The required capacity
of a rain barrel is a function of the rooftop surface area that drains to
it, the inches of rainfall required to fill the barrel, and water losses,
due mainly to evaporation. A general rule of thumb to utilize in the sizing
of rain barrels is that 1 inch of rainfall on a 1000 square foot roof will
yield approximately 600 gallons.
Sample
Calculation
Rain barrel volume can
be determined by calculating the roof top water yield for any given
rainfall, using the following general equation:
1
Equation 1. V=A2xRx 0.90 x 7.5
gals./
ft.3 where:
V
=
volume of rain barrel (gallons)
A2
=
surface area roof (square feet)
R
=
rainfall (feet)
0.90
=
losses to system (no units)
7.5
=
conversion factor (gallons per cubic foot)
Example: one 60-gallon
barrel would provide runoff storage from a rooftop area of approximately 215
square feet for a 0.5 inch (0.042 ft.) of rainfall.
60 gallons=
215 ft.2
x
0.042 ft. x 0.90 x 7.5 gallons/ft.3
Cisterns
can be sized following the same general formula given above for rain
barrels.Due to the size of rooftops and the amount of
imperviousness of the drainage area, increased runoff volume and peak
discharge rates for commercial or industrial sites may require large
capacity cisterns. Individual cisterns can be located beneath each
downspout, or the desired storage volume can be provided in one large,
common cistern that collects rainwater from several sources.
Pre-manufactured residential-use cisterns come in sizes ranging from 100 to
1,400 gallons. Cisterns designed for more than just supplemental use, for
full time domestic use, should be sized based upon a minimum of 30 gallons
per day per person when considering all potential domestic water uses.2Cisterns
should be located for easy maintenance or replacement.
The
water collected with a rain barrel or cistern can also be used for
potable (drinking) or other domestic water use if sufficient treatment
is provided and the system meets the local plumbing code. Water
treatment techniques for rainwater catchment systems include:
Screening.
The use of strainers and leaf screens located in the gutters and
downspouts are designed to prevent debris, like leaves, from entering
the tank.
Settling.
Sedimentation within the tank is necessary to settle out any potential
particulate matter and solids.
Filtering.
The use
of filters can include in-line multi cartridge systems, activated
charcoal, reverse osmosis, mixed media systems and slow sand filters;
all designed to remove potential contaminants either at the pump, tank
or tap.
Disinfecting.
The use
of boiling/distilling, chemical treatment (chlorine, iodine),
ultraviolet light and/or ozonation are all designed to kill
microorganisms, usually directly within the tank.
While rainwater
catchment systems are largely unregulated in many areas, local
regulations may require that plumbing and health codes are be met.
Some jurisdictions may require periodic testing of water for fecal
coliform bacteria, as is required for any private water system.
Maintenance
requirements for rain-fed potable water systems are higher than
those of systems designed for irrigation or stormwater control.
Typical maintenance activities consists of keeping gutters and
cistern screens clean as well as periodic inspection and replacement
of any water treatment components and equipment. The tank also
needs thorough cleaning, usually in the summer when its water levels
tend to be lower. Backflow prevention devices also require annual
inspection.
