New information is available about rainwater harvesting from composition roofs. The slow sand filter described on slowsandfilter.org has been shown to remove petroleum hydrocarbons from water down to less than 1 part per ten million by weight; and this is when it is operating at its LEAST efficient rate of purification (at 32 degrees F). Also, petroleum hydrocarbons are present in roof water from the composition roof in the study in potentially harmful amounts if consumed over a long period of time (2.9 parts per million by weight). If a first flush diverter is used the, concentration is reduced to almost acceptable levels (less than 1 part per million by weight). The hydrocarbons may, in fact, be originating partially from local air pollution, because diesel fuel traces were found along with heavy oils. The implication here is that composition roofing is indeed a viable means of collecting rainwater, if proper filtering is used. A first flush diverter can be built easily with some pvc and recycled containers, and a slow sand filter can also be put together for minimal cost using recycled containers.
Another important fact uncovered is that rainwater from a roof may indeed vary WIDELY in the amount of Coliform bacteria present. The study has shown that water from a roof can contain about 60 cfu/100ml of coliform bacteria (relatively harmless) to 50,000 cfu/100ml (very likely to contain harmful pathogens and not safe to use), and that a slow sand filter will vary somewhat in its ability to remove pathogens; and also that a first flush diverter will remove a considerable number of the pathogens, but not enough to be safe. The slow sand filter has been shown repeatedly, by other studies (see the lit cited page on the above mentioned website) to remove coliform, ecoli, and fecal coliform to a safe level – from none detectable, in the best case, to 10 cfu/100ml in the worst case. In all cases the water from the filter is very non-turbid and should work well in conjunction with a UV filter
followed by an epa approved point of use tap water filter.