Backflow Incident at a Car Wash on a Dead-End Main
A car wash on a 2-inch service off a dead-end 8-inch main has a reduced-pressure-zone (RPZ) assembly. During a main break repair two blocks away, the main was shut down and the car wash lost pressure for about 2 hours. The car wash's booster pump and chemical-injection system (soap and wax) kept running on a small bladder tank. After service was restored, a customer on the same dead-end reported soapy-tasting water. The RPZ's relief port was found discharging continuously, and the assembly's test tag shows it was last tested 14 months ago. The utility's cross-connection program requires annual testing and lists this service as a containment assembly.
1. Explain the failure sequence and identify what the RPZ did and did not protect.
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Sequence:
- Main shutdown dropped pressure at the car wash below the car wash's internal pressure. The booster pump and chemical injection kept running on the bladder tank, so the car wash side stayed pressurized.
- A sustained pressure reversal (backpressure/backsiphonage) developed across the service. An RPZ is designed to open its relief port and dump to atmosphere when the differential across the first check reverses - that is the continuous discharge observed. So the RPZ did protect the public main by relieving the reversed flow.
- However, the relief discharge and the soapy taste indicate the assembly was passing or the relief was overwhelmed. A 14-month-old test means the assembly's condition was unverified; a failed first or second check, or a fouled relief, can allow some passage before/while relieving.
What the RPZ protected: the public main from a direct cross-connection. What it did not do: stop the car wash from generating backpressure in the first place, and it cannot protect if it is overdue for test or has failed. Containment at the service does not protect the car wash's own internal fixtures - that is the car wash owner's responsibility (isolation).
2. State the immediate actions, the notification obligations, and the follow-up you would require.
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Immediate:
- Treat the dead-end main as potentially contaminated: flush the main and affected services, and collect samples (bacteriological and, if chemical contamination is suspected, appropriate chemical tests) before declaring it safe.
- Notify affected customers; if contamination is confirmed or cannot be ruled out, issue the required public notice per the utility's cross-connection/emergency response plan and state rules.
- Shut down or isolate the car wash service until the RPZ is repaired/tested and the car wash's internal protection is verified.
- Have a certified tester test/repair the RPZ; document the relief-port discharge and the failed test.
Notification: report the incident to the state primacy agency as required by the cross-connection program, and notify the car wash owner in writing.
Follow-up:
- Audit the cross-connection program: the 14-month gap is a program failure. Reconcile the testing list and schedule.
- Require the car wash to install/verify internal isolation (e.g., at the chemical injection and booster) so a service shutdown cannot create backpressure into the building.
- Review the main-repair procedure to include notifying high-hazard customers before shutdowns on dead-end mains.