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Food and shelter are crucial for living, but nobody can survive for very long without water. 's why, since the beginning of history, civilizations have lived near abundant sources of H20. But it's not enough just to have plenty of it. The same water that gives life can also make people sick or even kill them, if it contains dangerous substances or disease-causing microbes. As a result, humans have been trying to purify water for thousands of years. As far back as 1500 B.C. Egyptians used the chemical alum to filter suspended sediment out of their dr
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