What treatment of the onsite source is necessary? Note that most alternative
water sources will require treatment of some kind, ranging from simple filtration
to full treatment in compliance with NSF International/American National Stan-
dards Institute (NSF/ANSI) 350,
Onsite Residential and Commercial Reuse Treatment
What are the basic design factors for capturing and delivering the onsite alterna-
tive water source to the end use? This includes the proximity of the source to the
end use and piping, tanks, and construction that may be necessary to convey the
Because facilities’ ability to capture and convey onsite alternative water sources var-
ies, carefully evaluate the feasibility of using each source to determine the cost im-
plications and payback periods.
Guidance on the construction, alteration, and repair
of alternative water source systems for non-potable water applications is provided in
the International Association of Plumbing and Mechanical Officials
Mechanical Code Supplement
Specific considerations for certain onsite alternative
water sources are provided below.
Facilities with large areas of impervious cover can capture rainfall for use in non-
potable applications. Rainwater that runs off of rooftops is typically of high quality,
making it suitable for many end uses. In most facilities, it is used to supplement or
replace irrigation water with little treatment or filtering.
To estimate the amount of rainwater that can be captured for each rain event, a good
rule of thumb is to assume that 0.62 gallons of water can be collected per square
foot of collection surface area per inch of rainfall. Most rainwater collection system
installers will assume a capture efficiency of 80 percent, because some of the rainwa-
ter is lost through evaporation, splashing, or other means.
Equation 8-1 provides a
calculation for rainfall capture potential.
Equation 8-1. Annual Rainfall Capture Potential (gallons per year)
= Roof Area x Annual Precipitation x Rainfall Capture per Roof Area
x Collection Efficiency (0.8)
Roof Area (square feet)
Annual Precipitation (inches per year)
Rainfall Capture per Roof Area (0.62 gallons per square foot per inch of
Collection Efficiency (80 percent)
Onsite Alternative Water Sources
EBMUD, op. cit.
International Association of Plumbing and Mechanical Officials. February 2010. Green Plumbing & Mechanical Code Supplement. Pages 13-26.