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Each day, millions of Americans use billions of gallons of water without knowing where it comes from or what might be in it. As populations grow, the combination of increased demand and increased pollution means many of us are using sources of water that are less than pristine. Contamination from sediment, bacteria, protozoa, heavy metals, and synthetic organic compounds shows up with alarming frequency. As a result, many municipalities are having to pre-treat drinking water. The first step in most municipal treatment systems involves gravity. If you've ever let a glass of chocolate milk stand for any length of time, you've probably noticed that much of the chocolate settles to the bottom of the glass. The same is true of sediment in water. When the water is allowed to stand in large pools, many of these suspended particles simply settle to the bottom where they are collected and disposed of. Next comes flocculation. Here, a chemical is added to the water that causes tiny suspended particles to clump together. Usually, these flocs settle out just like large-sized sediment. But if they escape, they are caught by filters farther down the line. To kill unwanted microbes, protozoans, and other living organisms, many municipalities chlorinate the water. Chlorine is a chemical that kills microorganisms. In drinking water, the concentrations are low enough that often you can't even taste it. The final stage in most municipal treatments is filtration and aeration. Water is pumped through large tanks filled with fine sand, called rapid sand filters. As the water flows through the spaces between the sand grains, suspended particles and dead microbes get trapped. Sometimes, crushed anthracite coal is used in addition to sand. Because the coal grains carry a charge on their surface, they act like tiny magnets attracting other charged contaminants. Aeration is exactly what it sounds like - adding air to water. When sprayed through large, fountain-like devices, water not only gains a great deal of oxygen, but any volatile compounds that may have been in the water escape into the atmosphere. In the end, all the water we use doesn't just go down the drain and disappear. It gets recycled by the evaporative power of the sun. When water evaporates, most of the contaminants stay behind, which in theory means it rains pure water back on the land. Unfortunately, because of gases in the air, much of the water that falls from the sky is coming down pre-polluted, so municipalities are going to have to work even harder in the future to treat the water we drink.