This document combines several documents related to filtration. Individual documents can be viewed by clicking on the appropriate link below.


  1. Overview for filtration
  2. Types of filtration
  3. Design criteria for filtration
  4. Construction specifications for filtration
  5. Assessing the performance of filtration
  6. Operation and maintenance of filtration
  7. Cost-benefit considerations for filtration
  8. References for filtration
  9. Fact sheet for filtration


Contents

Overview

image

Filtering practices include media filters (surface, underground, perimeter), vegetative filters (filter strips, grass channels), and combination media/vegetative filters (dry swales). Media and media/vegetative filters operate similarly and provide comparable water quality capabilities as bioretention. Vegetative filters are generally more suitable as pre-treatment practices, but in some situations can be used on a stand alone basis.

Filtering practices have widespread applicability and are suitable for all land uses, as long as the contributing drainage areas are limited (e.g., typically less than 5 acres). Media filters are not as aesthetically appealing as bioretention, which makes them more appropriate for commercial or light industrial land uses or in locations that will not receive significant public exposure. Media filters are particularly well suited for sites with high percentages of impervious cover (e.g., greater than 50 percent). Media filters can be designed with an underdrain, which makes them a good option for treating potential stormwater hotspots (PSHs). They can also be installed underground to prevent the consumption of valuable land space (often an important retrofit or redevelopment consideration). Vegetative filters can be incorporated into landscaped areas, providing dual functionality.

Function within stormwater treatment train

Media filtration systems are designed primarily as off-line systems for stormwater quality and typically are used in conjunction with other structural controls in the stormwater treatment train. Vegetative filters, designed as grass channels or swales, may be the main form of conveyance between or out of BMPs, as well as providing treatment for stormwater runoff.

MPCA permit applicability

One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. These standards must be applied in all projects in which at least 1 acre of new impervious area is being created, and the permit stipulates certain standards for various categories of stormwater management practices.

For regulatory purposes, filtration practices fall under Section 17 (Filtration systems) of the MPCA CGP. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the filtration practice will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.

The following terms are thus used in the text to distinguish various levels of filtration practice design guidance:

  • REQUIRED:Indicates design standards stipulated by the MPCA CGP (or other consistently applicable regulations).
  • HIGHLY RECOMMENDED: Indicates design guidance that is extremely beneficial or necessary for proper functioning of the filtration practice, but not specifically required by the MPCA CGP.
  • RECOMMENDED: Indicates design guidance that is helpful for filtration practice performance but not critical to the design.

Of course, there are situations, particularly retrofit projects, in which a filtration facility is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular filtration facility, depending on where it is situated both jurisdictionally and within the surrounding landscape.

Retrofit suitability

The use of filters as a retrofit practice primarily depends on existing infrastructure and the compatibility of existing storm drain inverts that need to connect to the filter underdrain outflow. In general, four to six feet of elevation above the existing collection system invert is needed for media filter retrofits (2 to 3 feet is needed for perimeter filters). Underground media filters are excellent for ultra-urban settings where space is at a premium.

Special receiving waters suitability

The following table provides guidance regarding the use of filtration practices in areas upstream of special receiving waters. This table is an abbreviated version of a larger table in which other BMP groups are similarly evaluated. The corresponding information about other BMPs is presented in the respective sections of this Manual.

Summary of design restrictions for special waters.
Link to this table

BMP Group receiving water
A Lakes B Trout Waters C Drinking Water D Wetlands E Impaired Waters
Filtration Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments RECOMMENDED RECOMMENDED ACCEPTABLE RECOMMENDED for non-nutrient impairments


Cold climate suitability

Various options for use of filtration are available for treating snowmelt runoff. Some of the installations are built below the frost line (trenches, sub-grade proprietary chambers) and do not need further adaptation for the cold. However, some special consideration is HIGHLY RECOMMENDED for surface systems.

The problem with filtration in cold weather is the ice that forms both over the top of the facility and within the soil interstices. To avoid these problems to the extent possible, it is HIGHLY RECOMMENDED that the facility be actively managed to keep it dry before it freezes in the late fall. This can be done by various methods, including limiting inflow, under-drainage, and surface disking.

Proprietary, sub-grade filtration systems provide an alternative to standard surface based systems. Essentially, these systems provide an insulated location for pre-treated snowmelt to be stored and slowly filtered, or simply filtered and drained away if ground water sensitivity is an issue. The insulating value of these systems adds to their appeal as low land consumption alternatives to ponds and surface infiltration basins.

Water quantity treatment

Filters are not typically a primary practice for providing water quantity control. They are normally either designed off-line using a flow diversion or configured to safely pass large storm flows while still protecting the filter bed. In limited cases, filters may be able to accommodate the channel protection volume, Vcp, in either an off- or on-line configuration, and in general they do provide some (albeit limited) storage volume. Vegetative filters, in particular, can help reduce detention requirements for a site by providing elongated flow paths, longer times of concentration, and volumetric losses from infiltration and evapo-transpiration. Generally, however, to meet site water quantity or peak discharge criteria, it is HIGHLY RECOMMENDED that another structural control (e.g., detention) be used in conjunction with a filter.

It is HIGHLY RECOMMENDED that vegetative filters have a maximum slope of 5 percent and a minimum slope of 1 percent.

Warning: It is REQUIRED that volume reduction practices, such as infiltration basins, are considered before filtration practices

Water quality treatment

Filters can be an excellent stormwater treatment practice with the primary pollutant removal mechanism being filtering and settling. Less significant processes can include evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption. Pollutant removal data for select parameters are provided for filtration BMPs in the table below. “Performance” can also be defined as the quality of the water flowing out of a treatment BMP. These outflow concentrations can be used to assess how well a BMP is performing and what its benefit to a down-gradient receiving water will be. The Pollutant concentrations for filtration BMPs table below contains information on typical expectations for outflow concentration. Please note that Appendix N contains additional explanation for the importance of evaluation outflows from a BMP, as well as how one would adjust performance numbers based on design and operational parameters.

Pollutant removal percentages for filtration BMPs.
Link to this table

Practice TSS Low-Med-High TP Low-Med-High TN4 Metals3 (average of Zn and Cu) Bacteria3 Hydrocarbons3
Media Filter1 75-85-90 30-50-55 35 80 50 80
Vegetative Filter2 see here see here 35 80 0 80

1 For example, sand, mixed sand/peat and other geologic media
2 Grass filter/swale
3 Not enough information given in databases to differentiate type of filter so both combined for this entry


Typical pollutant effluent concentrations, in milligrams per liter, for filtration BMPs.
Link to this table

Practice TSS Low-Med-High4 TP Low-Med-High4 TN5 Cu5 Zn5
Media Filter2 5-11-16 0.06-0.10-0.19 1.1 0.008 0.060
Vegetative Filter3 13-20-44 0.15-0.24-0.36 1.1 0.008 0.060

1 All concentration values in mg/L which equals parts per million
2 For example, sand, mixed sand/peat and other geologic media
3 Grass filter/swale
4 See Appendix N discussion
5 Not enough information given in databases to differentiate type of filter so both combined for this entry


While it is possible to design media filters to discharge a portion of the effluent to the groundwater, they are typically designed as enclosed systems (i.e., no “infiltration”). Vegetative filters, on the other hand, can readily be designed as an effective infiltration/recharge practice, particularly when parent soils have good permeability (> ~ 0.5 inch per hour). Consult the credits section for more guidance on how to use filters to meet water quality and recharge criteria. Note that the vegetative filters might not meet the 80 percent TSS removal required by the Construction permit.

As noted in the discussion of BMP selection, the benefits associated with filtration BMPs should only be accrued based on the amount of water actually passing through the BMP. Excess runoff beyond that designed for the BMP should not be routed through the system because of the potential for hydraulic and particulate over-loading, both of which will adversely impact the life and operation of the BMP.

For example, a filtration device designed to treat the first 0.5 inch of runoff from a fully impervious surface will catch about 30 percent of the volume of runoff in the Twin Cities. This means that 70 percent of the runoff volume should be routed around the filtration system and will not be subject to the removals reflected in the above tables. Attributing removal to all runoff just because a BMP is in place in a drainage system is not a legitimate claim.

Limitations

The following general limitations should be recognized when considering installation of a filtration practice.

  • Nitrification of water in media filters may occur where aerobic conditions exist.
  • Filtration offers limited water quantity control.
  • The potential to create odors exists

It is HIGHLY RECOMMENDED that media filters be equipped with a minimum 8 inches diameter underdrain in a 1 foot gravel bed.



Types of Infiltration trench

Green Infrastructure: Infiltration practices can be an important tool for retention and 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.
image

Best Management Practices that infiltrate stormwater runoff into underlying soil include, but are not limited, to

  • infiltration basins,
  • infiltration trenches (includes dry wells),
  • underground infiltration systems,
  • bioinfiltration,
  • permeable pavements,
  • tree trenches and tree boxes, and
  • dry swale with check dams.

These are discussed briefly below. Additional information about these BMPs can be found in the following tables.

Infiltration basin

photo of an infiltration basin
Photo of an infiltration basin. Source: Clark County, Washington, with permission.
Applications and treatment capabilities for infiltration basins
Applications Treatment capabilities3, 4, 5
Residential Yes TSS High6
Commercial Yes TN Medium/high
Ultra-urban Limited1 TP Medium/high
Industrial Yes2 Chloride Low
Highway/road Limited Metals High
Recreational Yes Oils and grease High
Pathogens High
1 Due to a size restriction; 2 Unless the infiltration practice is located in an industrial area with exposed significant materials or from vehicle fueling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 3Underground infiltration systems may have different (likely lower) pollutant removal capabilities than what is provided in this table. These systems may have a wider application range. 4 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 5 Low = < 30%; Medium = 30-65%; High = 65 -100%); 6 Assumes adequate pre-treatment

Sources: Schueler, 1987, 1992; USEPA 1993a, 1993b; Maniquiz et al., 2010; NPRPD, 2007; California Stormwater Manual, 2009; Pennsylvania Stormwater Manual, 2006

An infiltration basin is a natural or constructed impoundment that captures, temporarily stores and infiltrates the design volume of water. Drawdown of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. The required drawdown time is 48 hours or less. Water that is stored but not infiltrated must leave the BMP, typically through an outlet, within the required drawdown time. In the case of a constructed basin, the impoundment is created by excavation or embankment. Infiltration basins are commonly used for drainage areas of 5 to 50 acres with land slopes that are less than 20 percent. Typical depths range from 2 to 6 feet, including bounce in the basin. The sizing is to control stormwater volumes at the regional or development scale as opposed to bioretention basins (rain gardens) that are designed at the site scale. Typical dimensions range from 1,000 square feet up to an acre. Infiltration basins are commonly constructed with plant species that can tolerate and thrive in this unique growing environment.

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

Infiltration trench

Photo of a Infiltration trench in Lino Lakes
Photo of a Infiltration trench in Lino Lakes
Applications and treatment capabilities for infiltration trenches, dry well, underground infiltration
Applications Treatment capabilities3, 4
Residential Yes TSS5 High5
Commercial Yes TN Medium/high
Ultra-urban Yes TP Medium/high
Industrial Yes1 Chloride Low
Highway/road 2 Metals High
Recreational Yes Oils and grease High
Pathogens High
1 Unless the infiltration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 2 Yes for infiltration trench, limited for underground infiltration, no for dry well; 3 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 4 Low = < 30%; Medium = 30-65%; High = 65 -100%); 5 Assumes adequate pre-treatment

Sources: Schueler, 1987, 1992; USEPA 1993a, 1993b; Maniquiz et al., 2010; NPRPD, 2007; California Stormwater Manual, 2009; Pennsylvania Stormwater Manual, 2006

An infiltration trench is a shallow excavated trench, typically 3 to 6 feet deep, that is backfilled with a coarse stone aggregate allowing for the temporary storage of runoff in the void space of the material. Drawdown of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. Infiltration trenches may be modified to become stormwater tree trenches and boxes where applicable with the addition of growing medium. All water captured by the BMP must be removed within 48 hours through infiltration and/or a drain. Trenches are commonly used for drainage areas less than 5 acres in size.

Caution: To avoid an infiltration trench being classified as a Class V injection well, it is strongly recommended that the length of the trench be at least 2 times greater than the depth of the trench.

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

Dry wells (a.k.a. infiltration tubes, french drains, soak-away pits or soak holes)

A dry well or soak away pit is a smaller variation of an infiltration trench. It is a subsurface storage facility (a structural chamber or an excavated pit backfilled with a coarse stone aggregate) that receives and temporarily stores stormwater runoff. Discharge of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. Due to their size, dry wells are typically designed to handle stormwater runoff from smaller drainage areas, less than one acre in size (e.g. roof tops).

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

Underground infiltration systems

Information: The MPCA is working with partners to determine the fate and transport of stormwater pollutants in underground storage systems. Information from these studies should be available in summer 2020.
This picture shows an infiltration system in native sandy soils
Underground TrueNorthSteel infiltration system in native sandy soils

Several underground infiltration systems, including pre-manufactured pipes, vaults, and modular structures, have been developed as alternatives to infiltration basins and trenches for space-limited sites and stormwater retrofit applications. Underground infiltration systems are occasionally the only stormwater BMP options on fully developed sites as they can be located under other land uses such as parking lots or play areas. These systems are similar to infiltration basins and trenches in that they are designed to capture, temporarily store and infiltrate the design volume of stormwater over several days. Underground infiltration systems are generally applicable to small development sites (typically less than 10 acres) and should be installed in areas that are easily accessible to routine and non-routine maintenance. These systems should not be located in areas or below structures that cannot be excavated in the event that the system needs to be replaced or invasive maintenance is required to maintain performance.

Underground infiltration systems and dry wells have been installed below parking lots and other impervious surfaces on sites where insufficient space exists for a surface infiltration system. They are designed to temporarily store stormwater runoff before slowly infiltrating the water into the subsurface (Connecticut, 2004). There is limited information on the effectiveness of these systems in removing pollutants.

One concern is that underground infiltration may meet the U.S. Environmental Protection Agency (EPA) definition of a Class V injection well. Class V injection wells are defined as any bored, drilled, or driven shaft, or any dug hole that is deeper than its widest surface dimension. Class V injection wells can also be an improved sinkhole, or a subsurface fluid distribution system (from U.S. EPA, June 2003). The U.S. EPA administers Class V injection well permits in Minnesota. Minimum requirements for installing, permitting, and operating a Class V well is defined by the USEPA.

A second concern pertains to the overall pollutant removal effectiveness of those underground infiltration systems that do not meet the definition of a Class V injection well. The document released by the Transport Research Synthesis titled “Issues of Concern Related to Underground Infiltration Systems for Stormwater Management and Treatment” provides a good overview of the concerns related to underground infiltration systems (MNDOT, 2009). Issues identified in this report include:

  • There is potential that an underground infiltration system meets the criteria of a Class V injection well.
  • There is insufficient knowledge of the fate of pollutants in the subgrade below the buried infiltration systems.
  • Roadways and parking lots with high volumes of traffic have higher concentrations of certain pollutants, including heavy metals and PAHs.
  • Underground systems do not allow for the pollutant removal that is accomplished through biological activity and vegetation uptake.
  • The minimum separation requirement of 3 feet between the bottom of the infiltration system and the seasonally high groundwater elevation may be insufficient for adequate pollutant removal. Additional study is recommended.
  • Maintenance of underground systems is critical for effective pollutant removal. However, access for maintenance is challenging. There are concerns that the difficult access is preventing owners from properly maintaining these systems.

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

Bioinfiltration basin

photo of a bioinfiltration BMP
Bioinfiltration basin (Source: CDM Smith)
Applications and treatment capabilities for bioinfiltration basins
Applications Treatment capabilities2, 3
Residential Yes TSS High4
Commercial Yes TN Low/Medium5
Ultra-urban Limited7 TP Medium/high6
Industrial Yes1 Chloride Low
Highway/road Yes Metals High
Recreational Yes Oils and grease High
Pathogens High
1 Unless the infiltration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 2 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 3 Low = < 30%; Medium = 30-65%; High = 65 -100%); 4 Assumes adequate pre-treatment; 5 This assumes no underdrain; 6 Certain soil mixes can leach P; 7 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

Bioinfiltration basins, often called rain gardens, use soil (typically engineered media or mixed soil) and native vegetation to capture runoff and remove pollutants. Both the media and underlying soil typically have high infiltration rates that allow captured water to infiltrate within a required drawdown time, usually 48 hours. Bioinfiltration systems, which lack an underdrain and are designed for infiltration, differ from biofiltration systems, which have an underdrain and are designed primarily for filtration. For more information, see the following pages in this Manual.

Permeable pavement

photo of permeable pavement
Permeable pavement (Source: CDM Smith)
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 infiltration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 2 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 3 Low = < 30%; Medium = 30-65%; High = 65 -100%); 4 Assumes adequate pre-treatment; 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 infiltration 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 through infiltration into the surrounding naturally permeable soil. 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

photo of tree box
Tree box (Source: CDM Smith)
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 infiltration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 2 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 3 Low = < 30%; Medium = 30-65%; High = 65 -100%); 4 Assumes adequate pre-treatment; 5 Certain soil mixes can leach P.

Source: see [1]

Tree trenches are a system of trees that are connected by an underground infiltration 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. Discharge of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. 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. Tree trenches and boxes drainage areas 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 check dams

photo concrete check dams
Dry swale with impermeable concrete check dams. Photo courtesy Limnotech.
Applications and treatment capabilities for dry swale with check dams
Applications Treatment capabilities2, 3
Residential Yes TSS High4
Commercial Yes TN Low/Medium
Ultra-urban Limited6 TP Low/Medium5
Industrial Yes1 Chloride Low
Highway/road Yes Metals High
Recreational Yes Oils and grease High
Pathogens Medium
1 Unless the infiltration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 2 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 3 Low = < 30%; Medium = 30-65%; High = 65 -100%); 4 Assumes adequate pre-treatment; 5 Certain soil mixes can leach P; 6 Due to a size restriction.

Source: see [2]

Similar to vegetated swales designed for stormwater conveyance, dry swales with check dams are designed as linear, multi-celled stormwater infiltration BMP’s. By incorporating earthen or structural check dams, runoff is retained and infiltrated along a series of narrow, shallow basins or cells. 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 one acre in size.

For more information see the following sections in the Minnesota Stormwater Manual.

High gradient stormwater step-pool swale with check dams

step pool photo
Step pool with impermeable check dam. Courtesy of Limnotech.
Applications and treatment capabilities for step pool with check dams
Applications Treatment capabilities3, 4
Residential Yes TSS Medium5
Commercial Yes TN Low
Ultra-urban Limited1 TP Medium6
Industrial Yes2 Chloride Low
Highway/road Yes Metals Medium
Recreational Yes Oils and grease Low
Pathogens Low
1 Due to size restriction; 2 Unless the infiltration practice is located in an industrial area with exposed significant materials or from vehicle fuelling and maintenance areas. Infiltration BMPs are PROHIBITED in these areas; 3 This is only for the portion of flow that enters the infiltration basin; by-passed runoff does not receive treatment; 4 Low = < 30%; Medium = 30-65%; High = 65 -100%); 5 Assumes adequate pre-treatment; 6 Certain soil mixes can leach P.

Source: see [3]

Stormwater step pools are defined by its design features that address higher energy flows due to more dramatic slopes than dry or wet swales. Using a series of pools, riffle grade control, native vegetation and a sand seepage filter bed, flow velocities are reduced, treated, and, where applicable, infiltrated. to shallow groundwater. The physical characteristics of the stormwater step pools are similar to Rosgen A or B stream classification types, where “bedform occurs as a step/pool, cascading channel which often stores large amounts of sediment in the pools associated with debris dams” (Rosgen, 1996). These structures feature surface/subsurface runoff storage seams and an energy dissipation design that is aimed at attenuating the flow to a desired level through energy and hydraulic power equivalency principles (Anne Arundel County, 2009). Stormwater step pools are designed with a wide variety of native plant species depending on the hydraulic conditions and expected post-flow soil moisture at any given point within the stormwater step pool.

For more information see the following sections in the Minnesota Stormwater Manual.

Enhanced turf

Infiltration can be enhanced on soils that have been improved or amended. This manual contains limited information on enhanced turf and does not provide guidance for design, construction, maintenance, and assessment of enhanced turf. Information on use of compost in soil and credits associated with improved turf can be found on the Turf page. A discussion of alleviating compaction from construction activities can be found here.

Warning: Enhanced turf cannot be used to meet requirements of the Construction Stormwater General permit because infiltrated water does not represent an instantaneous volume.

Unit processes for different infiltration BMPs

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

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

Control Infiltration basin Infiltration trench Bioinfiltration Permeable pavement Tree box/tree trench Enhanced turf
Peak flow attenuation X X X X
Runoff volume reduction X X X X X
Infiltration X X X X X X
Dispersion
Evapotranspiration X X
Runoff collection and usage X1 X1
Sedimentation X X X
Flotation X X
Laminar separation
Swirl concentration
Sorption X X X X
Precipitation X X X X
Coagulation X X X X
Filtration X
Plant metabolism X X X
Nitrification/denitrification X X X
Organic compound degradation X X X X
Pathogen die off X X X
Temperature reduction X X X X
Disinfection X X X X

1 If underdrain is present


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 infiltration BMP.

Stormwater infiltration BMPs - overview
Link to this table

Stormwater BMP General Overview Illustration
Infiltration Basin A natural or constructed impoundment that captures, temporarily stores and infiltrates the design volume of water into the surrounding naturally permeable soil over several days. In the case of a constructed basin, the impoundment is created by excavation or embankment.
Infiltration basin icon.png
Bioinfiltration Basin Often called rain gardens, bioinfiltration basins use engineered or mixed soils and plantings to capture and infiltrate runoff. Pollutants are removed using highly permeable soils that are able to draw the basin down in less than 48 hours.
Bioinfiltration icon.png
Infiltration Trench Synonym: Infiltration Gallery A shallow excavated trench that is backfilled with a coarse stone aggregate allowing for the temporary storage of runoff in the void space of the material. Discharge of this stored runoff occurs through infiltration into the surrounding naturally permeable soil.
Infiltration trench icon.png
Dry Well Synonym: Infiltration Tube, French Drain, Soak‐Away Pits, Soak Holes A smaller variation of an infiltration trench. It is a subsurface storage facility (a structural chamber or an excavated pit backfilled with a coarse stone aggregate) that receives and temporarily stores stormwater runoff. Discharge of this stored runoff occurs through infiltration into the surrounding naturally permeable soil. Due to their size, dry wells are typically designed to handle stormwater runoff from smaller drainage areas.
Dry well icon.png
Underground Infiltration Several underground infiltration systems, including pre‐manufactured pipes, vaults, and modular structures, have been developed as alternatives to infiltration basins and trenches for space‐limited sites and stormwater retrofit applications. These systems are similar to infiltration basins and trenches in that they are designed to capture, temporarily store and infiltrate the design volume of stormwater over several days. Discharge of this stored runoff occurs through infiltration into the surrounding naturally permeable soil.
Underground infiltration icon.png
Dry Swale with Check Dams Similar to vegetated swales designed for stormwater conveyance, dry swales with check dams are designed as linear, multi‐celled stormwater infiltration BMPs. By incorporating earthen, structural or rock check dams, runoff is retained and infiltrated along a series of narrow, shallow basins or cells. 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.
Swale check icon.png
Permeable Pavement Permeable pavements are paving surfaces that allow stormwater runoff to filter through surface voids into an underlying stone reservoir for infiltration and/or storage. 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 through infiltration into the surrounding naturally permeable soil.
Permeable pavement icon.png
Tree Trench/Tree Box A system of trees that are connected by an underground infiltration 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 this stored runoff occurs through infiltration into the surrounding naturally permeable soil.
Tree trench icon.png


Contributing drainage area table

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

Contributing area is defined as the total area, including pervious and impervious surfaces, contributing to a 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. 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 Table 2-2a, page 17, at this link.

Stormwater infiltration BMPs - contributing drainage area
Link to this table

Stormwater BMP Recommended contributing area Notes
Infiltration Basin 50 acres or less A natural or constructed impoundment that captures, temporarily stores and infiltrates the design volume of water into the surrounding naturally permeable soil over several days. In the case of a constructed basin, the impoundment is created by excavation or embankment.
Bioinfiltration Basin 5 acres or less Bioinfiltration 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 bioinfiltration 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.
Infiltration Trench 5 acres or less
Dry Well Synonym: Infiltration Tube, French Drain, Soak‐Away Pits, Soak Holes 1 acre or less (rooftop only)
Underground Infiltration 10 acres or less Though feasible, larger underground infiltration systems may cause groundwater contamination as water is not able to infiltrate through a surface cover. In addition, wind flocculation, UV degradation, and bacterial degradation, which provide additional treatment in surface systems, do not occur in underground systems. Because performance research is lacking for larger features, it is HIGHLY RECOMMENDED that the contributing drainage area to a single device not exceed 10 acres.
Dry Swale with Check Dams 5 acres or less
Permeable Pavement 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 up to 0.25 acres per tree

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 infiltration BMPs - treatment properties
Link to this table

Stormwater BMP Illustration Pollutant Removal Mechanism Location in Treatment Train Pollutant Removal 1,2 Potential Application 1
Infiltration Basin
Infiltration basin icon.png
Sedimentation / Infiltration End TSS: High

TN: Medium/High TP: Medium/High Chloride: Low Metals: High Oils and Grease: High

Pathogens: High
Residential: Yes

Commercial: Yes Ultra Urban: Limited Industrial: Limited Retrofit: Yes

Highway/Road: Limited
Bioinfiltration Basin
Bioinfiltration icon.png
Sedimentation / Infiltration Beginning TSS: High

TN: Low/Medium TP: Medium/High Chloride: Low Metals: High

Oils and Grease: High
Residential: Yes

Commercial: Yes Ultra Urban: Limited Industrial: Limited Retrofit: Yes

Highway/Road: Limited
Infiltration Trench
Synonym: Infiltration Gallery
Infiltration trench icon.png
Infiltration nd TSS: High

TN: Medium/High TP: Medium/High Chloride: Low Metals: High Oils and Grease: High

Pathogens: High
Residential: Yes

Commercial: Yes Ultra Urban: Yes Industrial: Limited Retrofit: Yes

Highway/Road: Yes
Dry Well
Synonym: Infiltration Tube, French Drain, Soak‐Away Pits, Soak Holes
Dry well icon.png
Infiltration throughout TSS: High

TN: Medium/High TP: Medium/High Chloride: Low Metals: High Oils and Grease: High

Pathogens: High
Residential: Yes

Commercial: Yes Ultra Urban: Yes Industrial: Limited Retrofit: Yes

Highway/Road: No
Underground Infiltration
Underground infiltration icon.png
Sedimentation / Infiltration / Flotation/Skimming End TSS: High

TN: Medium/High TP: Medium/High Chloride: Low Metals: High Oils and Grease: High

Pathogens: High
Residential: Yes

Commercial: Yes Ultra Urban: Yes Industrial: Limited Retrofit: Yes

Highway/Road: Limited
Dry Swale with Check Dams
Swale check icon.png
Sedimentation / Infiltration Throughout TSS: High

TN: Low/Medium TP: Low/Medium Chloride: Low Metals: High Oils and Grease: High

Pathogens: Medium
Residential: Yes

Commercial: Yes Ultra Urban: Limited Industrial: Yes Retrofit: Limited

Highway/Road: Yes
Permeable Pavement
Permeable pavement icon.png
Infiltration Beginning TSS: High

TN: Medium/High TP: Medium/High Chloride: Low Metals: High

Oils and Grease: High
Residential: Yes

Commercial: Yes Ultra Urban: Yes Industrial: Limited Retrofit: Yes

Highway/Road: Limited
Tree Trench/Tree Box
Tree trench icon.png
Infiltration, Transpiration Throughout TSS: High

TN: Medium/High TP: Medium/High Chloride: Low Metals: High Oils and Grease: High

Pathogens: High
Residential: Limited

Commercial: Yes Ultra Urban: Yes Industrial: Limited Retrofit: Yes

Highway/Road: Limited

1 Treatment Capabilities and Potential Applications referenced from Manual Section BMP's for stormwater infiltration
2 Low = < 30%; Medium = 30‐65%; High = 65‐100%


Selection considerations table

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

Stormwater infiltration BMPs – selection considerations
Link to this table

Stormwater BMP Illustration Cost Maintenance Requirements 3 Pre‐treatment 4 Habitat Quality 5
Infiltration Basin
Infiltration basin icon.png
Low $0.5‐$1.3 CF Simple‐Intensive Needed Oil/Water Separator, Vegetated Filter, Sediment Basin, Water Quality Inlets Low
Bioinfiltration Basin
Bioinfiltration icon.png
Low $0.5‐$1.3 CF Simple‐Intensive Needed Oil/Water Separator, Vegetated Filter, Sediment Basin, Water Quality Inlets Medium‐High
Infiltration Trench Synonym: Infiltration Gallery
Infiltration trench icon.png
Low $1‐$4 CF Medium Needed

Oil/Water Separator, Vegetated Filter, Sediment

Basin, Water Quality Inlets
None
Dry Well Synonym: Infiltration Tube, French Drain, Soak‐Away Pits, Soak Holes
Dry well icon.png
Low $1‐$4 CF Medium Needed

Oil/Water Separator, Vegetated Filter, Water

Quality Inlets
None
Underground Infiltration
Underground infiltration icon.png
High 14 CF Medium Needed Oil/Water Separator, Water Quality Inlets None
Dry Swale with Check Dams
Swale check icon.png
Low $.5‐$1.3 CF Simple‐Medium Needed Vegetated Filter, Water Quality Inlets Low‐Medium
Permeable Pavement
Permeable pavement icon.png
Medium 3‐10 CF Medium No Pretreatment Required None
Tree Trench/Tree Box
Tree trench icon.png
High

$1.80 ‐ $12.70 CF based on recommended soil volume

of 1,414 CF per tree
Intensive Needed Oil/Water Separator, Water Quality Inlets 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


See Infiltration Summary Table

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

Related pages for stormwater infiltration

Related pages for infiltration BMPs



Types of filtration

As filtration becomes a more common tool in stormwater management, and as the number of design variants increases, so does the number of names for each of these variants. For example:

  • Sand filters are also referred to as filtration basins, filter systems, first-flush filtration, or media filtration systems.
  • Grass channels are also referred to as biofilters. (Seattle METRO, 1992 from CWP)
  • Dry swales are also referred to as grassed or vegetated swales.

The following types of filtration systems are appropriate for Minnesota, depending upon project scale and site conditions.

Media filters

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.

Surface sand filter

For a surface sand filter, a flow splitter is used to divert runoff into an off-line sedimentation chamber. The chamber may be either wet or dry, and is generally used for pre-treatment. Runoff is then distributed into the second chamber, which consists of a sand filter bed (~18 inches) and temporary runoff storage above the bed. Pollutants are trapped or strained out at the surface of the filter bed. The filter bed surface may have a sand or grass cover. A series of perforated pipes located in a gravel bed collect the runoff passing through the filter bed, and return it to the stream or channel at a downstream point. If underlying soils are permeable, and groundwater contamination unlikely, the bottom of the filter bed may have no lining, and the filtered runoff may be allowed to infiltrate. See Computer-aided design and drafting (CAD/CADD) drawings for design drawing.

Underground sand filter

The underground sand filter was adapted for sites where space is at a premium. In this design, the sand filter is placed in a 3 chamber underground vault accessible by manholes or grate openings. The vault can be either on-line or off-line in the storm drain system. The first chamber is used for pre-treatment and relies on a wet pool as well as temporary runoff storage. It is connected to the second sand filter chamber by an inverted elbow, which keeps the filter surface free from trash and oil. The filter bed is 18 inches in depth and may have a protective screen of gravel or permeable geotextile to limit clogging. During a storm, the water quality volume is temporarily stored in both the first and second chambers. Flows in excess of the filter’s capacity are diverted through an overflow weir. Filtered runoff is collected, using perforated underdrains that extend into the third “overflow” chamber. See Computer-aided design and drafting (CAD/CADD) drawings for design drawing.

Perimeter sand filter

Schematic of Delaware sand filter
Schematic of Delaware sand filter

The perimeter sand filter consists of two parallel trench-like chambers that are typically installed along the perimeter of a parking lot (See schematic of Delaware sand filter). Parking lot runoff enters the first chamber, which has a shallow permanent pool of water. The first trench provides pre-treatment before the runoff spills into the second trench, which consists of a sand layer (12 inches to 18 inches). During a storm event, runoff is temporarily ponded above the normal pool and sand layer, respectively. When both chambers fill up to capacity, excess parking lot runoff is routed to a bypass drop inlet. The remaining runoff is filtered through the sand, and collected by underdrains and delivered to a protected outflow point. See Computer-aided design and drafting (CAD/CADD) drawings for design drawing.

Vegetative filters

Vegetative filters provide removal of sediment, nutrients, or pollutants by plant structures


Grass channels

Grass channels are designed to meet a runoff velocity target for a water quality storm as well as the peak discharge from a 2-year design storm. The runoff velocity should not exceed 1.0 feet per second (fps) during the water quality storm. Grass channels can be designed to pass larger storms and serve as conveyance tools. Pre-treatment can be created by placing checkdams across the channel below pipe inflows, and at various other points along the channel. Grass channels do not provide adequate pollutant removal benefits to act as a stand-alone BMP.

Dry swales

In dry swales, the entire water quality volume is temporarily retained by checkdams during each storm. Unlike the grass channel, the filter bed in the swale is 30 inches of prepared soil. Water is filtered through the sandy loam to underdrains and the swale is quickly dewatered. In the event that surface soils clog, the dry swale has a pea gravel window on the downstream side of each checkdam to route water to the underdrain. Dry swales are often preferred in residential areas because they prevent standing water. See Computer-aided design and drafting (CAD/CADD) drawings for design drawing.

Wet swales

Wet swales occur when the water table is located very close to the surface. This wet swale acts as a very long and linear shallow wetland treatment system. Like the dry swale, the entire water quality treatment volume is stored within a series of cells created by checkdams. Cells may be planted with emergent wetland plant species to improve pollutant removal.

Filter strips

Filter strips rely on the use of vegetation to slow runoff velocities and filter out sediment and other pollutants from urban stormwater.

Photo of vegetated swale city of Wayzata
Photo of vegetated swale in the City of Wayzata

To be effective, however, filter strips require the presence of sheet flow across the entire strip. Once flow concentrates to form a channel, it effectively short-circuits the filter strip. In the most common design, runoff is directed from a parking lot into a long filtering system composed of a stone trench, a grass strip and a longer naturally vegetative strip. The grass portion of the filter strip provides pre-treatment for the wooded portion. In addition, a stone drop can be located at the edge of the parking lot and the filter strip to prevent sediments from depositing at this critical entry point. The filter strip is typically an on-line practice, so it must be designed to withstand the full range of storm events without eroding. Filter strips do not provide adequate pollutant removal benefits to act as a stand-alone BMP. See Computer-aided design and drafting (CAD/CADD) drawings for design drawing.

Other filters not approved for Minnesota

The following filters are not recommended for use in Minnesota due to high probability of failure under cold climate conditions. They are included here for informational purposes only.

Organic filter

The organic filter functions in much the same way as the surface sand filter, but uses leaf compost or a peat/sand mixture as the filter media instead of sand (compost and peat should not be used when the target pollutant for removal is a dissolved nutrient. The organic material enhances pollutant removal by providing adsorption of heavy metals. In an organic filter, runoff is diverted with a flow splitter into a pre-treatment chamber, from which it passes into one or more filter cells. Each filter bed contains a layer of leaf compost or the peat/sand mixture, followed by a filter fabric and perforated pipe and gravel. Runoff filters through the organic media to the perforated pipe and ultimately to the outlet. The filter bed and subsoils can be separated by an impermeable polyliner to prevent movement into groundwater.

It is HIGHLY RECOMMENDED that the facility be actively managed to keep it dry before it freezes in the late fall.

Pocket filter

Pocket sand filters are intended as an inexpensive variation of a sand filter where sediment loads do not warrant a sedimentation chamber and can suffice with a grass filter strip and a plunge pool. The filter bed is comprised of a shallow basin containing the sand filter medium. The filter surface is a layer of soil and a grass cover. In order to avoid clogging the filter has a pea gravel “window” which directs runoff into the sand and a cleanout and observation well. Typically the filtered runoff is allowed to exfiltrate to ground water, although underdrains may be needed if the soils are not suitably permeable.

Submerged gravel wetland

Submerged gravel filters consist of a series of cells that are filled with crushed rock or gravel. The standpipe from each cell is set at an elevation that keeps the rock or gravel submerged. Wetland plants are rooted in the media, where they can directly take up pollutants. The anaerobic conditions on the bottom of the filter can foster the de-nitrification process. Submerged gravel wetlands are not recommended for stormwater quality in cold climate conditions, although they do have been used in Minnesota for effluent polishing of wastewater.



Design criteria for filtration

image

The following terminology is used throughout this design section:

Warning: REQUIRED - Indicates design standards stipulated by the MPCA Construction General Permit (CGP)(or other consistently applicable regulations)

HIGHLY RECOMMENDED - Indicates design guidance that is extremely beneficial or necessary for proper functioning of the filtration practice, but not specifically required by the MPCA CGP.

RECOMMENDED - Indicates design guidance that is helpful for filtration practice performance but not critical to the design.

Design phase maintenance considerations

Caution: Maintenance considerations are an important component of design

Implicit in the design guidance is the fact that many design elements of infiltration and filtration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include

For more information on design information for individual infiltration and filtration practices, link here.

Major design elements

Physical feasibility initial check

Before deciding to use a filtration device for stormwater management, it is helpful to consider several items that bear on the feasibility of using such a device at a given location. The following list of considerations will help in making an initial judgment as to whether or not a filtration device is the appropriate BMP for the site.

Drainage Area: Five acres maximum RECOMMENDED (0.5 to 2 acres is ideal). Surface sand filters can sometimes be sized for drainage areas up to 10 acres, but only with rigorous and committed maintenance schedules, among other design and O&M considerations. For more information on contributing area, see Contributing drainage area to stormwater BMPs.
Site Topography and Slopes: It is RECOMMENDED that sloped areas immediately adjacent to the practice be less than 33 percent but greater than 1 percent, to promote positive flow towards the practice.
Soils: No restrictions for media filters with underdrains. Vegetated filters should be sized assuming no losses to infiltration.
Depth to Water Table and Bedrock: A separation distance of at least 3 feet is REQUIRED under the MPCA CGP between the bottom elevation of vegetative filters and the elevation of the seasonally high water table. For purposes of this guidance, it is assumed that the definition of a stormwater pond includes a stormwater filtration system.
Warning: A separation distance of at least 3 feet is REQUIRED under the state CGP between the bottom elevation of vegetative filters and the elevation of the seasonally high water table (does not apply to wet swales)
Site Location/Minimum Setbacks:
Warning: A minimum setback of 50 feet between a stormwater pond and a water supply well is REQUIRED by the Minnesota Department of Health Rule 4725.4350.
Karst: It is HIGHLY RECOMMENDED that underdrains and an impermeable liner be used for sand filters in Karst terrain.

Conveyance

It is Highly Recommended that the designer provides non-erosive flow velocities at the outlet point to reduce downstream erosion. During the 10-year or 25-year storm (depending on local drainage criteria), discharge velocity should be kept below 4 feet per second for established grassed channels. Erosion control matting or rock should be specified if higher velocities are expected.

Common overflow systems within the structure consist of a yard drain inlet, where the top of the yard drain inlet is placed at the elevation of the shallow ponding area. A stone drop of about 12 inches or small stilling basin could be provided at the inlet of filtration areas where flow enters the practice through curb cuts or other concentrated flow inlets. In cases with significant drop in grade this erosion protection should be extended to the bottom of the facility.

Underdrains

The following are RECOMMENDED for filtration practices with underdrains.

  • The minimum pipe diameter is 4 inches.
  • Install 2 or more underdrains for each practice system in case one clogs. At a minimum provide one underdrain for every 1,000 square feet of surface area.
  • Include at least 2 observation /cleanouts for each underdrain, one at the upstream end and one at the downstream end. Cleanouts should be at least 4 inches diameter vertical non-perforated schedule 40 PVC pipe, and extend to the surface. Cap cleanouts with a watertight removable cap.
  • Construct underdrains with Schedule 40 or SDR 35 smooth wall PVC pipe.
  • Install underdrains with a minimum slope of 0.5 percent, particularly in HSG D soils (Note: to utilize Manning’s equation the slope must be greater than 0).
  • Include a utility trace wire for all buried piping.
  • For underdrains that daylight on grade, include a marking stake and animal guard;
  • For each underdrain have an accessible knife gate valve on its outlet to allow the option of operating system as either bioinfiltration, biofiltration system or both. The valve should enable the ability to make adjustments to the discharge flow so the sum of the infiltration rate plus the under-drain discharge rate equal a 48 hour draw-down time.
  • Perforations should be 3/8 inches. Use solid sections of non-perforated PVC piping and watertight joints wherever the underdrain system passes below berms, down steep slopes, makes a connection to a drainage structure, or daylights on grade.
  • Spacing of collection laterals should be less than 25 feet.
  • Underdrain pipes should have a minimum of 3 inches of washed #57 stone above and on each side of the pipe (stone is not required below the pipe). Above the stone, two inches of choking stone is needed to protect the underdrain from blockage.
  • Avoid filter fabric.
  • Pipe socks may be needed for underdrains imbedded in sand. If pipe socks are used, then use circular knit fabric.

The procedure to size underdrains is typically determined by the project engineer. An example for sizing underdrains is found in Section 5.7 of the North Carolina Department of Environment and Natural Resources Stormwater BMP Manual.

Pretreatment

Pretreatment refers to features of a filtration area that capture and remove coarse sediment particles.

Warning: To prevent clogging of the infiltration or filtration system with trash, gross solids, and particulate matter, use of a pretreatment device such as a vegetated filter strip, vegetated swale, small sedimentation basin (forebay), or water quality inlet (e.g., grit chamber) to settle particulates before the stormwater discharges into the infiltration or filtration system is REQUIRED.

For applications where runoff enters the filtration area through sheet flow, such as from parking lots, or residential back yards, a grass filter strip with a pea gravel diaphragm is the preferred pretreatment method. The width of the filter strip depends on the drainage area, imperviousness and the filter strip slope. The minimum RECOMMENDED vegetated filter strip width is 3 feet. The width should increase with increasing slope of the filter strip. Slopes should not exceed 8 percent. Pretreatment filter strips greater than 15 feet in width will provide diminishing marginal utility on the installation cost.

For retrofit projects and sites with tight green space constraints, it may not be possible to include a grass buffer strip. For example, parking lot island retrofits may not have adequate space to provide a grass buffer. For applications where concentrated (or channelized) runoff enters the filtration area, such as through a slotted curb opening, a grassed channel with a pea gravel diaphragm is the preferred pretreatment method.

The filtration area should be inspected semi-annually to determine if accumulated sediment needs to be removed. Accumulated sediment should be removed from the gravel verge (if applicable) and vegetated filter strip as needed. If the watershed runoff is especially dirty, this frequency may need to be monthly or quarterly. Trash removal should occur in conjunction with removal of debris from the filtration area. During maintenance, check for erosion in the filter strip. If it is visible, it should be repaired with topsoil and re-planted. Vegetation of the filter strip should be designed at least 2 inches below the contributing impervious surface. If, over time, the grade of the vegetated filter strip rises above the adjacent impervious surface draining into it, the grade of the vegetated filter strip needs to be lowered to ensure proper drainage.

In lieu of grass buffer strips, pretreatment may be accomplished by other methods such as sediment capture in the curb-line entrance areas. Additionally, the parking lot spaces may be used for a temporary storage and pretreatment area in lieu of a grass buffer strip. Local requirements may allow a street sweeping program as an acceptable pretreatment practice. It is HIGHLY RECOMMENDED that pretreatment incorporate as many of the following as are feasible:

  • grass filter strip;
  • vegetated swale;
  • gravel diaphragm;
  • mulch layer;
  • forebay;
  • flow-through structures; and
  • up flow inlet for storm drain inflow.

Treatment

The following guidelines are applicable to the actual treatment area of a filtration facility:

Space Required: Function of available head at site for surface filters. Underground filters generally have little or no surface space requirements except for access.
Slope: The surface slope of media filters should be level to promote even distribution of flow throughout the practice. It is HIGHLY RECOMMENDED that vegetative filters have a maximum slope of 5 percent and a minimum slope of 1 percent.
Depth: The RECOMMENDED elevation difference at a site from the inflow to the outflow is 4 to 6 feet for most sand filters, and 2 to 3 feet for perimeter filters.
Warning: Ground Water Protection: Infiltration of untreated PSH runoff into ground water is PROHIBITED. At confirmed hotspots, it is REQUIRED that the filter is lined and is discharged in a manner that will not mobilize pollutants.
Aesthetics: Vegetative filters can be effectively integrated into the site planning process, and aesthetically designed as attractive green spaces. Media filters are less conducive to site aesthetics, but surface media filters can be designed with turf cover vegetation if desired.

Landscaping

It is HIGHLY RECOMMENDED that impervious area construction be completed and pervious areas established with dense and healthy vegetation (see Minnesota plant lists or Plants for Stormwater Design) prior to introduction of stormwater into a filtration practice.

Surface filters can have a grass cover to aid in pollutant adsorption. The grass should be capable of withstanding frequent periods of inundation and drought.

Safety

  • No building structures should be constructed on top of underground filters.
  • The risk of creating mosquito breeding areas can be minimized by following the recommendations within the mosquito control section.
  • It is HIGHLY RECOMMENDED that swale side slopes be set at 1:3 (V:H) or flatter.
  • It is HIGHLY RECOMMENDED that perimeter sand filter grates be sufficiently heavy that they cannot be removed easily.
  • Fencing may be desirable in surface sand filter designs with significant vertical walls for the sedimentation and/or filter chamber. Fencing can also protect the filter from vandalism and limit animal access.
Warning: It is REQUIRED that underground media filters only be accessed by individuals with appropriate confined space entry training.
Warning: If a dry well or infiltration trench is greater than five feet deep, it is REQUIRED that OSHA health and safety guidelines be followed for safe construction practices.

Additional information on safety for construction sites is available from OSHA.

Design procedure: media filters

The following steps outline a recommended design procedure for media filters. Except where indicated, procedures are consistent with requirements for compliance with the MPCA CGP.

Design steps

Step 1. Preliminary judgment of site conditions and identify the function of the filter.

Make a preliminary judgment as to whether site conditions are appropriate for the use of a surface or perimeter sand filter, and identify the function of the filter in the overall treatment system.

A. Consider basic issues for initial suitability screening, including:
  • Site drainage area
  • Site topography and slopes
  • Regional or local depth to groundwater and bedrock
  • Site location/minimum setbacks
  • Presence of active karst
B. Determine how the media filter will fit into the overall stormwater treatment system.
  • Decide whether the filter is the only BMP to be employed, or if are there other BMPs addressing some of the treatment requirements.
  • Decide where on the site the media filter is most likely to be located.

Step 2. Confirm design criteria and applicability.

A. Determine whether the media filter must comply with the MPCA CGP. To determine if permit compliance is required, see Permit Coverage and Limitations.
B. Check with local officials, watershed organizations, and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply.

Step 3. Select design variant based on physical suitability evaluation

Once the Physical feasibility initial check is complete, apply the better site design principles in sizing and locating the filtration practice(s) on the development site. Given the drainage area, select the appropriate filtration practice for the first iteration of the design process.

Note: Information collected during the Physical feasibility initial check (see Step 2) should be used to explore the potential for multiple filtration practices versus relying on a single facility. The use of smaller filtration practices dispersed around a development is usually more sustainable than a single regional facility that is more likely to have maintenance problems (Source: Wisconsin Department of Natural Resources Conservation Practice Standards, 2004).

Step 4. Compute runoff control volumes

Calculate the Water Quality Volume (Vwq).

Warning: If the media filter is being designed to meet the requirements of the MPCA CGP, the REQUIRED treatment volume is the water quality volume of 1 inch of runoff from the new impervious surfaces created from the project and must be calculated as an instantaneous volume above the media filter.

If part of the overall Vwq is to be treated by other BMPs, subtract that portion from the Vwq to determine the part of the Vwq to be treated by the filter.

Step 5. Compute Vwq peak discharge (Qwq)

A flow regulator (or flow splitter diversion structure) should be supplied to divert the Vwq to the sand filter facility. This is generally accomplished by setting the bypass weir within the diversion to the elevation of the water quality volume within the practice. Please refer to the adjustable diversion detail found in the Computer-aided design and drafting (CAD/CADD) drawings section.

Size low flow orifice, weir, or other device to pass Qwq.

Step 6. Size filtration basin chamber

image showing BMP terms
Schematic illustrating some of the terms and dimensions used in the Stormwater Manual.

To meet requirements of the Stormwater General Permit (CSW permit), the surface area (AS, in square feet) of a media filter practice is given by

<math>A_S = V_w / (D_O) </math>

Where:
Vw is the the water treatment volume of the area contributing runoff to the practice, in cubic feet; and
DO is the the storage depth of ponded water in the practice, in feet.

Size the depth of the practice the meet the 48 hour draw down time. The maximum recommended depth of a media filter practice is 4 feet

Warning: The water treatment volume must drain with 48 hours (24 hours is RECOMMENDED if discharges from the practice are to a trout stream) per the MPCA CGP

The water treatment volume is given by

<math>V_w = 0.0833 A_c</math>

Where
0.0833 = one inch, converted to feet, of runoff captured for filtration, as required by the permit; and
Ac = the impervious surface area contributing to the practice, in square feet.

The entire water treatment volume is assumed to instantaneously pond in the media filter practice.

For a filtration BMP with sloped sides, the surface area (As) of the practice is the average area of the BMP, given by

<math> A_S = (A_O + A_M) / 2 </math>

Where
AO is the surface area at the overflow, in square feet; and
AM is the surface area at the top of the filtration media, in square feet.

Set preliminary dimensions of media filter basin chamber. The following guidelines are HIGHLY RECOMMENDED.

  • The filter media should consist of an 18-inch layer of clean washed medium sand (meeting ASTM C-33 concrete sand) on top of the under-drain system.
  • For surface sand filters, if an optional three inches of topsoil are placed over the sand bed, place permeable filter fabric between the topsoil and sand to prevent mixing of the layers. To prevent clogging of the underdrain, use either gravel or a permeable filter fabric.
  • Underground sand beds should be protected from trash accumulation by a wide mesh geotextile screen to be placed on the surface of the sand bed. The screen is to be rolled up, removed, cleaned and re-installed during maintenance operations.

Step 7. Pretreatment

Pre-treatment refers to features of a filtration system that capture and remove coarse sediment particles.

Warning: To prevent clogging of the infiltration or filtration system with trash, gross solids, and particulate matter, use of a pretreatment device such as a vegetated filter strip, vegetated swale, small sedimentation basin (forebay), or water quality inlet (e.g., grit chamber) to settle particulates before the stormwater discharges into the infiltration or filtration system is REQUIRED.

For applications where runoff enters the filtration system through sheet flow, such as from parking lots, or residential back yards, a vegetated filter strip with a pea gravel diaphragm is the preferred pre-treatment method. The width of the filter strip depends on the drainage area, imperviousness and the filter strip slope. The minimum RECOMMENDED vegetated filter strip width is 3 feet. The width should increase with increasing slope of the filter strip. Slopes should not exceed 8 percent. Pretreatment filter strips greater than 15 feet in width will provide diminishing marginal utility on the installation cost.

For retrofit projects and sites with tight green space constraints, it may not be possible to include a grass buffer strip. For example, parking lot island retrofits may not have adequate space to provide a grass buffer. For applications where concentrated (or channelized) runoff enters the filtration system, such as through a slotted curb opening, a vegetated filter strip with a pea gravel diaphragm is the preferred pre-treatment method.

The filtration system should be inspected semi-annually to determine if accumulated sediment needs to be removed. Accumulated sediment should be removed from the gravel verge (if applicable) and vegetated filter strip as needed. If the watershed runoff is especially dirty, this frequency may need to be monthly or quarterly. Trash removal should occur in conjunction with removal of debris from the filtration system. During maintenance, check for erosion in the filter strip. If it is visible, it should be repaired with topsoil and re-planted. Vegetation of the filter strip should be designed at least 2 inches below the contributing impervious surface. If, over time, the grade of the vegetated filter strip rises above the adjacent impervious surface draining into it, the grade of the vegetated filter strip needs to be lowered to ensure proper drainage.

The type of vegetation in the bioretention cell determines the appropriate flow velocity for which the pre-treatment device should be designed. For tree-shrub-mulch bioretention cells, velocity through the pre-treatment device should not exceed 1 foot per second, which is the velocity that causes incipient motion of mulch. For grassed bioretention cells, flow velocity through the pre-treatment device should not exceed 3 feet per second. In all cases, appropriate maintenance access should be provided to pre-treatment devices.

In lieu of grass buffer strips, pre-treatment may be accomplished by other methods such as sediment capture in the curb-line entrance areas. Additionally, the parking lot spaces may be used for a temporary storage and pre-treatment area in lieu of a grass buffer strip. If bioretention is used to treat runoff from a parking lot or roadway that is frequently sanded during snow events, there is a high potential for clogging from sand in runoff. Local requirements may allow a street sweeping program as an acceptable pre-treatment practice. It is HIGHLY RECOMMENDED that pre-treatment incorporate as many of the following as are feasible:

  • vegetated filter strip;
  • vegetated swale;
  • gravel diaphragm;
  • mulch layer;
  • forebay;
  • flow-through structures; and
  • up flow inlet for storm drain inflow.

Step 8. Design inlets, pre-treatment facilities, under-drain system, and outlet structures

  • Pre-treatment of runoff in a sand filter system must be provided. Inlets to surface sand filters must be provided with energy dissipaters. Exit velocities from the pretreatment device must be non-erosive.
  • An emergency or bypass spillway must be included in the surface sand filter to safely pass flows that exceed the design storm flows. The spillway prevents filter water levels from overtopping the embankment and causing structural damage. The emergency spillway should be located so that downstream buildings and structures will not be impacted by spillway discharges.
  • The allowable materials for sand filter construction are detailed in the table below.

This table shows sand material specifications.
Link to this table

Parameter specification Size Notes
Sand clean AASHTO M-6 or ASTM C-33 concrete sand 0.02” to 0.04” Sand substitutions such as Diabase and Graystone #10 are not acceptable. No calcium carbonated or dolomitic sand substitutions are acceptable. Rock dust cannot be substituted for sand.
Underdrain Gravel AASHTO M-43 1.5” to 3.5”
Geotextile Fabric (if required)

ASTM D-4833 (puncture strength - 125 lb.) ASTM D-1117 (Mullen Burst Strength - 400 psi)

ASTM D-4632 (Tensile Strength - 300 lb.)
0.08” thick equivalent opening size of #80 sieve Must maintain 125 gpm per sq. ft. flow rate. Note: a 4” pea gravel layer may be substituted for geotextiles meant to separate sand filter layers.
Impermeable Liner (if required)

ASTM D-4833 (thickness) ASTM D-412 (tensile strength 1,100 lb., elongation 200%) ASTM D-624 (Tear resistance - 150 lb./in)

ASTM D-471 (water adsorption: +8 to -2% mass)
30 mil thickness Liner to be ultraviolet resistant. A geotextile fabric should be used to protect the liner from puncture.
Under-drain Piping ASTM D-1785 or AASHTO M-278 minimum 4” rigid schedule 40 PVC 3/8” perf. @ 6” on center, 4 holes per row; minimum of 3” washed #57 stone over pipes; not necessary underneath pipes


Step 9. Compute overflow weir sizes

Surface sand filter:

  • If appropriate, plan outlet protection for overflow from sedimentation chamber.
  • Size bypass weir at elevation in filtration chamber above stand pipe to handle surcharge of flow through filter system within 48 hours.

Perimeter sand filter:

  • Size overflow weir at end of pretreatment to handle excess inflow, set at Vwq elevation

Follow the design procedures identified in the section on Unified sizing criteria to determine the volume control and peak discharge requirements for water quality, recharge (not required), channel protection, overbank flood and extreme storm. Adapt these values to local regulations, if any exist.

Model the proposed development scenario using a surface water model appropriate for the hydrologic and hydraulic design considerations specific to the site (see also the section on stormwater modeling). This includes defining the parameters of the filtration practice defined above: pond elevation and area (defines the pond volume), filtration rate and method of application (effective filtration area), and outlet structure and/or flow diversion information. The results of this analysis can be used to determine unintended consequences upstream (i.e. flooding).

Warning: The following item is specifically REQUIRED by the MPCA Permit:
Volume: The Permittee(s) must design infiltration or filtration systems that provide a water quality volume (calculated as an instantaneous volume) of one (1) inch of runoff (or one (1) inch minus the volume of stormwater treated by another system on the site) from the new impervious surfaces created by the project.
Drawdown: filtration practices shall discharge through the soil or filter media in 48 hours or less. Additional flows that cannot be infiltrated or filtered in 48 hours should be routed to bypass the system through a stabilized discharge point.

Experience has demonstrated that, although the drawdown period is 48 hours, there is often some residual water pooled in the infiltration practice after 48 hours. This residual water may be associated with reduced head, water gathered in depressions within the practice, water trapped by vegetation, and so on. The drawdown period is therefore defined as the time from the high water level in the practice to 1 to 2 inches above the bottom of the facility. This criterion was established to provide the following: wet-dry cycling between rainfall events; unsuitable mosquito breeding habitat; suitable habitat for vegetation; aerobic conditions; and storage for back-to-back precipitation events. This time period has also been called the period of inundation.

Step 10. Prepare vegetation and landscaping plan

See Major design elements section for guidance on preparing vegetation and landscaping management plan.

Step 11. Prepare operations and maintenance (O&M) plan

See Operation and maintenance section for guidance on preparing an O&M plan.

Step 12. Prepare cost estimate

See Cost considerations section for guidance on preparing a cost estimate that includes both construction and maintenance costs.

Design procedure: vegetative filters

The following steps outline a recommended design procedure for vegetative filters in compliance with the MPCA Permit for new construction. Design recommendations beyond those specifically required by the permit are also included and marked accordingly.

Design steps

Step 1: Make a preliminary judgment

Make a preliminary judgment as to whether site conditions are appropriate for the use of a vegetative filter, and identify the function of the filter in the overall treatment system

A. Consider basic issues for initial suitability screening, including:

  • Site drainage area
  • Site topography and slopes
  • Regional or local depth to ground water and bedrock
    • Dry Swale: Bottom of facility to be at least three feet above the seasonably high water table.
    • Wet Swale: The seasonally high water table may inundate the swale; but not above the design bottom of the channel.
  • Site location/minimum setbacks.
  • Presence of active karst.

B. Determine how the vegetative filter will fit into the overall stormwater treatment system

  • Decide whether the filter is the only BMP to be employed, or if are there other BMPs addressing some of the treatment requirements.
  • Decide where on the site the vegetative filter is most likely to be located.

Step 2. Confirm design criteria and applicability

A. Determine whether the vegetative filter must comply with the MPCA Permit.

B. Check with local officials, watershed organizations, and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply.

Step 3. Select design variant based on physical suitability evaluation

Once the physical suitability evaluation is complete, it is HIGHLY RECOMMENDED that the better site design principles be applied in sizing and locating the filtration practice(s) on the development site. Given the drainage area, select the appropriate filtration practice for the first iteration of the design process.

Note: Information collected during the physical suitability evaluation (see Step 1) should be used to explore the potential for multiple filtration practices versus relying on a single facility. The use of smaller filtration practices dispersed around a development is usually more sustainable that a single regional facility that is more likely to have maintenance problems (Source: Wisconsin Department of Natural Resources Conservation Practice Standards, 2004)

Step 4. Compute runoff control volumes and other key design parameters

Calculate the Water Quality Volume (Vwq), Channel Protection Volume (Vcp), Overbank Flood Protection Volume (Vp10), and the Extreme Flood Volume (Vp100).

If the vegetative filter is being designed to meet the requirements of the MPCA Permit, the REQUIRED treatment volume is the water quality volume of 1 inch of runoff from the new impervious surfaces created from the project. If part of the overall Vwq is to be treated by other BMPs, subtract that portion from the Vwq to determine the part of the Vwq to be treated by the filter.

For filter strips, compute the following design parameters:

a. Calculate the maximum discharge loading per foot of filter strip width

<math> q = (0.00236/n)Y^{1.67}S^{0.5} </math>

Where:

q = discharge per foot of width of filter strip, from Manning’s equation (cfs/ft);
Y = allowable depth of flow (inches) (3 to 4 inch maximum);
S = slope of filter strip (percent) (2 to 6 percent); and
n = Manning’s “n” roughness coefficient (use 0.15 for short prairie grass, 0.25 for dense grasses such as bluegrass, buffalo grass, blue grama grass and other native grass mixtures).

b. Use a recommended hydrologic model to compute Qwq

c. Minimum Filter Width (in feet) = Qwq / q

Where:

Qwq = the water quality peak discharge (cubic feet per second)

Step 5. Determine pre-treatment method

Warning: Pre-treatment for vegetative filters is REQUIRED

One alternative is a level spreader that allows coarse sediment to settle and evenly distributes flow across the full width of the filter. Pre-treatment could be provided with plunge pools where concentrated flows enter and with level spreaders where lateral flows enter. Additional pre-treatment measures include filter strips and street/parking lot sweeping. Street/parking lot sweeping may be considered pre-treatment in the case of a parking lot island or other area where spatial limitations make structural pre-treatment measures unfeasible.

Storage volume created for pre-treatment counts toward the total Vwq requirement, and should be subtracted from the Vwq for subsequent calculations.

Step 6. Preliminary design

Wet and dry swales:

  • Size bottom width, depth, length, and slope necessary to store Vwq with less than 18 inches of ponding at the downstream end.
  • Slope should not exceed 5 percent (1 to 2 percent recommended)
  • Bottom width should range from 2 to 8 feet
  • Ensure that side slopes are no greater than 3:1 (4:1 recommended)

If the system is on-line, channels should be sized to convey runoff from the overbank flood event (Vp10) safely with a minimum of 6 inches of freeboard and without damage to adjacent property. The peak velocity for the 2-year storm must be nonerosive for the soil and vegetative cover provided.

The channel and under-drain excavation should be limited to the width and depth specified in the design. The bottom of the excavated trench shall not be loaded in a way that causes soil compaction, and scarified prior to placement of gravel and permeable soil. The sides of the channel shall be trimmed of all large roots. The sidewalls shall be uniform with no voids and scarified prior to backfilling.

Step 7. Compute number of check dams (swales) or berms (filter strip)

Wet and Dry Swales: Checkdams

  • Design to contain entire Vwq.
  • Channel slopes between 1 and 2 percent are recommended unless topography necessitates a steeper slope, in which case 6- to 12-inch drop structures can be placed to limit the energy slope to within the recommended 1 to 2 percent range. Energy dissipation will be required below the drops. Spacing between the drops should not be closer than 50 feet. Depth of the Vwq at the downstream end should not exceed 18 inches.

Filter Strips: Berms

  • Size outlet pipes to ensure that the bermed area drains within 24 hours.
  • Specify grasses resistant to frequent inundation within the shallow ponding limit.
  • Berm material should be of sand, gravel and sandy loam to encourage grass cover(Sand: ASTM C-33 fine aggregate concrete sand 0.02 to 0.04 inch, Gravel: AASHTO M-43 ½ to 1 inch)
  • Size filter strip to contain the Vwq within the wedge of water backed up behind the berm.
  • Maximum berm height should be 12 inches.
  • Pervious berms to be a sand/gravel mix (35 to 60 percent sand, 30 to 55 percent silt, and 10 to 25 percent gravel). Berms are to have overflow weirs with 6-inch minimum head.

Step 8: Calculate draw-down time

Dry swale: The bed of the dry swale consists of a permeable soil layer of at least 30 inches in depth, above an 8-inch diameter perforated PVC pipe (AASHTO M 252) longitudinal under-drain in a 12-inch gravel layer. The soil media should have an infiltration rate of at least 0.5 feet per day (fpd) with a maximum of 1.5 fpd and contain a high level of organic material to facilitate pollutant removal. A permeable filter fabric is placed between the gravel layer and the overlying soil. Dry swale channels are sized to store and filter the entire Vwq and allow for full filtering through the permeable soil layer.

Step 9. Check 2-year and 10-year velocity erosion potential and freeboard

Check for erosive velocities and modify design as appropriate based on local conveyance regulations. Provide 6 inches of freeboard.

Step 10. Design low flow control at downstream headwalls and checkdams

Design control to pass Vwq in 48 hours.

Step 11. Design inlets, sediment forebay(s), and under-drain system (dry swale)

Inlets to swales must be provided with energy dissipaters such as riprap or geotextile reinforcement. Pre-treatment of runoff in both a dry and wet swale system is typically provided by a sediment forebay located at the inlet. Enhanced swale systems that receive direct concentrated runoff may have a 6-inch drop to a pea gravel diaphragm flow spreader at the upstream end of the control. A pea gravel diaphragm and gentle side slopes should be provided along the top of channels to provide pre-treatment for lateral sheet flows. The under-drain system should discharge to the storm drainage infrastructure or a stable outfall. For a wet swale, do not use an under-drain system.

Step 12. Check volume, peak discharge rates and drawdown time against State, local and watershed organization requirements (Note: Steps are iterative)

Follow the design procedures identified in the Unified Sizing Criteria section of the Manual to determine the volume control and peak discharge requirements for water quality, recharge (not required), channel protection, overbank flood and extreme storm.

Model the proposed development scenario using a surface water model appropriate for the hydrologic and hydraulic design considerations specific to the site. This includes defining the parameters of the filtration practice defined above: pond elevation and area (defines the pond volume), filtration rate and method of application (effective filtration area), and outlet structure and/or flow diversion information. The results of this analysis can be used to determine whether or not the proposed design meets the applicable requirements. If not, the design will have to be re-evaluated.

Warning: The following items are specifically REQUIRED by the MPCA Permit:

A. Volume: Filtration systems shall be sufficient to filter a water quality volume of 1 inch of runoff from the new impervious surfaces created by the project. If this criterion is not met,increase the storage volume of the filtration practice or treat excess water quality volume (Vwq) in an upstream or downstream BMP (see Step 5).

B. Drawdown: Filtration practices shall discharge through the soil or filter media in 48 hours or less. Additional flows that cannot be infiltrated or filtered in 48 hours should be routed to bypass the system through a stabilized discharge point.

Experience has demonstrated that, although the drawdown period is 48 hours, there is often some residual water pooled in the infiltration practice after 48 hours. This residual water may be associated with reduced head, water gathered in depressions within the practice, water trapped by vegetation, and so on. The drawdown period is therefore defined as the time from the high water level in the practice to 1 to 2 inches above the bottom of the facility. This criterion was established to provide the following: wet-dry cycling between rainfall events; unsuitable mosquito breeding habitat; suitable habitat for vegetation; aerobic conditions; and storage for back-to-back precipitation events. This time period has also been called the period of inundation.

Step 13: Prepare Vegetation and Landscaping Plan

A landscaping plan for a dry or wet swale should be prepared to indicate how the enhanced swale system will be stabilized and established with vegetation. Landscape design should specify proper grass species and wetland plants based on specific site, soils and hydric conditions present along the channel.

This table shows sand material specifications.
Link to this table

Parameter specification Size Notes
Sand clean AASHTO M-6 or ASTM C-33 concrete sand 0.02” to 0.04” Sand substitutions such as Diabase and Graystone #10 are not acceptable. No calcium carbonated or dolomitic sand substitutions are acceptable. Rock dust cannot be substituted for sand.
Underdrain Gravel AASHTO M-43 1.5” to 3.5”
Geotextile Fabric (if required)

ASTM D-4833 (puncture strength - 125 lb.) ASTM D-1117 (Mullen Burst Strength - 400 psi)

ASTM D-4632 (Tensile Strength - 300 lb.)
0.08” thick equivalent opening size of #80 sieve Must maintain 125 gpm per sq. ft. flow rate. Note: a 4” pea gravel layer may be substituted for geotextiles meant to separate sand filter layers.
Impermeable Liner (if required)

ASTM D-4833 (thickness) ASTM D-412 (tensile strength 1,100 lb., elongation 200%) ASTM D-624 (Tear resistance - 150 lb./in)

ASTM D-471 (water adsorption: +8 to -2% mass)
30 mil thickness Liner to be ultraviolet resistant. A geotextile fabric should be used to protect the liner from puncture.
Under-drain Piping ASTM D-1785 or AASHTO M-278 minimum 4” rigid schedule 40 PVC 3/8” perf. @ 6” on center, 4 holes per row; minimum of 3” washed #57 stone over pipes; not necessary underneath pipes


Step 14. Prepare Operation and Maintenance (O&M) Plan

See Operation and Maintenance section for guidance on preparing an O&M plan.

Step 15. Prepare Cost Estimate

See Cost Considerations section for guidance on preparing a cost estimate that includes both construction and maintenance costs.



Construction specifications for filtration

Construction details and specifications

CADD based details for filtration systems can be accessed at this link. The drawings provide details and specifications for the following filtration systems.



Cost-benefit considerations for filtration

Cost estimates that site planners can use to compare the relative construction and maintenance costs for structural best management practices are excellent for purposes of comparison. However, it is recommended that construction and maintenance budgets should be based on site specific information. Utilizing the table below and cost estimation worksheets for either bioretention or surface sand filter will allow designers to more accurately estimate the cost of a filtration BMP.

Cost components for filtration practices.
Link to this table

Implementation Stage Primary Cost Components Basic Cost Estimate Other Considerations
Site Preparation Tree & plant protection Protection Cost ($/acre) x Affected Area (acre) Removal of existing structures, topsoil removal and stockpiling
Topsoil salvage Salvage cost ($/acre) x Affected Area (acre)
Clearing & grubbing Clearing Cost ($/acre) x Affected Area (acre)
Site Formation Excavation / grading X-ft Depth Excavation Cost ($/acre) x Area (acre) Soil & rock fill Hauling material material, tunneling
Hauling material offsite Excavation Cost x (% of Material to be hauled away)
Structural Components Under-drains Under-drain cost ($/lineal foot) x length of device Pipes, catchbasins, manholes, valves, vaults
Vault structure (for media filters) ($/structure)
Media (for media filters) Media cost ($/cubic yard) X filter volume (cubic yard)
Inlet structure (for vegetative filters ($/structure)
Outlet structure (for vegetative filters) ($/structure)
Site Restoration Filter strip Sod cost ($/square foot) x filter strip area Tree protection, soil amendments, seed bed preparation, trails
Soil preparation Topsoil or amendment cost ($/acre) x Area (acre)
Seeding Seeding Cost ($/acre) x Seeded Area (acre)
Planting / transplanting Planting Cost ($/acre) x Planted Area (acre)
Annual Operation, Maintenance, and Inspection Debris removal Removal Cost ($/acre) x Area (acre) x Frequency (2x / 1yr) Vegetation maintenance, cleaning of structures
Sediment removal Removal Cost ($/acre) x Area (acre) x Frequency (1x / 5yr)
Gate / valve operation Operation Cost ($) x Operation Frequency (2x / 1 yr)
Inspection Inspection Cost ($) x Inspection Frequency (6x / 1 yr)
Mowing (for some vegetative filters) Mowing Cost ($) x Mowing Frequency (4x / 1 yr)




References for filtration

References



Fact sheet for filtration

Filtration practices are structural stormwater controls that capture, temporarily store, and route stormwater runoff through a filter bed to improve water quality.

Photo of a swale at Lino Lakes city hall
Photo of a swale at Lino Lakes city hall
Photo of a Swale city of Woodbury MN
Photo of a Swale city of Woodbury MN

Design criteria

  • Ensure adequate space for Filtration system
  • Some installations require 2 to 6 feet of head
  • Removal potential of the key pollutant
  • Parent material and potential for ground water contamination

Benefits

  • Good for highly impervious areas with low sediment/high pollutant load (e.g. urban land use and retrofit scenarios)
  • High pollutant removal rates
  • May be used in a variety of soil types
  • Good for the treatment of hotspots because it can be isolated from groundwater by using a liner if contamination concerns exist

Limitations:

  • Higher maintenance requirements
  • Some installations (media filters) have higher construction costs
  • Minimal treatment of soluble nutrients
  • Potential for nitrification in media filters where anaerobic conditions exist

Description

Filtration systems vary in their operation and applicability, but all can be described as structural BMPs that function mainly to enhance water quality by passing stormwater through a media. The media can be made of sand, peat, soil, or compost and should be assigned on a case-by-case basis. Filters can be off-line systems or designed as pre-treatment before discharging to other stormwater features.

Media filters can be located on the surface, underground, along the perimeter or an area, or in what is called a pocket design.

Drawing of the start of Storm Event - Initial runoff & storage
Drawing of the start of Storm Event - Initial runoff & storage
Drawing of the Duration of Storm Event - Storage & filtration/infiltration
Drawing of the Duration of Storm Event - Storage & filtration/infiltration
Drawing of the Following Storm Event - Remaining storage drawdown
Drawing of the Following Storm Event - Remaining storage drawdown

Mechanisms

Pollutant removal

Pollutant removal varies with the design, construction and maintenance of the BMP. Values below are approximately mid-range removals for a standard designed BMP that is properly constructed and maintained.

Media filter

  • 85 percent Total Suspended Solids (TSS)
  • 50 percent/35 percent Nutrients - Total Phosphorus /Total Nitrogen
  • 40 to 80 percent Metals - Cadmium, Copper, Lead, and Zinc (will vary with metal)
  • 35 percent Coliform, Streptococci, E. Coli
  • 80 percent Hydrocarbon

Vegetative filter

  • 68 percent Total Suspended Solids (TSS)
  • 40 percent/35 percent Nutrients - Total Phosphorus /Total Nitrogen
  • 40 to 80 percent Metals - Cadmium, Copper, Lead, and Zinc (will vary with metal)
  • 0 percent Coliform, Streptococci, E. Coli
  • 80 percent Hydrocarbon

Site factors

  • The RECOMMENDED maximum drainage area is typically 5 acres, but can be greater if the discharge to the basin has received adequate pretreatment and the basin is properly designed, constructed, and maintained.
  • 20 percent maximum Site Slope
  • 3 feet minimum Depth to Bedrock unless lined
  • 3 feet minimum Depth to Seasonally High Water Table unless lined
  • A,B,C,D - NRCS Soil Type
  • Poor to Good Freeze/ Thaw Suitability
  • Suitable with impermeable liner - Hotspot Runoff

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