Distribution System Operations, Storage, and Cross-Connection Control
Keep pressure, quality, and separation
The distribution system is where treatment either holds or unravels. Maintain at least 20 psi everywhere under peak demand and fire flow; low pressure invites contamination and backflow. Keep water age down with looping, strategic flushing, and proper tank turnover - long detention burns residual, feeds nitrification in chloraminated systems, and raises DBPs.
Storage tanks need circulation (inlet/outlet placement or mixing), routine inspection for sediment, corrosion, and intrusion, tight hatches and vents with screens, and an overflow that cannot admit vermin. Chambers and vaults are confined spaces.
New and repaired mains must be disinfected, flushed, and bacteriologically sampled before service. Coliform monitoring follows the Revised Total Coliform Rule: positives trigger repeat samples, and E. coli or a routine/ repeat pair brings assessment and, if confirmed, public notice.
Cross-connection control is heavily tested. Understand backpressure (downstream pressure exceeds supply) versus backsiphonage (negative supply pressure). Recognize the air gap (best protection), the reduced-pressure principle assembly (health hazard, protects against both conditions), the double check valve assembly (non-health hazard only), the pressure vacuum breaker, and the atmospheric vacuum breaker (no downstream shutoff, no continuous pressure). California requires testable assemblies to be tested on installation and annually by a certified tester.
Traps
- Using a double check valve where a health hazard requires an RPZ.
- Confusing containment (protects the public main at the service connection) with isolation (protects inside plumbing).
- Ignoring thermal expansion after installing a backflow device on a closed system.
Rule of thumb
If a contaminant could cause illness and could be pushed or drawn back into the main, only an air gap or an RPZ is acceptable.