Chlorination is the last safety barrier in most drinking-water systems, from municipal plants to factory canteens, hostels and apartment complexes. Getting it right means enough chlorine to kill pathogens and keep the water safe through the pipes, without the taste and odour complaints that come from overdosing.
Three numbers to understand
- Chlorine demand: the chlorine used up reacting with organic matter, iron, ammonia and microbes in the water. It varies with season and source.
- Dose: the total chlorine you add. Dose = demand + the residual you want left over.
- Free residual chlorine: what remains after the demand is met. This is what protects the water in the distribution system.
India's drinking-water standard, IS 10500, calls for a minimum free residual chlorine of 0.2 mg/L at the consumer end when the water is chlorinated. Measure it at the farthest tap, not just at the tank outlet.
Contact time
Chlorine needs time to work. Plants usually provide a contact tank or sump so the water holds the chlorine for a set time before it reaches users. If you dose straight into a line that reaches consumers in seconds, the disinfection is incomplete even when the residual looks fine.
Clear water first
Chlorine works far better on clear water. Turbid water shields microbes and uses up chlorine, so coagulation and filtration come first. IS 10500 sets an acceptable turbidity of 1 NTU (5 NTU permissible). If your turbidity is high, look at your coagulant and filters before raising the chlorine dose. See How to Run a Jar Test.
Which chlorine source?
- Bleaching powder: economical and stores well, but needs dissolving and leaves insoluble residue. Common in rural and municipal systems.
- Sodium hypochlorite: a liquid that is easy to dose with a pump, but loses strength over time, especially in heat.
- High-strength calcium hypochlorite and TCCA: more concentrated solid sources, convenient for smaller systems and tanks.
- Chlorine gas: used by large municipal plants with trained staff and gas-handling safety systems.
Our guide Bleaching Powder vs Sodium Hypochlorite compares the two most common options.
Day-to-day checks
- Test free residual chlorine daily with a DPD test kit at the tank outlet and the farthest tap.
- Adjust the dose when the source water changes, for example during the monsoon.
- Store chlorine chemicals dry, shaded and away from acids and organics.
We supply bleaching powder, sodium hypochlorite and TCCA and SDIC across South India.