Intermittent Water Supply: The Intrusion Volume per Outage Computed from First Principles, and a Household Break-Tank Design That Defeats It

SIMULATED, NOT MEASURED. For a stated reference network (10.0 km of mains, 10,000 people, stated leak census totalling 68.33 mm2), integrating the orifice equation over a stated drain-down profile (30.0 m head falling to a -5.0 m trough, 30 min hold, 30 min recovery, soil head 1.0 m) gives an intrusion of 1.49 m3 of soil water per outage: 2.98 m3/day and 1,087 m3/year at two outages per day, 0.30% of the 1,000 m3 daily supply; negative pressure accounts for a factor of 2.11 against a soil-head-only control. Volumes only are reported; no health-risk conversion is attempted. Continuous pressurisation would prevent intrusion at an extra 9.01 kWh/day pumping energy while raising leakage from 14.32 to 85.94 m3/day. The household alternative is a nominal 75 L break tank (63.75 L usable, 40 mm air-gap inlet excluding back-siphonage structurally, gravity feed at 20.11-24.93 kPa) bridging two 2 h supply windows at 50 L/day essential demand with a 50% reserve margin, with a chlorine budget giving re-dose intervals of 6.0 days at 20 C down to 2.0 days at 35 C. All computations are reproducible from the deposited compute.py and results.json.