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

# Overview

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 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

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 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

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

## 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

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

$A_S = V_w / (D_O)$

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

$V_w = 0.0833 A_c$

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

$A_S = (A_O + A_M) / 2$

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

$q = (0.00236/n)Y^{1.67}S^{0.5}$

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)

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

## 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

This page was last edited on 8 April 2013, at 19:14.

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