Drinking Water Distribution: Residual, Pressure, and Backflow
Water quality does not stop at the plant gate
Distribution acts like a reactor: pipe material, biofilm, storage, and residence time all shift chlorine residual, turbidity, and lead/copper. The core controls are minimum pressure (most states require at least 20 psi at all points under maximum demand), tank turnover, valve and hydrant exercise, systematic flushing, cross-connection control, and corrosion control through pH adjustment and inhibitors such as orthophosphate.
Backflow protection ranks by hazard: an air gap is the highest protection, a reduced-pressure-zone assembly serves high-hazard connections and must be tested annually by a certified tester, and a double check valve is limited to low hazard.
Traps
- A sample from the plant or a freshly flushed hydrant shows a falsely healthy residual; compliance samples represent what the customer receives.
- Chloraminated systems nitrify at dead ends: falling residual, falling pH, rising nitrite.
- Stagnation, not low tank level, ages water; long turnover times drive disinfection by-product formation.
- Distribution pressure, not chlorine dose, explains many customer complaints about taste and intermittent discoloration.
- Total coliform positives trigger repeat samples within 24 hours at the same site plus immediate upstream and downstream taps.
Worked example
Volume of 500 ft of 8-inch main: diameter = 8 ÷ 12 = 0.667 ft; area = 0.785 × 0.667² = 0.349 ft²; volume = 0.349 × 500 = 175 ft³; gallons = 175 × 7.48 ≈ 1310 gal. To dose 25 mg/L: 1310 gal × 8.34 lb/gal = 10,930 lb of water; 10,930 × 25 ÷ 1,000,000 ≈ 0.27 lb chlorine. Hold the required contact time and verify residual before placing the main in service.