Reverse osmosis explained
Reverse osmosis forces water under pressure through a membrane with pores small enough to block dissolved substances — salts, metals, nitrates, fluoride, most things. What comes through is close to pure water.
It is the most thorough filtration available domestically, and it comes with trade-offs that no other product on this site has: it wastes water, it is slow, it takes up real space, and it removes beneficial minerals along with everything else.
How it works
Osmosis is water moving naturally across a membrane from a less concentrated solution to a more concentrated one. Reverse osmosis applies pressure to push it the other way — leaving the dissolved substances behind.
A domestic system runs four to six stages. Sediment pre-filter, carbon pre-filter (which also protects the membrane from chlorine, which damages it), the membrane itself, a post-carbon polish, and often a remineralisation cartridge to put back some of what was removed.
Because the process is slow, the treated water collects in a small pressurised tank under the sink, and the dedicated tap draws from that.
What it removes
Effectively: dissolved salts, heavy metals including lead, nitrates, fluoride, and the great majority of anything else dissolved. Combined with its pre-filters it also handles sediment, chlorine and taste.
It also removes calcium and magnesium — so RO water will not scale a kettle. That does not make it a limescale solution for a property, because it only treats what comes out of the RO tap. The boiler, the shower and the washing machine are unaffected.
What it does not remove reliably on its own is bacteria and viruses. The membrane blocks most of them, but RO is not a disinfection technology and should not be relied on as one. On a private supply, UV does that job.
The waste water question
RO rejects concentrated water to drain. Older systems reject three or four litres for every litre produced. Modern high-efficiency systems have brought that down considerably, some to around one to one.
Check the ratio before buying, particularly if you are on a water meter or live in a water-stressed area — which in the UK increasingly means the South East and East Anglia, exactly where hard water makes people consider RO in the first place.
Do you actually need it in the UK?
For most households on mains water, no. UK drinking water is treated and monitored to a high standard, and the complaints people actually have — chlorine taste, general taste, smell — are handled perfectly well by a carbon filter at a fraction of the cost and none of the waste.
RO earns its place where there is a specific reason: a private well or borehole, nitrates in a rural supply, a specific contaminant you have tested for, an aquarium, or specialist coffee where you want to build water from scratch.
It is also a perfectly legitimate choice for someone who simply wants the most thorough option available and is content with the trade-offs. That is a preference, and there is nothing wrong with it — but it should be a choice made knowing the alternative was cheaper, simpler and probably sufficient.