Rainwater Harvesting and Stormwater Reuse for Arid Environments - Design with the Desert: Conservation and Sustainable Development - page 367

FIGURE 21.2
Residential cistern painted to look like a turtle. (From Kinkade-Levario, H., Forgotten Rain, Rediscovering
Rainwater Harvesting , Granite Canyon Publications, Forsyth, MO, 2004.)
The quality of the captured rainwater depends, in part, upon catchment texture as the best
water quality comes from the smoother, more impervious catchment or roofing materials.
Captured rainwater quality is also determined by rainfall pattern and frequency. Both the
greater the storm event—i.e., the rainfall extent and the quantity of rain that falls—and
the shorter the time between storms will affect the catchment surface condition. The larger
the quantity of rain and the more often it rains means the catchment area will be cleaner
and fewer pollutants will be transported to the first-flush device or to the storage unit dur-
ing subsequent rainfall events.
If a catchment area is insufficient for quantities required to meet a water demand,
supplemental water sources can be added to the rainwater/stormwater quantities. These
water sources can include cooling tower blowdown water, air-conditioning condensate,
pool water backwash, and gray water. For treatment of any nonpotable water, local codes
should be reviewed (Figure 21.2).
21.3.2 Conveyance
A commonly used rainwater conveyance system is comprised of gutters with downspouts
and/or rainchains. Gutters and downspouts direct rain from rooftop catchment surfaces
to cisterns or storage tanks. Gutters and downspouts can be easily obtained as a standard
construction material, or they can be specifically designed to enhance a building facade
and maximize the amount of harvested rainfall.
21.3.3 Roof Washing
Roof washing is the initial process in reducing the debris and soluble pollutants that may
enter a rainwater harvesting system. Roof washing systems may use one or several com-
ponents to filter or collect debris and soluble pollutants, including gutter leaf guards, rain
heads, screens, and/or first-flush devices.
Roofs, like other large, exposed areas, continuously receive deposits of debris, leaves,
silt, and pollutants on their surfaces. All rainwater dislodges and carries away some
of these deposits, but, during any given rainfall, the stormwater that falls first carries