Bottled-water marketing throws “RO + UV + Ozone” around like a magic phrase. The reality is more interesting: each of those three stages removes something the others cannot. Skip any one and you have a gap in the wall.
Here is a plain-English walkthrough of what each stage actually does — and why a real water plant uses all three.
Reverse osmosis (RO) — removing what is dissolved
RO is the workhorse. Tap water is pushed under pressure against a semipermeable membrane with pores around 0.0001 microns — fine enough to block dissolved salts, heavy metals (lead, arsenic, mercury), and most organic compounds. Pure water molecules slip through; almost everything else is left behind as concentrated reject water.
What RO is good at:
- Total dissolved solids (TDS) — drops them by 95–99%.
- Heavy metals — lead, arsenic, mercury, chromium.
- Nitrates, fluorides, sulphates.
- Most pesticide and industrial residues.
What RO does not handle well:
- Live microbiology — most bacteria are larger than the membrane pores, but viruses can occasionally slip through if the membrane is damaged. Never rely on RO alone for biological safety.
- Volatile organic compounds (VOCs) and chlorine — these pass through unless a carbon stage precedes the membrane.
In Nepal specifically, the biggest RO job is TDS. Kathmandu groundwater and municipal supply frequently test above 300–500 ppm TDS. After a good RO stage, that drops below 20 ppm — clear water with almost nothing dissolved in it. The remineralisation stage that follows adds back only what should be there: calcium, magnesium, and trace minerals in measured amounts.
Ultraviolet (UV) — killing what is alive
UV-C light at 254 nanometres breaks the DNA of bacteria, viruses, and protozoa as water passes through a quartz sleeve around the lamp. The microorganisms are still present in the water after UV — they just can no longer reproduce or infect.
What UV is good at:
- Bacteria — E. coli, salmonella, coliforms.
- Viruses — hepatitis A, rotavirus.
- Protozoa — giardia, cryptosporidium.
- Real-time disinfection with zero chemicals in about 10 milliseconds.
What UV cannot do:
- Anything to dissolved solids, metals, or chemicals — UV ignores them completely.
- Treat turbid water — sediment casts shadows that protect microorganisms. UV needs visually clear water entering the lamp chamber, which is why it sits late in the process, after RO and the polishing filter.
UV is the most critical stage for biological safety in Nepal. Tap water contamination events in Kathmandu and the valley towns almost always involve coliform bacteria — exactly what UV destroys.
Ozone — the disinfecting finisher
Ozone (O₃) is oxygen with a third atom — extremely reactive, extremely short-lived. It is generated on demand from ambient air, bubbled or injected into the water, and rapidly oxidises any remaining biological material. After roughly 20 minutes it decomposes back into ordinary oxygen, leaving no taste, no smell, and no residual chemistry.
What ozone is good at:
- Final-stage disinfection inside the bottle or jar — long after the UV lamp is no longer in contact with the water.
- Oxidising lingering organic taste and odour compounds from trace residuals.
- Sanitising the inside of the container at fill — particularly important for refillable 20L jars that return from customers.
The risk: too much ozone can degrade the plastic in cheap bottles, or react with bromide ions in the water to form bromate, a regulated disinfection by-product. Good plants run ozone at a tightly-controlled dose with a bromate verification step before sealing.
RO handles dissolved chemistry. UV handles live biology in real time. Ozone handles biology in the sealed container after filling. Remove any one stage and you cover only part of the risk. A label that says only RO water is incomplete — ask where the UV and ozone steps are.
How to read a water brand label
When you pick up a 20L jar or a 500ml bottle in Kathmandu, here is what the process description actually tells you:
- RO water only — no biological safety stage mentioned. Ask why.
- RO + UV — chemistry and real-time biology covered. No protection inside the sealed bottle over time.
- RO + UV + Ozone — all three gaps closed. This is the minimum standard you want for bottled drinking water.
- ISO 22000 certified — the process has been independently audited against an international food-safety standard. Stronger assurance than self-reported claims.
- Lab report published — the brand tests finished product and makes the results available. If they do not publish one, ask for it.
Frequently asked questions
Is RO water safe to drink without UV?
It depends on the source. RO removes most bacteria by physical filtration, but a damaged membrane or a system that has not been sanitised can allow biological contamination through. UV provides a guaranteed disinfection step regardless of membrane condition. For packaged drinking water, UV is required under Nepal Food Safety standards.
Does ozone change the taste of water?
At correct dosing, no. Ozone decomposes fully before you open the container. Some people notice a very faint clean smell from a freshly opened jar — that is residual oxygen, not ozone. If the water tastes or smells of anything unusual, that is a different problem: carbon or polishing filter failure, not ozone.
What does TDS actually mean for drinking water?
TDS (total dissolved solids) measures everything dissolved in the water in parts per million — salts, minerals, metals, and organic compounds all combined into one number. WHO guidelines suggest below 300 ppm is acceptable, below 100 ppm is ideal for drinking water. After RO, a well-run plant delivers 5–15 ppm. Remineralisation then targets 50–80 ppm, adding back only beneficial minerals.
Why do cheaper brands skip remineralisation?
Remineralisation adds cost and requires careful calibration. Brands that skip it deliver water at 5–15 ppm TDS — technically very pure, but flat in taste and missing the calcium and magnesium that make water feel clean and contribute (modestly) to daily mineral intake. It is not a safety issue but it is a quality difference that you can taste.
If you only remember one thing: RO water alone is not a complete safety claim. The full picture is RO for chemistry, UV for biology, ozone for the sealed container — plus a published lab report you can actually read.