Credit: Erik Christensen via Wikipedia
One might think that green roofs and rainwater harvesting were made for each other, that the sand, soil and filter cloth would help clean the water as it made it's way to the drain. But apparently not; in a new study by the Texas Water Development Board (TWDB), investigators found that water from green roofs had high levels of dissolved organic carbon.
With funding from TWDB, Cockrell School faculty and students conducted an in-depth study - recently published in the academic journal Water Research - examining the effects of conventional and alternative roofing materials on the quality of harvested rainwater. The study, led by civil, architectural and environmental engineering Assistant Professor Mary Jo Kirisits, showed that, of the five roofing materials tested, metal (specifically Galvalume®), concrete tile and cool roofs produce the highest harvested rainwater quality for indoor domestic use. The study also showed that rainwater from asphalt fiberglass shingle roofs and increasingly popular "green" roofs contain high levels of dissolved organic carbon (DOC). Although other potential pollutants can be significantly lower on green roofs (turbidity and aluminum), the high DOCs are significant where these roofs would be used for potable rainwater collection.
So green roofs actually do filter out some things, like dirt and aluminum, but add others.
Dissolved organic carbon is commonly found in surface water and lakes; it is:
Organic material from plants and animals broken down into such a small size that it is "dissolved" into water. Some DOC molecules have a recognizable chemical structure that can easily be defined (such as fats, carbohydrates, and proteins) however most have no readily identifiable structure and are lumped under the term humic or tannin substances.
But the investigators note that when DOCs are mixed with chlorine, it can form potentially cancer-causing byproducts. But they note that any harvested rainwater is going to have to be filtered and treated if it is going to be used for drinking, and most is used for irrigation. So it shouldn't be a reason not to use a green roof.
More in Physorg.
More on Rainwater Harvesting
Los Angeles May Get Mandatory Rainwater Harvesting Law : TreeHugger
Rainwater Harvesting Poised To Grow In US West : TreeHugger