Answer: There are several factors that can affect the quality of water harvested off a roof, including the type of roofing material and environmental and climatic conditions. These factors can contribute to water contamination and may affect how the collected water is used. While many rainwater collection systems are designed for drinking water, water tests have shown that high levels of bacteria, heavy metals (primarily zinc, lead, chromium, and arsenic), and polyaromatic hydrocarbons can affect plant health from water that is collected and used to irrigate plants. In addition, many roofs are treated to resist algae, moss, and lichen and these chemicals can be toxic. It is also important to note that flashing, such as around a skylight or vent, can leach lead in to the water.
There are many types of filtering devices that are used in purifying harvested rainwater, from gutter guards and debris traps to advanced filtration systems. Some systems are even designed to divert the first flush of harvested water away from the catchment system. Studies have shown that as much as 80% of contaminates from the roof or gutter can be diverted in a first flush. That said, several factors, such as the type and slope of the roof, amount of rainfall, and amount of debris, must be considered in determining how much water to divert.
One technology worth considering is a slow sand filter. This filter is designed for small-scale systems and test results from the past five years have shown that it is producing safe levels of drinking water. For more information, visit www.slowsandfilter.org.
For more specific information on filtering rainwater from roofs, consult the following resources.
Kinkade-Levario, Heather. 2009. Design for Water: Rainwater Harvesting, Stormwater Catchment & Alternative Water Use. Gabriola Island, BC. New Society Publishers.
Lancester, Brad. 2005. Rainwater Harvesting for Drylands and Beyond, Vol.1: Guiding Principles. Tucson, AZ. Rainsource Press.
Lancester, Brad. 2012. Rainwater Harvesting for Drylands and Beyond, Vol.3: Roof Catchment and Cistern Systems. Tucson, AZ. Rainsource Press.
Ludwig, Art. 2005. Water Storage: Tanks, Cisterns, Aquifers, and Ponds. Santa Barbara, CA. Oasis Design.
www.harvestingrainwater.com – website of author mentioned above, Brad Lancester. Contains information and resources on rainwater collection.
www.arcsa.org – the American Rainwater Catchment System Association. Promotes rainwater catchment systems in the U.S. Contains information on rainwater catchment, suppliers, and additional resources.
www.nsf.org – NSF International. Organization supports standards for safe food, water, and consumer goods. Contains consumer information on rainwater collection.
www.harvesth2o.com - online community and resources that supports sustainable water management.
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