PME - August 2009 - (Page 14)
HOW TO DESIGN DECENTRALIZED RAINWATER HARVESTING SYSTEMS By Ed Crawford and Sarah Lawson W orldwide consumption of water is rising at double the population growth rate. Many municipalities are struggling with supplying water to meet the needs of desired growth. At the same time, federal stormwater regulations are causing municipalities to consider various options in order to reduce reliance on aging water and stormwater infrastructures. Decentralized rainwater harvesting systems collect and supply water on-site and reduce reliance on potable water supplies. Decentralized rainwater harvesting systems can also have an impact on energy demands. Efforts to reduce our energy consumption have overlooked the cost of energy required to supply water to end users. Treating and distributing drinking water in the U.S. accounts for 4% of the total energy used. Almost 80 cents of every dollar spent by municipalities on water treatment and distribution is for electricity (Electrical Power Research Institute, 2002). Much of this potable water is used for non-potable water needs. An alternative to this energy-intensive process of supplying people with water is falling from the sky: rainwater. Harvesting rainwater has a plethora of environmental, social and economic benefits. Why should you take a closer look at rainwater harvesting? The reasons are many: • Save money on utility bills • Manage stormwater runoff • Reduce non-point source pollution • Reduce reliance on aging infrastructures • Attain points for LEED certification • Recharge local aquifers with tank overflows • Reduce detention pond collection requirements • Increase building resale value Did you know that one inch of rain on a 1,000-sq.ft. roof is 623 gallons of water? Think of all this water flowing off the roof. Rather than becoming pollutant carrying stormwater, rainwater can be stored and re-used on-site — reducing the volume of stormwater reaching rivers and streams and supplying potable and non-potable needs in and around the building. Non-potable uses of water include: Toilet flushing, irrigation, water features, laundry washing, vehicle washing, cooling towers, fire suppression, pool/pond filling and manufacturing processes. When considering a rainwater harvesting system, local authorities should be consulted regarding regulations or codes pertaining to rainwater. The use of rainwater is fairly new to most localities. Others may have codes and regulations in place. Most municipalities appear focused on cross connections and air-gaps. Rainwater systems can be applied to single-family and multi-family residential, commercial and industrial projects. When designing a system for new construction, incorporating a rainwater harvesting system can reduce or eliminate the requirements for detention ponds. Civil and plumbing engineers should work together during this process to accomplish their goals and meet regulatory requirements. As Easy As 1-2-3-4 When designing a rainwater harvesting system, four basic steps should be followed and implemented. While these steps will often be a part of the civil engineer’s design, the result will be a low-maintenance, high water quality system with reduced risk of failure, decreasing the need for additional treatment and concern from the plumbing designer. Systems should be designed to protect and enhance the naturally occurring biofilm in the tank. Diagram of a vortex filter. 14 08.09 Umbrella photo courtesy of ©iStockphoto.com/Steven Foley. All other graphics courtesy of Ed Crawford.
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