What's in my water
Turbidity — cloudy water, and why it disables your UV purifier
Turbidity is the cloudiness caused by suspended particles, measured in nephelometric turbidity units (NTU), and it is the parameter that determines whether UV disinfection can work at all.
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 — acceptable limit | 1 NTU | — |
| BIS IS 10500:2012 — permissible in absence of alternate source | 5 NTU | — |
| WHO | Below 1 NTU for effective disinfection; ideally below 0.2 NTU where disinfection is critical | WHO frames turbidity primarily as a treatment-performance and disinfection-efficacy parameter. |
Where it comes from
Monsoon runoff
Surface water sources spike in turbidity during and after heavy rain. The classic seasonal cause in India.
Pipeline disturbance
Repairs, pressure changes and restored supply after an interruption dislodge accumulated sediment in the mains.
Tank sediment
Disturbed sediment in an overhead or underground tank, often after a refill.
Oxidised iron and manganese
Produces coloured turbidity — orange for iron, black for manganese — rather than neutral cloudiness.
Air
Not turbidity at all, but the most common cause of a 'cloudy water' complaint. Dissolved air coming out of solution as pressure drops.
Why it matters
- Particles physically shield bacteria and viruses from UV light and from chlorine, so disinfection efficacy falls sharply as turbidity rises.
- Turbidity consumes chlorine, so a residual that would have been adequate is depleted before it reaches the tap.
- It rapidly clogs sediment filters, carbon cartridges and RO pre-filters, which is why filter life collapses during monsoon.
- Turbidity itself is generally not a direct health hazard — its significance is entirely about what it prevents from working.
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? | Directly visible. The simple diagnostic: fill a clear glass and watch it. Clearing from the bottom upwards within a minute or two is air. Cloudiness that persists, or particles that settle to the bottom, is genuine turbidity. |
| Does a TDS meter show it? | Nothing useful. Suspended particles are not dissolved and contribute little to conductivity — visibly muddy water can read low. |
| Field / home test | Turbidity tubes and handheld nephelometers exist; for household purposes the glass test plus a white background is adequate to decide whether you need pre-filtration. |
| Laboratory test to ask for | Turbidity by nephelometry — APHA 2130 B (indicative ₹100–₹300) |
When to pay for a laboratory test
- Persistent turbidity from a municipal supply that used to be clear — worth reporting to the utility, as it usually indicates a network problem.
- Coloured turbidity: orange suggests iron, black suggests manganese, and both need different treatment from ordinary sediment.
- You run a UV purifier on visibly cloudy water. Test for coliforms, because the UV may not be doing anything.
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.
- Sediment filtration (spun or pleated cartridge)Effective
The standard, cheap answer. A 20 µm cartridge ahead of a 5 µm one lasts much longer than a 5 µm alone.
Caveat: Consumable. During monsoon on a turbid supply, expect to change it several times more often than the schedule on the box.
- Multigrade sand filtrationEffective
The correct whole-house answer where turbidity is persistent. Backwashable, so it is not a cartridge you keep buying.
- SettlingPartial
Letting water stand in a tank drops the heavier fraction. Free, effective for coarse sediment, does nothing for fine colloidal turbidity.
- Ultrafiltration (UF)Effective
Removes turbidity completely along with bacteria.
Caveat: Fouls quickly on genuinely turbid feed. Needs sediment pre-filtration ahead of it, or the UF module becomes the consumable.
- CoagulationEffective
Alum dosing aggregates fine colloidal particles so they can settle or be filtered. Standard municipal practice, and available domestically as a sachet product for emergencies.
What does not work
Each of these is commonly assumed — or actively marketed — to help with turbidity. None of them do.
UV
The relationship runs the other way — turbidity disables UV, not the reverse. Treating turbid water with UV alone gives you an indicator light and very little disinfection.
Activated carbon
Not a turbidity technology, and it fouls quickly if asked to be one.
Water softener
Turbidity fouls softener resin. Sediment filtration goes before a softener, not after.
Boiling
Kills organisms but leaves the particles. The water is safer and still cloudy.
Air or turbidity? The thirty-second test
Fill a clear glass from the tap and set it on a table with a white background behind it.
If the cloudiness clears from the bottom upwards over 30 to 120 seconds, leaving clear water, that is dissolved air coming out of solution. It is harmless, common after supply interruptions or pressure changes, and needs no treatment at all.
If the water stays uniformly cloudy, or particles settle to the bottom over a few hours, that is genuine turbidity and needs sediment filtration.
This test costs nothing and resolves a large share of household 'my water looks wrong' concerns, most of which turn out to be air.
Tip
Frequently asked questions
Why is my tap water cloudy?
Is cloudy water safe to drink?
What is NTU in water?
How do I fix muddy water after monsoon?
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.