A cross connection between
potable and nonpotable water sources poses a high potential risk for
contamination. Cross connections can occur through a direct piping
linkage of a nonpotable water source to a potable water source. In
many cases, however, the cross connection results from a backflow.
Backflow can occur from either backpressure or backsiphonage. Both
types of backflow are due to a pressure differential and result in
the flow of water from a water system user's premises or
establishment toward the potable water distribution system.
Backpressure backflow occurs when the water system
user's water pressure is greater than the potable water system's
pressure. When this happens, potentially contaminated water from the
higher pressure in the user's pipeline will flow into the potable
water system pipeline. Increased pressure on the user side of the
system can result from a booster pump, an elevation differential, a
water storage tank, and even from temperature differences (e.g., as
in a boiler).
Backsiphonage backflow results in pressure in the distribution
system falling below atmospheric pressure (i.e., 14.7 psi at sea
level). Potentially contaminated water in the user's pipelines
is "sucked" into the potable water system because the supply
pressure is less than the atmospheric pressure. A reduce
pressure or partial vacuum can occur when high water flow
conditions exist. This condition can result from a water line
break or a peak demand on the system.
Backflow will occur through any unprotected cross connection
whenever backpressure or backsiphonage conditions exist.
Both SDWA and 64E-8 prohibit cross connections. The
American Water Works Association Publishes a manual entitled
Recommended Practice for Backflow Prevention and Control that
may be ordered through their web site.
This page was last modified on: 10/2/2012 10:34:34