DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 12: FILTERING PRACTICES
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 12
FILTERING PRACTICES
VERSION 1.5
Note to Reviewers of the Stormwater Design Specifications
The Virginia Department of Conservation and Recreation (DCR) has developed an updated set of non-proprietary BMP standards and specifications for use in complying with the Virginia Stormwater Management Law and Regulations. These standards and specifications were developed with assistance from the Chesapeake Stormwater Network (CSN), Center for Watershed Protection (CWP), Northern Virginia Regional Commission (NVRC), and the Engineers and Surveyors Institute (ESI) of Northern Virginia. These standards and specifications are based on both the traditional BMPs and Low Impact Development (LID) practices. The advancements in these standards and specifications are a result of extensive reviews of BMP research studies incorporated into the CWP's National Pollution Removal Performance Database (NPRPD). In addition, we have borrowed from BMP standards and specifications from other states and research universities in the region. Table 1 describes the overall organization and status of the proposed design specifications under development by DCR.
Table 1: Organization and Status of Proposed DCR Stormwater Design Specifications:Status as of 9/24/2008
# / Practice / Notes / Status 1
1 / Rooftop Disconnection / Includes front-yard bioretention / 2
2 / Filter Strips / Includes grass and conservation filter strips / 2
3 / Grass Channels / 2
4 / Soil Compost Amendments / 3
5 / Green Roofs / 1
6 / Rain Tanks / Includes cisterns / 2
7 / Permeable Pavement / 1
8 / Infiltration / Includes micro- small scale and conventional infiltration techniques / 2
9 / Bioretention / Includes urban bioretention / 3
10 / Dry Swales / 2
11 / OPEN
12 / Filtering Practices / 2
13 / Constructed Wetlands / Includes wet swales / 2
14 / Wet Ponds / 2
15 / ED Ponds / 2
1 Codes: 1= partial draft of design spec; 2 = complete draft of design spec;
3 = Design specification has undergone one round of external peer review as of 9/24/08
Reviewers should be aware that these draft standards and specifications are just the beginning of the process. Over the coming months, they will be extensively peer-reviewed to develop standards and specifications that can boost performance, increase longevity, reduce the maintenance burden, create attractive amenities, and drive down the unit cost of the treatment provided.
Timeline for review and adoption of specifications and Role of the Virginia’s Stormwater BMP Clearinghouse Committee:
The CSN will be soliciting input and comment on each standard and specification until the end of 2008 from the research, design and plan review community. This feedback will ensure that these design standards strike the right balance between prescription and flexibility, and that they work effectively in each physiographic region. The collective feedback will be presented to the BMP Clearinghouse Committee to help complement their review efforts. The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR.
The revisions to the Virginia Stormwater Management Regulations are not expected to become finalized until late 2009. The DCR intends that these stormwater BMP standards and specifications will be finalized by the time the regulations become final.
The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR, which is vested by the Code of Virginia with the authority to determine what practices are acceptable for use in Virginia to manage stormwater runoff.
As with any draft, there are several key caveats, as outlined below:
· Many of the proposed design standards and specifications lack graphics. Graphics will be produced in the coming months, and some of graphics will be imported from the DCR 1999 Virginia Stormwater Management (SWM) Handbook. The design graphics shown in this current version are meant to be illustrative. Where there are differences between the schematic and the text, the text should be considered the recommended approach.
· There are some inconsistencies in the material specifications for stone, pea gravel and filter fabric between ASTM, VDOT and the DCR 1999 SWM Handbook. These inconsistencies will be rectified in subsequent versions.
· While the DCR 1999 SWM Handbook was used as the initial foundation for these draft standards and specifications, additional side-by-side comparison will be conducted to ensure continuity.
· Other inconsistencies may exist regarding the specified setbacks from buildings, roads, septic systems, water supply wells and public infrastructure. These setbacks can be extremely important, and local plan reviewers should provide input to ensure that they strike the appropriate balance between risk aversion and practice feasibility.
These practice specifications will be posted in Wikipedia fashion for comment on the Chesapeake Stormwater Network’s web site at http://www.chesapeakestormwater.net, with instructions regarding how to submit comments, answers to remaining questions about the practice, useful graphics, etc. DCR requests that you provide an email copy of your comments, etc., to Scott Crafton (). The final version will provide appropriate credit and attribution on the sources from which photos, schematics, figures, and text were derived.
Thank you for your help in producing the best stormwater design specifications for the Commonwealth.
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 12
FILTERING PRACTICES
VERSION 1.5
SECTION 1: DESCRIPTION OF PRACTICE
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 sand or an organic filter media.
Because they consume very little surface land area and have few site restrictions, stormwater filters are a versatile option that offers moderate pollutant removal performance at small sites where space is limited. Sand filters, however, have limited or no runoff reduction capability. Therefore, filtering practices should only be considered after all upland opportunities to install runoff reduction practices have been exhausted, or special treatment is needed 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.
SECTION 2: PERFORMANCE CRITERIA
Table 1: Summary of Stormwater Functions Provided by Filtering PracticesStormwater 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 can be subtracted from the CPv.
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 and will be higher than these percentages, as calculated using the Runoff Reduction Spreadsheet Methodology.
Sources: CSN (2008), CWP, 2007
SECTION 3: PRACTICE APPLICATIONS AND FEASIBILITY
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 the following:
Available Hydraulic 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 2-10 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 two 2 feet of head.
Depth to Water Table and Bedrock: The standard separation distance of at least 2 feet between 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 2-3% of the contributing drainage area, while perimeter sand filters typically consume less than 1%. Underground stormwater filters generally consume no surface area except for their manholes.
SECTION 4: ENVIRONMENTAL AND COMMUNITY CONSIDERATIONS
Stormwater filters have 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.
SECTION 5: DESIGN APPLICATIONS AND VARIATIONS
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 filters 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 Tv is directed to the filter for treatment, although in some cases they can be installed on the bottom of a dry Extended Detention Pond (see Wet Pond Design Specification, No. 14).
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.
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.
Figure 1. A Hybrid Filter
SECTION 6: SIZING AND TESTING GUIDANCE
6.1: Overall Sizing
Filtration devices are sized to accommodate a specified water quality 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 storage volume.
(1) Required Treatment Storage Volume for Filter Practices
Vs = VWQ - [(k)(A)(t)] - [(Qu)(t)]
where
Vs = the volume of storage (cu. ft.)
VWQ = the water quality volume (cu. ft.)
k = the infiltration rate (ft./day)
A = the area of the filter surface (sq. ft.)
t = the drawdown time (days)
Qu = the Outflow through the underdrain (cu. ft./day)
The above-ground storage volume must drain through the filter section in a maximum of 24 hours. Equation 2 computes the drain time for the filter device. In this equation, assume that the rainfall event has ended and the ponding depth is at the maximum elevation prior to the initiation of drawdown. The rate of drawdown is typically limited by the media rather than the underdrain system. For this reason, the infiltration rate (k) for the filter media should be used for the variable k in Equation 2, to estimate the above ground drain time.