected by an underground filtration structure. The system consists of a stormwater tree trench or box lined with geotextile fabric with structural stone, gravel or soil boxes in which the trees are placed. Tree systems consist of an engineered soil or rock layer designed to treat stormwater runoff via filtration through plant and soil/rock media, and through evapotranspiration from trees. Water that is filtered through the engineered media largely passes to an underdrain and is returned to the storm sewer system. Some infiltration occurs into the underlying soil. The amount of water that infiltrates can be maximized by raising the underdrain above the native soil or using an upturned elbow in the underdrain. Water captured by the BMP must drawdown within 48 hours. Tree species are carefully selected to survive both inundation and drought conditions in urban environments where they will be potentially affected by chloride and other traffic concerns. The drainage area to tree trenches and boxes should be less than five acres depending on the size of each trench. Irrigation, whether manual or automated, is strongly encouraged during the tree’s establishment period.

For more information, see the following pages in this Manual.

Trees - general

Tree boxes/tree trenches

Dry swale (with underdrains)

Dry swale (no check dams)

Applications and treatment capabilities for dry swale with check dams
Applications		Treatment capabilities^{2, 3}
Residential	Yes	TSS	High⁴
Commercial	Yes	TN	Low/Medium
Ultra-urban	Limited	TP	Low/Medium⁵
Industrial	Yes¹	Chloride	Low
Highway/road	Yes	Metals	High
Recreational	Yes	Oils and grease	High
		Pathogens	Medium
¹ Unless the filtration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. ² This is only for the portion of flow that enters the filtration basin; by-passed runoff does not receive treatment; ³ Low = < 30%; Medium = 30-65%; High = 65 -100%); ⁴ Assumes adequate pretreatment; ⁵ Certain soil mixes can leach P. Source: see [2]

Similar to vegetated swales designed for stormwater conveyance, dry swales with underdrains are designed as linear, multi-celled stormwater filtration BMP’s. By incorporating earthen or structural check dams, runoff is retained along a series of narrow, shallow basins or cells. This runoff is filtered through engineered media or soil and largely captured by an underdrain and returned to the storm sewer system. Some infiltration occurs into the underlying soil. The amount of water that infiltrates can be maximized by raising the underdrain above the native soil or using an upturned elbow in the underdrain. Water captured by the BMP must drawdown within 48 hours. Coarse vegetation such as decorative plantings or even turf grass slow runoff movement. This system is designed to move, store, and infiltrate runoff from impervious surfaces such as linear roadways or parking lots. Dry swales are best designed for sites under 5 acres in size.

For information, see the following pages.

Wet swales

Wet swale (no check dams)

Applications and treatment capabilities for wet swale with check dams
Applications		Treatment capabilities^{2, 3}
Residential	Yes	TSS	Medium⁴
Commercial	Yes	TN	Low
Ultra-urban	Limited	TP	Low⁵
Industrial	Yes¹	Metals	Low
Highway/road	Yes	Oil and grease	Low
Recreational	Yes	Pathogens	Low
		Chloride	Low
¹ Unless the filtration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. ² This is only for the portion of flow that enters the filtration basin; by-passed runoff does not receive treatment; ³ Low = < 30%; Medium = 30-65%; High = 65 -100%); ⁴ Assumes adequate pretreatment; ⁵ Certain soil mixes can leach P. Source: see [3]

Wet swales occur when the water table is located very close to the surface or water does not readily drain out of the swale. A wet swale acts as a very long and linear shallow biofiltration treatment system. The entire water quality treatment volume is stored within a series of cells created by check dams. Cells may be planted with emergent wetland plant species to improve pollutant removal. Wet swales are commonly used for drainage areas less than 5 acres in size.

For more information, see the following pages in this manual.

Green roofs

Green roof on the Target Center in Minneapolis Minnesota. Note the vegetation free zones. Image Courtesy of The Kestrel Design Group, Inc.

Applications and treatment capabilities for wet swale with check dams
Applications		Treatment capabilities¹
Residential	Yes	TSS	Medium
Commercial	Yes	TN	Low
Ultra-urban	Yes	TP	Low²
Industrial	Yes	Metals	Low
Highway/road	No	Oil and grease	Low
Recreational	No	Pathogens	Low
		Chloride	Low
¹ Low = < 30%; Medium = 30-65%; High = 65 -100%); ² Certain soil mixes can leach P.

Green roofs typically occur at the beginning of stormwater treatment trains. Green roofs provide filtering of suspended solids and pollutants associated with those solids, although total suspended solid (TSS) concentrations from traditional roofs are generally low. Green roofs provide both volume and rate control, thus decreasing the stormwater volume being delivered to downstream Best Management Practices (BMPs).

For more information, see the following pages in this manual.

Unit processes for different filtration BMPs

The following table provides a summary of unit processes for the different infiltration BMPs.

Unit processes of stormwater treatment techniques for filtration practices (Adapted from WEF, 2008)
Link to this table

Control	Dry swale or step pool	Wet swale	Biofiltration	Permeable pavement	Tree box/tree trench	Green roof
Peak flow attenuation	X	X	X	X		X
Runoff volume reduction	X		X	X	X	X
Infiltration	X		X	X	X
Dispersion	X
Evapotranspiration	X	X	X		X	X
Runoff collection and usage			X	X
Sedimentation	X	X	X
Flotation			X
Laminar separation
Swirl concentration
Sorption	X	X	X	X		X
Precipitation			X	X
Coagulation			X	X
Filtration	X	X	X	X	X	X
Plant metabolism			X		X	X
Nitrification/denitrification		X	X
Organic compound degradation	X	X	X	X		X
Pathogen die off				X
Temperature reduction	X	X	X	X		X
Disinfection			X	X

Information tables

The following tables describe and differentiate different characteristics of stormwater infiltration BMPs.

Overview table

The following table provides a brief description and schematic of each stormwater filtration BMP.

Stormwater filtration BMPs - overview

Contributing drainage area table

The following table provides a summary of recommended contributing drainage area for each stormwater filtration BMP.

Contributing drainage area is defined as the total area, including pervious and impervious surfaces, contributing to a best management practice (BMP). It is assumed that in most cases, with the exception of green roofs and many permeable pavement systems, impervious surfaces will constitute more than 50 percent of the contributing area to the BMP and that most of this impervious is directly connected impervious. The recommended contributing area to a BMP may be modified for the following conditions.

The recommended contributing area may be increased if pervious surfaces constitute the majority of the contributing area and soils are hydrologic soil group (HSG) A or B
The recommended contributing area should be decreased if impervious surfaces contribute more than 80 percent of the contributing area or if the entire impervious surface is directly connected and routed to the BMP
The recommended contributing area should be decreased or may be increased based on pollutant loading (decrease with higher pollutant loads)

Runoff coefficients may be calculated for an area contributing to a BMP. Runoff coefficients greater than about 0.55 are typical of urban areas having 50 percent or more impervious surface. Typical runoff coefficients are shown on these pages ([4], [5]) and discussed here. To see how runoff curve number is associated with impervious percentages, see at this link.

Stormwater filtration BMPs - contributing drainage area
Link to this table

Stormwater BMP	Recommended contributing area	Notes
Dry swale or step pool	5 acres or less	A dry swale or step pool must be able to drawdown within 48-hours in order to preserve vegetation.
Wet swale	5 acres or less	The boundary between upland and wetland type vegetation should be considered in the design. Different plant species will be tolerant of different periods of inundation.
Biofiltration Basin	5 acres or less	Biofiltration basins must meet the required 48 hour drawdown time and must be sized in order to allow for adequate maintenance. It is HIGHLY RECOMMENDED that biofiltration basins be designed to prevent high levels of bounce as submerging vegetation may inhibit plant growth. A maximum wet storage depth of 1.5 feet is HIGHLY RECOMMENDED.
Permeable Pavement (with underdrain)	It is RECOMMENDED that external contributing drainage area not exceed the surface area of the permeable pavement. It is HIGHLY RECOMMENDED that external contributing drainage area not exceed twice the surface area of the permeable pavement	It is RECOMMENDED that external drainage area be as close to 100% impervious as possible. Field experience has shown that drainage area (pervious or impervious) can contribute particulates to the permeable pavement and lead to clogging. Therefore, sediment source control and/or pretreatment should be used to control sediment run-on to the permeable pavement section.
Tree Trench/Tree Box (with underdrain)	up to 0.25 acres per tree
Green roof	Not applicable	Contributing drainage from conventional roofs must be no larger than the surface area of the green roof

References: Virginia, North Carolina, West Virginia, Maine, Lake Tahoe, Connecticut, Massachusetts, New York, Wisconsin, Vermont, New Hampshire, Ontario, Pennsylvania

Treatment properties table

The following table provides information on pollutant removal mechanism(s), location in the stormwater treatment train, general pollutant removal, and potential applications for each of the stormwater BMPs.

Stormwater filtration BMPs - treatment properties
Link to this table

Stormwater BMP	Pollutant Removal Mechanism	Location in Treatment Train	Pollutant Removal ¹	Potential Application
Dry swale or step pool	Sedimentation / Infiltration	Throughout	TSS: Medium TN: Low TP: Medium Chloride: Low Metals: Medium Oils and Grease: Low Pathogens: Low	Residential Commercial Industrial Highway/Road Recreational
Wet swale	Sedimentation / Transpiration	Throughout	TSS: Medium TN: Low TP: Low Chloride: Low Metals: Low Oils and Grease: Low Pathogens: Low	Residential Commercial Industrial Highway/Road Recreation
Biofiltration Basin	Sedimentation / Filtration	Beginning	TSS: High TN: Low/Medium TP: Medium/High² Chloride: Low Metals: High Oils and Grease: High	Residential Commercial Retrofit
Permeable pavement with underdrain	Filtration	Beginning	TSS: High TN: Medium/High TP: Medium/High Chloride: Low Metals: High Oils and Grease: High	Residential Commercial Ultra Urban Retrofit
Tree Trench/Tree Box (with underdrain)	Filtration, Transpiration	Throughout	TSS: High TN: Medium/High TP: Medium/High Chloride: Low Metals: High Oils and Grease: High Pathogens: High	Commercial Ultra Urban Retrofit
Media filter (sand filter)	Filtration	Throughout	TSS: High TN: Medium TP: Medium/High Chloride: Low Metals: Medium/High Oils and Grease: High Pathogens: High	Residential Commercial Industrial Recreation
Green roof	Filtration	Throughout	TSS: High TN: Low TP: Low Chloride: Low Metals: High Oils and Grease: High Pathogens: High	Residential Commercial Ultra Urban Industrial Retrofit

¹² Low = < 30%; Medium = 30‐65%; High = 65‐100%
²Some media mixes can leach phosphorus.

Selection considerations table

The following table provides information on general cost, maintenance requirements, pretreatment needs, and habitat quality for each of the stormwater filtration BMPs.

Stormwater filtration BMPs – selection considerations
Link to this Stormwater filtration BMPs – selection considerations

Stormwater BMP	Cost	Maintenance Requirements ³	Space needed	Pretreatment ⁴	Habitat Quality ⁵
Dry swale or wet pool	Low to medium (depends on components)	Simple‐Intensive	Small	Oil/Water Separator Sediment Basin (depending on watershed soil types, contributing drainage area, and potential for system fouling/plugging)	Low
Wet pool	Low	Simple	Small	Oil/Water Separator	Low
Biofiltration Basin	Low $0.5‐$1.3 CF	Simple‐Intensive	Small to medium	Needed Oil/Water Separator, Vegetated Filter, Sediment Basin, Water Quality Inlets	Medium‐High
Permeable Pavement (with underdrain)	Medium 3‐10 CF	Medium	Small to medium	No Pretreatment Required	None
Tree Trench/Tree Box (with underdrain)	High $1.80 ‐ $12.70 CF based on recommended soil volume of 1,414 CF per tree	Intensive	Small to large	Needed Oil/Water Separator, Water Quality Inlets	Medium
Media filter (sand filter)	Low	Simple-moderate	Small	Needed Oil/Water Separator, Vegetated Filter, Sediment Basin	Low
Green roof	High	Moderate-intensive	Small	Low	Medium

¹ Maintenance requirements to be addressed and updated in future section
² Pretreatment requirements to be revised as per updated section
³ Habitat quality refers to the possible diversity of plantings commonly installed with each BMP

Infiltration Summary Table

To see all information contained in the previous tables in a single table, click on the following link: Filtration Summary Table

Related pages

The following practices typically employ an underdrain, which captures most of the runoff that enters the BMP. For information on these BMPs with no underdrain, see Stormwater infiltration Best Management Practices.

Other filtration BMPs, including pre-treatment practices

Manufactured devices

Filtration devices

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Difference between revisions of "BMPs for stormwater filtration" BMPs for stormwater filtration