Exact Stormwater Management

A stormwater sand filter is a type of filtration system that uses twelve or more inches of sand as a filtration media to capture pollutants from stormwater runoff before it enters natural water bodies such as rivers, lakes, or oceans. Sand is a course media that traps sediments contained in runoff as it passes through the filter, making sand filters a common and effective system to improve the quality of stormwater discharges and keep the downstream ecosystem clean and healthy.

Sand filters come in many different shapes and sizes, but most systems fall into one of two categories: surface sand filters or below ground sand filters. A surface sand filter uses a thick layer of sand that sits within a shallow depression in the landscape, and usually has grass growing over the surface. Runoff can flow across the landscape into the system, or it can be piped in directly from rooftops or upstream drainage inlets. Below ground sand filters are usually housed inside large pipes or concrete vaults where the sand beds are contained in one section of the vault, and most of the runoff is piped into the system.

Both types of sand filters typically have a settling area to pre-treat runoff that removes the larger sediment particles and debris, which helps extend the effective life of the sand. They also typically have an underdrain which collects the treated water and pipes it to a nearby waterway or into the municipal drainage system.

Sand filters require regular inspections find problems before they become severe. Highly polluted runoff will cause sand filters to become clogged within a few months of normal rainfall, but under normal conditions sand filters should function for 1-3 years with only routine maintenance. For underground structures you should be able to carry out the inspection from the surface, but be sure to follow all Confined Space, OSHA, and local requirements when physically entering the chambers. All inspections should be conducted within 24 hours following a storm that exceeds 1/2 inch of rainfall to evaluate the condition and performance of the system, including checking for the following:

• Check to see if sediment accumulation in the sedimentation chamber has exceeded 6 inches. If so, schedule a cleanout.

• Check to see if inlets and flow splitters are clear of debris and are operating properly.

• Check the dry sediment chamber and sand filter bed for any evidence of standing water or ponding more than 48 hours after a storm

• Inspect whether the contributing drainage area to the filter is stable and not a source of sediment.

• Ensure that the filter bed is level and remove trash and debris from the filter bed. Sand or gravel covers should be raked to a depth of 3 inches. Filters with a turf cover should have 95% vegetative cover.

• Dig a small test pit in the sand filter bed to determine whether the first 3 inches of sand are visibly discolored and need replacement.

• For surface sand filters, check whether turf on the filter bed and buffer is more than 12 inches high, and schedule necessary mowing operations.

• Check the integrity of observation wells and cleanout pipes.

• Check concrete structures and outlets for any evidence of spalling, joint failure, leakage, corrosion, etc.

Maintaining your sand filter is essential to ensure its continued effectiveness in treating stormwater runoff and protecting your watershed. If your system is a surface sand filter, maintenance will be fairly easy. But maintaining an underground sand filter will require confined space entry and special training and equipment necessary to perform the work safely. Here are some general maintenance tasks typically involved in keeping a stormwater sand filter in good working condition:

• Remove accumulated sediment from the pre-treatment chamber or forebay regularly to prevent it from clogging the filter bed.

• Check the underdrain system for any signs of damage, blockages, or malfunction.

• Clean the underdrain pipes and outlets to ensure proper drainage and prevent backups.

• Keep detailed records of maintenance activities, including inspection reports, sediment removal schedules, and filter media replacement.

For Surface Sand Filters:

• Remove any trash, debris, or litter that accumulates in or around the filter.

• Control vegetation growth around the filter to prevent roots from infiltrating the filter bed or obstructing inlet and outlet structures.

• Trim vegetation as needed.

For Underground Sand Filters:

• Remove accumulated sediment and other materials above the filter bed, including the geotextile and stone, if your system uses them.

• It’s a good practice to remove and replace the top layer of sand (3-6 inches), which traps and retains fine sediment and will reduce the efficiency of your system.

• Once the sand layer has been restored, replace the geotextile panel (if included in your system) with new fabric and replace the stone layer that holds it in place.

By following these maintenance practices, you can help ensure that your sand filter remains effective in removing pollutants from stormwater runoff and protecting the quality of downstream water bodies. Regular maintenance is crucial for maximizing the lifespan and performance of your sand filter.

Installing an underground sand filter will typically involve setting very large concrete structures that weigh thousands of pounds using heavy machinery. If you need to install an underground sand filter, let Exact or another qualified company help you. If you’re installing a surface sand filter, you may be able to use smaller equipment and complete the project yourself.

Start by locating the area where your sand filter will be located and stake out the footprint of the system. Excavate both the filter area and the forebay area to the required depth. Install the underdrain pipe, and connect it to your outlet structure or wherever you want the treated water to go. Backfill the pipes with clean, washed stone (typically AASHTO #57 stone) and grade the surface as flat as you can. Most sand filters use a geotextile to separate the underdrain stone from the sand above, so install that over the stone and up the sides of the excavation to prevent the native soils from contaminating your filter material. With the geotextile in place, you can now install your filter material. Not all sand filters use 100% sand, so follow your plans or local guidelines closely to make sure your filter meets the requirements. Make sure any inlet pipes leading to the system are stabilized and won’t erode, and ensure the surrounding areas or berm are in place to hold water above the system until it has time to pass through the filter media. Some sand filters use a thin topsoil or compost layer over the filter layer to help get grass established. Add grass seed over the top, and you’re almost done. Keep watering that grass seed, and wait until your grass is fully established before activating the system.

Surface sand filters can take on different shapes and sizes with different components, so be sure to follow your plan or local codes for a successful installation. And if you need professional help, Exact is ready to get to work for you.