💧 Water guide

How Does an RO Water Purifier Actually Work?

8 min readPublished 25 March 2026Updated 1 August 2026

Osmosis, and why this is the reverse of it

In natural osmosis, water moves across a semi-permeable membrane from the less concentrated side to the more concentrated side, diluting it. It is how plant roots draw water from soil, and it happens without any external energy.

Reverse osmosis does the opposite by applying pressure to the concentrated side, pushing water molecules through the membrane against the natural gradient. Water passes; the dissolved sodium, calcium, magnesium, nitrate, arsenic, fluoride and lead ions do not, because they are physically larger than the membrane's pores once hydrated. They leave down the reject line as concentrate.

What is actually inside the box

Component	Job	Replacement interval
Sediment filter	Removes silt, rust and turbidity that would abrade the membrane	6-9 months
Pre-carbon block	Removes chlorine, which destroys a polyamide RO membrane	6-9 months
Booster pump	Raises inlet pressure to the 5-7 bar the membrane needs	3-5 years
RO membrane	The separation itself — removes 90-98% of dissolved solids	18-30 months
TDS controller	Blends a controlled amount of pre-RO water back in for taste and minerals	Adjust, not replace
UV lamp (if fitted)	Inactivates microbes, usually after the storage tank	12 months
Post-carbon polisher	Final taste and odour polish before the tap	9-12 months
Storage tank	Holds 6-10 L so purified water is available on demand	Sanitise at every service

Important

The pre-carbon block is the component people economise on, and it is the one that costs the most to skip. Chlorine in municipal supply will oxidise a polyamide RO membrane within months, and a membrane costs three to four times what the carbon block does.

Why it needs a pump and a drain

Indian mains pressure is typically 1-3 bar and frequently lower on upper floors. An RO membrane needs roughly 5-7 bar to work properly, which is what the booster pump provides. A purifier running on inadequate pressure produces less water, wastes more of it, and fouls its membrane faster — which is why low inlet pressure is one of the most common causes of premature membrane failure.

The drain line exists because the salts have to go somewhere. Everything the membrane rejects leaves in a concentrated stream, typically 1-3 litres for every litre purified. That ratio is a design characteristic, not a fault, though high-recovery designs have improved it considerably.

The TDS controller, and why your water tastes flat without it

An RO membrane is indiscriminate. It removes calcium and magnesium along with the arsenic, and it removes the bicarbonates that buffer water's pH. Output at 20-30 ppm tastes flat and slightly acidic, which is the source of most complaints about RO water.

A TDS controller is a small valve that lets a measured amount of pre-membrane water bypass and blend back into the output. Set sensibly, it lands the final reading at 60-100 ppm — enough mineral content to taste like water rather than nothing.

Many units ship set aggressively low so the demonstration TDS meter reads impressively at installation. If your RO water tastes empty, that is the first thing to check, and adjusting it costs nothing.

How RO units actually fail

Chlorine damage to the membrane, from a pre-carbon block left past its interval. The symptom is output TDS climbing steadily over a few months.
Fouling from sediment, on units fitted without an adequate sediment stage or on very turbid supply. The symptom is falling output flow.
Scaling on hard water, where calcium precipitates on the membrane surface. This is why very hard supply benefits from softening ahead of the RO rather than a bigger membrane.
Pump failure, usually after three to five years, and usually preceded by louder or more frequent cycling.
Tank bladder failure, which shows up as reduced usable capacity and a purifier that runs almost continuously.
Biofilm in the storage tank on units that are serviced late — the reason a UF stage or post-tank UV is worth having.

Frequently asked questions

How does reverse osmosis work in simple terms?

A pump pushes water against a membrane whose pores are around 0.0001 microns. Water molecules pass through; dissolved salts, heavy metals, arsenic, fluoride and microbes are too large and are flushed away down a drain line as concentrate. It is a physical separation, which is why it removes chemical and biological contaminants alike.

How many stages does an RO purifier need?

Functionally, four to six: sediment, pre-carbon, the RO membrane, a TDS controller, and usually UV plus a post-carbon polisher. Ignore headline stage counts of 10 or 12 — Indian marketing counts cosmetic cartridges. Confirm which technologies are physically present instead.

Why does an RO purifier waste water?

Because the dissolved salts the membrane rejects have to leave in something. That concentrate stream is typically 1-3 litres per litre purified. High-recovery and zero-waste designs reduce the ratio or redirect the reject to an overhead tank, but some rejection is intrinsic to how RO works.

How often should the RO membrane be replaced?

Typically every 18-30 months on normal supply, sooner on very high TDS or hard water. The clearest indicator is output TDS: measure it at installation and every few months afterwards, and replace when it has climbed substantially. Timers and indicator lights count hours, not actual condition.

Does an RO purifier work without electricity?

No. RO requires the booster pump to reach the pressure the membrane needs, so it stops entirely in a power cut — you have only whatever is in the storage tank. If power cuts are routine where you live, tank capacity matters more than any other specification.