Drinking Water Distribution
Core concepts
Distribution is where treated water quality can degrade. Track three things: pressure, disinfectant residual, and water age. Keep at least 20 psi everywhere at all times; any negative pressure is a contamination risk requiring corrective action and notification. Maintain a detectable residual, commonly ≥0.2 mg/L free chlorine, and recognize that residual decays with age, warm temperature, and high organic or sediment load.
Know cross-connection control: air gaps, testable backflow assemblies, annual testing, and the priority of protection (containment, isolation, premises). Know new-main and repair disinfection - chlorinate, hold, flush until the residual matches the system, then collect bacteriologic samples. Know unidirectional flushing, storage tank turnover, valve and hydrant exercise, and lead/copper corrosion control through pH and alkalinity adjustment.
Traps Questions Love
Free versus total chlorine. Assuming pressure is maintained during a main break. Percent slope versus ft/ft. Sampling at dead ends or at a customer's kitchen tap instead of a designated site. Forgetting that a bigger storage tank means longer water age unless turnover is managed.
Worked Example
A 1,000-ft, 8-inch main holds: area = π/4 × (0.667 ft)² ≈ 0.349 ft²; volume = 349 ft³; gallons = 349 × 7.48 ≈ 2,610 gal. Use the same math to size a chlorine slug for new-main disinfection - commonly 50 mg/L held 24 hours, then flushed to system residual.