Backflow Prevention & Cross-Connection Control

Controlling potentially hazardous cross-connections is required by Washington Administrative Code (WAC) 246-290-490. Seattle Public Utilities (SPU) does this by protecting our drinking water quality from its source to every water fixture within our customers' properties.

Something called a "cross-connection" within private water piping systems can threaten the quality of the customer's drinking water first and then threaten the city's drinking water as well. A cross-connection is where substances other than potable drinking water may enter the water pipes and come in contact with your drinking water. SPU requires that any actual or potential cross-connection be either eliminated or have approved backflow prevention installed.

Some common places around your home or property that can result in a hazardous cross-connections include:

  • Irrigation or lawn sprinkler systems
  • Lawn chemical application equipment
  • Boilers
  • Pools and spas
  • Water or other fluid pumps
  • Auxiliary water supplies (drilled wells, water storage tanks, etc.)
  • Gray water systems
  • Fire sprinkler systems (not including approved flow-through systems)

Backflow of contaminants into the drinking water supply can occur due to back-pressure or back-siphonage. Back-pressure occurs when the pressure at the point of use becomes higher than the water supply pressure from the city's water system. Sources of back-pressure include booster pumps, boilers, elevated piping and interconnections with auxiliary systems. Back-siphonage can occur when the water system's pressure falls, such as during water main breaks, firefighting efforts, or as a result of water system construction and maintenance.


Contact Us

For answers to specific cross-connection control questions, lists of approved assemblies or certified testers, or to request an inspection, please contact or call (206) 684-3536.


Public Utilities

Andrew Lee, General Manager and CEO
Address: 700 5th Avenue, Suite 4900, Seattle, WA, 98104
Mailing Address: PO Box 34018, Seattle, WA, 98124-5177
Phone: (206) 684-3000

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Seattle Public Utilities (SPU) is comprised of three major direct-service providing utilities: the Water Utility, the Drainage and Wastewater Utility, and the Solid Waste Utility.