Media filters are used to provide further treatment of septic tank effluent. Media, such as sand, acts as a filter. The media is placed above a liner of impermeable material such as plastic or concrete. They can be designed to pass the effluent once or multiple times through the media bed. Because of the fluctuating flow of wastewater the upper region of the sand layer alternates between aerobic and anaerobic conditions. Under these conditions, a significant part of nitrogen in the wastewater can be removed by nitrification and denitrification. In addition, if materials that can remove phosphate are mixed with the sand, phosphorus in the wastewater is also removed. One material, that has been found to remove phosphate effectively with a capacity for phosphorus removal for several years, is bauxite-refining residue (red mud). (UNEP 2000 & EPA 2004)