A recent report from the Texas Water Development Board entitled: Effect of Roof Material on Water Quality for Rainwater
Harvesting Systems looked at the effect that different roof treatments have on the water quality of collected rainwater.
Materials studied included asphalt-fiberglass shingles, Galvalume® metal panels, and concrete tiles. In addition to
monitoring runoff from these three roof materials, runoff from a green roofing module as well as a white roof were
The study followed best practices for collecting rainwater, fitting all collectors with first flush systems to capture and sequester the dirtiest water before allowing additional water to flow into the two subsequent storage tanks. Such first flush systems are a standard feature on most rainwater harvesting systems because the initial surge of rainfall across the roof washes any contaminants on the roof into the collection system. The collected water from each treatment was tested for a number of water quality indicators, which are discussed below.
Turbidity / TSS
Turbidity is one measure of how many suspended solids are floating around in a fluid. These solids, in high enough concentrations, will cause a clear fluid like water to appear cloudy or opaque. The green roof runoff had the lowest turbidity out of any roofing treatment tested. The results were similar for Total Suspended Solids (the measure of how much stuff collects on a filter when a sample of water is poured through it); the green roof showed the lowest values.
Nitrates in water come from a number of sources, and have potential health implications especially for very young children. While health effects in adults are mild and rare, the presence of nitrates in water usually indicates anthropogenic contamination of some kind in the water.
Because the water samples in this study were collected from roof surfaces, they are less susceptible to the more common vectors of nitrate contamination (fertilizer, human/animal waste). Almost all of the samples were below the EPA limit, and the green roof had the lowest values of all treatments tested.
Dissolved Organic Carbon (DOC)
DOC tests measure the amount of organic material in a solution. Organic materials from plants and animals can break down to such small sizes that they can become dissolved in water. Because DOC is derived from living organisms, it makes sense that the water collected from the green roof had the highest levels of DOC’s.
The big implication here is on the use of chlorine to disinfect collected rain water. Chlorine reacts with DOC to form undesirable chlorinated byproducts such as trihalomethanes and haloacetic acids. To avoid this problem all together, collected rainwater from a green roof should be treated with a disinfectant other than chlorine before use. UV light and ozone are proven alternatives to Chlorine.
Runoff from the green roof was lowest in both Total Coliform and Fecal Coliform compared to the other treatments, but both were present in all treatments after the first flush. Coliform can be eliminated with a UV or ozone disinfectant system.
Roof runoff was also tested for Aluminum (Al), Iron (Fe), Copper (Cu), Zinc (Zn), Lead (Pb), and Chromium (Cr).
Rainwater harvested after the first flush from the green roof consistently showed the lowest concentrations of Al, Fe, Cr, and Cu. The highest Zn concentrations were seen in the harvested rainwater after the first flush from the green and metal roofs.
there is some evidence that green roofs can contribute to a light coloring of the rainwater. This can create an aesthetic problem for users if the water is employed for indoor applications like toilet flushing. Ozone would be a good choice for such situations since it is both a disinfectant and color removal agent.