is covered with coarse gravel. Further layers of sand are placed on top followed by a thick layer of fine sand. The whole depth of filter material may be more than 1 metre in depth, the majority of which will be fine sand material. On top of the sand bed sits a supernatant layer of unpurified water. Advantages As they require little or no mechanical power, chemicals or replaceable parts, and they require minimal operator training and only periodic maintenance, they are often an appropriate technology for poor and isolated areas. Slow sand filters, due to their simple design, may be created DIY. DIY-slow sand filters have been used in Afghanistan and other countries to aid the poor. Slow sand filters are recognized by the World Health Organization, Oxfam, and the United States Environmental Protection Agency as being superior technology for the treatment of surface water sources. According to the World Health Organization, "Under suitable circumstances, slow sand filtration may be not only the cheapest and simplest but also the most efficient method of water treatment." Disadvantages Due to the low filtration rate, slow sand filters require extensive land area for a large municipal system. Many municipal systems in the U.S. initially used slow sand filters, but as cities have grown, and because of their need to treat high-turbidity source waters, they subsequently installed rapid sand filters, due to increased demand for drinking water. See also Notes References "UN High Commissioner for Refugees (UNHCR) Water Manual for Refugee Situations", Geneva, November 1992. Slow sand filters recommendations listed on, p. 38. "Small System Compliance Technology List for The Surface Water Treatment Rule", United States Environmental Protection Agency, EPA 815-R-97-002 August 1997. Slow sand filtration is listed on, p. 24. Water filters Appropriate technology Environmental soil science DIY culture Sand