The chlorine residual is the free chlorine still in the water after the dose has done its work, and it is the one number that proves disinfection happened: the target is 0.5 milligrams per liter after 30 minutes of contact and 0.2-0.5 milligrams per liter at the tap. Measure it with a DPD test kit, not by smell, and never confuse it with the dose, which is about ten times larger and is what you add; a residual reading mistaken for a dose under-treats the water by a factor of ten.
Part of Community Water Testing
A chlorine residual test measures the disinfectant remaining in treated water, at the end of contact time and at the tap, to confirm that the dose was enough and that protection reaches the user.
The Residual Proves The Dose Worked, And Shows Where Protection Ends
Chlorine added to water is used up first by organic matter and other substances, which is the demand. What is left after the demand is met is the residual, and only the residual keeps killing germs. The treatment page sets 0.5 mg/L free chlorine after 30 minutes, and 0.2-0.5 mg/L at the point of consumption (Water Treatment at Scale). If there is no residual, the dose was consumed and the water is not reliably disinfected.
Three kinds of chlorine number must be kept apart: the dose added, about 5 mg/L on the treatment page; the residual measured after contact time; and the shock concentration used to disinfect tanks, wells or new pipework, far higher than either (Dosing Calculation). The test here measures only the residual. Free chlorine is the strong, available form. Total chlorine includes combined forms such as chloramines, which are much weaker. A kit that reads total chlorine can look reassuring while the free part is low. Combined chlorine forms when ammonia is present in the water, which can itself be a sign of sewage or fertiliser reaching it.
What You Need
- A DPD test kit, tablets or powder with a colour comparator, such as sold for swimming pools, reading free chlorine
- Clean, rinsed sample bottles
- A watch and the log
- Shade, since sunlight destroys chlorine in the sample and in the comparator
Some older pool kits use orthotolidine (OTO), which reads total chlorine and which I recall is toxic and suspected to cause cancer; if that is all you have, use it with gloves and ventilation, and do not let any sample or reagent near food.
Method: Sample, Read At Once, Place It In Time
Step 1: Rinse the sample bottle with the water to be tested, then fill it.
Step 2: Test immediately. Chlorine falls with time and light. Add the reagent as the kit directs and compare the colour in the shade.
Step 3: Read free chlorine against the scale.
Step 4: Record the value with the place and the time since dosing.
Step 5: Where to test: at the outlet of the contact tank after the contact time, at a tap near the tank, and at the farthest tap. The first morning draw is the most revealing, since water has stood in the pipe all night.
Step 6: If the colour flashes and fades, or reads zero while the water smells strongly of chlorine, the sample may be too strong for the reagent. As I recall, high chlorine can bleach DPD and give a false zero. Dilute the sample with clean water in a known ratio, re-test, and multiply the reading back by the dilution. Do not respond to a false zero by adding more chlorine.
Worked Example: Where The Residual Goes
Chlorine decays as it travels and stands. If a reading falls from 0.6 mg/L at the tank outlet to 0.2 mg/L at a far tap in 6 hours, treat the decay as first-order: C = C0 × e^(-kt). Then k = ln(0.6 ÷ 0.2) ÷ 6 = 1.0986 ÷ 6 = 0.183 per hour.
- After 3 hours: 0.6 × e^(-0.183 × 3) = 0.6 × 0.577 = 0.35 mg/L.
- After 12 hours, such as overnight in a dead end: 0.6 × e^(-0.183 × 12) = 0.6 × 0.111 = 0.067 mg/L.
- To have 0.2 mg/L left after 12 hours, you would have to start with 0.2 × e^(0.183 × 12) = 0.2 × 9 = 1.8 mg/L.
So the first draw of the morning can have almost no residual even when the daytime readings pass. The sensible responses are to flush the morning draw and to treat water that has stood overnight with suspicion; any change in the dose itself follows the treatment procedure and is judged by the reading.
Dose against residual. If a dose of 5 mg/L gives a residual of 0.5 mg/L, the demand is 5 - 0.5 = 4.5 mg/L. A reading of 0.5 does not mean “add 0.5”: it means the water used 4.5 of the 5. The dose is also 5 ÷ 0.5 = 10 times the residual, which is why mistaking one for the other under-treats by a factor of ten.
What A Good Reading Does And Does Not License
A residual in range at a tap tells you that free chlorine was there at that moment. It supports the idea that disinfection worked, but only for organisms that chlorine kills: it says nothing about Cryptosporidium, which it does not kill, or about chemicals. In cloudy water the reading can be good while germs sit shielded inside particles. Combine it with the turbidity check and the bacteria test.
Common Mistakes
- Reading by smell. People differ and noses tire.
- Testing in the sun, or after the sample has stood.
- Using a total-chlorine kit and calling it free.
- Testing only at the plant. The far tap and the morning draw are where it fails.
- Taking a zero at face value after a heavy dose.
Safety
Reagents are chemicals; chlorine and acid make gas
Keep tablets and reagents from children and out of food containers; never swallow a test sample. Never pour chlorine solution into anything containing acids, vinegar, rust or toilet cleaner, or ammonia products: chlorine or chloramine gas results. Throw reagent wastes away from wells and springs.
What’s Next
- Learn next: Contact Time — what the 30 minutes depends on
- Combine with: Test Log — recording the time since dosing with each reading