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How does reverse osmosis work?
Osmosis is a natural phenomenon you probably remember from biology class. Two liquids, separated by a semi-permeable membrane. Water moves of its own accord from the side with fewer dissolved substances to the side with more, until both sides are in balance.
Reverse osmosis does exactly the opposite. You apply pressure and force the water the other way: from dirty to clean. The water gets through; most of what was in it doesn't.
Why 0.0001 micron makes such a difference
A micron is a thousandth of a millimetre. The pores of a reverse osmosis membrane measure 0.0001 micron — ten thousand times smaller.
For comparison:
| Particle | Size (micron) |
|---|---|
| Particulate matter PM10 | 10 |
| Virus | 0.02 – 0.3 |
| Dissolved salts and metals | 0.0001 – 0.001 |
| RO membrane | 0.0001 |
A carbon filter, like the one in a filter jug, works by adsorption: substances stick to the surface of the carbon. That works well for chlorine, odour and flavour. It doesn't work for dissolved substances that are smaller than the pores in the carbon.
An RO membrane works by size exclusion. Anything larger than the pore can't get through. That's a fundamentally different principle, and the reason RO is used so widely as a filtration technique — from water treatment to seawater desalination.
The layers of the Osmora
A membrane on its own isn't enough. It's fine, and therefore delicate. Coarser particles would clog it within a few weeks. That's why there are layers in front of it:
01 · PP cotton
Catches sand, rust, sediment and loose particles from the pipes. This is the coarsest stage, and it spares everything that comes after it.
02 · Activated carbon
Reduces chlorine and the substances that affect odour and flavour. Chlorine also damages the RO membrane itself, so this stage doubles as a buffer for the membrane.
03 · Carbon block
More densely compressed than loose carbon, with a larger contact surface. Captures organic contaminants and chemical substances.
04 · Reverse osmosis
The main stage. Filters down to 0.0001 micron.
05 · Post-filter
The final stage, for flavour. Water that has just passed through a membrane tastes flat; this layer makes it pleasant to drink again.
On the Osmora Premium · Remineralisation
The Osmora Premium has a remineralisation layer as its final stage. It gives the filtered water back calcium and magnesium, so it tastes full rather than stripped. Here's how the remineralisation works.
The order isn't random. Each layer looks after the next, and the membrane sits in position four on purpose — only after everything that could damage it has been caught.
Why there's water left over
This is the question we get asked most, and the answer is simple: the membrane holds substances back, and those substances have to go somewhere.
When you press water against a membrane, some of it passes through and some stays behind. That leftover water contains everything the membrane has held back, in concentrated form. We call it residual water, or concentrate.
If that residual water weren't drained away, the membrane would clog within a short time and stop working. So draining residual water isn't an inefficiency — it's how the technology works. Every reverse osmosis system in the world does this, from household appliances to industrial installations.
What differs is the ratio. The higher the pressure and the better the membrane, the more favourable it is.
What it isn't
Reverse osmosis adds nothing. No chemicals, no agents, no treatment. It's a physical separation: water on one side, everything else on the other. The remineralisation in the Osmora Premium is a separate stage after the membrane.
That also makes it easy to check. A TDS meter measures the total dissolved solids in water, before and after filtration. You can read the difference straight off the display — no need to take anyone's word for it.
Want to know more about what's in Dutch tap water? Take a look at our sources page, featuring research from RIVM, KWR and WHO.
