Bottomless sand filters are similar in several ways to the single-pass sand filter. However, BSF media may be finer textured and less uniform. Typically bottomless sand filter media have an effective size of 0.33 mm and a uniformity coefficient of 2.5 to 4. In addition, they lack a bottom filter liner and underdrain.
Wastewater is applied under low pressure to the top of a 2 to 3 foot deep bed of sand media through a distribution manifold and lateral system. The manifold and laterals are surrounded by pea gravel which extends to the filter surface (free access). Wastewater trickles down in unsaturated thin-film flow through the sand media in a time-dosed mode. BSF effluent then percolates directly into the soil under the filter.
Because of a higher hydraulic loading rate and oxygen and gas exchange requirement, BSFs are usually not buried as are single-pass sand filters. BSFs can be designed at-grade, where just 6 to 10 inches of the filter projects above existing grade, or above grade, with the top 24 to 30 inches of the filter above the ground surface. The technology is usually used for residential systems where soil and site conditions exist that make the use of conventional or LPP drainfields impractical or not economical.