Design and Optimization of Two Recirculating Sand Filters for the Removal of Nitrogen and Organic Chemicals from Domestic Wastewater

Abstract: Removal of nitrogen from domestic wastewater prior to discharge to a soil absorption system can help to prevent nitrate concentrations in groundwater from exceeding the drinking water standard. This is of increasing importance in Wisconsin and elsewhere, as rural development increases and people demand high quality drinking water.

This study involved the design, installation, and optimization of two sand filter denitrification systems on private residences in Central Wisconsin. The recirculating sand filter design involved the use of 7,571 | (2,000 gal) septic tanks with wooden covers. The systems had foam insulation to conserve heat during Wisconsin’s cold winters. The sand filter contained a rock storage layer consisting of 41 cm (16 in) of 2.5 to 3.8 cm (1 to 1.5 in) limestone. The rock storage layer was fully saturated to promote denitrification in the sand filter. The filter media overlaying the rock storage layer consisted of 7.6 cm (3 in) of pea gravel and 61 cm (24 in) of coarse sand. The sand filter also contained a pump chamber with a pump and flow regulation equipment to recirculate various amounts of effluent to the top of the sand filter, back to the septic tank for denitirification, and to the soil absorption system for disposal.