Your kitchen purifier pushes water so hard that it squeezes out of the salt, through a skin with no real holes. A home RO purifier chains a sediment filter, activated carbon, a booster pump, the RO membrane, a carbon polish, a UV lamp and UF fibres, a mineral cartridge and a storage tank.
Your kitchen purifier pushes water so hard that it squeezes out of the salt, through a skin with no real holes. Take an RO purifier apart in 3D, flip osmosis backwards, and see where the other two litres go.
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A row of filters and one fine membrane
A home RO purifier chains a sediment filter, activated carbon, a booster pump, the RO membrane, a carbon polish, a UV lamp and UF fibres, a mineral cartridge and a storage tank. Each stage catches something different: dirt, chlorine, dissolved salts, germs.
Osmosis, run backwards
Water on its own flows through a membrane towards the saltier side. The push that stops it is the osmotic pressure, about 0.8 bar for water with 1,000 ppm of salt and 27 bar for seawater. A pump pushing harder than that forces fresh water out of the salty side instead.
No holes, just a very thin skin
The membrane is a polyamide skin about 0.1 to 0.2 micrometres thick, rolled round a tube. Water dissolves into it and diffuses across; ions in their shell of water barely do. It holds back 95 to 99% of dissolved salts, and germs thousands of times bigger than a water molecule.
TDS: less is not always better
RO takes 1,000 ppm borewell water down to about 50 ppm. India's BIS standard allows up to 500. A TDS controller blends some pre-filtered water back for taste, but that water skipped the membrane, so any fluoride or arsenic in it comes back too.
Most of the water goes down the drain
The rejected salt has to be washed away, so a typical home unit sends 2 to 3 litres to the drain for every litre you drink. Recover more and the reject gets so salty that scale forms on the membrane. Reject water is fine for mopping and flushing.
UV kills, UF strains, neither desalts
UV-C light at 254 nm scrambles germs' DNA; the dose falls if the water runs faster or the lamp ages. UF fibres with 0.01 µm pores strain out bacteria and cysts. Both leave salts alone, so low-TDS city water needs only UV or UF, not RO.
Eight stages on the wall, one tiny membrane doing the hard part.
An RO purifier is a row of filters, each catching something different, joined by thin plastic pipes. Follow the water from the wall:
A sediment pre-filter traps sand and rust. An activated carbon filter soaks up chlorine and smells, which would otherwise damage the membrane. A booster pump then squeezes the water to about 5 bar, five times the push of the air around you.
The star is the RO membrane, a sheet wrapped round a tube. Only water squeezes through it. Dissolved salts, most germs and metals stay behind and are washed away down the reject line, together with most of the water. The clean permeate then passes a post-carbon polish, a UV lamp and a UF filter to kill or catch any germs, and a mineral cartridge, before it waits in the storage tank for the tap.
A solenoid valve shuts the inlet when the pump stops, and a float switch in the tank turns everything off when it is full.
Water sneaks towards salt. Push hard enough and it goes the other way.
Put fresh water on one side of a special skin and salty water on the other. The skin is a semi-permeable membrane: water molecules can get through, but dissolved salt cannot.
Water moves on its own towards the salty side, as if trying to dilute it. This is osmosis. It is how plant roots drink and why a cucumber goes limp in salty pickle. The salty side rises until the extra weight of water pushes back just as hard. That push is the osmotic pressure.
The chemist van 't Hoff found a simple rule: π = iMRT. Double the salt and you double the pressure. Water with 1,000 ppm of salt has about 0.8 bar; seawater, about 27 bar.
Now push on the salty side harder than π. The flow flips: fresh water is squeezed out of the salty water. That is reverse osmosis. A home purifier's pump gives about 5 bar, plenty for well water but far too little for the sea, where plants push 55 to 70 bar.
A sheet thinner than a soap bubble, rolled up like a carpet.
Cut open an RO cartridge and you find a spiral-wound element: flat envelopes of membrane rolled round a perforated tube, with a plastic mesh, the feed spacer, between the layers.
The membrane is a thin-film composite. A strong spongy backing holds up a polyamide skin only about 0.1 to 0.2 micrometres thick, a few hundred times thinner than a hair. That skin does all the work.
It has no real holes. Water molecules dissolve into the polymer and hop across; bigger, charged ions wrapped in their shell of water are held back. So RO removes about 95 to 99% of dissolved salts, and viruses and bacteria, which are hundreds to thousands of times bigger, have no chance.
Feed water sweeps along the surface, not straight into it. This cross-flow keeps washing the rejected salt away. Only a part of the water soaks through, then spirals inward along the permeate carrier to the central tube.
The membrane strips almost everything; a small valve puts some back.
Even clear water carries dissolved stuff: calcium, magnesium, sodium, chloride, bicarbonate and more. Together they are the total dissolved solids, or TDS, measured in ppm (the same as mg per litre).
India's drinking-water standard, BIS IS 10500, says up to 500 mg/L is acceptable, and up to 2,000 mg/L is allowed only where there is no other source. City tap water is often 100 to 300 ppm; borewells in many places give 500 to 1,500 or more.
An RO membrane removes about 95% of TDS, so 1,000 ppm becomes about 50. That is safe, but can taste flat. So many purifiers have a TDS controller: a small valve that blends a little pre-filtered water back in for taste and minerals.
Careful: that blended water has not been through the membrane. If your borewell has fluoride or arsenic, which RO removes well, the controller lets some of it straight back into your glass.
For every glass you drink, two or three go down the drain.
The membrane cannot keep the salt it rejects. Something has to carry it away, or the surface would clog within minutes. That something is the reject water, also called concentrate or brine.
The share of the incoming water that ends up in your glass is the recovery. A typical home purifier recovers only about 25 to 40%, so for every litre you drink, about 2 to 3 litres go down the drain. A small flow restrictor on the reject line sets this balance and keeps the pressure up on the membrane.
Why not recover more? The less water that leaves as reject, the saltier it gets. Push too far and hardness salts crystallise as scale on the membrane, and it dies early. Better membranes and designs now reach 50 to 60% or more, and India's National Green Tribunal has pushed makers towards 60% and above.
Reject water is too salty to drink, but it is fine for mopping, flushing toilets and washing cars. Use it on plants only now and then: the salt builds up in the soil.
One scrambles germs’ DNA, the other strains them out. Neither touches salt.
RO already holds back germs, but seals can leak and water can sit in the tank. So most purifiers add a germ-killing stage or two.
A UV lamp glows with UV-C light at 254 nanometres, the colour DNA absorbs best. It scrambles the germs' genes so they cannot multiply. What matters is the dose: brightness × time, in mJ/cm². Good home units give at least 40 mJ/cm² at their rated flow. Run water faster, or let the lamp age, and the dose falls.
UF (ultrafiltration) uses bundles of hollow fibres with pores about 0.01 µm across. Bacteria and cysts are far too big to get in; most viruses are held back too. Dissolved salts slip straight through, and UF works without electricity.
So which do you need? If your water is city water with low TDS, under about 300 ppm, UV and UF are enough and waste no water. If it is salty borewell water, only RO removes the dissolved salts.
Why does the carbon filter come before the RO membrane?
Chlorine would damage the thin membrane. Most home RO membranes are polyamide, which chlorine attacks. The carbon removes it first.
What does the booster pump do?
Pushes water hard against the membrane. City and borewell water is too weak to force water through the membrane, so the pump raises it to about 5 bar.
Where do the salts go?
Down the reject line with the extra water. The membrane holds them back and the flowing reject water carries them to the drain.
In osmosis, which way does water move?
From the fresh side to the salty side. Water moves towards where it is "less concentrated", the salty side, as if to dilute it.
What makes it reverse osmosis?
Pushing on the salty side harder than the osmotic pressure. Once the push beats π, the flow flips and fresh water is squeezed out of the salty side.
Seawater has an osmotic pressure of about 27 bar. Why can’t a home purifier turn it into drinking water?
Its pump gives only about 5 bar. Below 27 bar, water would actually flow into the seawater. Desalination plants use 55 to 70 bar.
How does water get through an RO membrane?
It dissolves into the thin polymer skin and diffuses across. The polyamide has no fixed pores. Water moves through it by solution and diffusion; ions barely do.
Why does the feed water flow along the surface?
To keep washing away the salt the membrane rejects. Cross-flow sweeps the rejected salts and dirt to the drain, so the membrane does not clog so quickly.
A bacterium is about 1 µm. How does that compare with a water molecule?
About 3,500 times bigger. 1 µm is 1,000 nm, and a water molecule is about 0.28 nm across.
Borewell water at 1,000 ppm goes through a membrane with 95% rejection. What comes out?
About 50 ppm. 95% is removed, so 5% of 1,000, about 50 ppm, is left.
What does the TDS controller do?
Blends some pre-filtered, un-membraned water back in. It lets a little of the incoming water bypass the membrane, raising TDS for taste.
Why can the TDS controller be risky with fluoride-rich borewell water?
The blended water skips the membrane, so its fluoride comes straight back. The membrane removes most fluoride, but the bypass water keeps all of it.
A purifier has 25% recovery. How much water goes to the drain for each litre you drink?
3 L. You need 4 L of feed for 1 L of product, so 3 L leave as reject.
Why not run at 90% recovery?
The reject would be so salty that scale forms and the membrane clogs. With little water left to carry the salt, it becomes very concentrated and crystallises on the membrane.
What is a good use for reject water?
Mopping floors and flushing. It is safe to touch but too salty to drink or use often on plants or fish.
What does UV light do to germs?
Damages their DNA so they cannot multiply. UV-C at 254 nm is absorbed by DNA and RNA and scrambles them. The germs stay in the water but are harmless.
If you double the flow through a UV chamber, what happens to the dose?
It halves. Each drop spends half as long near the lamp, so it gets half the dose.
Your city water is 150 ppm TDS. What do you need?
UV and/or UF is usually enough. The TDS is already low, so the job is killing or catching germs, and UV and UF waste no water.
Words worth knowing
Reverse osmosis
Pushing water through a semi-permeable membrane harder than the osmotic pressure, so fresh water leaves the dissolved salts behind.
Osmotic pressure
The pressure needed to stop water flowing into a salty solution through a membrane. For salt water, π = iMRT.
Semi-permeable membrane
A skin that lets water molecules through but blocks dissolved ions.
Thin-film composite
An RO membrane made of an ultra-thin polyamide layer on a porous support, rolled into a spiral-wound element.
TDS
Total dissolved solids: everything dissolved in water, in mg per litre (ppm).
Salt rejection
The share of dissolved salt a membrane holds back, usually 95 to 99%.
Recovery
The share of the incoming water that comes out purified. The rest leaves as reject.
UV dose
UV light intensity multiplied by time, in mJ/cm². It sets how many germs are inactivated.
Ultrafiltration
Filtering through pores about 0.01 µm wide: it catches bacteria and cysts but not dissolved salts.
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