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9611 SE 36th Street
Mercer Island, WA 98040
Phone: 206.275.7783
Fax: 206.275.7787
Email: mifinance@mercergo. . .
Hours: 8:30 am - 5:00 pm (Mon - Fri)
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City of Mercer Island / Finance / Backflow Information

Cross Connections & Backflow Prevention Devices


At the 1933 Chicago World's Fair, 98 people died from Dysentery due to sewage from the toilets that backed up into the water system. More recently, the neighbors to a food processing company discovered that their water was tainted with apple juice!

These events and many others like them are the result of cross connections. A cross connection is any potential or actual physical connection between potable water and a non potable fluid. These connections can range in severity from extreme hazards (such as sewage or chemicals) to aesthetic problems (e.g., juice, dirt, etc.).

Testing a backflow assembly
Without protective devices, (referred to as backflow prevention assembly devices), the potable water supply can become contaminated by any customer in the system. A Cross Connection Control Program is required by the following entities to protect the public from contaminated water:

a) The United States Environmental Protection Agency (through the Safe Drinking Water Act)
b) Washington State Department of Health (WAC 246-290-490)
c) The City of Mercer Island (Ordinance 15C-09)

The program calls for facility inspections to identify and eliminate cross connections, as well as annual testing of backflow prevention assemblies by a State-certified backflow tester.

Does This Apply To You?

Learn more about cross connections here, and whether you need a backflow prevention device, in this simple WA State Dept of Health brochure.

Mercer Island Backflow Program

The customer is responsible for the initial, annual, and any other tests required of the backflow assembly by a certified backflow tester.  The City maintains a database of the water or irrigation systems that have backflow assemblies.  As required by State law, an annual letter will be distributed requesting inspection of the backflow assembly.  The annual Test Report is to be filled out and submitted by the backflow assembly tester via the City’s online test portal.

New Installations
If the assembly is a NEW installation, the backflow assembly tester can submit the first Test Report (see form below) via email to the following City Email address: -- any questions regarding the City’s Cross-connection Program can also be emailed to this same address.

A courtesy list of Backflow Assembly Testers, as well as a Backflow Test Report Form are provided below:

Learn more

- Images of common backflow situations, and protection devices
Educational videos showing how backflow can occur:
- Residential backflow scenario (garden hose to water mains)
- Commercial backflow scenario (industrial pump to water mains)
- Recreational/boat lift scenario (garden hose to water-operated boat lift)
- Backflow flyer for kids (coming soon)

Reference Materials:
- 50 Questions Answered about backflow and cross-connection devices
     (no endorsement by the City is implied)
- Simple overview handbook of cross-connection scenarios and devices [7 Mb]
     (no endorsement by the City is implied)

Download File


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