Reverse osmosis: what it removes and what it does not
Reverse osmosis forces water through a semipermeable membrane that retains most dissolved salts, heavy metals and organic compounds, but it also removes beneficial minerals and produces a reject stream that goes to drain.
gelisted editorial team · Updated 15 August 2026
The short version
- The membrane retains over 90 per cent of dissolved salts.
- Units with a tank reject 2 to 4 litres per litre produced.
- A domestic unit costs 250 € to 900 € installed.
- Pre filters are changed every 6 to 12 months.
It is the domestic system that removes most and also the one that wastes most water. Both statements are true and both belong in the decision.
How it works
Water is driven against a membrane with very small pores, which lets water molecules through and retains most of what is dissolved. What is retained is carried away by a reject stream that goes to drain. Ahead of the membrane sit sediment and carbon filters that remove particles and chlorine, since chlorine damages the membrane.
| Element | Retention | Note |
|---|---|---|
| Dissolved salts | over 90 % | lowers conductivity |
| Heavy metals | high | |
| Nitrates | high | relevant for well water |
| Chlorine and taste | high | via the carbon pre filter |
| Calcium and magnesium | high | including beneficial minerals |
| Dissolved gases | low | they pass the membrane |
| Water rejected | 2 to 4 litres per litre | units with a tank |
| Unit installed | 250 to 900 € |
The reject stream
This is the most debated point. A domestic unit with a tank sends 2 to 4 litres to drain for every litre it produces. Direct flow units with a pump improve that ratio to around 1 to 1 or 1,5 to 1, at the cost of a higher price and some electricity use. On an island where water is a limited resource and demand peaks in summer, that figure is not incidental.
Remineralisation
Osmosis water leaves with very few minerals, which changes the taste and removes the calcium and magnesium contribution of the water. Many units include a remineralising cartridge afterwards that restores part of it. It is not essential nutritionally, since the main dietary source of those minerals lies elsewhere, but it improves the taste noticeably and costs 25 € to 60 € added to the unit.
That cartridge is replaced on the same schedule as the filters.
Maintenance and when it makes sense
Pre filters every 6 to 12 months at 30 € to 90 € a set, and the membrane every 2 to 4 years at 50 € to 150 €. It makes sense where mains water has a marked taste, where it comes from a well with parameters that justify it, or to stop buying bottled water: a family drinking 6 litres a day spends 250 € to 500 € a year on bottles, which pays for the unit within 1 or 2 years.
Frequently asked
+What does reverse osmosis remove?
Over 90 per cent of dissolved salts, plus heavy metals, nitrates and organic compounds.
+How much water does it waste?
Two to four litres per litre produced in units with a tank, less in direct flow units.
+Does it remove good minerals too?
Yes, which is why many units add a remineralising cartridge costing 25 € to 60 €.
+What maintenance does it need?
Filters every 6 to 12 months at 30 € to 90 €, and the membrane every 2 to 4 years at 50 € to 150 €.
+When is it worth installing?
With marked taste, with well water, or to stop buying bottles, paying back within 1 or 2 years.
Sources
- 1Ministerio de Sanidad, Gobierno de España (August 2026)
- 2Govern de les Illes Balears, Govern de les Illes Balears (August 2026)