The species, briefly
When chlorine (gas, granules or hypochlorite solution) dissolves in water it forms hypochlorous acid — the active disinfectant. This working fraction is called free chlorine. When free chlorine reacts with ammonia and organic nitrogen in the water it forms chloramines (combined chlorine) — much weaker disinfectants that famously cause the "chlorine smell" and eye sting wrongly blamed on too much chlorine.
Total chlorine = free + combined. A test kit that only reads total chlorine can flatter badly contaminated water: high total with low free means the chlorine has been used up fighting contamination and little disinfecting power remains.
Targets worth memorising
| Point | Free chlorine |
|---|---|
| Municipal water at the tap (SANS 241 typical) | 0.2 – 1.0 mg/L residual |
| Shock/superchlorination of tanks and boreholes | 10+ mg/L, then dechlorinate |
| Ahead of polyamide RO membranes | < 0.1 mg/L — effectively zero |
| Ahead of ion exchange resin | < 0.5 mg/L, lower for long life |
Disinfection performance depends on concentration × contact time (the CT concept) and falls off sharply as pH rises above 7.5 — the same dose does half the work at pH 8.
The RO warning, in bold
Polyamide membranes — every modern spiral-wound element, including the full DuPont FilmTec range — have no meaningful chlorine tolerance. Oxidation damage is cumulative and irreversible: the membrane loses rejection permanently, and the damage is measured in a few hundred ppm-hours, meaning 0.5 mg/L of free chlorine can noticeably degrade an element within weeks. It is the most expensive commissioning mistake in small RO.
Protection is standard practice, not optional: a granular activated carbon or carbon block stage ahead of the membrane (carbon catalytically destroys free chlorine), or sodium metabisulphite (SMBS) dosing on larger plants. Chloramines are harder to remove than free chlorine — they need catalytic carbon and more contact time — so if the municipality chloraminates, size the carbon stage for it.
Where measurement catches people
- Test the right species. DPD No. 1 tablets read free chlorine; DPD No. 3 completes the total. Cheap single-reagent kits often read total only.
- Carbon beds are not immortal. Chlorine capacity is consumed; a carbon cartridge that has been in service a year is protection in name only. Test free chlorine after the carbon, not before it.
- Residual matters at the far end. In long reticulation or standing tanks, dose so a measurable free residual survives to the furthest tap — chlorine that has decayed to nothing protects nothing.
Rule of thumb: free chlorine is the number that matters — keep a residual of 0.2 – 0.5 mg/L where people drink, and effectively zero where membranes live. Verify the zero with a test after the carbon stage, every service visit.