Andrea Kirsh had never heard of permeable pavement before, and storm water runoff wasn't exactly high on her list of dinner conversation topics.

Yet this art historian from University City is now conversant with both subjects and among the biggest boosters of Rain Check, a Philadelphia Water Department program designed to help homeowners manage storm water.

"Why throw away something - water, in this case - when you can keep it and reuse it? And you're doing something good for the environment," says Kirsh. Through Rain Check, the University City resident got a price break on rain-absorbing pavers and a 55-gallon rain barrel that collects rain water from a downspout attached to the roof to be reused on lawns, gardens, and cars.

Philadelphia was one of the first cities in the country to develop a sewer system; in the oldest neighborhoods, such as South and West Philadelphia, Northern Liberties, and parts of Germantown, that system combines waste water and storm water, which means that heavy rains put diluted sewage into the Delaware River and the Schuylkill.

According to the Water Department's Maggie Dunn, the idea behind Rain Check, which is part of Green City, Clean Waters, Philadelphia's 25-year storm water management plan, "is to use natural systems to collect some of that rain water before it even enters the sewer system, to lower the amount the treatment plants have to clean, so they can clean the waste water that's there."

Besides free rain barrels, those "natural systems" include the subsidized removal of impenetrable concrete or asphalt surfaces, and the installation of permeable pavers, rain gardens, and downspout planters.

Since 2012, Rain Check - which is managed by the Pennsylvania Horticultural Society, in partnership with the Sustainable Business Network - has given away more than 3,500 rain barrels and shared the cost of 113 downspout planters; 22 rain gardens; 31 masonry jobs.

Rain barrels (one per household) are available to any city resident who attends a free workshop. For the other offerings, you must live in one of the combined sewer areas.

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