What's in my water
Nitrate in drinking water — the agricultural contaminant a TDS meter hides
Nitrate is the oxidised form of nitrogen, entering groundwater principally from fertiliser leaching and sewage, and it is the most widespread agricultural contaminant in Indian groundwater.
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 | 45 mg/L (as NO₃) | No relaxation permitted. This is unusual and deliberate. |
| WHO guideline value | 50 mg/L (as NO₃) | WHO's value is expressed as nitrate ion; a value expressed as nitrate-nitrogen (NO₃-N) is roughly 4.4 times smaller for the same water. Check which basis a report uses before comparing. |
Where it comes from
Nitrogenous fertiliser
The dominant source in India. Urea applied in excess of crop uptake leaches to groundwater over seasons, which is why nitrate rises gradually rather than appearing suddenly.
Septic tanks and soak pits
A major cause of nitrate in shallow wells in unsewered areas. Proximity of a well to a soak pit is the strongest single predictor.
Livestock waste
Concentrated manure storage and dairy operations produce highly localised contamination.
Natural soil nitrogen
A small background contribution — but a well reading above about 20 mg/L is not a natural background level.
Why it matters
- In infants under six months, ingested nitrate is converted to nitrite in the gut and binds haemoglobin, reducing oxygen transport. WHO documents this as methaemoglobinaemia, or blue baby syndrome.
- This is one of the very few water contaminants with a documented acute risk to a specific group, rather than only a long-term one.
- Bottle-fed infants are at highest risk because formula is made up with tap water, so the entire fluid intake carries the contaminant.
- For adults, WHO's guideline value is set principally to protect infants; adult health effects at concentrations near the limit are not well established.
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? | None. Nitrate is completely undetectable by taste, smell or appearance. |
| Does a TDS meter show it? | Nitrate at the 45 mg/L limit does contribute to a TDS reading — but so does an equal mass of harmless calcium bicarbonate. The meter cannot tell them apart, so a 400 ppm reading gives you no information about whether nitrate is present or absent. |
| Field / home test | Nitrate test strips are cheap and give a usable band. They are adequate for deciding whether to send a sample to a lab, and inadequate for confirming that water is safe for an infant. |
| Laboratory test to ask for | Nitrate by ion chromatography or UV spectrophotometry — APHA 4110 B or 4500-NO₃⁻ B (indicative ₹250–₹700) |
When to pay for a laboratory test
- There is a bottle-fed infant in the household and the supply is any form of groundwater. Test before making up formula with it. This is the clearest single testing recommendation on this site.
- Your well is shallow — under about 30 m — and within roughly 50 m of a septic tank or soak pit.
- You are in an intensively farmed area, particularly the wheat, rice and sugarcane belts of Punjab, Haryana, western Uttar Pradesh, Rajasthan, Maharashtra and Andhra Pradesh.
- Nitrate has been detected before. It tends to rise slowly over years rather than resolve, so a single old result is not reassurance.
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.
- Reverse osmosis (RO)Effective
85–95%
The practical household answer. Nitrate is charged and is rejected well by the membrane.
Caveat: Rejection is somewhat lower than for divalent ions like sulphate. In a high-nitrate well, verify the treated water rather than assuming a specification figure.
- Anion exchange (nitrate-selective resin)Effective
>90%
Standard at community and commercial scale. Nitrate-selective resins avoid the sulphate competition that limits standard anion resins.
Caveat: Requires salt regeneration and produces a nitrate-rich brine that has to be disposed of somewhere sensible.
- DistillationEffective
Effective but impractical at household scale.
- Biological denitrificationEffective
Used at municipal and industrial scale. Not a household technology.
What does not work
Each of these is commonly assumed — or actively marketed — to help with nitrate. None of them do.
Boiling
The most dangerous misconception on this page. Boiling removes water and leaves nitrate behind at a higher concentration — and it is often done specifically to make water 'safe' for an infant, which is precisely the case where the concentration matters most.
Activated carbon
Carbon has essentially no capacity for nitrate. Every household carbon cartridge sold in India is irrelevant to this contaminant.
UV
Inactivates organisms. No effect on nitrate.
UF and sediment filters
Pore size is vastly larger than a nitrate ion.
Standard water softener
Cation resin removes calcium and magnesium. Nitrate is an anion and is untouched.
Read the units before you read the number
Nitrate results are reported on two different bases and they differ by a factor of about 4.4. A result of 45 mg/L as NO₃ is at the BIS limit. A result of 45 mg/L as NO₃-N is roughly 200 mg/L as nitrate — over four times the limit.
Indian laboratories usually report as NO₃, matching BIS. Some international kits and instruments report as NO₃-N. Reading one as the other is a real and repeated error, and it goes wrong in the dangerous direction.
Important
Why nitrate contamination almost never improves
Bacterial contamination is episodic — a pipeline break, a monsoon ingress event, a contaminated tanker. Fix the cause and the problem goes.
Nitrate is not like that. It arrives by decades of slow leaching through soil into the aquifer, and the nitrogen already in transit through the unsaturated zone will keep arriving for years after any change in surface practice. A well that reads 60 mg/L this year will, absent a change of source, read something similar or higher next year.
The practical consequence: nitrate is a permanent-treatment problem, not a boil-and-wait problem. Budget for it as an ongoing running cost rather than a one-off fix.
Frequently asked questions
Does RO remove nitrate from water?
Is boiling water enough for nitrate?
What is blue baby syndrome?
Why does BIS allow no relaxation for nitrate?
Can a normal water filter remove nitrate?
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
Ground Water Quality in Shallow Aquifers — annual and state reports
Central Ground Water Board, Ministry of Jal Shakti, Government of India · checked 2026-08-01
District-level groundwater quality reporting. Coverage and sampling year vary by state — always check the report date for your district rather than assuming a national figure applies.
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.