Final Dispersal Option: Bottomless Sand Filter

Wastewater is applied under low pressure to the top of a 2foot deep bed of sand media through a PVC-pipe distribution system. The distribution network is embedded in peastone to protect it. Wastewater percolates down through the sand media and into the understory soil. Media filters with an open bottom are specifically intended to disperse wastewater into the soil immediately under and adjacent to their footprint. Unlike single-pass sand filters, the bottomless version is not buried, in order to allow for sufficient gas exchange between the filter and the atmosphere. The filter can either be installed at-grade, with 6 to 10 inches of the filter above the ground surface, or above grade, with 2 to 3 feet of filter above grade.

 A treatment train containing a bottomless sand filter

Wastewater leaves the home through the building sewer (A) flowing by gravity to the processing tank (B), where solids, fats, oils and greases are trapped and stored. Wastewater is pumped to the textile media filter (C), where the textile media and bacteria reduce the strength of the wastewater before it is recirculated back to the inlet-end of the processing tank, where nitrogen removal occurs. Recirculation continues until the effluent level in the processing tank is high; at this point the effluent is routed to a drainfield pump chamber (D) and pumped to the final dispersal technology: a bottomless sand filter (E). The effluent is dispersed onto and filters through specific sand media in the bottomless sand filter before it reaches the shallow surface soils where further nutrient removal and pathogen reduction may occur. Shallow narrow drainfields may also be used as a final dispersal option with this technology.

Treatment performance observed for bottomless sand filters in RI
% TN Reduction
Fecal Coliform CFU
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For more information on bottomless sand filters:

Web Links: This drainfield option has been studied in great detail, with the above-referenced results summarizing some of the data obtained. The construction of a BSF must adhere to Rhode Island DEM regulations as per the BSF Guidance Document, which can be found on the RI DEM ISDS site.