Design Blueprint: Household Break Tank — Build Specification Derived from the Intermittent Water Supply Intrusion Paper

DESIGN BLUEPRINT — SIMULATED, NOT MEASURED — NOT BUILT. A complete build specification for a household break tank that provides backflow-safe storage for an intermittent water supply, derived from the source paper: https://doi.org/10.5281/zenodo.24109065. Design basis: in the source paper's stated reference network (10.0 km of mains, stated leak census totalling 68.33 mm2), each supply outage draws 1.49 m3 of soil water into the network — 2.98 m3/day and 1,087 m3/yr at two outages per day — and the household unit defeats the household side of that threat structurally: a nominal 75 L vertical cylindrical tank (0.42 m diameter, 0.65 m overall height, 63.75 L usable) filled through a 15 mm inlet terminating across a 40 mm air gap (stated minimum 30 mm), so no continuous liquid path exists and back-siphonage into the main is impossible under any suction. The tank bridges the source pattern of two 2 h supply windows per day at 50 L/day essential demand, covering the governing 10.0 h gap by a factor of 2.04 with 30.6 h of autonomy — one full missed supply day is bridged (56.25 L demanded against 63.75 L usable), two are not — and feeds a ground-floor tap by gravity from a base 2.0 m above the tap at 20.11 kPa to 24.93 kPa. Chlorine budget: inlet residual 0.50 mg/L defended above a 0.20 mg/L floor with re-dose intervals of 6.0 days at 20 C, 4.0 days at 25 C, 3.0 days at 30 C and 2.0 days at 35 C, a full re-dose being 19.13 mg of chlorine as 1.91 mL of a 1% solution. The blueprint gives dimensions and level stacking (overflow spill at 0.541 m above the interior base, inlet outlet at 0.581 m), a bill of materials in generic specification classes, an installation sequence centred on measuring the air gap, the operating envelope, the air gap as the single safety-critical dimension, failure modes including a bridged air gap and high-temperature chlorine decay, a maintenance schedule, and the measurements a first installation must make. Nothing has been built or measured; every derived quantity is reproducible from the accompanying compute.py.