Chlorination disinfects clear water by adding a dose of about 5 mg/L, which is 1 milliliter of 5% household bleach per 10 liters, and then waiting at least 30 minutes, so that about 0.5 mg/L of free chlorine is still left at the end, which is what proves the dose was enough. Never mix bleach or pool chlorine with acids, vinegar, toilet cleaner or ammonia products, because they release chlorine or chloramine gas, and never rely on chlorine in cloudy water, where particles shield germs and use up the chlorine.
Part of Water Treatment at Scale
Chlorination is the chemical disinfection of water by adding a chlorine compound, usually bleach or pool chlorine, which kills bacteria and most viruses and leaves a protective residual in the pipes.
Three Different Chlorine Numbers Are Used, And They Must Not Be Mixed Up
Chlorine figures come in three kinds that are not interchangeable.
| Kind | What it is | Figure in this chain | Measured when |
|---|---|---|---|
| Dose | Chlorine added to the water | About 5 mg/L | At the moment of adding |
| Residual | Free chlorine still present after the water’s demand has used its share | 0.5 mg/L after 30 minutes; 0.2-0.5 mg/L at the point of consumption | After the contact time |
| Shock | A very high concentration for disinfecting a tank, a well or new pipework | Far above the dose; not set here | During a cleaning or commissioning procedure |
The carrier states the trap in plain words: do not mistake the residual target for a dose and under-treat by a factor of ten. A reader who adds only 0.5 mg/L, believing that is the dose, ends with no residual at all, because the water’s organic demand consumes it. Shock chlorination is a separate procedure with its own strength and time (Dosing Calculation).
Where Chlorine Comes From
- Household bleach, sodium hypochlorite, 3-8% strength, the most common
- Calcium hypochlorite, pool shock, 65-70% available chlorine, which stores better than liquid bleach
- Salt electrolysis, in which an electric current through salt water makes hypochlorite; small solar-powered units exist
See Chlorine Sources for the catalogue and shelf life.
What You Need
- Clear water, settled and filtered first (Treatment Chain)
- Unscented household bleach, at the strength you have confirmed, or dry calcium hypochlorite and a way to make a stock solution
- A measuring dropper or spoon
- A contact tank or container where the water can stand 30 minutes
- A chlorine test kit to read the residual (Chlorine Residual)
Method: Dose, Wait, Measure
Step 1: Start from clear water. Cloudy water needs settling first. The carrier doubles the dose for turbid water as a stopgap, but clearing the water is the proper remedy.
Step 2: Add the dose. For 5% bleach add 1 ml, about 20 drops, per 10 liters.
Step 3: Mix well, then leave it to stand for 30 minutes. In cold water below 10 °C, allow 60 minutes (Contact Time).
Step 4: Check the residual. Measure free chlorine with a test kit. The target is 0.5 mg/L after 30 minutes. A very faint chlorine smell is a hint, not a measurement.
Step 5: Keep a residual in the pipe, 0.2-0.5 mg/L at the point of consumption, because that is what protects the water between the plant and the tap.
Worked Example: Dose And Residual For 10 Liters
A 5% bleach, taking the 5% as available chlorine, holds 5 g of chlorine in each 100 ml, which is 50 g per liter, or 50,000 mg/L. Labels differ in what the percentage means, so confirm the strength you have.
- Dose: 1 ml of that bleach holds 50 mg of chlorine. Spread in 10 L, that is 50 ÷ 10 = 5 mg/L.
- Residual after 30 minutes: 0.5 mg/L.
- Consumed by the water’s demand: 5 - 0.5 = 4.5 mg/L. That is nine tenths of the dose.
For a 1,000 L tank the same dose is 1,000 ÷ 10 = 100 ml of bleach; for 10,000 L it is 1 liter.
If the bleach is weaker than the label, or the water dirtier, more chlorine is consumed and less is left. If you add only the residual figure, 0.5 mg/L, the demand takes all of it and nothing remains to protect anyone.
Common Mistakes
- Adding the residual target as the dose. The water ends with no protection: a tenfold under-treatment.
- Chlorinating cloudy water. Particles shield germs and use up chlorine (Sedimentation).
- Using scented, splash-less or cleaner-added bleach. The additives are unsuitable for drinking water.
- Skipping the wait. Chlorine needs time to work.
- Using old bleach at full-strength assumptions. Liquid bleach loses potency with time.
Safety
Chlorine chemicals release toxic gas when mixed and can ignite materials
Never mix bleach or pool chlorine with acids, vinegar, toilet-bowl or rust cleaners, or with ammonia or products containing ammonia, including urine: they release chlorine gas or chloramine gas, which damages the lungs. This happens most often when cleaning tanks and pipes. Calcium hypochlorite is an oxidiser that can ignite on contact with oil, fuel, cloth, wood or organic matter; store it dry and apart from other supplies. Wear eye protection and gloves, use it in open air, and never put dry granules straight into drinking water. Shock chlorination of a tank, well or pipework uses concentrations far above those here and must follow a written procedure.
What’s Next
- Learn next: Dosing Calculation — working out how much to add for any volume and strength
- Combine with: Contact Time — how long the water must stand before use