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There are numerous roof material options from asphalt to metal to concrete roof tiles and more. If you’re considering collecting rainwater, it’s important to consider roof material for rainwater harvesting and the material’s impact on water quality.
A study from the Texas Water Development Board titled: Effect of Roof Material on Water Quality for Rainwater Harvesting Systems looked at the effect that different roof materials 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 included.
The study followed best practices for collecting rainwater, fitting all collectors with first flush diversion systems to capture and sequester the dirtiest water before allowing additional water to flow into the two subsequent storage tanks. First flush diversion 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 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.
Roof Material Winner: Green Roof showed lowest values.
Nitrates in rainwater 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 tested were below the EPA limit, and the green roof had the lowest values of all treatments tested.
Roof Material Winner: All roof materials tested below EPA limit; Green Roof had lowest values.
Dissolved organic carbon 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. Other roof materials generally showed a decrease in DOC concentrations after the first flush, but the asphalt shingle roof showed an increase in DOC concentration from the first flush.
The big implication here is on the use of chlorine to disinfect collected rainwater. 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.
Roof Material Winner: Varies.
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 Material Winner: Green Roof at first flush.
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.
Roof Material Winner: Green Roof, except Zinc (after first flush).
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.
According to the study, none of the roofing materials emerged as clearly superior to the others in terms of the quality of the rainwater harvested after the first flush. The quality of the rooftop-harvested rainwater generally increased with roof flushing indicating the importance of having an effective first flush diverter. The study also indicated that harvested rainwater must be treated prior to potable use. Green roofs showed better results in some cases, but roofing material is just one factor that affects rainwater harvesting. It’s important to note that green roofs are not the best candidates for rainwater harvesting for indoor domestic use if the water is disinfected with chlorine. Lastly, while metal and tile roofs are commonly recommended for rainwater harvesting in developed countries, the data suggest that asphalt fiberglass shingle and cool roofs also might be considered for this purpose.
There are several factors to consider for rainwater harvesting. If you have questions about rainwater collection and harvesting or would like Spartan to evaluate your rainwater treatment objectives, please contact us or e-mail directly at info@spartanwatertreatment.com. We look forward to helping you.
Spartan Environmental Technologies, LLC
PO Box 1270
Mentor, OH 44061-1270