Used in conjunction with pre-treatment methods such as septic tanks and soil absorption fields. An intermittent sand filter receives and treats effluent from the septic tank before it is distributed to the leaching field. The sand filter consists of a bed (open or buried) of granular material 60 to 90 cm deep. The material is usually 0.35 to 1.0 mm in diameter. The bed of granular material is underlain with graded gravel and collector drains. These systems have been shown to be effective for nitrogen removal, however, this process is dependent on temperature. Water loading recommendations for intermittent sand filters are typically between 4 and 10 L/m2/day but may be higher, depending on wastewater characteristics. Primary failure of sand filters is due to clogging, and maintenance is recommended to keep the system performing properly, resting the bed, raking the surface layer, or removing the top surface medium and replacing it with clean medium. In general, the filters should be inspected every 3 to 4 months to ensure that they are operating properly.
Intermittent sand filters are used for small commercial and institutional developments as well as individual homes. The size of the facility is limited by land availability. The filters should be buried in the ground, but they may be constructed above ground in areas of shallow bedrock or high-water tables. Covered filters are required in areas with extended periods of subfreezing weather. Excessive, long-term rainfall and runoff may be detrimental to filter performance, requiring measures to divert water away from the system.