Soil filters mimic permeable upland areas that soak up and cleanse runoff as it travels through the soil toward groundwater. The soil acts as a filter by removing sediment and other pollutants. Oxygen inside the soil filter aerates the wastewater and fuels the microbes that break down pollutants.

Soil filters have been widely used since the 1970’s to treat household wastewater at sites where soil conditions (high water table and slowly permeable clays) hinder the performance of a standard septic system. Under these circumstances, wastewater is pumped from the septic tank into the soil filter, may require a second pass through the septic tank and filter system to remove nitrogen.

Two types of soil filters are :

  • Sand filters are framed by wood, masonry or soil and filled with medium to coarse sand. Wastewater is pumped intermittently into the upper sand layers and collects temporarily within the filter. The system is wetter than a mound and has less surface are for air infiltration.
  • Mounds are small hills of soil and gravel similar to sand dunes. Wastewater is pumped intermittently into the top of the mound, allowing for dry periods. This makes oxygen plentiful for removal of Biochemical Oxygen Demand (BOD), sediment and the first step of nitrogen treatment. Phosphorus removal depends on sand chemistry.