the sand in gravel. Then construction fabric and additional soil are placed on top of the gravel to help keep the pipes from freezing. The top layer of soil also allows the mound to be planted with grass or non-woody plants to control erosion The primary waste liquids cleaning and purification actions in a drain field are performed by a biofilm in the loose fill surrounding the perforated drain tile. If the soil permeability is too low, the liquid is not absorbed fast enough. If the soil permeability is too high or is exposed to fractured bedrock, the wastewater reaches the water table before the biofilm has time to purify the water, contaminating the aquifer. In either situation, the mound system provides an ideal habitat for the biofilm and has the correct permeability to assure slow absorption of effluent into the mound before exiting as purified water into the surrounding environment. When installing a mound system, the soil in the area where the mound is to be placed will be compacted or disturbed. Any trees that in the mound area are cut away, and the roots and stumps retained. The surface of the area for the mound is then roughened with a chisel plow. This prepares the area for the sand. Work is done from upslope of the mound area so that the ground downslope of the mound does not get compacted. Tyler tables are used to help determine the mound size. Time dosing is another important aspect of the functioning of the mound system. Short frequent doses of effluent onto sand filters with orifices that are closely spaced helps to improve effluent quality. By contrast, demand dosing releases large amounts of effluent at once, which rapidly passes through the sand. This does not give the biota the proper amount of time to clean the effluent. See also Blackwater Biofilter (also called a Trickle filter) Bioreactor Cesspit Drain-waste-vent system Ecological sanitation Grease interceptor Latrine Composting toilet Outhouse Percolation test (for the capacity of soil to absorb water) Pit toilet Plumber Plumbing Potable cold and hot water supply Traps, drains, and vents Rainwater, surface, and subsurface water drainage Fuel gas piping Sepsis Septage Sewage treatment Sewer Waste disposal Wastewater References Further reading Solomon, C., P. Casey, C. Mackne, and A. Lake. 1998. Mound Systems. National Small Flows Clearinghouse. 1-2. 10 Oct. 2007. Link. National Small Flows Clearinghouse, 1999. MOUNDS: a SEPTIC