Water purifiers are usually sold on brand and on the number of stages. Neither tells you whether the unit suits your water. The three main technologies solve genuinely different problems, and fitting the wrong one is both wasteful and, in one specific case, actively counterproductive.
The starting point is a test, not a catalogue.
What TDS actually tells you
TDS — total dissolved solids — measures the dissolved mineral and salt content of your water. Indian standards treat up to 500 mg/L as acceptable for drinking water, with under 300 mg/L generally regarded as good. Many people find water somewhere in the 50–150 range tastes best.
But TDS is a single number describing a mixture. It tells you how much is dissolved, not what. Water can have a perfectly reasonable TDS reading and still be microbiologically unsafe, which is exactly why the number alone should not decide your purifier.
Reverse osmosis (RO)
RO forces water through a semi-permeable membrane that removes dissolved salts along with a great deal else. Where TDS is genuinely high — common with borewell water in some areas — it is the only one of the three technologies that meaningfully brings it down.
It has two costs. It rejects some water as concentrate, which is inherent to how the membrane works, and it needs adequate inlet pressure to perform. It also removes beneficial minerals along with the unwanted ones, which is why RO on already-low-TDS water is a genuine mistake rather than merely an overspend.
Ultraviolet (UV)
UV deactivates bacteria and viruses by disrupting their DNA as water passes a lamp. It removes nothing dissolved, wastes no water, and leaves the mineral content untouched.
That makes it the right fit where the chemistry is fine but the microbiology is not — which describes a great deal of piped supply. It needs electricity, and the lamp is a consumable that must be replaced on schedule; an expired lamp still glows while doing very little.
Ultrafiltration (UF)
UF uses a membrane fine enough to physically strain out bacteria and suspended matter, but coarse enough to let dissolved salts through. It needs no electricity and wastes no water, which makes it useful where power is unreliable.
Like UV, it does not reduce TDS. It suits low-TDS water that needs protection from microbiological contamination and turbidity.
- High TDS — RO is the only one that reduces it
- Low TDS with bacterial risk — UV or UF, keep the minerals
- No reliable power — UF works without electricity
- Cloudy or sediment-heavy water — filtration ahead of anything else
Why most real systems combine stages
In practice a single technology rarely covers a whole supply. A typical installation puts sediment and carbon pre-filters ahead of an RO membrane to protect it, then UV after it to handle anything that survives storage.
This is also where the pre-filters earn attention. Membranes are expensive and are destroyed by exactly the things pre-filters remove. Neglecting cartridge changes is the most common reason a purifier quietly stops performing long before anyone notices.
Before you buy
- Get your water tested, including TDS and hardness
- Ask which technology the result points to, and why
- Check inlet pressure if RO is proposed
- Ask for the filter-change schedule in writing
- Find out where the RO reject line will be routed
The short version
Test first. High TDS points to RO; low TDS with microbiological risk points to UV or UF. Fitting RO to water that never needed it wastes water and strips minerals for no benefit. And whichever you choose, the maintenance schedule matters as much as the initial specification.