Why chlorine is used
Water can carry bacteria, viruses and cysts that cause illnesses such as typhoid, cholera and dysentery. Chlorine kills or inactivates these germs. It has been used in public water supply for more than a century, and it remains popular because it is effective, affordable and easy to dose automatically.
Chlorine has one advantage that UV light and boiling do not. A small amount stays in the water after treatment. This residual chlorine keeps protecting the water as it travels through tanks and pipes, which matters in long networks and overhead tanks where water may sit for hours.
What makes chlorination work well
Adding chlorine is not enough on its own. How well it disinfects depends on a few conditions, and a well-run system controls each of them. If any one is off, germs can survive even when the chlorine reading looks acceptable, so operators should understand all four rather than relying on dose alone.
Operators often talk about free chlorine and combined chlorine. Free chlorine is the active, fast-acting form. Combined chlorine forms when chlorine reacts with ammonia in the water and is a much weaker disinfectant. A test kit or analyser that reads free chlorine gives the clearest picture of whether the water is properly protected.
- Contact time: chlorine needs time with the water before it is used. Contact tanks are sized to provide it.
- pH: chlorine works best in slightly acidic to neutral water and loses strength as pH rises.
- Temperature: disinfection is slower in cold water, so more contact time may be needed.
- Clarity: particles can shield germs and use up chlorine, which is why water is filtered first.
Disinfection by-products
When chlorine meets natural organic matter in water, such as decaying leaves or algae, it can form small amounts of compounds called disinfection by-products. The two best known groups are trihalomethanes (THMs) and haloacetic acids (HAAs). Drinking water standards around the world set limits for them and expect regular testing at larger supplies.
This does not mean chlorinated water is unsafe. The risk of disease from undisinfected water is far greater. It does mean that dose matters. The aim is to use enough chlorine to disinfect and keep a residual, and no more, and to remove as much organic matter as possible before chlorine is added.
Why dose control and monitoring matter
Too little chlorine leaves germs alive and no protection in the network. Too much gives a strong taste and smell, more by-products and complaints from users. Manual dosing with a bucket or a fixed-rate pump rarely gets this right, because flow and water quality change through the day.
Automatic dosing pumps paced to flow, together with residual chlorine analysers, keep the dose steady. Operators should also check residual chlorine at the far end of the network, not just at the plant, and keep simple records so trends are noticed early.
When dechlorination is needed
Sometimes chlorine has to be removed after it has done its job. Reverse osmosis membranes and ion-exchange resins can be damaged by chlorine, so water is passed through activated carbon or dosed with a dechlorinating chemical before reaching them. Some process uses, such as food and beverage production, also need chlorine-free water.
Treated effluent that has been chlorinated may also need dechlorination before discharge, because residual chlorine can harm aquatic life. If you are unsure how your system is dosed, an engineer can review the set-up and test the water at key points.