Getting clean water; atmosphere and hydrosphere review

Even when tap water has been cleaned many times, it may still contain a few dangerous microbes or rust from pipes. So people boil it, filter it, or buy clean water in bottles. Boiling destroys microbes; only adults work with boiling water. Even clear mountain water must be boiled before drinking: it may contain invisible microbes. Muddy water can be cleaned with a simple filter: layers of cloth, sand and charcoal are put in a container; water is poured in at the top and comes out cleaner at the bottom — but it still must be boiled afterwards. Finding water in a desert is hard; sometimes morning dew can be collected with a polyethylene sheet, but it is a method for real need only. A person needs about 30 ml of water per kilogram of body weight. Making sea water fresh means taking out the salt; it is expensive, so it is used only where water is truly scarce. Never drink untreated water from an unknown canal, lake or stream.