“Ah, but what about the air the rain falls through?” you ask. “What if it’s polluted?” Well, first off, aren’t you breathing that air? Seems like that should be your first concern. But, yes, rainwater can be contaminated by air pollution that plagues industrial areas, heavily populated cites, agricultural areas where crop dusting is common, and anywhere downwind from any of the sites mentioned above. And even in areas where falling rain is free of these hazards, your rainwater collection surface (i.e.: your roof) may also harbor contaminants like bacteria, molds, algae, bird poop, and squirrel pee. But what exactly do you think is in the raw source of many municipal water supplies? Austin, Texas, for example, taps Town Lake, a veritable stew of urban rubbish, for its water supply.
As for your own personal water supply, when collected rainwater is used solely for watering the yard, no treatment is required. To render rainwater potable, a series of filters and an ultraviolet light will do the trick. And for the truly persnickety or for kidney dialysis, nothing beats reverse osmosis, the same process a lot of those fancy bottled water companies use to turn tap water into “spring” water.
Answer excerpted from Richard Heinicken’s Rainwater Collection For the Mechanically Challenged , available from rainwater.org.
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