Rainwater Harvesting and Stormwater Reuse for Arid Environments - Design with the Desert: Conservation and Sustainable Development - page 369

FIGURE 21.3
Commercial office building with a butterfly roof hidden behind the parapet directs rainwater to a rainchain and
retention basin. (From Kinkade-Levario, H., Forgotten Rain, Rediscovering Rainwater Harvesting , Granite Canyon
Publications, Forsyth, MO, 2004.)
The outlet leads to the distribution system. Some systems—especially if they are a sole
source of water or if landscape irrigation requires supplemental water—may have an inlet
pipe from an alternate, makeup water (water supplied to compensate for losses) source
such as a municipal water supply. Whenever an alternate, makeup water source is used
with a storage system, an air gap (a typical size is 14 in.) must be maintained between the
high water line or highest flood line in the storage cistern/tank and the inlet of the alternate
water. The overflow line should be placed to maintain the maximum high water line in the
cistern/tank. An additional security to avoid contamination of the alternate water supply is
a reduced pressure double backflow device which should be installed in the alternate water
line prior to it reaching the storage container. A reduced pressure double backflow device
also may be required on the service line from mains/municipality (Figures 21.3 and 21.4).
21.3.5 Distribution
Stored rainwater may be conveyed or distributed by gravity or by pumping. If a tank is
located uphill or above the area proposed for irrigation, gravity may be sufficient for the
system. Most plumbing fixtures and appliances including drip irrigation systems require
at least 20 lb per square inch (psi) for proper operation, while standard municipal water
supply pressures are typically in the 40-80 psi range. Pumps, rather than elevated tanks,
are typically used to extract both below-grade and at-grade cistern- or tank-stored water.
Submersible or at-grade pumps may be used in any rainwater storage system. Self-priming
pumps with floating filter intakes and automatic shutoffs—for times when water levels are
insufficient—are optimal equipment. The bottom few inches of stored water will typically
contain very fine sediment and should be avoided if possible. A storage system may