What's in my water
Chlorine in tap water — the contaminant that is protecting you
Chlorine is a disinfectant deliberately added to municipal water and maintained as a residual through the distribution network, so that water contaminated after treatment is still protected.
Limits and standards
BIS publishes two figures for many parameters: an acceptable limit, and a permissible limit that applies only where no alternate source of water exists. The second is a fallback for communities without a choice — not a target to design a purchase around.
| Standard | Limit | What it means |
|---|---|---|
| BIS IS 10500:2012 — minimum residual free chlorine | 0.2 mg/L at consumer end | Stated as a minimum where water is chlorinated. BIS notes that a residual of 0.5 mg/L is desirable where the risk of viral contamination exists. |
| WHO guideline value | 5 mg/L | WHO notes this is conservative and that the taste and odour threshold is well below it — most people detect chlorine long before any health-relevant concentration. |
| BIS — total trihalomethanes | Individual THMs limited separately; e.g. chloroform 0.2 mg/L, bromoform 0.1 mg/L | Disinfection by-products, formed when chlorine reacts with organic matter. |
Where it comes from
Municipal disinfection
Added at the treatment plant and often boosted at intermediate points to maintain a residual across the network.
Tanker and tank chlorination
Bleaching powder dosed into overhead tanks and tankers. Dose control is often poor, which is why tanker water sometimes smells strongly.
Household disinfection
Chlorine tablets or bleach used during and after monsoon contamination events.
Why it matters
- Chlorine's presence is protective. Its absence in a supply that is supposed to be chlorinated is the actual warning sign, because it means the residual has been consumed by something.
- Taste and odour complaints are the main practical issue, and they start at concentrations far below any health concern.
- Chlorine reacts with natural organic matter to form disinfection by-products such as trihalomethanes, which BIS limits individually. WHO's position is consistent and clear: the risk from by-products is small compared with the risk of not disinfecting at all.
- For equipment, chlorine is a hazard. It oxidises polyamide RO membranes and destroys them, which is why every RO system has a carbon stage before the membrane.
Note
Can you detect it yourself?
This is the section most purifier buying decisions skip. A handheld TDS meter is the instrument almost every Indian household is shown, and for most contaminants that matter it reads exactly the same whether the contaminant is present or not.
| Question | Answer |
|---|---|
| Can you sense it? | Highly detectable. Most people smell chlorine at 0.3 to 0.5 mg/L, well below any concentration of concern. That gap between the detection threshold and the health threshold is the reason for most chlorine complaints. |
| Does a TDS meter show it? | Nothing useful. Free chlorine barely affects conductivity. |
| Field / home test | DPD test tablets or strips give a free-chlorine reading in under a minute for a few rupees per test. Genuinely worth keeping in a household on intermittent municipal supply — a zero residual after a supply interruption is a real signal. |
| Laboratory test to ask for | Residual chlorine by DPD colorimetry; THMs by GC-MS — APHA 4500-Cl G for chlorine; 6232 for trihalomethanes (indicative ₹150–₹2,500) |
When to pay for a laboratory test
- Your municipal supply has no detectable chlorine residual at the tap. That is worth reporting to the utility — it may mean the residual has been consumed by contamination in the network.
- A persistent strong chemical smell that is not the usual chlorine odour, particularly a medicinal or antiseptic note, which can indicate chlorophenols from organic contamination.
What removes it
Effectiveness ratings below describe the technology working correctly, within its service life, on water or air that suits it. “Depends” means there is a condition in the caveat that decides the outcome — read it before buying.
- Activated carbonEffective
>95% of free chlorine
Carbon reduces chlorine chemically and by adsorption. Fast, cheap and complete — the clearest case in household water treatment where an inexpensive filter fully solves the problem.
Caveat: Capacity is finite and there is no visible indication of exhaustion. An old carbon cartridge silently stops working and can become a bacterial growth site.
- Catalytic carbonEffective
Required for chloramine, which standard carbon handles poorly. Ask specifically if your utility uses chloramine rather than free chlorine.
- Standing / aerationPartial
Free chlorine dissipates from an open container over several hours. Free and effective for taste; useless for chloramine, which is far more stable.
- BoilingPartial
Drives off free chlorine within a few minutes of boiling. One of the few contaminants on this site that boiling genuinely helps with.
Caveat: Also concentrates every non-volatile contaminant in the same pot.
- Vitamin C (ascorbic acid)Effective
Neutralises both chlorine and chloramine instantly. Used in aquarium and brewing practice; not a household drinking-water technology, but chemically real.
What does not work
Each of these is commonly assumed — or actively marketed — to help with chlorine. None of them do.
UV
No effect on chlorine at doses used for disinfection. Very high UV doses do dechlorinate, but that is an industrial process, not a household UV purifier.
Sediment filters
Remove particles, not dissolved gases.
RO membrane alone
This inverts the relationship. Chlorine attacks the membrane, so carbon must come before RO — never rely on the membrane to handle chlorine.
The chlorine paradox
Chlorine is the only entry in this encyclopedia that people want removed precisely because they can detect it, and that they are usually better off having.
The reasoning is straightforward. Water leaving a treatment plant may be perfectly clean; the pipeline between there and your tap is not sealed, is often intermittently pressurised, and in many Indian cities runs alongside drains. A chlorine residual is what makes an ingress event survivable.
So the correct order of operations is: keep the chlorine in the distribution system, and remove it at the point of use if you dislike the taste. A carbon filter at the kitchen tap does exactly that. Campaigning for less chlorination in the mains does the opposite of what anyone wants.
Note
Chlorine and your RO membrane
Thin-film composite polyamide — the material of every domestic RO membrane — is oxidised by chlorine. The damage is cumulative, measured in ppm-hours, and irreversible.
A membrane exposed to chlorinated feed water loses rejection progressively: the unit keeps producing water, but the TDS of that water climbs. Because output volume does not fall, nobody notices until someone checks the meter.
This is the practical reason the carbon pre-filter has a replacement schedule that matters more than the sediment filter's. When carbon is exhausted, chlorine reaches the membrane, and an exhausted ₹600 cartridge takes a ₹2,500 membrane with it.
Frequently asked questions
Why does my tap water smell like a swimming pool?
Is chlorine in drinking water harmful?
Does boiling remove chlorine?
What removes chlorine from water?
Why does RO need a carbon filter before the membrane?
Sources
Every limit and standard quoted on this page comes from one of the documents below. Where a figure is not in one of these, we have not published it.
Bureau of Indian Standards · published 2012-05-01 · checked 2026-08-01
BIS states two figures for many parameters: an 'acceptable limit' and a 'permissible limit in the absence of an alternate source'. The second is a fallback for communities with no other supply, not a target.
Guidelines for Drinking-water Quality, 4th edition incorporating the 1st addendum
World Health Organization · published 2017-04-01 · checked 2026-08-01
This page states what published standards say and what the physics of each technology allows. It does not state what is in your supply — no page can. Where the two questions meet is a test result, and every entry in this library says which test to ask for.