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　　The provision of safe water necessitates one of the rnajor expenditures of manpower and revenue in our modern cities. The purification of water is basically a two-step or three-step process carried out under the strict supervision of public health scientists and engineers. As the first step, natural water from the least contaminated source is allowed to stand in large reservoirs, where most of the mud, clay, and silt settle out; this is called "sedimentation". Often in water with high mud content, lime and aluminum sulfate are added to the water in the

　　settling reservoirs. These chemicals react in the water to form aluminum hydroxide, which settles slowly and carries much of the suspended material, including most of carries much of the suspended material, including most of the bacteria, to the bottom of the reservoirs. As the second step, the water is filtered through beds of sand and grovel, which remove other impurities and chemicals in it. During or after filtration, chemicals are ordinarily added to the water to kill any remaining harmful bacteria. Chlorine is one of the most common chemicals used for this purpose. A third step taken by some rmnicipalities is adding to the water other

　　beneficial chemicals such as fluoride to make tooth enamel hard, and soda ash to make the water itself soft. The water purification process, carried out with little variation from one large city to another, is perhaps the biggest factor in the prevention of major outbreaks of disease in this country.

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