A recent report from the Texas Water Development Board entitled: Effect of Roof Material on Water Quality for Rainwater Harvesting Systems was recently published. As the title mentions, the study centered on the effect that different roof material have on the quality of collected rainwater. A survey of the most common roofing materials in Texas was completed, and found that the three most common were asphalt-fiberglass shingles, Galvalume® metal panels, and concrete tiles. In this post we will look at two specific issues in the study, dissolved organic carbon (DOC) and coliform contamination. The study looked at the impact of the first flush through third flush of water off the surface. As expected, the first flush contains the most contaminants.
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 disinfection system, but a green roof goes a long way towards eliminating it before filtration.
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 all sorts of undesirable 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.