Chlorine in the pool: which form to choose and how to add it
Chlorine only disinfects while there is free chlorine in the water, usually 1–3 mg/l. The form you choose (tablets, granules or liquid) affects the pH, the stabilizer level and how you add it to a liner pool.
Free, combined and total chlorine
When you add chlorine to the water, part of it stays “free”, meaning ready to neutralize germs and algae. The rest reacts with sweat, urine, sunscreen and other organic matter and becomes “combined” chlorine, the well-known chloramines. Combined chlorine barely disinfects at all, and it is what smells and irritates the eyes.
- Free chlorine: the part that counts for disinfection. The target is usually 1–3 mg/l.
- Total chlorine: free plus combined.
- Combined chlorine: total minus free. We want it close to zero; when it rises, the answer is shock chlorination.
That is why a kit that just shows “chlorine” is not always enough. DPD tests and photometers measure free and total chlorine separately (see how to test the water). A strong chlorine smell usually means chloramines, not an overdose; more on this in the article on chlorine smell.
Tablets, granules or liquid: the differences
| Form | Active ingredient (usually) | Stabilizer | Effect on pH | Best suited to |
|---|---|---|---|---|
| Slow-dissolving tablets | Trichloroisocyanuric acid (trichlor) | Yes, they add it | Lower it | Steady, everyday chlorination |
| Fast-dissolving granules | Dichloroisocyanurate (dichlor) | Yes, they add it | Almost neutral | Quick corrections |
| Calcium granules or tablets | Calcium hypochlorite | No | Raise it | Shock; they add calcium, so take care with hard water |
| Liquid chlorine | Sodium hypochlorite | No | Raises it | Shock, pools with a dosing pump |
Stabilized forms protect chlorine from the sun, but they leave behind cyanuric acid, which builds up over the season. If you chlorinate only with tablets, also test the chlorine stabilizer from time to time. Liquid chlorine loses strength when stored for long, especially in the heat, so buy it in quantities that will last a few weeks.
How to add chlorine without harming the liner
A PVC liner can handle chlorine dissolved in the water, but not concentrated contact with the product. Most discolouration comes from chlorine that has settled in one spot.
- Never put granules straight into the pool, where they sink and sit on the liner. Dissolve them first in a bucket of water (product into water, not the other way round) and pour the solution in front of the return inlets with the pump running.
- Tablets go in a floating dispenser or an automatic in-line chlorinator, never loose in the water.
- The floating dispenser should not stay stuck in a corner or by the ladder; if necessary, tether it so it moves around in open water.
- Pour liquid chlorine in slowly, near the return inlets, with the circulation running.
- Add chlorine in the late afternoon or evening, so the sun does not break it down straight away.
Never mix different chlorine products with each other, or with pH− and other acids. The storage rules are in the article on safe chemical storage.
How much chlorine you need and what affects it
Chlorine consumption is not constant. It rises with heat, sun, lots of swimmers, rain and dust, and falls when the pool is covered at night. Its effectiveness also depends on pool pH: with a pH above 7.6, the same chlorine works noticeably more slowly.
The dose per cubic metre is printed on the packaging, because strength varies from product to product. So you need the pool's volume: length × width × average water depth. A 4×8 m pool with about 1.40 m of water holds around 45 m³. At the start of the season, test often so you learn how much chlorine your own pool uses in a hot week.
Questions
How often should I test the chlorine?
In summer at least two or three times a week, and always after a storm or a day with lots of swimmers. In spring and autumn once a week is usually enough.
Do I need chlorine if I have salt chlorination?
Yes; the chlorine is simply produced inside the unit from the salt in the water. For a shock or a quick correction you may occasionally need packaged chlorine as well; see salt chlorination.
See also
Pool pH
Pool pH: why we keep it at 7.2–7.6, how to raise or lower it with pH+ and pH−, what happens to chlorine and…
Shock chlorination
Pool shock chlorination: when you need it (storms, many swimmers, cloudy water), how to do it without harming…
Chlorine stabilizer
Chlorine stabilizer (cyanuric acid): how it protects chlorine from the sun, what level to aim for, what…