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
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.
Applications and treatment capabilities for biofiltration basins | |||
Applications | Treatment capabilities2, 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.
Applications and treatment capabilities for permeable pavement | |||
Applications | Treatment capabilities2, 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.
Applications and treatment capabilities for tree box/tree trench | |||
Applications | Treatment capabilities2, 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.
Applications and treatment capabilities for dry swale and step pools with check dams and underdrains | |||
Applications | Treatment capabilities2, 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. 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.
Applications and treatment capabilities for wet swale with check dams | |||
Applications | Treatment capabilities2, 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.
Applications and treatment capabilities for media filters | |||
Applications | Treatment capabilities2, 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.
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.
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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.
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.
Results of the analysis are summarized below.
The analysis indicates significant increase in volume retained, moderate increase in phosphorus retention, and a small increase in TSS retention.
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 |
The following tables describe and differentiate different characteristics of stormwater filtration BMPs.
The following table provides a brief description and schematic of each stormwater filtration BMP.
Stormwater filtration BMPs - overview
Link to this 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.
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
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
12 Low = < 30%; Medium = 30‐65%; High = 65‐100%
2Some media mixes can leach phosphorus.
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
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
To see all information contained in the previous tables in a single table, click on the following link: Filtration Summary Table
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