Drinking Water Distribution Systems and Storage
Pressure, storage, and water quality in the pipe
Minimum pressure in the distribution system is 20 psi under the Safe Drinking Water Act; anything lower triggers public notification and boil-water consideration. Storage falls into elevated tanks, ground-level reservoirs, and hydropneumatic tanks. Tanks provide fire flow and equalization but also create water age, which drives disinfectant decay, nitrification in chloraminated systems, and biofilm growth. Calculate turnover by dividing tank volume by average daily demand.
Maintain a detectable disinfectant residual throughout, monitor for total coliform and E. coli under the Revised Total Coliform Rule, and respond to positive samples with repeat samples and an assessment. Corrosion control is separate chemistry: the Lead and Copper Rule uses a 90th percentile comparison against action levels of 0.015 mg/L lead and 1.3 mg/L copper, so pH, alkalinity, and inhibitor dosing must be stable. DBP compliance uses locational running annual averages of 80 ug/L for TTHM and 60 ug/L for HAA5, and sites are chosen where water age is highest.
Cross-connection control is the highest-consequence program: backflow prevention devices, air gaps, and annual testing, plus containment at the service connection.
Traps: pressure and fire flow questions often hinge on psi versus feet of head (1 psi equals 2.31 ft of water); hypochlorite decays faster in warm storage; dead ends and oversized mains are water-quality problems, not just hydraulic ones; flushing programs must not exceed chlorine or DBP limits in discharge permits.
Worked example
A 0.5 MG elevated tank serves 1.2 MGD average demand: turnover = 0.5 / 1.2 = 0.42 day, about 10 hours. That is healthy. If demand dropped to 0.25 MGD, turnover becomes 2 days - expect low residuals and nitrification and plan a turnover or mixing fix.