Sand Filter
A sand filter is a stormwater treatment practice that captures and treats runoff by passing it through a filtering medium. These systems are typically designed as a two-chamber structure. The first chamber provides pretreatment by allowing coarse sediments to settle out, while the second chamber contains a bed of sand or other media that filters finer particles and associated pollutants. As stormwater flows through the filter bed, pollutants are trapped, adsorbed, or processed within the media.
Several design variations exist to adapt the basic sand filter concept to different site conditions or to enhance pollutant removal. Common types include the surface sand filter, underground sand filter, and perimeter sand filter. Other variations substitute or amend the sand with organic materials like peat or compost to improve the removal of certain pollutants. While effective for water quality treatment, sand filters are primarily designed for this purpose and do not provide significant flood control or channel protection benefits.
Applicability
Sand filters are versatile and can be adapted for many regions and site types, though some constraints exist. They are particularly effective in treating runoff from highly impervious areas. When selecting a stormwater practice, the site’s drainage area, available head, and land use are critical factors. The BMP selector tool can help evaluate whether a sand filter is a suitable option for a given project.
Regional Suitability
Sand filters can be implemented in most climates, including cold and arid regions, with appropriate design modifications. In cold climates, the filter bed should be located below the frost line where possible, and conveyance structures must be designed to prevent freezing. In arid climates, which often have high sediment loads, the pretreatment settling chamber may need to be enlarged to handle the increased volume of coarse material.
Ultra-Urban Areas
Sand filters are an excellent choice for ultra-urban landscapes where space is limited. Underground and perimeter sand filters are especially well-suited for these environments because they require no surface footprint, allowing for treatment in densely developed areas like parking lots and commercial districts.
Stormwater Hotspots
A stormwater hotspot is a land use that generates runoff with higher-than-normal pollutant concentrations. Examples include vehicle maintenance facilities, fueling stations, and industrial sites. Because standard sand filters are sealed systems with underdrains, they do not infiltrate runoff into the native soil. This design prevents the potential contamination of groundwater, making them a preferred practice for treating runoff from hotspot locations.
Retrofit Applications
Stormwater retrofits involve installing treatment practices on previously developed sites to improve water quality. The compact design of sand filters makes them a practical option for retrofitting existing urban areas where space is scarce. While they are effective for treating small drainage areas, using them to manage runoff from an entire watershed can be costly compared to larger, centralized practices like ponds or wetlands.
Cold Water Streams
Protecting cold water streams requires stormwater practices that minimize thermal impacts on runoff. Since most sand filter designs do not have a large permanent pool of water exposed to sunlight, they have a low potential for warming stormwater. Underground and perimeter filters are particularly effective in this regard, as the runoff is not exposed to solar radiation. For surface designs, minimizing the temporary ponding time can further reduce the risk of thermal pollution.
Design Criteria
Proper design is essential for the effective and long-term performance of sand filters. Key considerations include site feasibility, conveyance of stormwater into and out of the system, pretreatment to remove coarse sediment, treatment chamber sizing, and landscaping of the surrounding area.
Feasibility
Site conditions must be evaluated before selecting a sand filter. Most designs require between two and six feet of hydraulic head, which is the elevation difference from the inflow to the outflow. The perimeter sand filter is an exception, capable of functioning with as little as one foot of head. Drainage areas are typically limited to 10 acres for surface filters and two to five acres for underground or perimeter designs to prevent premature clogging. There should be at least two feet of separation between the bottom of the filter and the seasonally high groundwater table.
Conveyance
Sand filters are typically designed as off-line systems. A flow splitter or diversion structure is used to direct the water quality volume (e.g., runoff from the first inch of rainfall) into the filter, while larger storm flows bypass the system and are routed directly to the downstream storm drain. An overflow must also be included within the filter to safely pass flows that exceed its capacity. After passing through the filter media, treated runoff is collected in a perforated underdrain pipe (minimum 4-inch diameter) and returned to the conveyance system.
Pretreatment
Pretreatment is a critical design element that protects the filter bed from clogging with coarse sediment and debris. This is typically achieved with a sedimentation chamber located upstream of the filter bed. The pretreatment chamber should have a volume equal to at least 25% of the total water quality volume (WQv) being treated. In cold climates or areas with high sediment loads, this volume should be increased to 40% of the WQv. Other pretreatment measures, such as a vegetated Filter Strip, can reduce sediment loads from the contributing drainage area.
Treatment and Sizing
The treatment system, including the pretreatment chamber and the volume above the filter bed, should be sized to temporarily store at least 75% of the WQv. The filter bed itself should have a minimum depth of 18 inches (12 inches for perimeter filters) and be composed of a medium sand meeting ASTM C-33 specifications.
The required surface area of the filter bed (Af) is calculated to ensure the WQv dewaters completely within a specified time, typically 40 hours. The calculation is based on Darcy’s Law:
Af = (WQv) (df) / ((k) (hf + df) (tf))
Where:
- Af = Surface area of filter bed (ft2)
- WQv = Water quality volume (ft3)
- df = Filter bed depth (ft)
- k = Coefficient of permeability of filter media (ft/day)
- hf = Average height of water above filter bed (ft)
- tf = Design filter drain time (days)
The coefficient of permeability (k) varies by media type, with typical values of 3.5 ft/day for sand and higher values for some organic media. Detailed sizing can be performed using the sand filter design calculator.
Design Variations
Several sand filter designs have been developed to accommodate different site constraints and treatment goals.
Surface Sand Filter
The surface sand filter is the most basic design, with both the sedimentation chamber and filter bed constructed at ground level. It is an off-line system that treats only the water quality volume. This is often the least expensive filter option but requires surface space and may not be considered aesthetically pleasing.
Underground Sand Filter
In this variation, all components are contained within a subterranean vault, typically made of precast concrete. Like the surface filter, it is an off-line system. While more expensive to construct, its primary advantage is that it consumes no surface land, making it ideal for highly developed sites.
Perimeter Sand Filter
Also known as the Delaware sand filter, this design consists of a long, narrow trench typically installed along the edge of a parking lot. Runoff enters through grates into a sedimentation chamber. The perimeter filter is unique in that it is an on-line system; all flows enter the practice, but larger storm events bypass the filter media through an overflow chamber. It requires very little hydraulic head, making it suitable for flat sites.
Organic Media Filter
Organic media filters are similar in design to surface sand filters but use a different filtering medium. The sand is either replaced with or amended by organic materials like a peat/sand mixture or leaf compost. The higher cation exchange capacity of the organic matter is intended to enhance the removal of nutrients, metals, and other pollutants. These systems are distinct from Bioretention, which incorporates a complex soil media and living plants into the treatment process.
Landscaping
The drainage area contributing to a sand filter must be fully stabilized with dense vegetation to prevent erosion and minimize sediment loads. For surface sand filters, a grass cover can be planted on the filter bed to aid in pollutant uptake and stabilize the surface. The selected grass species should be tolerant of both wet and dry conditions.
Pollutant Removal
Sand filters are highly effective at removing total suspended solids (TSS) and associated pollutants. They also achieve moderate to high removal of phosphorus, metals, and bacteria. However, monitoring data indicates that sand filters often export nitrate nitrogen, likely due to the mineralization of organic nitrogen within the filter bed. The performance of organic filters is generally similar to sand filters, though more data is needed for a definitive comparison. For a broader look at BMP performance, consult the national pollutant removal database.
| Pollutant | Sand Filters1 | Organic Filter |
|---|---|---|
| Total Suspended Solids (TSS) | 86 | 88 |
| Total Phosphorus (TP) | 59 | 61 |
| Total Nitrogen (TN) | 38 | 412 |
| Nitrate-Nitrogen (NOx) | -14 | -15 |
| Metals (Cd, Cu, Pb, Zn) | 49 – 88 | 53 – 85 |
| Bacteria | 37 | NA |
| Source: Winer, 2000. | ||
| 1 Excludes vertical sand filters. | ||
| 2 Data based on fewer than five data points. | ||
Construction and Cost Considerations
Construction costs for sand filters vary widely based on the design type, site conditions, and regional economic factors. A 1997 study found that the total installed cost typically ranged from $2.50 to $7.50 per cubic foot of stormwater treated, with an average of about $5.00 per cubic foot. Underground and perimeter filters are generally more expensive than surface filters due to the cost of excavation and structural vaults, but their value increases in urban areas where surface land is at a premium.
| Region (Design Type) | Cost per Impervious Acre |
|---|---|
| Delaware (Perimeter) | $10,000 |
| Alexandria, VA (Perimeter) | $23,500 |
| Austin, TX (Surface, <2 acres) | $16,000 |
| Washington, DC (Underground) | $14,000 |
| Denver, CO | $30,000 – $50,000 |
Cost data from the 1990s should be adjusted for inflation. However, the relative cost differences between filter types remain a useful guide for planning purposes.
Maintenance
Frequent and consistent maintenance is crucial for the proper functioning of sand filters. The primary tasks involve removing accumulated sediment and debris from the pretreatment chamber and ensuring the filter surface remains permeable. Because underground and perimeter filters are not visible, they are at a higher risk of being forgotten, making a formal maintenance schedule and inspection program essential.
| Activity | Frequency |
|---|---|
| Inspect inlets, outlets, and filter surface for debris and clogging. | Monthly |
| Ensure contributing drainage area is stabilized and mowed. | Monthly |
| Check for standing water on filter bed more than 48 hours after a storm. | Monthly |
| Inspect concrete structures for cracking or spalling. | Annually |
| Remove sediment from the pretreatment chamber when it accumulates to a depth of six inches. | Annually, or as needed |
| Remove and replace the top few inches of the filter media when the surface becomes clogged and infiltration rates decrease significantly. | As needed |
| Repair or replace any damaged structural components. | As needed |
Limitations
While sand filters are effective for water quality treatment in constrained sites, they have several limitations. They do not provide flood control or stream channel protection, as they are not designed to manage large storm volumes or reduce peak flow rates. Standard designs also offer no groundwater recharge benefits, unlike practices such as an Infiltration Trench. The practice requires a significant amount of hydraulic head to function, which can make it infeasible on flat sites. Finally, sand filters require diligent maintenance to prevent clogging, and the aesthetic quality of surface sand filters is often considered poor.
Frequently Asked Questions
What is the primary function of a sand filter?
The primary function of a sand filter is to improve water quality by removing pollutants from stormwater runoff. It achieves this by first settling out larger sediment particles in a pretreatment chamber and then filtering the water through a bed of sand or other media. This process is highly effective at capturing suspended solids, heavy metals, and bacteria. Sand filters are not designed for flood control or to reduce the volume of stormwater runoff, but specifically to treat the “first flush” of a storm, which typically carries the highest pollutant load.
How does a sand filter handle large storms?
Most sand filters are designed as “off-line” systems. This means a flow splitter or diversion weir is used to direct only a specific amount of runoff—the water quality volume—into the filter for treatment. During larger storms, flows exceeding this volume bypass the sand filter entirely and are safely conveyed to the downstream storm drain system. This design prevents the filter from being overloaded or scoured by high flows, protecting its internal components and ensuring its long-term effectiveness for water quality treatment. The on-line perimeter filter is an exception, but it includes an internal overflow for large events.
What is the difference between a surface and an underground sand filter?
The main difference is their location. A surface sand filter is built at grade and is visible, resembling a shallow basin. An underground sand filter contains all the same components—a settling chamber and a filter bed—but they are housed within a subterranean concrete vault. While underground filters are more expensive to construct, they are invaluable in dense urban areas because they require no surface land. Surface filters are less costly but require a dedicated footprint on the site and are often considered less aesthetically appealing.
Why is pretreatment necessary for a sand filter?
Pretreatment is essential to protect the sand filter bed from premature clogging. Runoff, especially from impervious surfaces, carries coarse sediment, trash, and debris. A pretreatment chamber, typically a small sedimentation basin, allows these larger materials to settle out before the water reaches the fine sand media. By removing this material first, the pretreatment chamber extends the functional life of the filter bed, reduces the frequency of major maintenance, and ensures the system operates at its designed capacity for a longer period.
Can sand filters be used in cold climates?
Yes, sand filters can be adapted for cold climates with specific design considerations. To prevent freezing, the filter bed should be placed below the frost line whenever feasible. Conveyance pipes and structures must also be designed to avoid ice formation that could cause blockages. The pretreatment chamber should be oversized to accommodate sediment from winter road sanding operations. In some cases, such as with certain organic media filters, the system may need to be taken offline during the winter months to prevent damage from freezing conditions.
Do sand filters provide groundwater recharge?
Standard sand filter designs do not provide groundwater recharge. They are engineered as sealed systems with an underdrain that collects the filtered water and discharges it back into the storm drain network. This design is intentional, making them suitable for treating runoff from stormwater hotspots without risking groundwater contamination. While some modifications called “exfilters” exist to allow for infiltration, the vast majority of sand filters are designed solely for water quality treatment through filtration and discharge, not for volume reduction or recharge.
What is an organic media filter?
An organic media filter is a variation of the standard sand filter where the filter medium is amended or replaced with organic material. Common media include a peat and sand mixture or leaf compost. The goal of using organic media is to enhance the removal of certain pollutants, such as dissolved metals and some nutrients, that are not as effectively captured by sand alone. The organic material provides a higher cation exchange capacity, allowing it to bind with and retain more pollutants as stormwater passes through.
How often does a sand filter need to be maintained?
Sand filters require frequent and routine maintenance to function properly. Monthly inspections are recommended to check for debris, clogging, and proper drainage. The most common major maintenance task is removing accumulated sediment from the pretreatment chamber, which is typically needed annually or whenever sediment depth exceeds six inches. Over time, the surface of the sand bed itself will become clogged. When this happens, the top few inches of discolored sand must be removed and replaced with fresh material to restore the filter’s hydraulic capacity.
Are sand filters a good option for residential lots?
Sand filters are generally not the best choice for individual residential lots. They are better suited for commercial, industrial, or high-density residential sites with large impervious areas and limited space. Practices like rain gardens, bioretention, or downspout disconnection are typically more cost-effective and practical for managing runoff on a single-family property. Sand filters require a significant amount of hydraulic head and have maintenance needs that are often beyond the scope of a typical homeowner. They are more effective when applied at a slightly larger, shared scale.
Why do sand filters sometimes increase nitrate levels?
Monitoring has shown that sand filters can sometimes be a net exporter of nitrate-nitrogen. This phenomenon occurs because the filter bed creates an aerobic environment where organic nitrogen, which is captured from the runoff, is converted into nitrate through a microbial process called mineralization or nitrification. Since the filter lacks the anaerobic conditions needed for denitrification (the conversion of nitrate to nitrogen gas), the newly created nitrate becomes dissolved in the filtered water and is discharged from the system, resulting in a higher concentration at the outlet than at the inlet.