We have been studying rain water harvesting / roof water harvesting here for 6 years. There are 3 rain water harvesting systems running here now that use water from a composition roof. We use slow sand water filters here. A biological sand water filter (often called a slow sand filter, or a biosand filter) will take nearly all of the petroleum pollutants (and 99.999 percent of pathogens) out of the water from a "tar" roof. Tar is actually used to seal reservoirs - it is basically insoluble in water below 50 degrees F. . These filters are totally sustainable. They don't need chemicals, or petrol or electricity to work. My websites have lots of info about these filters and much more along with over 50 scholarly references and documentation of over 60 epa certified tests showing how well these filters work. I just cannot believe that more people don't know about these filters. They use the naturally occurring life found in all water to form a biofilm in the top layers of sand contained inside filter. These microbes in the biofilm (usually called the "schmutzdecke"), actually eat the pathogens and break down petroleum residuals in water. I've thousands of hours of info on my websites. Its all free and licensed under the creative commons, and, sadly pretty much ignored by most people stateside. Check out slowsandfilter.org, roofwaterharvesing.org, and enlight-inc.com/blog/
