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

Latest revision as of 15:04, 11 January 2023

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 (BMPs) 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,
sand filter; and
green roofs.

Raising an underdrain above the bottom of the filtration practice increases infiltration. However, the practice still functions as a filtration practice. For more information on practices with a raised underdrain, link here.

1 Biofiltration basin (bioretention with an underdrain)
2 Permeable pavement (with an underdrain)
3 Tree box/Tree trench (with an underdrain)
4 Dry swale and stormwater step-pool (with check dams and underdrains)
5 Wet swales
6 Media (sand) filter
7 Green roofs
8 Practices with a raised underdrain or internal water storage
9 Unit processes for different filtration BMPs
10 Information tables
11 Related pages

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 and stormwater step-pool (with check dams and underdrains)

Dry swale (no check dams)

Applications and treatment capabilities for dry swale and step pools with check dams and underdrains
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]

Stormwater step pool. Courtesy of Limnotech.

Similar to vegetated swales designed for stormwater conveyance, dry swales with underdrains are designed as linear, multi-celled stormwater filtration BMP’s. For high gradient conditions, stormwater step-pools can be utilized. 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 on dry swales, see the following pages.

For information on step pools, 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.

Media (sand) filter

Iron enhanced sand filter basin, Maplewood, MN. Photo courtesy of Barr Engineering.

Applications and treatment capabilities for media filters
Applications		Treatment capabilities^{2, 3}
Residential	Yes	TSS	High⁴
Commercial	Yes	TN	Medium
Ultra-urban	Limited	TP	Medium
Industrial	Yes¹	Metals	Medium
Highway/road	Yes	Oil and grease	High
Recreational	Yes	Pathogens	High
		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. Source: see [4]

Media filters treat stormwater through a variety of different filtering materials whose purpose is to remove pollution from runoff. Variants include surface sand filters, underground sand filters and perimeter sand filters. Most media filters have an underdrain and are effective at removing sediment and pollutants associated with sediment. Media filters may be enhanced with iron or other material to facilitate removal of dissolved pollutants, such as dissolved phosphorus. For more information on media filters, see the following sections in the Minnesota Stormwater 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.

Practices with a raised underdrain or internal water storage

Biofiltration device with elevated underdrain

Biofiltration device with internal water storage

A filtration practice with a raised underdrain provides a storage area below the invert of the underdrain discharge pipe. This area provides a recharge zone and quantity control can also be augmented with this storage area. The storage area is equal to the void space of the material used. Since the practice utilizes both infiltration and an underdrain, considerations include those for both infiltration practices and filtration with an underdrain at the bottom. These include an assessment of infiltration constraints and media.

A filtration practice with internal water storage is not commonly used in the Midwestern United States but is widely used in some places on the east coast, such as North Carolina. The use of an upturned elbow in this practice allows water to be retained within the practice, leading to increased pollutant removal, increased infiltration, and increased evapotranspiration. The practice is particularly effective at removing nitrogen through denitrification. The media should be 3 feet or more thick to allow water to be drawn down below the root zone. Underlying soils should be permeable enough to allow water stored within the practice to infiltrate. Hunt provides an overview of this practice.

Raising an underdrain or utilizing internal water storage enhances infiltration, but the practice still functions as a filtration practice. Pollutant removal for the infiltrated portion is 100 percent. We conducted simple calculations utilizing the Minimal Impact Design Standards (MIDS) calculator to illustrate the effect of a raised underdrain on volume and pollutant removal. The following inputs were used in the analysis.

Site zip code = 55155
1 acre of impervious and 1 acre of pervious managed turf
Biofiltration with a raised underdrain, sized to hold the MIDS performance goal of 1.1 inches
Basin dimensions 3993 square feet by 1 foot ponding depth
Media Mix C (low phosphorus media)
3 feet of media
Underlying soil = Hydrologic Soil Group C
Three scenarios
- Underdrain at bottom
- Underdrain raised 0.5 feet
- Underdrain raised 0.8 feet (Note: this is the maximum depth below the underdrain to meet the 48 hour drawdown requirement)

Results of the analysis are summarized below.

Underdrain at bottom: 27% of performance goal achieved; 45% annual volume removal; 74% annual total phosphorus removed; 89% annual TSS removed
Underdrain raised 0.5 feet: 39% of performance goal achieved; 58% annual volume removal; 80% annual total phosphorus removed; 92% annual TSS removed
Underdrain raised 0.8 feet: 47% of performance goal achieved; 64% annual volume removal; 83% annual total phosphorus removed; 93% annual TSS removed

The analysis indicates significant increase in volume retained, moderate increase in phosphorus retention, and a small increase in TSS retention.

Unit processes for different filtration BMPs

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

Overview table

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

Stormwater filtration BMPs - overview
Link to this table

Stormwater BMP	General Overview	Illustration
Dry swale or step pool	Dry swales, sometimes called grass swales, are similar to bioretention cells but are configured as shallow, linear channels. Dry swales function primarily as a conveyance BMP but may incorporate check dams into the design, thus allowing water to be stored behind the check dam and filtered through the underlying soil or media. Discharge of filtered runoff occurs primarily through an underdrain, though some infiltration into the underlying soil occurs below the underdrain.
Wet swale	Wet swales act as very long and linear shallow biofiltration or linear wetland treatment systems. Wet swales do not provide volume reduction and have limited treatment capability. Incorporation of check dams into the design allows treatment of a portion or all of the water quality volume within a series of cells created by the check dams.
Biofiltration Basin	Often called rain gardens, biofiltration basins use engineered or mixed soils and plantings to capture and filter runoff. Discharge of filtered runoff occurs primarily through an underdrain, though some infiltration into the underlying soil occurs below the underdrain.
Permeable pavement with underdrain	Permeable pavements are paving surfaces that allow stormwater runoff to filter through surface voids into an underlying stone reservoir. The most commonly used permeable pavement surfaces are pervious concrete, porous asphalt, and permeable interlocking concrete pavers (PICP). All permeable pavements have a similar structure, consisting of a surface pavement layer, an underlying stone aggregate reservoir layer, optional underdrains and geotextile over uncompacted soil subgrade. Discharge of this stored runoff occurs primarily through an underdrain, though some infiltration into the underlying soil occurs below the underdrain.
Tree Trench/Tree Box (with underdrain)	A system of trees that are connected by an underground storage structure. The system consists of a trench lined with geotextile fabric with structural stone, gravel or soil boxes in which the trees are placed. Tree systems consist of an engineered soil layer designed to treat stormwater runoff via filtration through plant and soil media, and through evapotranspiration from trees. Discharge of filtered runoff occurs primarily through an underdrain, though some infiltration into the underlying soil occurs below the underdrain.
Media filter (sand filter)	Runoff is captured and stored in a filter media consisting of a sand filter bed (~18 inches), with temporary runoff storage above the bed. Pollutants are trapped or strained out at the surface of the filter bed. Discharge of filtered runoff occurs primarily through an underdrain.
Green roof	Green roofs consist of a a layer or layers of media that filter suspended solids and pollutants associated with those solids. Water passes through the media to an underlying impermeable layer that conveys the filtered water from the roof.

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 ([5], [6]) 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

@@ Line 1: / Line 1: @@
 {{alert|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.|alert-success}}
-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
+Filtration practices treat stormwater by [https://stormwater.pca.state.mn.us/index.php?title=Glossary#F filtering] pollutants. They are effective at attenuating [https://stormwater.pca.state.mn.us/index.php?title=Total_Suspended_Solids_(TSS)_in_stormwater suspended sediment] and pollutants associated with that sediment. Most filtration practices utilize an [https://stormwater.pca.state.mn.us/index.php?title=Glossary#U underdrain] and [https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_bioretention#Materials_specifications_-_filter_media engineered soil media] to attenuate pollutants and can be sized, in appropriate conditions, to treat the [https://stormwater.pca.state.mn.us/index.php?title=Glossary#W water quality volume]. Some infiltration and/or loss of water through [https://stormwater.pca.state.mn.us/index.php?title=Glossary#E evapotranspiration] occurs in most filtration practices. Filtration [https://stormwater.pca.state.mn.us/index.php?title=Glossary#B best management practices] (BMPs) include, but are not limited, to
-*biofiltration (bioretention with an underdrain),
+*biofiltration ([[Bioretention|bioretention]] with an underdrain),
-*permeable pavement with an underdrain,
+*[[Permeable pavement|permeable pavement]] with an underdrain,
-*tree trenches and tree boxes with an underdrain,
+*[[Trees|tree trenches and tree boxes]] with an underdrain,
-*dry swale with an underdrain (with or without check dams),
+*[https://stormwater.pca.state.mn.us/index.php?title=Dry_swale_(Grass_swale) dry swale] with an underdrain (with or without check dams),
-*wet swales, and
+*[https://stormwater.pca.state.mn.us/index.php?title=Wet_swale_(wetland_channel) wet swales],
-*green roofs.
+*[https://stormwater.pca.state.mn.us/index.php?title=Filtration sand filter]; and
+*[[Green roofs|green roofs]].
+Raising an underdrain above the bottom of the filtration practice increases infiltration. However, the practice still functions as a filtration practice. For more information on practices with a raised underdrain, [https://stormwater.pca.state.mn.us/index.php?title=BMPs_for_stormwater_filtration#Practices_with_a_raised_underdrain link here].
 ==Biofiltration basin (bioretention with an underdrain)==
@@ Line 59: / Line 62: @@
 |}
-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.
+[[Bioretention|Biofiltration basins]], often called rain gardens, use soil (typically [https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_bioretention#Materials_specifications_-_filter_media 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 [[Design infiltration rates|low infiltration rate]] (C or D soils). Therefore an [https://stormwater.pca.state.mn.us/index.php?title=Glossary#U 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 [http://chesapeakestormwater.net/wp-content/uploads/downloads/2014/03/Internal-Water-Storage-for-Bioretention-2009.pdf 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.
 *[[Bioretention terminology]] (including types of bioretention)
@@ Line 130: / Line 133: @@
 |}
-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.
+[[Permeable pavement|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 [https://stormwater.pca.state.mn.us/index.php?title=Types_of_permeable_pavement 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 [http://chesapeakestormwater.net/wp-content/uploads/downloads/2014/03/Internal-Water-Storage-for-Bioretention-2009.pdf 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.
@@ Line 196: / Line 199: @@
 |}
-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.
+[[Trees|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 [https://stormwater.pca.state.mn.us/index.php?title=Design_guidelines_for_soil_characteristics_-_tree_trenches_and_tree_boxes 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 [http://chesapeakestormwater.net/wp-content/uploads/downloads/2014/03/Internal-Water-Storage-for-Bioretention-2009.pdf upturned elbow in the underdrain]. Water captured by the BMP must drawdown within 48 hours. [https://stormwater.pca.state.mn.us/index.php?title=Design_guidelines_for_tree_quality_and_planting_-_tree_trenches_and_tree_boxes 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.
@@ Line 219: / Line 222: @@
 *[[Requirements, recommendations and information for using trees as a BMP in the MIDS calculator]]
-==Dry swale (with underdrains)==
+==Dry swale and stormwater step-pool (with check dams and underdrains)==
 [[file:dry swale.jpg|300px|thumb|left|alt=dry swale photo|<font size=3>Dry swale (no check dams)</font size>]]
 {| class="wikitable" style="float:right; margin-left: 10px; width:500px;"
 |-
-| colspan="4" style="text-align: center;"| '''Applications and treatment capabilities for dry swale with check dams'''
+| colspan="4" style="text-align: center;"| '''Applications and treatment capabilities for dry swale and step pools with check dams and underdrains'''
 |-
 | colspan="2" style="text-align: center;" | '''Applications'''
@@ Line 269: / Line 272: @@
 |}
-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
+[[File:Step pool.jpg|300px|thumb|left|alt=image of step pool|<font size=3>Stormwater step pool. Courtesy of Limnotech.</font size>]]
+Similar to vegetated swales designed for stormwater conveyance, [https://stormwater.pca.state.mn.us/index.php?title=Dry_swale_(Grass_swale) dry swales with underdrains] are designed as linear, multi-celled stormwater filtration BMP’s. For high gradient conditions, stormwater step-pools can be utilized. By incorporating [https://stormwater.pca.state.mn.us/index.php?title=Check_dams_for_stormwater_swales 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 [http://chesapeakestormwater.net/wp-content/uploads/downloads/2014/03/Internal-Water-Storage-for-Bioretention-2009.pdf 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.
+For information on dry swales, see the following pages.
 *[[Overview for dry swale (grass swale)]]
 *[[Design criteria for dry swale (grass swale)]]
@@ Line 278: / Line 283: @@
 *[[Operation and maintenance of dry swale (grass swale)]]
 *[[Assessing the performance of dry swale (grass swale)]]
-*[[Cost-benefit considerations for dry swale (grass swale)]]
+*[https://stormwater.pca.state.mn.us/index.php?title=Cost_considerations_for_dry_swale_(grass_swale) Cost considerations]
 *[[Case studies for dry swale (grass swale)]]
-*[[External resources for dry swale (grass swale)]]
+*[[External resources for swales|External resources for dry swale (grass swale)]]
 *[[References for dry swale (grass swale)]]
 *[https://stormwater.pca.state.mn.us/index.php?title=Requirements,_recommendations_and_information_for_using_swale_without_an_underdrain_as_a_BMP_in_the_MIDS_calculator Requirements, recommendations and information for using dry swale (grass swale) without an underdrain in the MIDS calculator]
 *[https://stormwater.pca.state.mn.us/index.php?title=Requirements,_recommendations_and_information_for_using_swale_with_an_underdrain_as_a_BMP_in_the_MIDS_calculator Requirements, recommendations and information for using dry swale (grass swale) with an underdrain in the MIDS calculator]
 *[[Requirements, recommendations and information for using swale side slope as a BMP in the MIDS calculator]]
-*[[Dry swale (grass swale) and interesting websites]]
+For information on step pools, see the following pages.
+*[[Terminology for swales|Terminology for high-gradient stormwater step-pool swale]]
+*[[Overview for high-gradient stormwater step-pool swale]]
+*[[Design criteria for high-gradient stormwater step-pool swale]]
+*[[Construction specifications for high-gradient stormwater step-pool swale]]
+*[[Operation and maintenance of high-gradient stormwater step-pool swale]]
+*[[Assessing the performance of high-gradient stormwater step-pool swale]]
+*[[Check dams for stormwater swales]]
+*[[Plants for swales]]
+*[[Calculating credits for high-gradient stormwater step-pool swale]]
+*[https://stormwater.pca.state.mn.us/index.php?title=Cost_considerations_for_dry_swale_(grass_swale) Cost considerations]
+*[[External resources for high-gradient stormwater step-pool swale]]
+*[[References for high-gradient stormwater step-pool swale]]
 ==Wet swales==
@@ Line 337: / Line 355: @@
 |}
-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.
+[https://stormwater.pca.state.mn.us/index.php?title=Wet_swale_(wetland_channel) 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 [https://stormwater.pca.state.mn.us/index.php?title=Check_dams_for_stormwater_swales check dams]. Cells may be planted with [https://stormwater.pca.state.mn.us/index.php?title=Plants_for_swales 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.
@@ Line 347: / Line 365: @@
 *[[Assessing the performance of wet swale (wetland channel)]]
 *[[Calculating credits for wet swale (wetland channel)]]
-*[[Cost-benefit considerations for wet swale (wetland channel)]]
+*[https://stormwater.pca.state.mn.us/index.php?title=Cost_considerations_for_dry_swale_(grass_swale) Cost considerations (Note this is for dry swales)]
 *[[External resources for wet swale (wetland channel)]]
 *[[References for dry swale (grass sawale)|References for wet swale (wetland channel)]]
 *[https://stormwater.pca.state.mn.us/index.php?title=Requirements,_recommendations_and_information_for_using_wet_swale_as_a_BMP_in_the_MIDS_calculator Requirements, recommendations and information for using wet swale in the MIDS calculator]
 *[[Requirements, recommendations and information for using swale side slope as a BMP in the MIDS calculator]]
-*[[Wet swale (wetland channel) and interesting websites]]
+==Media (sand) filter==
+[[File:Sand infiltration 4.jpg|thumb|left|300px|alt=photo of an iron ehanced sand filter basin|<font size=3>Iron enhanced sand filter basin, Maplewood, MN. Photo courtesy of Barr Engineering.</font size>]]
+{| class="wikitable" style="float:right; margin-left: 10px; width:500px;"
+|-
+| colspan="4" style="text-align: center;"| '''Applications and treatment capabilities for media filters'''
+|-
+| colspan="2" style="text-align: center;" | '''Applications'''
+| colspan="2" style="text-align: center;" | '''Treatment capabilities<sup>2, 3</sup>'''
+|-
+| Residential
+| Yes
+| TSS
+| High<sup>4</sup>
+|-
+| Commercial
+| Yes
+| TN
+| Medium
+|-
+| Ultra-urban
+| Limited
+| TP
+| Medium
+|-
+| Industrial
+| Yes<sup>1</sup>
+| Metals
+| Medium
+|-
+| Highway/road
+| Yes
+| Oil and grease
+| High
+|-
+| Recreational
+| Yes
+| Pathogens
+| High
+|-
+|
+|
+| Chloride
+| Low
+|-
+| colspan="4" style="text-align: center;" | <font size=1><sup>1</sup> Unless the filtration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. <sup>2</sup> This is only for the portion of flow that enters the filtration basin; by-passed runoff does not receive treatment; <sup>3</sup> Low = < 30%; Medium = 30-65%; High = 65 -100%); <sup>4</sup> Assumes adequate pretreatment.<br>
+Source: see [http://stormwater.pca.state.mn.us/index.php/File:Trees_Tasks_2_and_13_Water_quality_benefits.docx]
+</font size>
+|}
+Media filters treat stormwater through a variety of different filtering materials whose purpose is to remove pollution from runoff. Variants include surface sand filters, underground sand filters and perimeter sand filters. Most media filters have an underdrain and are effective at removing sediment and pollutants associated with sediment. Media filters may be [https://stormwater.pca.state.mn.us/index.php?title=Soil_amendments_to_enhance_phosphorus_sorption enhanced with iron or other material] to facilitate removal of dissolved pollutants, such as [https://stormwater.pca.state.mn.us/index.php?title=Phosphorus_in_stormwater dissolved phosphorus]. For more information on media filters, see the following sections in the Minnesota Stormwater Manual.
+*[[Overview for filtration]]
+*[[Types of filtration]]
+*[[Design criteria for filtration]]
+*[[Construction specifications for filtration]]
+*[[Assessing the performance of swales]]
+*[[Assessing the performance of sand (media) filters]]
+*[[Operation and maintenance of filtration]]
+*[[Calculating credits for sand filter]]
+*[[Cost-benefit considerations for filtration]]
+*[[References for filtration]]
 ==Green roofs==
@@ Line 403: / Line 483: @@
 |}
-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).
+[[Green roofs|Green roofs]] typically occur at the beginning of [https://stormwater.pca.state.mn.us/index.php?title=Using_the_treatment_train_approach_to_BMP_selection 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.
@@ Line 421: / Line 501: @@
 *[[Green roof fact sheet]]
 *[[Requirements, recommendations and information for using green roofs as a BMP in the MIDS calculator]]
+==Practices with a raised underdrain or internal water storage==
+[[File:Biofiltration with elevated underdrain.png|thumb|300px|alt=image of biofiltration device with elevated underdrain|<font size=3>Biofiltration device with elevated underdrain</font size>]]
+[[File:Biofiltration with internal water storage.png|thumb|300px|alt=image of biofiltration device with internal water storage|<font size=3>Biofiltration device with internal water storage</font size>]]
+A filtration practice with a raised underdrain provides a storage area below the invert of the underdrain discharge pipe. This area provides a recharge zone and quantity control can also be augmented with this storage area. The storage area is equal to the void space of the material used. Since the practice utilizes both infiltration and an underdrain, considerations include those for both infiltration practices and filtration with an underdrain at the bottom.  These include an assessment of [[Stormwater infiltration and constraints on infiltration|infiltration constraints]] and [[design criteria for bioretention#Materials specifications - filter media|media]].
+A filtration practice with internal water storage is not commonly used in the Midwestern United States but is widely used in some places on the east coast, such as North Carolina.  The use of an upturned elbow in this practice allows water to be retained within the practice, leading to increased pollutant removal, increased infiltration, and increased evapotranspiration.  The practice is particularly effective at removing nitrogen through denitrification. The media should be 3 feet or more thick to allow water to be drawn down below the root zone. Underlying soils should be permeable enough to allow water stored within the practice to infiltrate.  [https://epa.ohio.gov/static/Portals/41/storm_workshop/lid/IWS.Dec10.pdf Hunt] provides an overview of this practice.
+Raising an underdrain or utilizing internal water storage enhances infiltration, but the practice still functions as a filtration practice. Pollutant removal for the infiltrated portion is 100 percent. We conducted simple calculations utilizing the [https://stormwater.pca.state.mn.us/index.php?title=MIDS_calculator Minimal Impact Design Standards (MIDS) calculator] to illustrate the effect of a raised underdrain on volume and pollutant removal. The following inputs were used in the analysis.
+*Site zip code = 55155
+*1 acre of impervious and 1 acre of pervious managed turf
+*Biofiltration with a raised underdrain, sized to hold the MIDS performance goal of 1.1 inches
+*Basin dimensions 3993 square feet by 1 foot ponding depth
+*Media Mix C (low phosphorus media)
+*3 feet of media
+*Underlying soil = [[Design infiltration rates|Hydrologic Soil Group C]]
+*Three scenarios
+**Underdrain at bottom
+**Underdrain raised 0.5 feet
+**Underdrain raised 0.8 feet (Note: this is the maximum depth below the underdrain to meet the 48 hour drawdown requirement)
+Results of the analysis are summarized below.
+*'''Underdrain at bottom''': 27% of performance goal achieved; 45% annual volume removal; 74% annual total phosphorus removed; 89% annual TSS removed
+*'''Underdrain raised 0.5 feet''': 39% of performance goal achieved; 58% annual volume removal; 80% annual total phosphorus removed; 92% annual TSS removed
+*'''Underdrain raised 0.8 feet''': 47% of performance goal achieved; 64% annual volume removal; 83% annual total phosphorus removed; 93% annual TSS removed
+The analysis indicates significant increase in volume retained, moderate increase in phosphorus retention, and a small increase in TSS retention.
 ==Unit processes for different filtration BMPs==
-The following table provides a summary of unit processes for the different infiltration BMPs.
+The following table provides a summary of unit processes for the different filtration BMPs.
 {{:Unit processes of stormwater treatment techniques - filtration practices}}
 ==Information tables==
-The following tables describe and differentiate different characteristics of stormwater infiltration BMPs.
+The following tables describe and differentiate different characteristics of stormwater filtration BMPs.
 ===Overview table===
 The following table provides a brief description and schematic of each stormwater filtration BMP.
-[[Stormwater filtration BMPs - overview]]
+{{:Stormwater filtration BMPs - overview}}
 ===Contributing drainage area table===
@@ Line 457: / Line 565: @@
 ==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]].'''
-*[[Permeable pavement]] ([[Permeable pavement combined|+]])
+*[[Permeable pavement]]
 *[[Trees]]
 *[[Bioretention|Bioretention - biofiltration]]
@@ Line 464: / Line 572: @@
 *[[Filtration|Sand filter]]
 *[[Iron enhanced sand filter (Minnesota Filter)]] ([[Iron enhanced sand filter combined|+]])
-*[[Modular treatment wetlands for stormwater management]]
 '''Other filtration BMPs, including pre-treatment practices'''
-*[[Green roofs]] ([[Green roofs combined|+]])
+*[[Green roofs]]
 *[[Turf]]
 *[[Vegetated filter strips]]
@@ Line 473: / Line 580: @@
 '''Manufactured devices'''
 *[[Filtration devices]]
+<noinclude>
+[[Category:Level 3 - Best management practices/Guidance and information/BMP types and terminology]]
+[[Category:Level 2 - Best management practices/Structural practices]]
+</noinclude>

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