Stormwater Management Fact Sheet: Sand and Organic Filter

Description

Sand filters are usually two-chambered stormwater treatment practices; the first chamber is for settling, and the second is a filter bed filled with sand or another filtering media. As stormwater flows into the first chamber, large particles settle out, and the finer particles and other pollutants are removed as stormwater flows through filtering media. There are several modifications of the basic sand filter design, including the surface sand filter, underground sand filter, perimeter sand filter, organic media filer, and the Multi-Chamber Treatment Train (MCTT). All of these filtering practices operate on the same basic principle. Modifications to the traditional surface sand filter were made primarily to fit sand filters into more challenging design sites (e.g., underground and perimeter filters) or to improve pollutant removal (e.g., organic media filter). For a detailed review of these practices, see Developments in Sand Filter Technology to Treat Stormwater Runoff, Article 105 and Further Developments in Sand Filter Technology, Article 106 in The Practice of Watershed Protection.

Applicability

Sand filters can be applied in most regions of the country, and on most types of sites. There are some site restrictions, however, that limit their use as a stormwater management practice (see Siting and Design Considerations).

Regional Applicability
Although sand filters can be used in both cold and arid climates, several design modifications may be necessary (see Siting and Design Considerations).

Ultra Urban Areas
Ultra urban areas are densely developed urban areas in which little pervious surface exists. Sand filters, in general, are good options for these areas because they consume little space. Underground and perimeter sand filters are particularly are well suited for ultra urban watersheds as they consume no surface space.

Stormwater Hotspots
Stormwater hotspots are land uses or activities that generate highly contaminated runoff (i.e., pollutant concentrations that exceed those typically found in "normal" stormwater. These areas include: commercial nurseries, auto recycle facilities, commercial parking lots, fueling stations, feet storage areas, industrial rooftops, marinas, outdoor container storage of liquids, outdoor loading/unloading facilities, public works storage areas, hazardous materials generators (if containers are exposed to rainfall), vehicle service and maintenance areas, and vehicle and equipment washing/steam cleaning facilities. Sand filters are an excellent option to treat runoff from stormwater hotspots because stormwater treated by sand filters has no interaction with, and thus no potential to contaminate groundwater. For more information see Multi-Chamber Treatment Train Developed for Stormwater Hot Spots, Article 111 in The Practice of Watershed Protection.

Stormwater Retrofit
A stormwater retrofit is a stormwater treatment practice installed after development has occurred, to improve water quality, protect downstream channels, reduce flooding, or meet other watershed restoration objectives. Sand filters are a good option to achieve water quality goals in retrofit situations where space is limited, because they consume very little surface space and have few site restrictions. It is important to note, however, that sand filters cannot treat a very large drainage area. Using a small site practice to retrofit an individual site may be the only option, but it is expensive to treat an entire watershed with many small practices (compared to using larger practices such as a pond).

Cold Water (Trout) Streams
The species in cold water streams, notably trout, are extremely sensitive to changes in temperature. In order to protect these resources, designers should avoid treatment practices that increase the temperature of the runoff they treat. Sand filters can be a good treatment option for cold water streams. Surface sand filters are typically not designed with a permanent pool, although there may be temporary ponding in the sedimentation chamber and above the sand filter. Designers may consider shortening the detention time in cold water watersheds. Underground and perimeter sand filter designs have little potential for warming because they are practice is not exposed to the sun.

Siting and Design Considerations

Designers need to carefully consider conditions at the site level before using a sand or organic filter, and should incorporate design features to improve the longevity and performance, as well as minimizing their maintenance burden.

Siting Considerations
Important considerations when selecting a sand or organic filter are the drainage area that needs treatment, the slopes at the location of the filter and draining to it, soil and subsurface conditions, and the depth of the seasonably high groundwater table. Although sand filters are highly versatile, some constraints, such as available head may limit their use at individual sites.

Drainage Area

Sand filters are best applied on small sites (maximum of 5 acres for surface sand filters, and maximum of 2 acres for perimeter or underground filters) (CWP, 1998). Filters have been used on larger drainage areas in the past (up to one hundred acres), but these systems often clog.

Slope

Sand filters can be used on sites with up to about 6% slopes. It is difficult to use sand filters in extremely flat terrain, as they require a significant drop in elevation (or head, about five to 8 feet) to allow runoff flow through the filter. One exception is the perimeter sand filter, which can be applied with as little as 2 feet of head.

Soils/Topography

Sand filters can be used on almost any soil, because they rely on a "made" soil for runoff treatment. They can be designed so that stormwater never infiltrates into the soil or interacts with the groundwater.

Groundwater

Designers should provide at least two feet of separation between the bottom of the filter and the seasonally high groundwater table. This prevents both structural damage to the filter and possible, groundwater contamination.

Design Considerations
Specific filter designs vary considerably, depending on site constraints or preferences of the designer or community. There are some features, however, that should be incorporated into most designs. These design features can be divided into five basic categories: pretreatment, treatment, conveyance, maintenance reduction, and landscaping (for more information see the Manual Builder Category for more information).

Pretreatment

Pretreatment is a critical component of any stormwater treatment practice. In sand filters, pretreatment is achieved by a sedimentation chamber that precedes the filter bed. In this chamber, the coarsest sediment particles settle out before they reach the filter bed. Pretreatment reduces the maintenance burden of sand filters by reducing the potential for clogging of the filter bed surface. Designers should provide at least 25% of the water quality volume in a dry or wet sedimentation chamber as pretreatment. The water quality volume is the amount of runoff that will be treated for pollutant removal in the practice. Typical water quality volumes are the runoff from a 1" storm or �" of runoff over the entire drainage area to the practice.

Treatment

Treatment design features help enhance the ability of a stormwater treatment practice to remove pollutants. In filters, designers should incorporate the following features to improve the treatment ability of a practice. In filtering systems, designers should provide at least 75% of the water quality volume in the practice (including both the sand chamber and the sediment chamber). In sand filters, designers should select a medium sand as the filtering medium.

Conveyance

Conveyance of stormwater runoff into and through a stormwater treatment practice is a critical element of design. Stormwater should be conveyed to and from practices safely and to minimize erosion potential.

Typically, filtering practices are designed as "off-line" systems, meaning that they only have the smaller water quality volume diverted to them during larger storms, using a flow splitter, which is a structure that bypasses larger flows to the storm drain system, or to a stabilized channel. One exception is the perimeter filter; in this design, all flows enter the system, but larger flows overflow to an outlet chamber, and are not treated by the practice.

Most filtering practices, are designed with an underdrain below the filtering bed. An underdrain is a perforated pipe system in a gravel bed, installed on the bottom of filtering practices, and used to collect and remove filtered runoff.

Maintenance Reduction

In addition to regular maintenance activities needed to maintain the function of stormwater practices, some design features can be incorporated to ease the maintenance burden of each practice. Designers should provide maintenance access to the filtering system. In underground sand filters, OSHA confined space rules need to be addressed.

Landscaping

Landscaping can add to both the aesthetic value and treatment ability of stormwater practices. In sand filters, little landscaping is generally used on the practice although surface sand filters and organic media filters may be designed with a grass cover on the surface of the filter. In all filters, designers need to ensure that the contributing drainage has dense vegetation to reduce sediment loads to the practice.

Design Variations
As mentioned earlier in this fact sheet, there are four basic stormwater filter designs, including: surface sand filter, underground filter, perimeter filter (also known as the "Delaware" filter), and the organic media filter. Other design variations can incorporate design features to recharge groundwater, or meet the design challenges of cold or arid climates. For more information, see Developments in Sand Filter Technology to Treat Stormwater Runoff, Article 105 and Stormwater Strategies for Arid and Semiarid Watersheds, Article 381 in The Practice of Watershed Protection.

Surface Sand Filter

The surface sand filter is the original sand filter design. In this practice both the filter bed and the sediment chamber are above ground. The surface sand filter is designed as an off-line practice, where only the water quality volume is directed to the filter. The surface sand filter is the least expensive filter option, and has been the most widely used (see Figure 1).

Underground Sand Filter

The underground sand filter is a modification of the surface sand filter, where all of the filter components are underground. Like the surface sand filter, this practice is an off-line system that receives only the smaller water quality events. Underground sand filters are expensive to construct, but consume very little space. They are well suited to highly urbanized areas (see Figure 2).

Perimeter Sand Filter

The perimeter 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 the only filtering option that is on-line, with all flows entering the system, but larger events bypassing treatment by entering an overflow chamber. One major advantage to the perimeter sand filter design is that it requires little hydraulic head, and thus is a good option in areas of low relief (see Figure 3).

Organic Media Filter

Organic media filters are essentially the same as surface filters, with the sand media replaced with or supplemented with another medium. Two examples are the peat/sand filter (Galli, 1990), and the compost filter system (CSF, 1996). The assumption is that these systems will have enhanced pollutant removal for many compounds due to the increased cation exchange capacity achieved by increasing the organic matter. For more information see, Performance of Delaware Sand Filter Assessed, Article 107 in The Practice of Watershed Protection (see Figure 4).

Multi-Chamber Treatment Train

The Multi-Chamber Treatment Train (Pitt et al., 1997) is essentially a "deluxe sand filter." This underground system consists of three chambers. Stormwater enters into the first chamber where screening occurs, trapping large sediments and releasing highly volatile materials. The second chamber provides settling of fine sediments and further removal of volatile compounds and also floatable hydrocarbons through the use of fine bubble diffusers and sorbent pads. The final chamber provides filtration by using a sand and peat mixed media for reduction of the remaining toxicants. The top of the filter is covered by a filter fabric that evenly distributes the water volume and prevents channelization. While this practice can achieve very high pollutant removal rates, it may be prohibitively expensive in many areas, and has only been implemented on an experimental basis.

Regional Variations

Arid Climates

Filters have not been widely used in arid climates. In these climates, however, it is probably necessary to increase storage in the sediment chamber to account for the high sediment loads in these regions. Designers should consider increasing the volume of sediment chamber to up to 40% of the water quality volume.

Cold Climates

In cold climates, filters can be used, but surface or perimeter filters will not be effective during the winter months, and may have unintended consequences from a frozen filter bed. Using alternative conveyance measures such as a weir system between the sediment chamber and filter bed may avoid freezing associated with the traditional standpipe. Where possible, the filter bed should be below the frost line. Some filters, such as the peat sand filter, should be shut down during the winter. These media will become completely impervious during freezing conditions. Using a larger underdrain system to encourage rapid drainage during the winter months may prevent freezing of the filter bed. Finally, the sediment chamber should be larger in cold climates to account for road sanding (up to 40% of the water quality volume).

Limitations

Sand filters can be used in unique conditions where many other stormwater management practices are inappropriate, such as in karst topography, or in highly urbanized settings. There are several limitations to these practices, however. Sand filters cannot control floods, and generally are not designed to protect stream channels from erosion or recharge the groundwater. In addition, sand filters require frequent maintenance, and underground and perimeter versions of these practices are easily forgotten because they are out of sight. Perhaps one of the greatest limitations to sand filters is that they cannot be used to treat large drainage areas. Finally, surface sand filters are generally not aesthetically pleasing management practices. Underground and perimeter sand filters are not visible, and thus do not add or detract from the aesthetic value of a site.

Maintenance Considerations

Intense and frequent maintenance and inspection practices are needed for filter systems. Table 1 outlines some these requirements.

Effectiveness

Structural stormwater management practices can be used to achieve four broad resource protection goals. These include: Flood Control, Channel Protection, Groundwater Recharge, and Pollutant Removal (see the Manual Builder Category for more information). Filtering practices are for the most part only adapted to provide pollutant removal.

Groundwater Recharge
In exfilter designs, some groundwater recharge can be provided. None of the other sand filter designs can provide recharge, however.

Pollutant Removal
Sand filters are effective stormwater treatment practices for pollutant removal. Removal rates for all sand filters and organic filters are presented below. With the exception of nitrate nitrogen, which appears to be exported from filtering systems, they perform reasonably well at removing pollutants. The export of nitrate from filters may be caused by mineralization of organic nitrogen in the filter bed.

From the few studies available, it is difficult to determine if organic filters necessarily have higher removal efficiencies than sand filters. The Multi-Chamber Treatment Train appears to have high pollutant removal for some constituents, although these data are also based only on a handful of studies. The siting and design criteria presented in this sheet reflect the best current information and experience to improve the performance of sand filters.

Cost Considerations

There is little consistent data on the cost of sand filters, largely because, with the exceptions of Austin, TX, Alexandria, VA, and Washington, DC, they have not been widely used. Furthermore, filters have such varied designs that it is difficult to assign a cost to filters in general. A study by Brown and Schueler (1997) was unable to find a statistically valid relationship between the volume of water treated in a filter and the cost of the practice, but typical total cost of installation ranged between $2.50 and $7.50 per cubic foot of stormwater treated, with an average cost of about $5 per cubic foot. (This estimate includes approximately 25% contingency costs beyond the construction costs reported). The cost per impervious acre treated varies considerably depending on the region and design used (see Table 3). It is important to note that, while underground and perimeter sand filters can be more expensive than surface sand filters, they consume no surface space, making them a relatively cost-effective practice in ultra-urban areas where land is at a premium.

References

Brown, W. and T. Schueler. 1997. The Economics of Stormwater BMPs in the Mid-Atlantic Region. Prepared for: Chesapeake Research Consortium. Edgewater, MD. Center for Watershed Protection. Ellicott City, MD.

Center for Watershed Protection (CWP), Environmental Quality Resources and Loiederman Associates. 1998. Maryland Stormwater Design Manual. Draft. Prepared for: Maryland Department of the Environment. Baltimore, MD. http://www.mde.state.md.us/environment/wma/stormwatermanual/mdswmanual.html

Center for Watershed Protection (CWP). 1997. Stormwater BMP Design Supplement for Cold Climates. Prepared for: US EPA Office of Wetlands, Oceans and Watersheds. Washington, DC.

Center for Watershed Protection (CWP). 1996. Design of Stormwater Filtering Systems. Prepared for: Chesapeake Research Consortium. Solomons, MD. and US EPA Region V. Chicago, IL.

City of Alexandria, VA. Unconventional BMP Design Criteria. In: Alexandria Supplement to the Northern Virginia BMP Handbook. Alexandria, VA.

City of Austin, TX. 1996. Design of Water Quality Controls. Austin, TX.

CSF Treatment Systems, Inc. 1996. Stormwater Management Promotional Brochure. Portland, OR.

Curran, T. 1996. Peat Sand Efficiency Calculations for McGregor Park. Unpublished data. Lower Colorado River Authority. Austin, TX.

Galli, F. 1990. Peat-Sand Filters: A Proposed Stormwater Management Practice for Urban Areas. Metropolitan Washington Council of Governments. Washington, DC.

Leif, T. 1999. Compost Stormwater Filter Evaluation. Snohomish County Department of Public Works. Everett, WA.

Schueler, T. 2000. Comparative Pollutant Removal Capability of Urban BMPs: A Reanalysis, Article 64 in The Practice of Watershed Protection. Center for Watershed Protection. Ellicott City, MD.

Shaver, E. and R. Baldwin. 1991. Sand Filter Design for Water Quality Treatment. Delaware Department of Natural Resources and Environmental Control. Dover, DE.

Stewart, W. 1992. Compost Stormwater Treatment System. W&H Pacific Consultants. Draft Report. Portland, OR.

Watershed Management Institute (WMI). 1997. Operation, Maintenance, and Management of Stormwater Management Systems. Prepared for: US EPA Office of Water. Washington, DC.

Washington State Department of Ecology (WSDE). 1992. Stormwater Management Manual for the Puget Sound Basin. Olympia, WA.

Winer, R. 2000. National Pollutant Removal Performance Database for Stormwater Treatment Practices: 2^nd Edition. Center for Watershed Protection.� Ellicott City, MD.