Sand Filters

Sand filters, like planter boxes, are structural reservoirs that filter pollutants from stormwater and slow water runoff.  Infiltration sand filters allow water to infiltrate through the sand and into the ground.  Flow through sand filters include the use of a waterproof lining, and include an overflow to an approved stormwater conveyance system.

Specifications

The minimum width is 18 inches, with at least 12 inches of storage depth and up to 6 feet storage depth, and filter slopes must be less than 0.5%.  Minimum setback from property lines is 5 feet, unless the facility is less than 30 inches high.  The sand layer shall be a minimum of 18 inches deep of clean medium to fine sand with no organic material, underlain by filter fabric and an 8” layer of ½” to 2” gravel.  Side slopes will be a maximum of 4:1.  Any shape sand bed may be used, provided that adequate treatment surface is provided.

An energy dissipater must be provided for water entering from a point source (pipe).  If stormwater with a high concentration of particles will be treated, treating water in a sediment basin prior to discharge into the sand filter may help extend the life of the sand filter.

A high-flow overflow must be provided to an approved conveyance system.  Sand filters shall be designed to pond water for less than 4 hours after each storm event.

Filter structural walls must not be made out of material that can leach toxic chemicals and contaminate stormwater.

Infiltration sand filters

Infiltration sand filters are appropriate for soils with a minimum infiltration rate of 2 inches per hour.  Infiltration filters must be set back at least 10 feet from building structures, to prevent damage to the foundation.