A solar still is a low-tech way of distilling water, powered by the heat of the sun (more precisely, the heat & humidity of the soil, and relative cool of the plastic). Two basic types of solar stills are box, and pit. In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through clear plastic. The pure water vapor (and any other included volatile solvent) condenses on the cool inside plastic surface and drips down off of the weighted low point, where it is collected and removed. The box type is more sophisticated.
Solar stills are used in cases where rain, piped, or well water is impractical, such as in remote homes or during power outages. In Florida and other hurricane target areas that frequently lose power for a few days, solar distillation can provide an alternate source of clean water.
Solar stills are occasionally used on a longer term basis in developing world settings. However, depending on environmental conditions, they usually produce a relatively small amount of water, and even less where the source is saline or brackish. If the source is inadequate, a compromise method is to mix the distilled water with the brackish or saline water purified with other methods - this gives a more adequate quantity and re-introduces the other source contaminants, while still lowering the salinity, and improving the taste. The Water Pyramid is a larger version, which uses an inflatable dome as the condensing surface and can be used in tropical, rural areas.
Knowing how to put together a solar still can prove to be a useful survival skill and could provide an important means of potable water in the event of a wilderness emergency. Nevertheless, under typical conditions makeshift solar stills rarely produce enough water for long-term survival, and the sweat expended in building one can easily exceed its daily output. Solar stills can extract water from moisture in the ground but to increase the amount of moisture available to a solar still, water (fresh or saline) can be added inside or along the edges of the still. Where no water sources are readily available, shredded vegetation , wet soil/sand, urine or covered feces, can be used inside the pit. If only the natural soil moisture is used, the pit must eventually be moved as the productivity drops. To prevent losing moisture by taking apart the still to retrieve collected water a length of plastic tubing can be used to sip water as it accumulates.
Besides using stills (which work by controlled evaporation and condensation of water), filtration, and reverse osmosis are more difficult but more productive. Water may thus be obtained from other larger sources (eg the sea as well as fauna and flora, ...) More information can be found at Drinking water.
Jackson, R. D.//van Bavel, C. H. M. (1965) Solar Distillation of Water from Soil and Plant Materials: A Simple Desert Survival Technique. Science 149(3690)1377-1378