Backflow After Main Shutdown at a Commercial Building
During a 6-hour emergency shutdown of a 10-in main, pressure in a commercial district drops to zero. A carpet-cleaning business has a domestic service with an RPZ assembly, but its irrigation system is teed off the service line upstream of the RPZ and has no backflow assembly. The irrigation booster pump is running when pressure returns. Two days later, customers in the building report discolored water and a chemical odor. The utility's repair crew used a chlorine spray on the new pipe but did not flush the service line before restoring pressure. The building owner asks whether the RPZ 'should have prevented this.'
1. What immediate actions should the utility and building owner take?
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Treat this as a possible backflow incident. Stop the irrigation booster pump and isolate the irrigation connection. Shut off or isolate the building service if contamination is suspected, and advise occupants not to drink or use the water until it is cleared. Notify the utility's cross-connection program and the primacy agency as required. Collect samples for bacteria and for the suspected chemical, inspect the RPZ and irrigation piping, and provide bottled water or an alternate supply if needed. Do not simply flush the building and reopen it without identifying the source and confirming that the water is safe.
2. Explain why the RPZ did not prevent the problem. What would you investigate?
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An RPZ protects only the piping downstream of its location. The irrigation tee is upstream of the RPZ, so the RPZ never saw the irrigation system. When main pressure dropped to zero, the running irrigation booster pump could push irrigation water backward through the service line into the building and the main. The missing irrigation backflow assembly is the key defect. Investigate the RPZ's test history and current condition, confirm the irrigation tie-in location, look for other unprotected connections, review the repair crew's disinfection and flushing steps, and sample both the building and the distribution main to determine the extent of contamination.
3. What corrective and preventive actions should follow?
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Relocate the irrigation connection downstream of the RPZ or install a dedicated, testable backflow assembly on the irrigation line. Test all assemblies and maintain annual testing records. Establish a premise-isolation policy and a shutdown SOP that requires flushing and disinfection of services after zero-pressure events. Train repair crews to flush service lines before restoring pressure and to document it. Inspect the building for other cross-connections, notify the regulator of the incident, and follow up with sampling before removing any use restrictions.