Stormwater filters are a useful practice to treat stormwater runoff from small, highly impervious sites. Stormwater filters capture, temporarily store, and treat stormwater runoff by passing it through an engineered filter media, collecting it in an underdrain and then returning it back to the storm drain system. The filter consists of two chambers; the first is devoted to settling, and the second serves as a filter bed consisting of a sand or organic filter media.

Stormwater filters are a versatile option that offers moderate pollutant removal performance at small sites where space is limited, because they consume very little surface land and have few site restrictions. Sand filters, however, have limited or no runoff reduction capability, so designers should consider using upgradient runoff reduction practices, which can also decrease the treatment volume (and size) of filtering practices.  Filtering practices are also suitable to provide special treatment at a designated stormwater hotspot. For a list of potential stormwater hotspots that merit treatment by filtering practices, consult the Infiltration Design Specification (No. 8).

Stormwater filters depend mainly on physical treatment mechanisms to remove pollutants from stormwater runoff including gravitational settling in the sedimentation chamber, straining at the top of the filter bed, and filtering and adsorption onto the filter media. Microbial films often form on the surface of the filter bed which can also enhance biological removal. Filters are usually designed only for water quality treatment.
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Table 1: Summary of Stormwater Functions Provided by Filtering Practices
Stormwater Function	Level 1 Design	Level 2 Design
Annual Runoff Reduction	0%	0%
Total Phosphorus Removal 1	60%	65%
Total Nitrogen Removal 1	30%	45%
Channel Protection	Limited. Treatment volume diverted off-line into a storage facility for treatment can be utilized in a Curve Number Adjustment per guidance.
Flood Mitigation	None. Most filtering practices are off-line and do not materially change peak discharges
1 Change in event mean concentration (EMC) through the practice. Actual nutrient mass load removed is the product of the removal rate and the runoff reduction rate. Sources: CWP and CSN (2008), CWP, 2007

SECTION 2: LEVEL 1 AND 2 DESIGN TABLES

The major design goal is to maximize nutrient removal. To this end, designers may choose to go with the baseline design (Level 1) or choose an enhanced Level 2 design that maximizes nutrient removal. To qualify for Level 2, the filter must meet all design criteria shown in the right hand column of Table 2.

SECTION 3. TYPICAL DETAILS

Figures 1 and 2 provide typical schematics for a surface sand filter and organic filter, respectively.

SECTION 4:  PHYSICAL FEASIBILITY & DESIGN APPLICATIONS

Stormwater filters can be applied to most types of urban land, although they are not always cost-effective, given their high unit cost and small area served. Design constraints for filtering practices include:

• Available Head: The principal design constraint for stormwater filters is available hydraulic head which is defined as the vertical distance between the top elevation of the filter and the bottom elevation of the existing storm drain system that accepts its runoff. The head required for stormwater filters ranges from 2 to 10 feet, depending on the design variant. Thus, it is difficult to employ filters in extremely flat terrain since they require gravity flow through the filter. The one exception is the perimeter sand filter, which can be applied at sites with as little as 2 feet of head.
• Depth to Water Table and Bedrock:The standard separation distance of at least 2 feet between for the seasonally high groundwater table and/or bedrock layer and the bottom invert of the filtering practice must be maintained.
• Contributing Drainage Area:Sand filters are best applied on small sites that are as close to 100% impervious as possible. A maximum contributing drainage area of 5 acres is recommended for surface sand filters, and a maximum contributing drainage area of 2 acres is recommended for perimeter or underground filters. Filters have been used on larger drainage areas in the past, but these tend to experience greater clogging problems.
• Space Required: The amount of space required for a filter practice depends on the design variant selected. Both sand and organic surface filters typically consume about 2 to 3% of the contributing drainage area, while perimeter sand filters typically consume less than 1%. Underground stormwater filters generally consume no surface area except their manholes.

Filters are particularly well suited to treat runoff from stormwater hotspots and smaller parking lots. Other applications include redevelopment of commercial sites or when existing parking lots are renovated or expanded. Filters can work on most commercial, industrial, institutional or municipal sites and can be located underground if surface area is not available

There are several design variations of the basic sand filter that enable designers to use a filter at challenging sites or to improve pollutant removal rates. The most common design variants include the following:

Non-Structural Sand Filter: The nonstructural sand filter is applied to sites less than 2 acres in size, and is essentially the same as a bioretention basin (see Design Specification No. 9), with the following exceptions:  

• The bottom is lined with an impermeable filter fabric and always has an underdrain
• The surface cover is sand, turf or pea gravel
• The filter media is 100% sand,
• The filter is not planted with trees, shrubs or herbaceous materials
• The filter has two cells, with a dry or wet sedimentation chamber preceding the sand filter bed.

The non-structural sand filter is the least expensive filter option for treating hotspot runoff. The use of bioretention areas is generally preferred at most other sites.

• Surface Sand Filter:The surface sand filter is designed with both the filter bed and sediment chamber located at ground level. In most cases, the filter chambers are created using pre-cast or cast in place concrete. Surface sand filters are normally designed off-line so that only the desired water quality or runoff reduction volume is directed to the filter for treatment, although in some cases they can be installed on the bottom of a dry extended detention (ED) pond (see Figure 1  and Stormwater Design Specification No. 15).
• Organic Media Filter: Organic media filters are essentially the same as surface filters, but the sand is replaced with an organic filtering medium. Two notable examples are the peat/sand filter and the compost filter system. Organic filters achieve higher pollutant removal for metals and hydrocarbons due to the increased cation exchange capacity of the organic media.
• Underground Sand Filter: The underground sand filter is modified to install the filtering components underground and is often designed with an internal flow splitter or overflow device that bypasses runoff from larger stormwater events around the filter .Underground sand filters are expensive to construct, but consume very little space and are well suited to ultra-urban areas (Figure 4).
• Perimeter Sand Filter: The perimeter sand filter also includes the basic design elements of a sediment chamber and a filter bed. In this design, however, flow enters the system through grates, usually at the edge of a parking lot. The perimeter sand filter is usually located on-line, with all flows entering the system, but larger events bypass treatment by entering an overflow chamber. One major advantage to the perimeter sand filter design is that it requires little hydraulic head and is therefore a good option for sites with low relief.
• Proprietary Filters: Proprietary filters utilize numerous filtering media and geometry to achieve filtration within a packaged structure. In some cases, these systems can provide excellent targeting of specific pollutants; However, designers must verify that the particular product has been reviewed and accepted by the Virginia BMP Clearinghouse: http://www.vwrrc.vt.edu/swc/

 

Figure 3: Hybrid Sand Filter in Detention Pond

5.1. Overall Sizing

Filtration devices are sized to accommodate a specified treatment volume.  The volume to be treated by the device is a function of the storage depth above the filter and the surface area of the filter itself.  The storage volume is the volume of ponding above the filter.  For a given treatment volume, Equation 1 is used to determine the required filter surface area:

(1)  Minimum Filter Surface Area for Filtering Practices

where:
Af   = area of the filter surface (ft2)
TV = Treatment Volume, volume of storage (ft3)
df   = Filter media depth (thickness), minimum 1 ft. (ft)
K   = Coefficient of permeability = 3.5 ft/day
hf   =  Average height of water above the filter bed, maximum 5ft/2 (ft)
tf    = Allowable drawdown time, 1.67 days

The coefficient of permeability (ft/day) is considered to be the condition of the sand media at the point in its operational life where it is in need of replacement or maintenance. Filtering practices are therefore sized to function within the desired constraints at the end of the media’s operational life cycle.

A storage volume of a least 75% of the design treatment volume, including the volume over top of the filter media, and the pretreatment chamber(s), as well as any additional storage is required in order to hold the volume prior to filtration. This reduced volume takes into account the varying filtration rate of the water through the media as a function of a gradually declining hydraulic head, 

(2)  Required Treatment Volume Storage for Filtering Practices

where:
Vs    = Volume of storage (ft3)
TV  =  Treatment Volume (ft3)

5.2. Soil Testing Requirements

At least one soil boring shall be taken at a low point within the footprint of the proposed filtering practice to establish water table and bedrock elevations and evaluate soil suitability.

5.3. Pretreatment

Adequate pretreatment is needed to prevent premature filter clogging and ensure filter longevity.

• Sedimentation chambers are typically used for pretreatment to capture coarse sediment particles before they reach the filter bed.
• Sedimentation chambers may be wet or dry but must comprise at least 25% of the total water quality or treatment volume (inclusive).
• Non-structural sand filters may use alternative pretreatment measures such as grass filter strip/check dam/level spreader combination. The filter strip must be a minimum length of 15 feet, have a slope of 3% or less, and contain compost amended soils (see Stormwater Design Specification No. 4). The check dam may be wooden or concrete, and may be installed so that it extends only two inches above the filter strip and has lateral slots to allow runoff to be evenly distributed across the filter.
• If proprietary devices are used for pretreatment, designers must confirm through the VA BMP Clearinghouse that the practice has capacity to effectively trap and retain particles down to 20 microns in size for the design flow rate.
• Sediment chambers should be designed as level spreaders such that inflows to the sand filter bed have near zero velocity and spread runoff evenly across the bed

5.4. Conveyance and Overflow

Most filtering practices are designed as off-line systems so that all flows enter the filter storage chamber until it reaches capacity, larger flows are then diverted or bypassed around the filter  an outlet chamber, and are not treated. Runoff from larger storm events should be bypassed using an overflow structure or a flow splitter. Claytor and Schueler (1996) and ARC (2001) provide design guidance for flow splitters for filtering practices.

Some underground filters will be constructed on-line. In these cases, designers must indicate how the device will safely pass the local design storm (e.g., 10 year event) without resuspending or flushing previously trapped material.

All stormwater filters should be designed to drain or dewater within 40 hours after a storm event to reduce the potential for nuisance conditions

Stormwater filters are normally designed with an impermeable liner and underdrain system following the minimum specifications outlined in Table 4.

5.5. Filter Media and Surface Cover

Type of Media: The normal filter media consists of clean, washed concrete sand with individual grains between 0.02 and 0.04 inches in diameter. Alternatively, organic media can be used, such as a peat/sand mixture or a leaf compost mixture. The decision to use organic media in a stormwater filter depends on which stormwater pollutants are targeted for removal. Organic media may enhance pollutant removal performance with respect to metals and hydrocarbons (Claytor and Schueler, 1996). Recent research, however, has shown that organic media can actually leach soluble nitrate and phosphorus, suggesting it is a poor choice when nutrients are the pollutant of concern.

• Type of Filter: The choice of which sand filter design to apply depends on available space and head, and the level of pollutant removal desired. In ultra-urban situations where surface space is at a premium, underground sand filters are often the only design that can be used. Surface and perimeter filters are often a more economical choice when adequate surface area is available.

• Surface Cover:The surface cover for structural and non-structural surface sand filters should consist of a three inch layer of topsoil, on top of a non-woven filter fabric above the sand layer. The surface may also have pea gravel inlets in the topsoil layer to promote filtration, The pea gravel may be located where sheet flow enters the filter, around the margins of the filter bed, or at locations in the middle of the filter bed

• Underground sand filters should have a pea gravel layer on top of a coarse non-woven fabric over the sand layer. The pea-gravel helps to prevent bio-fouling or blinding of the sand surface. The fabric serves to facilitate removing the gravel during maintenance operations.

• Depth of Media: The depth of the filter media plays a role in how quickly stormwater moves through the filter bed and how well it removes pollutants. Recent design guidance recommends that a minimum filter bed depth ranging from 12 to 18 inches. Greater depths can be used in order to facilitate the removal of 1 to 3 inches of sand during maintenance, withough having to necessarily replace  

• Impervious Drainage Area: The contributing drainage area should be as close to 100% impervious as possible in order to reduce the risk that eroded sediments will clog the filter.

5.6. Maintenance Reduction Features

Several maintenance issues should be addressed during filter design to reduce future maintenance problems, including the following:

• Observation Wells and Cleanouts: Surface sand filters should include an observation well consisting of a six-inch diameter non-perforated PVC pipe fitted with a lockable cap. It should be installed flush with the ground surface to facilitate periodic inspection and maintenance. In most cases, the cleanout pipe will be tied into the end of all underdrain pipe runs. The portion of the cleanout pipe/observation well in the underdrain layer should be perforated. At least one cleanout pipe shall be provided every 2000 square feet of filter surface area

• Access: Good maintenance access is needed to allow crews to perform regular inspections and maintenance activities. This is operationally defined as the ability to get a vacuum truck or similar equipment close enough to the sedimentation chamber and filter to enable cleanouts.

• Manhole Access (Underground filters):Access to the headbox and clearwell of underground filters shall be provided by at least 30 inch diameter manholes, along with steps to the areas where maintenance will occur.

• Visibility: Stormwater filters should be clearly visible at the site so inspectors and maintenance crews can easily find them. Adequate signs or markings should be provided at manhole access points for underground filters.

• Confined Space Issues: Underground filters are often classified as an underground confined space. Consequently, special OSHA rules and training are needed to protect the workers that access them. These procedures often involve training on confined space entry, venting and the use of gas probes. 

5.7. Filtering Material Specifications   

The basic material specifications for filtering practices are outlined in Table 3. 

 

SECTION 6: REGIONAL AND SPECIAL CASE DESIGN ADAPTATIONS

6.1. Karst Terrain  

Stormwater filters are a good option in active karst areas since they are not connected to groundwater and therefore minimize the risk of sinkhole formation and groundwater contamination. Construction inspection should certify that the filters are indeed water tight, and that excavation will not extend into an active karst layer

6.2. Coastal Plain

The flat terrain, low head and high water table of the coastal plain make several filter designs difficult. The perimeter sand filter and the non-structural sand filter generally have low head requirements and can work effectively at many small coastal plain sites.

• The combined depth of the underdrain and sand filter bed can be reduced to 24 to 30 inches.
• Designers may wish to maximize the length of the stormwater filter or provide treatment in multiple connected cells.
• The minimum depth to the seasonally high water table may be relaxed to 1 foot, as long as the filter is equipped with an large diameter underdrain (e.g., 6 inches) that is only partially efficient at dewatering the bed
• It is important to maintain at least a 0.5% slope in the underdrain to ensure drainage and to tie it into the ditch or conveyance system

6.3. Steep Terrain 

The gradient of contributing slopes to sand filters can be increased to 15% in areas of steep terrain, as long as a two cell, terraced design is used to dissipate erosive energy prior to filtering. The drop in slope between cells should be limited to a foot, be armored with river stone or suitable equivalent.

6.4. Winter Performance:

Surface or perimeter filters may not always be effective during the winter months. The main problem is ice that forms over and within the filter bed. Ice formation may briefly cause nuisance flooding if the filter bed is still frozen when spring melt occurs. To avoid these problems, filters should be inspected before the onset of winter (prior to the first freeze) to dewater wet chambers and scarify the filter surface. Other measures to improve winter performance include the following:

• Place a weir placed between the pretreatment chamber and filter bed to reduce ice formation as a more effective substitute than a traditional standpipe orifice.
• Extend the filter bed below the frost line to prevent freezing within the filter bed
• Oversize the underdrain to encourage more rapid drainage and to minimize freezing of the filter bed
• Expand the sediment chamber to account for road sanding. Pretreatment chambers should be sized for up to 40% of the water quality or treatment volume.

6.5. Linear Highway Sites

Non-structural sand filters are a preferred practice for constrained highway right of ways when designed as a series of individual on-line or off-line cells. In these situations, the final design closely resembles that of dry swales. Salt tolerant grass species should be selected if the contributing roadway will be salted in the winter

SECTION 7: FILTER PRACTICE CONSTRUCTION SEQUENCE AND INSPECTION

7.1. Construction Sequence

The following is a typical construction sequence to properly install a structural sand filter. This sequence can be modified to reflect different filter designs, site conditions, and the size, complexity and configuration of the proposed filtering application.

Step 1. Use of Filtering Practices as an E&S Control. The future location of a filtering practice can be used as a temporary sediment basin or trap during site construction, with design elevations should be set with final cleanout and conversion in mind. The bottom elevation of the filtering practice should be lower than the bottom elevation of the temporary sediment basin. Appropriate procedures should be implemented to prevent discharge of turbid waters when the temporary basin is converted to a filtering practice

Step 2. Stabilize Drainage Area. Filtering practices should only be constructed after the contributing drainage area to the pond is completely stabilized so that sediment from these areas does not flow into and clog the filter.  If the proposed filtering area is used as a sediment trap or basin during the construction phase, the construction notes should clearly indicate that the facility will be dewatered, dredged and re-graded to design dimensions after construction is complete.

Step 3. Install filtering practice E&S controls.  Stormwater should be diverted around the filter practices as they are being constructed. This is usually not difficult to accomplish for off-line filter practices. It is extremely important to keep runoff and eroded sediments away from the sand filter throughout the construction process. Silt fence or other sediment controls should be installed around the perimeter of the sand filter, and erosion control fabric may be needed during construction on exposed side-slopes with gradients exceeding 4: 1. Exposed soils in the vicinity of the filtering practice should be rapidly stabilized by hydro-seed, sod, mulch or other locally approved method of soil stabilization. 

Step 4. Assemble construction materials on-site, make sure they meet design specifications and prepare any staging areas

Step 5. Clear and strip the project area to the desired sub-grade

Step 6. Excavate/grade until the appropriate elevation for the bottom of the filtering practice is reached and desired contours and acceptable side slopes are achieved.

Step 7. Install the filter structure and check all design elevations (concrete vaults for surface, underground and perimeter sand filters). Upon completion of the filter structure shell, inlets and outlets should be temporarily plugged and the structure filled with water to the brim to demonstrate water tightness. Maximum allowable leakage is 5% of the water volume in a 24 hour period. If the structure fails the test, repairs shall be performed to make the structure watertight before and sand is placed into it.

Step 8. Install the gravel, underdrains, and choker layer to the filter

Step 9. Spread sand across the filter bed in 1 foot lifts up to the design elevation. Backhoes or other equipment can deliver the sand from outside the sand filter structure. Sand should be manually raked. Clean water is then added until the sedimentation chamber and filter bed are completely full. The facility is then allowed to drawdown, hydraulically compacting the sand layers. After 48 hours of drying, refill the structure to the final top elevation of the sand filter bed.

Step 10. Install the permeable filter fabric over the sand, add a 3-inch topsoil layer and pea gravel inlets, and immediately seed with the permanent grass species. The grass should be watered, and the facility should not be turned on-line until a vigorous grass cover has become established

Step 11. Stabilize any exposed soils on the perimeter of the structure with temporary seed mixtures appropriate for the pond buffer. All areas above the normal pool should be permanently stabilized by hydroseeding or seeding over straw.

Step 12. Conduct final construction inspection (see Section 9.2).

7.2. Construction Inspection

Multiple construction inspections are critical to ensure that stormwater filters are properly constructed. Inspections are recommended during the following stages of construction: 

• Pre-construction meeting
• Initial site preparation (including installation of project E&S controls)
• Excavation/grading to design elevations
• Installation the filter structure including the watertight test
• Installation of the underdrain and sand filter bed
• Check off that turf cover is vigorous enough to turn on the facility

• Final Inspection (after a rainfall event to ensure that it drains properly, all pipe connections are watertight, develop a punch list for facility acceptance and login the GPS filter coordinates to enter into the local maintenance tracking database)

.A construction inspection form for filtering practices can be accessed at CWP website at: www.cwp.org/postconstruction (Tool #6).

SECTION 8: FILTERING PRACTICE MAINTENANCE

8.1. Maintenance Agreements

Section 4VAC 50-60-124 of the regulations specifies a maintenance agreement to be executed between the owner and the local program.  The section requires a schedule of inspections, compliance procedures if maintenance is neglected, notification of the local program upon transfer of ownership, and right-of-entry for local program personnel.    

8.2. Maintenance Inspections 

Regular inspections are critical to schedule sediment removal operations, replace filter media and relieve any surface clogging. Frequent inspections are especially needed for underground and perimeter filters since they are out of sight and can be easily forgotten. Depending on the level of traffic or the particular land uses, a filter system may become clogged within a few months of normal rainfall, or could possibly last several years with only routine maintenance. Maintenance inspections should be conducted within 24 hours of a rainfall event that exceeds 0.5 inch to evaluate the condition and performance of the filtering practice (see Table 4). The results of the inspection will then determine the level of maintenance required (routine or major; see Table 5) An annual maintenance inspection form for filtering practices can be accessed at CWP website at: www.cwp.org/postconstruction (Tool #6). 

8.3. Routine Maintenance Tasks

A cleanup should be scheduled at least once a year to remove trash and floatables that accumulate in the pretreatment cells and filter bed. Frequent sediment cleanouts in the dry and wet sedimentation chambers are recommended every 2-3 years to maintain the function and performance of the filter. If the filter treats runoff from a stormwater hotspot, crews may need to test sediments before disposing of trapped sediments or filter bed media. Sediment testing is not needed if the filter does not receive runoff from a designated stormwater hotspot, and can be safely disposed by either land application or land filling.

SECTION 9: COMMUNITY AND ENVIRONMENTAL CONCERNS

Stormwater filters have a few community and environmental concerns. Their main drawback is their appearance - many filtering practices are imposing concrete boxes that tend to accumulate a lot of trash and debris. Designers should focus on aesthetics to make sure they are integrated into the landscape. There is a small risk that underground and perimeter filters may create a potential habitat for mosquitoes to breed. If this is a community concern, designers should shift to dry rather than wet sedimentation chambers.

 SECTIO 10: DESIGN REFERENCES

Claytor, R. and T. Schueler. 1996. Design of Stormwater Filtering Systems. Chesapeake Research Consortium and Center for Watershed Protection. Ellicott City, MD  http://www.cwp.org/PublicationStore/special.htm

CWP. 2007. National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection, Ellicott City, MD.

District of Columbia Stormwater Management Guidebook
http://dchealth.dc.gov/DOH/site/default.asp?dohNav=|33110|

Maryland Department of Environment. Maryland Stormwater Design Manual
http://www.mde.state.md.us/Programs/WaterPrograms/SedimentandStormwater/stormwater_design/index.asp

Minnesota Stormwater Steering Committee (MSSC). 2005. The Minnesota Stormwater Manual.
http://www.pca.state.mn.us/water/stormwater/stormwater-manual.html

Northern Virginia Regional Commission. 2007. Low Impact Development Supplement to the Northern Virginia BMP Handbook. Fairfax, Virginia

Schueler et al  2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection. Ellicott City, MD.

Schueler, T. 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD www.chesapeakestormwater.net