Drinking Water Distribution
Pressure and hydraulics
A minimum of 20 psi must be maintained throughout distribution under all conditions, including fire flow; normal operation is roughly 40-80 psi. Pressure, not flow, is the regulatory concern. Know the hydraulic grade line, head loss, and C-factor, and why a large main break drops pressure and triggers a boil-water advisory: loss of pressure means potential contamination.
Residual and water quality
Maintain a measurable disinfectant residual throughout the system, either free chlorine or chloramines. Chloraminated systems are prone to nitrification: residual falls, nitrite rises, and the response is breakpoint chlorination, flushing, or basin turnover. Storage tanks need turnover and mixing to prevent stagnation. Disinfection byproducts form from chlorine and natural organic matter; control by removing TOC, adjusting dose, or changing the setpoint.
Cross-connections and repair
Backflow protection is matched to hazard: air gap, RPZ, or double check. New and repaired mains are disinfected, flushed, and sampled bacteriologically before return to service. Flushing should reach about 2.5 ft/s to scour the pipe.
Sampling traps
Total coliform routine samples per month depend on population served. A positive triggers repeat samples and state notification. Traps: confusing sample site with source; assuming a chlorine residual excuses a bacteriological positive; forgetting that a positive changes monitoring frequency.
Worked example
A dead-end line shows 0.2 mg/L free chlorine while the plant finishes 1.0 mg/L, so decay is 0.8 mg/L. To hold 0.5 mg/L at the dead end, the finish residual must be 0.5 + 0.8 = 1.3 mg/L, or add a booster or flush the dead end.