STRUCTURAL MEASURES: CONSTRUCTION, OPERATION, AND MAINTENANCE - Low Impact Development and Sustainable Stormwater Management - page 185

4 months, so the storage capacity must equal that demand. Again, if potential
capture is insufficient for total seasonal needs, public supply must augment the
system.
2. Where potable use is anticipated, the storage may be coupled with public
service, following guidelines established by purveyor and local health department
regulations. Filtration and disinfection units must be approved by all regulatory
agencies.
3. Some designs may identify opportunities where treated wastewater can
be reused for irrigation. It is recommended that such systems be isolated from
rooftop capture systems and stored separately, because of the potential for con-
tamination of potable supply.
4. The estimate of irrigation water demand will be based on the selection of
landscape materials. As discussed elsewhere in the topic, a number of possible
plant mixes can be designed that minimize the need for irrigation. Every effort
should be made to utilize the guidance shown to reduce this demand as much
as possible, assuming that in the near future, the use of potable water for this
purpose will be restricted. Based on the acreage in landscape and the minimum
use, the designer must determine how much water will be needed to achieve this
goal and how often the storage unit will be refilled via precipitation.
5. For residential design, rain barrels and cisterns should be positioned to
receive rooftop runoff directly, especially with roof designs that have multiple
panels. If only irrigation use is anticipated, the site design should be configured
so that the planted beds are in close proximity to the downspouts. For potable use,
single tank storage is most efficient for filtration, disinfection, and pressurization.
The design capacity should probably be at least 2,000 gallons per dwelling unit,
and greater if possible. External storage below grade may be most appealing
from an aesthetic consideration.
6. Provide for the discharge or release of unconsumed stored water between
storm events so that the necessary stormwater storage volume will be available
and fully mitigate any runoff rate increase. Any release should, where possible,
be directed to an infiltration system.
7. Consider household water demands when sizing a system to supplement
residential gray water use. Every effort should be made to minimize potable water
usage, including low or ultralow flow fixtures and other techniques that reduce
demand.
8. Discharge points or storage units should be clearly labeled “Caution:
Untreated Rainwater: Do Not Drink ” unless an approved disinfection unit is
installed in line with the service. In some countries, such as New Zealand,
rooftop runoff is used for all residential demands following ultraviolet
disinfection (Figure 8-9). It is appropriate that this example should be the
last image in this topic, because it represents the greatest potential concept in
stormwater management: reuse for potable purposes.
9. Screens should be used to prevent roof debris from entering storage units.