The air was held in a reservoir and when the pressure reached 100 PSI, a modified irrigation valve was triggered to launch a tennis ball that had been stuffed down the barrel.

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Used water is treated and then either safely disposed of, reused for industrial purposes or air-conditioning, or mixed with reservoir water for drinking.

ECONOMIST: Every drop counts | The

One forest fire near Denver last summer dumped tons of mud and soot into the city's largest reservoir for drinking water and created the worst air pollution in the area's history.

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