Water Distribution
Distribution questions test whether you can keep treated water safe, adequately pressured, and free of contamination from the plant to the last customer.
Pressure and storage
Know minimum pressure requirements. Systems are generally expected to maintain at least 20 psi under all conditions, including fire flow, with many systems designed for 35-50 psi in normal service. Storage provides equalization, fire volume, and emergency supply. Stored water ages, so tanks need turnover and routine monitoring to keep residual from decaying or from driving nitrification in chloraminated systems.
Residual and monitoring
Disinfectant residual must be maintained and monitored throughout the system, with boosters where needed. Coliform monitoring under the Revised Total Coliform Rule distinguishes routine samples, repeats, and Level 1 or Level 2 assessments triggered by positives or monitoring failures.
Cross-connection control
The classic trap is matching the wrong backflow device to the hazard. An air gap is the only absolute protection. A reduced-pressure zone assembly suits high-hazard service; a double check valve suits lower-hazard, non-toxic connections. Atmospheric vacuum breakers require no downstream shutoff valve.
Main repair
After repair, mains are disinfected, commonly with about 25 mg/L free chlorine held for 24 hours, then flushed and sampled for coliforms; the standard also permits higher concentrations for shorter contact. Confirm the edition your state adopts before citing.
Worked example
Disinfect a 500-ft, 12-inch main to 50 mg/L using 1% sodium hypochlorite (10,000 mg/L). Volume = 0.7854 ft² × 500 ft = 392.7 ft³ × 7.48 = 2,937 gal. Hypochlorite needed = 2,937 gal × (50 ÷ 10,000) = 14.7 gal.