Sand and gravel may seem unlikely choices for materials to use to construct a water filter. Yet water has been filtering through these substances underground for millions of years. The result is spring water, some of the cleanest water on Earth.

Spring water is pure precisely because it has filtered through porous layers of sediment. These natural filters are much deeper and more complex than a bit of sand in a plastic cup. Pure spring water results from filtration through hundreds of feet of sediment, sand, and gravel — a thorough and extremely slow process. Experts estimate that water moves through most natural sediment filters at a rate of less than 0.1 gallon (0.4 liter) per minute for every square foot of area. As it moves through these layers, solids suspended in the water are trapped in the sediments. Disease-causing microorganisms, such as bacteria, are also trapped, and either die naturally or become food for microorganisms that live in the sediment. In the end, the water that bubbles out of artesian springs or is pumped from aquifers several hundred feet below the surface is on average more than 99.9 percent free of contaminants.

Of course, pure spring water is not always easy to find. Surface water is much more readily available, but it often contains contaminants, including disease-causing organisms and toxic chemicals. In many places, groundwater is similarly contaminated. To make water from these sources safe to drink, most treatment facilities use processes and materials similar to those that remove contaminants in natural sediment filters. The objective of artificial filters, however, is to speed the filtration process and decrease the amount of space required for purification. Modern facilities use substances called coagulants to chemically trap particles and organisms, which are then easily skimmed off. Afterward, more fine-grained filtration and treatment with chemicals kill any remaining microorganisms.

In coastal desert regions, where freshwater of any kind is scarce, people have turned to the sea for their drinking water. Making seawater drinkable, however, requires more than simple filtration. One method is called distillation. In this process, seawater is evaporated, leaving the dissolved salt behind. The water vapor is condensed, at which time it is collected as pure, fresh water. Another method, reverse osmosis, uses high pressure to force water through a very fine-grained filter, called a semi-permeable membrane. This membrane allows water molecules to pass through, but not the larger sodium and chlorine ions. Both reverse osmosis and distillation require a large amount of energy to turn seawater into drinking water and are consequently very expensive.

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