Chlorine Decay in School Water Storage: A Dosing Bound

Free chlorine in a school storage tank decays, so a morning dose must leave a protective residual at the last draw without exceeding a taste ceiling at the first. This paper models decay as an immediate demand followed by first-order decay (temperature factor 1.07 per degree C), using published decay ranges stated explicitly, for 500-2000 L tanks. Floor 0.2 mg/L, ceiling 1.0 mg/L. For the reference case (covered opaque tank, moderate-demand water, 25 C, k=0.541/day) the 24-hour morning dose is 0.844 mg/L, the re-dosing interval at the ceiling is 40.7 h, and the rule is a measured post-demand residual of 0.27-0.55 mg/L from 15 to 35 C (6.7 mL of 12.5% sodium hypochlorite per 1000 L at 25 C), costing US$0.32 per child per year, almost all of it residual testing. The bound fails for sunlit translucent tanks (no 24-hour dose within the ceiling), for high-demand source water (immediate demand 1.5 mg/L alone exceeds the ceiling), and across weekends at 20 C and above in the reference case, where dumping and re-dosing on return is required. SIMULATED, NOT MEASURED.