What's in my water
Viruses in drinking water — smaller than bacteria, and harder to filter out
Waterborne enteric viruses — rotavirus, norovirus, hepatitis A and E among them — are 10 to 100 times smaller than bacteria, which is why filtration that reliably stops bacteria does not reliably stop viruses.
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 | No separate viral parameter | Microbiological safety is verified through E. coli and coliform indicators, which stand for the entire faecal contamination pathway including viruses. |
| WHO | Health-based targets expressed as log reduction | WHO frames virus control as a treatment performance requirement — typically 4-log (99.99%) reduction for enteric viruses — rather than as a concentration limit in finished water. |
Where it comes from
Human sewage
Enteric viruses are shed in very high numbers by infected people. Human faecal contamination is essentially the only significant source.
Sewage ingress into distribution
The same intermittent-supply mechanism that causes bacterial contamination, with the same causes and the same fixes.
Shallow groundwater near sanitation
Viruses travel further through soil than bacteria do, so the safe separation distance from a soak pit is greater than most people assume.
Post-treatment handling
Contaminated hands and storage containers reintroduce viruses after treatment.
Why it matters
- Hepatitis A and E, rotavirus and norovirus are all transmitted through contaminated water and are documented causes of outbreaks in India.
- The infectious dose for several enteric viruses is very low — orders of magnitude below what is typically required for bacterial infection — which means a treatment that removes 99% may still leave an infective dose.
- Rotavirus is a leading cause of severe diarrhoeal disease in young children, which puts households with infants in the group that should treat this most seriously.
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 whatsoever. |
| Does a TDS meter show it? | Nothing. Viruses do not affect conductivity in any measurable way. |
| Field / home test | There is no household virus test. E. coli and H₂S indicator tests are the practical proxy — they detect the contamination pathway rather than the organism. |
| Laboratory test to ask for | Enteric virus detection by RT-PCR — Specialist laboratories only, generally in outbreak investigation rather than routine testing (indicative ₹4,000–₹15,000) |
When to pay for a laboratory test
- A jaundice or hepatitis case in the neighbourhood — hepatitis A and E are waterborne and cluster geographically. Test for coliforms and boil in the meantime.
- Repeated gastrointestinal illness in a household using a UF-only gravity purifier on a suspect supply.
- Any suspected sewage ingress into the supply.
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.
- UV disinfectionEffective
>99.99% at adequate dose in clear water
UV-C damages viral nucleic acid effectively. The practical household answer alongside boiling.
Caveat: Adenoviruses are notably more UV-resistant than most enteric viruses and need a higher dose. The same clear-water, clean-sleeve, in-life-lamp conditions apply.
- BoilingEffective
Complete inactivation
A rolling boil for one minute inactivates enteric viruses. Immediate and free.
- Reverse osmosis (RO)Effective
>99.99%
RO membrane pore size is well below viral dimensions, so exclusion is effective.
Caveat: Storage tank recontamination downstream remains the weak point, as with bacteria.
- ChlorinationEffective
Free chlorine at adequate dose and contact time inactivates enteric viruses, and leaves a residual that continues protecting the water.
Caveat: Requires correct dosing. BIS notes 0.5 mg/L residual as desirable where viral contamination risk exists — higher than the 0.2 mg/L general minimum, and that difference exists precisely because of viruses.
- Ultrafiltration (UF)Partial
Partial — varies widely by membrane cut-off
UF gives useful but incomplete virus retention. Tighter membranes do better; the nominal 0.1 µm end of the range does poorly.
Caveat: This is the key limitation of UF-only gravity purifiers. They are excellent against bacteria and cysts and should not be relied on alone where viral contamination is likely.
What does not work
Each of these is commonly assumed — or actively marketed — to help with viruses. None of them do.
Sediment filtration
Off by three orders of magnitude. A 5 µm cartridge is 250 times larger than a 20 nm virus.
Activated carbon
No meaningful virus removal.
'Mineral' and 'alkaline' stages
No disinfection function.
TDS reduction
Unrelated to microbiological safety in every respect.
Why size is the whole argument
Read the fourth row carefully, because it is the one that decides whether a gravity UF purifier is adequate for your supply.
A UF membrane with a nominal 0.01 µm cut-off overlaps the size range of enteric viruses. Some are retained, some are not, and the fraction depends on the specific membrane and on how the virus is associated with particles in the water. 'Partially' is an honest description and a bad basis for confidence where sewage contamination is plausible.
| Organism / particle | Typical size | Stopped by UF (0.01–0.1 µm)? | Stopped by RO? |
|---|---|---|---|
| Sand, silt, rust particles | > 1 µm | Yes | Yes |
| Protozoan cysts (Giardia, Cryptosporidium) | 3–15 µm | Yes | Yes |
| Bacteria (E. coli) | 0.5–5 µm | Yes | Yes |
| Viruses (rotavirus, hepatitis A) | 0.02–0.1 µm | Partially | Yes |
| Dissolved salts, fluoride, arsenic | < 0.001 µm | No | Yes |
Note
Frequently asked questions
Does UF remove viruses from water?
Does UV kill viruses in water?
How do I test my water for viruses?
Can hepatitis spread through drinking water?
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.