Distribution System Operation and Maintenance
The distribution domain covers pressure, storage, cross connections, flushing, main repair, and recordkeeping - the daily work that keeps treated water safe from plant to tap.
Pressure and residual
Massachusetts requires a minimum of 20 psi throughout the system under all conditions, including fire flow and main breaks. A disinfectant residual must be maintained throughout the system; a result below 0.2 mg/L is treated as loss of residual requiring action, and the maximum residual disinfectant level is 4.0 mg/L. Pressure below 20 psi or a lost residual triggers corrective action and possible public notice.
Storage, mains, and cross connections
- Tanks: floating or elevated; manage turnover to limit water age, inspect interiors and vents, keep hatches locked and screened.
- Main work: disinfection per AWWA C651, flushing, and two consecutive sets of satisfactory bacteriological samples before a main goes into service.
- Cross-connection control: containment assemblies at service connections, annual testing of reduced-pressure zone and double-check assemblies, and surveys by certified personnel.
- Flushing, leak detection, and water loss audits (AWWA M36) control stagnation and unbilled water.
- Exercise valves on a schedule; inspect and drain hydrants so barrels do not freeze.
Worked example
How much water fills 1,000 feet of 12-inch main? Area = 0.785 × (1 ft)² = 0.785 ft²; volume = 785 ft³; × 7.48 gal/ft³ = 5,872 gallons. That figure sets the chlorine dose and flush volume after a repair.
Traps
Quoting feet of head when a customer complains in psi (1 psi = 2.31 ft). Returning a repaired main to service without disinfection and sampling. Confusing a single-device service protection with system-wide containment.