Chlorine dosing is a mass balance: the milligrams of chlorine to add equal the dose in mg/L times the volume in liters, and the volume of product equals that mass divided by its strength. The carrier’s dose of about 5 mg/L needs 50 g of chlorine for 10,000 liters, which is 1 liter of 5% bleach, and it leaves a residual of 0.5 mg/L only if the water’s chlorine demand is 4.5 mg/L; a residual figure must never be used as the dose, and shock chlorination for tanks and wells is a far higher third figure.
Part of Water Treatment at Scale
Dosing calculation is the arithmetic that turns a target chlorine concentration into an amount of bleach or calcium hypochlorite for a given volume of water, and checks the result against the residual that remains.
Dose, Demand And Residual Are Linked By One Subtraction
The carrier’s figures are a residual of 0.5 mg/L free chlorine after 30 minutes, with about 5 mg/L going in. The relation is
dose = demand + residual
where demand is what the water consumes by reacting with organic matter and other substances. With the carrier’s figures, demand = 5 - 0.5 = 4.5 mg/L. Raise the demand, by dirtier water, and the same dose leaves less residual, or none. This page uses the carrier’s dose as given; the dose to use is set by the written treatment procedure and judged by the residual, and nothing here changes it.
Three numbers with the same unit are not interchangeable:
- Dose: mg/L added.
- Residual: mg/L left after contact time. It is measured, not added.
- Shock concentration: a far higher concentration for disinfecting a tank, a well or new pipework, held for hours. Published procedures use values on the order of tens of mg/L; I am recalling that, so take the figure and the hold time from the procedure you follow and not from here.
Units That Cause Errors
- 1 mg/L = 1 g/m³ = 1 part per million by mass, in dilute water.
- 1% = 10 g/L = 10,000 mg/L. A 5% bleach is therefore 50,000 mg/L, and a 0.5% solution is 5,000 mg/L. Confusing 0.5% with 0.5 mg/L is an error of a factor of 10,000.
- Handy rule: 1 mg/L over 1 m³ (1,000 L) is exactly 1 g of chlorine. So grams of chlorine = dose in mg/L × volume in m³.
- Labels differ in whether the percentage is sodium hypochlorite or available chlorine. Confirm what your product states.
What You Need
- The volume of water to be treated, measured, not guessed
- The strength of the product, in % available chlorine, checked against its age
- A chlorine test kit to read the residual (Chlorine Residual)
- A graduated container, a dropper and a scale if using dry powder
Method: Mass, Then Volume Of Product
Step 1: Target dose in mg/L, from the treatment procedure.
Step 2: Mass of chlorine = dose (mg/L) × volume (L), in mg. Or in grams: dose × volume in m³.
Step 3: Volume of product = mass ÷ strength in mg/L. For liquid bleach of p%, strength = p × 10,000 mg/L.
Step 4: For calcium hypochlorite, grams of compound = grams of chlorine ÷ fraction available (65% is 0.65). Dissolve it in a stock solution; never put dry granules in drinking water.
Step 5: Measure the residual after the contact time and adjust.
Worked Examples
A. 10 liters with 5% bleach. Mass = 5 × 10 = 50 mg. Strength = 50,000 mg/L. Volume = 50 ÷ 50,000 = 0.001 L = 1 ml, which is the carrier’s 20 drops.
B. 10 liters with a calcium hypochlorite stock. The carrier’s stock is 8 g in 1 L of 65% compound: 8 × 0.65 = 5.2 g = 5,200 mg of chlorine per liter. For 50 mg: 50 ÷ 5,200 = 0.0096 L = 9.6 ml, which is the carrier’s 10 ml. The dose delivered by 10 ml is 10 × 5.2 = 52 mg in 10 L = 5.2 mg/L.
C. A 10,000 liter tank, which is 10 m³. Chlorine = 5 × 10 = 50 g. As 5% bleach: 50,000 mg ÷ 50,000 mg/L = 1 L. As 65% calcium hypochlorite: 50 ÷ 0.65 = 76.9 g of compound.
D. Continuous feed for 2,000 L/day (83.3 L/h). Chlorine per day = 5 × 2 m³ = 10 g. As 5% bleach that is 10,000 ÷ 50,000 = 0.2 L/day = 200 ml/day = 8.3 ml/h. At 20 drops per ml that is 167 drops/h, about 2.8 drops per minute. A drip that slow is hard to hold steady, so dilute the bleach tenfold, to 0.5% (5,000 mg/L): the feed becomes 83 ml/h, 1,667 drops/h, about 28 drops per minute. Make dilute stocks in small batches; they weaken, and the residual check will show it.
E. Ageing bleach. The phase-0 chemical treatment page puts liquid bleach’s loss at roughly 20% a year (Chemical Treatment). Taking that as a rule: 5% × 0.8 = 4.0% after 1 year, 5% × 0.64 = 3.2% after 2 years, 5% × 0.512 = 2.56% after 3. The same 1 ml per 10 L then gives 4.0, 3.2 and 2.6 mg/L. To restore 5 mg/L (50 mg), use 50 ÷ 40 = 1.25 ml, 50 ÷ 32 = 1.56 ml and 50 ÷ 25.6 = 1.95 ml per 10 L. Date every container, and check the residual to confirm.
F. Finding the demand by jar test. Fill five jars with the same water, dose them at 3, 4, 5, 6 and 7 mg/L, wait 30 minutes and read the residual in each. If the readings are 0, 0, 0.5, 1.5 and 2.5 mg/L, demand is 4.5 mg/L and the lowest dose reaching 0.5 mg/L residual is 5 mg/L. If the residual stays near zero however far the dose rises, something in the water, such as sewage or ammonia, is consuming the chlorine and the source needs investigating (Source Investigation).
Why The pH Matters
Free chlorine exists as hypochlorous acid, the strong disinfectant, and hypochlorite ion, the weak one. Their balance depends on pH, with the two equal near pH 7.5 (a value I am recalling). The fraction as hypochlorous acid is 1 ÷ (1 + 10^(pH - 7.5)): about 0.76 at pH 7, 0.50 at 7.5, 0.24 at 8, 0.09 at 8.5 and 0.03 at 9. The WHO guideline I am recalling gives 0.5 mg/L after at least 30 minutes for water with pH below 8.0 and turbidity under 1 NTU. Above pH 8 the same residual disinfects much less, and you need more contact time or a higher residual.
Common Mistakes
- Using 0.5 mg/L as the dose. It leaves nothing: the tenfold under-treatment.
- Treating the percentage as milligrams per liter. A factor of 10,000.
- Applying a dose designed for clear water to cloudy water.
- Forgetting that old bleach is weaker, or that stock solutions decay.
- Using one dose for shock chlorination. It is a different procedure.
Safety
Wrong arithmetic poisons or under-treats; wrong mixing makes gas
Under-dosing leaves pathogens alive, and over-dosing makes water unpalatable so that people drink from untreated sources instead. Check every dose by the residual, not by the arithmetic alone. Never mix hypochlorite with acids, vinegar, toilet-bowl cleaners or ammonia products, and never put dry granules into drinking water: the first releases chlorine or chloramine gas, the second dissolves unevenly. Calcium hypochlorite can ignite on contact with oil, fuel, cloth or wood. Shock chlorination uses far stronger solutions and requires a written procedure, eye protection, gloves and open air.
What’s Next
- Learn next: Contact Time — how long the dose must act
- Combine with: Chlorine Residual — the measurement that proves the dose worked