GChlorine Dose Calculation for a Rural Water Supply
Disinfection is only as good as the dose. Working the dose from flow and chlorine demand, and checking the residual, is what makes chlorination protect people instead of just reassuring them.
Dose, demand and residual
The chlorine a supply needs is the sum of the chlorine demand of the water — what is consumed reacting with organic matter, iron and ammonia — plus the free residual that must survive to the far end of the distribution to guard against recontamination. If the demand is, say, 1.5 mg/l and you want 0.5 mg/l of free residual at the tail, the applied dose is about 2.0 mg/l. The residual, not the dose, is the number you actually verify in the field.
The dose is set at the point of application — usually at the reservoir inlet or outlet — and the free residual is checked with a simple orthotolidine or DPD test at the far points of the network. A residual that disappears before the tail means the dose is too low or the contact arrangement is poor.
Turning the dose into a daily quantity
To order and feed the right amount, the dose in mg/l is converted to a daily mass from the flow. Mass of chlorine per day (in grams) is the flow in litres per day multiplied by the dose in mg/l and divided by 1000. Where bleaching powder is used, that chlorine mass is divided by the available-chlorine fraction of the powder — commonly around 30 per cent for stored stock — to give the weight of powder to dissolve each day.
Because stored bleaching powder loses strength, the available-chlorine percentage should be checked periodically and the quantity adjusted; feeding a fixed scoop of weak powder is a common reason a residual test comes back empty.
What is the difference between dose and residual?
The dose is the chlorine applied; the residual is the free chlorine that survives after the water’s chlorine demand is met. The applied dose roughly equals demand plus the target residual, and the residual is what you verify in the field.
How is the daily bleaching-powder quantity found?
Convert the dose to a daily chlorine mass (flow in litres/day × dose in mg/l ÷ 1000 = grams/day), then divide by the available-chlorine fraction of the powder (often about 30%). Check the powder strength periodically as it weakens in storage.