SAND FILTERs
What Is a Stormwater Sand Filter?
A stormwater sand filter is a treatment system that removes pollutants from runoff by passing it through a layer of sand before it is discharged into natural water bodies or drainage systems. Sand filters are notable for their compact size. They are typically installed underground in a rectangular chamber, making them well-suited for urban sites where space is limited.
Despite their relatively small footprint, sand filters are just as effective as many larger stormwater systems and typically require less frequent maintenance to remain healthy. As runoff moves toward the system, it usually passes through a forebay or sedimentation basin that serves as pre-treatment, capturing larger particles and debris before the water enters the main filter.
Once the water enters the main chamber, it flows through a layer of sand. As it percolates through, physical straining, adsorption, and biological processes remove pollutants before the treated water exits through the underdrain system and flows to a discharge point. The specifications and design of each system are unique to the site where it is installed. While sand filters are most commonly installed underground, they can also be built above ground to function similarly to other stormwater systems such as bioretention facilities.
How Do I Inspect My Sand Filter?
Sand filters require regular inspections to ensure continued treatment effectiveness. A visual inspection can identify most maintenance needs early, before they become more serious problems.
Start by checking the inlet. Look for blockages or debris accumulation and confirm that water flows freely into the filter. In the pre-treatment area, inspect the sedimentation basin or forebay for sediment buildup and remove any trash or debris.
Next, check the surface of the sand filter. Look for signs of clogging, standing water, or poor drainage. Ensure the sand layer is even and has not been eroded or displaced. Check the outlet pipes for obstructions or damage and confirm that water is exiting the filter properly.
Finally, inspect the surrounding area for erosion or other signs of damage. If the system uses vegetation, ensure it is maintained and not obstructing the filter. These inspections should be performed monthly to catch potential issues early.
How Do I Maintain My Sand Filter?
Sand filter maintenance includes both routine tasks that can be performed by facility staff and more involved work that requires professional equipment and expertise.
Routine maintenance: Regularly remove leaves, trash, and other debris from the inlet and pre-treatment areas to prevent clogging. Periodically remove accumulated sediment from the forebay or pre-treatment basin. Gently rake the top layer of the sand filter to break up any crust that forms on the surface, which improves filtration and prevents clogging. Trim overgrown vegetation around the system that might block inlets or outlets, and remove invasive plants that can damage the filter.
Professional maintenance: Replacing the sand layer when it becomes clogged or contaminated, repairing structural components of the filter chamber, and inspecting and repairing the underdrain system should all be handled by stormwater professionals. Exact Stormwater has extensive experience maintaining a wide variety of sand filter systems regardless of the design configuration.
How Does a Sand Filter Get Installed?
Installation of a sand filter begins with excavation to the required depth, including space for the pre-treatment area and underdrain system. Sedimentation basins, forebays, or other pre-treatment structures are installed first to capture larger particles before they reach the filter.
The underdrain system, typically consisting of perforated pipes laid in a gravel bed, is installed next. This system collects and conveys filtered water to the outlet. If necessary, an impermeable liner is installed along the bottom of the excavation to prevent groundwater contamination or to compensate for unsuitable soil permeability.
The primary sand layer is deposited on top of the underdrain system, with the sand evenly distributed at the correct depth specified in the design. Depending on the design, a surface layer of gravel or vegetation is added to protect the sand filter and enhance filtration. Inlet and outlet structures are installed to direct stormwater into and out of the filter and connect to the site's drainage or discharge point.
Frequently Asked Questions
-
Sand filters should be inspected monthly and receive professional maintenance at least annually. The sand surface should be raked periodically to prevent crust formation. The pre-treatment forebay should be cleaned whenever sediment accumulates to a level that could impede flow. Full sand replacement is typically needed every 5 to 10 years depending on the sediment load and maintenance history. Exact Stormwater provides comprehensive sand filter maintenance services. Call (804) 302-5151 to schedule.
-
Yes. While most stormwater sand filters are installed underground in concrete or fiberglass chambers, above-ground sand filters can be designed to look and function similarly to bioretention facilities. The choice between above-ground and underground installation depends on available space, site topography, aesthetics, and the engineering design requirements for the project.
-
Sand filters effectively remove suspended sediments, particulate-bound metals, hydrocarbons, and some nutrients through physical straining, adsorption, and biological processes. They are particularly effective at removing total suspended solids (TSS) and can achieve removal rates of 80% or higher when properly maintained. They are less effective at removing dissolved pollutants compared to water quality filters with specialized media.
Last Updated: April 2026