Dose Past the Demand, Then Mind the Calendar: Chlorine, Contact Time, and Re-Dosing for Stored Roof Water

Households that drink stored roof water are commonly told a single chlorine dose — a fixed number of drops or millilitres per container — with no reference to the water's chlorine demand, the contact time the dose needs, or how long the protection lasts. This paper separates the three quantities. Dose must first satisfy demand before any free residual exists to disinfect: in water with a stated demand of 1.5 mg/L, representative of a dirty roof or an open tank, a 1.0 mg/L dose leaves no free chlorine at all, and disinfection does not begin however long the water stands. In the typical case examined — demand 0.8 mg/L, dose 2.0 mg/L, leaving a residual of 1.2 mg/L — the stated bacterial CT target of 15 mg·min/L is reached in 12.5 minutes, using 40 mL of 5 percent household bleach per 1,000 L. Contact time turns out to be nearly insensitive to storage conditions, because decay over minutes is negligible; the calendar is not. The same dose's protective residual, down to a floor of 0.2 mg/L, lasts 35.8 days in a cool, dark, covered tank, 11.9 days in a warm covered tank, and 4.5 days in a warm tank admitting light. Re-dosing is therefore a property of the store, not of the dose, and any dosing instruction that does not ask how the water is stored is incomplete. All results are simulated, not measured.