Sand filters can be used for storm water quality control and managing storm water runoff volumes. Sand filters are composed of at least two components: a sedimentation chamber and a filtration chamber. The sedimentation chamber removes floatables and heavy sediments, while the filtration chamber removes additional pollutants by filtering flow through a sand bed. Treated filtrate is normally diverted back to the storm drainage system via an underdrain system or pipe network. Pollutants such as suspended solids, biochemical oxygen demand (BOD), total phosphorus, and fecal coliform bacteria are effectively removed from storm water flows when treated by a sand filter system. Other pollutants removed include phosphorus and metals. Sand filter designs include the surface sand filter basin (AKA Austin sand filter), the underground vault sand filter (Washington, DC sand filter), the double trench sand filter (Delaware sand filter), the stone reservoir trench sand filter, and the peat sand filter system. Modifications are often made to these designs based on site-specific conditions.

Sand filters provide a highly effective means of removing pollutants from storm water while remaining flexible in application to allow for modifications in basic design structure to accommodate site-specific criteria. Modifications to the basic structure arise due to site differences, including drainage area served, filter surface areas, land requirements, and quantity of runoff treated. Sand filters are currently popular best management practices (BMPs) used in Delaware; Florida; Austin, Texas; Alexandria, Virginia; and Washington, DC.

The Austin sand filter was designed to detain runoff in a sedimentation chamber where heavy sediments and floatables are removed. Estimates of pollutant removal efficiencies for various Austin sand filters, based on the preliminary findings of the City’s storm water monitoring program, are as follows. In addition, data from an intermittent sand filter (Delaware sand filter), constructed to treat runoff from a commercial parking lot near Ronald Reagan National Airport in Alexandria, Virginia, are provided below.

The percentages listed for the Austin sand filter include partial and full sedimentation systems with different drainage areas. Current monitoring data from the Austin sand filters indicates phosphorous removal efficiencies of up to 60 percent. The Austin sand filter also has been used in Alexandria, Virginia; monitoring of these units indicated a phosphorus removal of up to 40 percent. Nitrate was not removed nor is it known what the removal efficiencies are for other dissolved pollutants..

Performance of sand filters may be sustained through frequent inspections and replacement of the filter fabric and the top of the media every three to five years, depending on the pollutant load being treated. One system has been reported to need filter changes two times per year due to heavy pollutant loads. Accumulated trash and debris should be removed from the sand filters every six months or as necessary. Performance also can be increased by stabilizing the drainage area to minimize sediment loading, ensuring that the sedimentation chamber adequately removes suspended solids and sediments prior to the filtration chamber and allowing for adequate detention times for both sedimentation and filtration.

The design of sand filters with impermeable chambers that prevent groundwater infiltration are preferred in situations where groundwater contamination is a concern. The Austin; Delaware; and Washington, DC, sand filters may substitute for water quality inlets when hydrocarbons are of concern. Due to the size of the Austin sand filter, it also can be used instead of wet ponds for treatment of contaminated run-off in areas where evaporation exceeds rainfall.

The use of a sand filter for treating stormwater runoff may help facilities meet requirements for implementation of stormwater runoff best management practices contained in stormwater permits and plans (40 CFR 122.26).

The compliance benefits listed here are only meant to be used as a general guideline and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g. the amount of workload involved.

No materials compatibility issues were identified.

Safety and health concerns depend on the types of contaminants in the stormwater. Metals and phosphorus, for instance, require caution in handling. They are skin irritants. Protective gear should be worn when handling contaminants such as fecal coliform. Proper personal protection equipment is, therefore, recommended. In addition, care should be taken when working with sealed systems, as gas may accumulate.

• Sand filters, in particular those mentioned previously, achieve high removal efficiencies for suspended solids, BOD, and fecal coliform bacteria, and total phosphorous.
• Hydrocarbons and nutrients also are removed by sand filters.
• Sand filters designed with impermeable basins limit the potential for groundwater contamination while treating storm water.
• Sand filters can be used in small sites (e.g., gas stations or other urban settings) where a wet pond is not possible due to spacial constraints.

• Nitrates are not removed.
• Sand filters are ineffective in removing dissolved pollutants except by adsorption.

Construction costs vary depending on the sand filter system being designed. The Austin sand filtration system costs approximately $18,500 for treatment of a 1-acre drainage area. In this instance, the cost decreases with increasing drainage area. Cost per acre decreases as the number of acres served increases. For example, the cost for a sand filter decreases to approximately $2,360/acre when treating 30 acres. The precast cost for one impervious acre for a Washington, DC, sand filter is approximately $25,000 to $30,000. Costs for the Delaware sand filter are $20,000 per impervious acre treated.

Assumptions:

• The following comparison uses an average cost of sand filters.
• Maintenance costs for a sand filter average 5 percent of the construction cost.
• Labor costs for operating a wet pond include mowing and debris removal.
• Labor costs for operating a sand filter include filter changing, gravel and sand replacement, and debris removal, estimated at approximately 3 hours/year.
• Material costs for the gravel layer, filter fabric, and top portion of sand based on the experience of the Washington, D.C., sand filter are approximately $1,700 annually.
• The figures in the table are based on one impervious acre treated.
• The cost per pound of pollutant removed equals $8.30 based on the experience for the Washington, D.C. sand filter.
• Labor costs $45/hour.
• Materials disposed are not hazardous. Solid waste disposal costs are $40/ton or $0.02/lb.
• Annual disposal of sediments is 350 lbs.

Annual Operating Cost Comparison for Wet Ponds and Sand Filter

Economic Analysis Summary

Annual Savings: -$215
Capital Cost for Equipment/Process: $20,000
Payback Period for Investment in Equipment/Process: N/A

Overall costs for installing and operating a sand filter system appear to be higher than that of using a wet pond system. However, in many urban situations, it is not feasible to install a wet pond. A sand filter is an effective alternative for treating stormwater runoff.

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Approval is controlled locally and should be implemented only after engineering approval has been granted. Major claimant approval is not required.