What's in my water
Bacteria in drinking water — the contaminant with a zero limit
Coliform bacteria and E. coli are indicator organisms — not usually dangerous themselves, but reliable evidence that water has been in contact with faecal material and may carry organisms that are.
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 — E. coli or thermotolerant coliform | Shall not be detectable in any 100 mL sample | — |
| BIS IS 10500:2012 — total coliform | Shall not be detectable in any 100 mL sample | — |
| WHO | E. coli must not be detectable in any 100 mL sample | WHO treats verification of microbial safety as the highest priority in drinking-water quality, ahead of every chemical parameter. |
Where it comes from
Sewage ingress into distribution
The dominant urban mechanism in India. Intermittent supply means pipes lose pressure, and a depressurised pipe near a drain draws contamination in through any defect.
Overhead and underground tanks
Uncleaned tanks accumulate sediment and biofilm. A tank that has not been cleaned in years is a contamination source regardless of the water entering it.
Tankers
Source, tank hygiene and handling are all variable and usually unverifiable by the customer.
Shallow wells and hand pumps
Particularly vulnerable to surface runoff during monsoon, and to nearby soak pits year-round.
Post-treatment handling
A clean supply stored in a dirty container is a contaminated supply. This is a common and entirely avoidable failure.
Why it matters
- Bacterial contamination is the fastest-acting water risk. Chemical contaminants act over years; a contaminated supply causes illness within days.
- The vulnerable groups — infants, elderly people and anyone immunocompromised — are affected at lower doses and more severely.
- It is also the most tractable. Unlike arsenic or fluoride, an outbreak has a cause that can usually be found and fixed.
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? | Effectively none. Contaminated water very often looks, tastes and smells completely normal. This is the single most important sentence on this page. |
| Does a TDS meter show it? | Zero information. Bacteria are not dissolved solids. Water teeming with E. coli reads exactly the same as sterile water of the same mineral content — and people buy purifiers on that reading every day. |
| Field / home test | H₂S vial presence/absence tests cost ₹50–150, take 24–48 hours, and give a clear yes/no on likely faecal contamination. For households on tanker or well supply this is the highest-value test available in India per rupee spent. |
| Laboratory test to ask for | Total coliform and E. coli by membrane filtration or MPN — APHA 9222 (membrane filtration) or 9221 (multiple-tube MPN) (indicative ₹300–₹900) |
When to pay for a laboratory test
- Any gastrointestinal illness in the household with no other obvious explanation. Test immediately and boil in the meantime.
- During and immediately after monsoon, if you are on well, tanker or intermittent municipal supply.
- After any supply interruption, pipeline repair or road excavation near your line.
- Your tank has not been cleaned in over a year, or you do not know when it last was.
- You have moved into a new property and have not established what the water is like.
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.
- BoilingEffective
Complete inactivation
A rolling boil for one minute inactivates bacteria, viruses and protozoa. Free, immediate, requires nothing to be installed.
Caveat: Does nothing for chemical contaminants and concentrates them. It is an emergency and interim measure, not a treatment system.
- UV disinfectionDepends
>99.99% at adequate dose in clear water
Standard household technology and genuinely effective when the conditions are met.
Caveat: The conditions are the whole story. UV needs low turbidity, a clean quartz sleeve, a lamp within its rated hours, and adequate contact time. Fail any one and the unit runs with an indicator light on while delivering almost nothing.
- Ultrafiltration (UF)Effective
>99.9% of bacteria
Pore sizes around 0.01–0.1 µm physically exclude bacteria and protozoan cysts. It works without electricity, which matters in areas with unreliable supply.
Caveat: Physical barrier only — it does not inactivate, so accumulated organisms on the membrane must be flushed. Viruses are smaller and are only partially retained.
- Reverse osmosis (RO)Effective
Membrane pore size excludes bacteria comfortably.
Caveat: Post-membrane recontamination in the storage tank is a real and common failure. This is why RO+UV exists as a configuration — the UV protects the stored water, not the incoming water.
- ChlorinationEffective
Effective, cheap, and it leaves a protective residual — the only treatment here that continues protecting the water after the treatment step.
Caveat: Requires correct dose and contact time. Protozoan cysts such as Cryptosporidium are highly chlorine-resistant.
What does not work
Each of these is commonly assumed — or actively marketed — to help with bacteria (e. coli & coliforms). None of them do.
Sediment filters
A 5 micron cartridge is fifty times larger than the bacteria you are trying to stop. It removes turbidity, which helps UV work, but it is not disinfection.
Activated carbon
Removes no bacteria, and an exhausted carbon cartridge becomes a bacterial growth site. Carbon must be changed on schedule for this reason as much as for taste.
'Alkaline' and 'mineral' cartridges
No disinfection function of any kind.
TDS reduction
Not a mechanism. Lowering TDS has no relationship to microbiological safety, and treating one as evidence of the other is the most common misconception in the Indian market.
What a positive coliform result actually means
A laboratory reporting coliforms has not found the organism that would make you ill. It has found evidence that the pathway exists.
Coliforms are used as indicators because they are abundant in faecal material, easy and cheap to culture, and survive in water at least as long as most bacterial pathogens. Testing for every possible pathogen individually would be slow and expensive; testing for the indicator is fast and cheap.
So a positive result means: faecal contamination has reached this water, and whatever else was in that contamination has probably reached it too. That is why the limit is zero rather than a low number — the standard is not tolerating a small amount of an unpleasant organism, it is refusing to accept evidence that the pathway is open.
Important
The UV failure mode nobody checks
A UV purifier gives no feedback about whether it is working. The lamp glows, the indicator is green, water comes out. All three of those remain true when the delivered dose has collapsed to a fraction of what is required.
Three things cause it. The lamp's UV-C output falls to roughly 60–70% of new by around 9,000 operating hours — about a year of continuous use — long before it visibly stops glowing. The quartz sleeve accumulates a film of iron, hardness and organic deposit that blocks transmission. And turbidity in the water shields organisms from the light entirely.
None of these produce a symptom, and a household that has run the same lamp for four years because it still lights up is a common finding. The lamp is a consumable with a date, not a bulb that works until it fails.
What to do right now if you suspect contamination
- Boil all drinking and cooking water at a rolling boil for one minute. This is immediate, complete for microbiological risk, and costs nothing.
- Use boiled or bottled water for brushing teeth and for washing produce that will be eaten raw.
- Order an H₂S vial test — ₹50 to ₹150, results in 24 to 48 hours — and a laboratory coliform test if the household includes an infant, an elderly person or anyone immunocompromised.
- Get the overhead and underground tanks cleaned, and check for any cross-connection between a supply line and a drain.
- Do not rely on an existing RO or UV unit until you have verified it. The failure you are experiencing may be the unit itself.
Frequently asked questions
Does UV kill bacteria in water?
Can a TDS meter detect bacteria in water?
How long should I boil water to make it safe?
What is an H2S test for water?
Is RO water safe from bacteria?
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.