Reverse osmosis pushes water through a membrane fine enough to strip out most dissolved contaminants, which is why it is the standard answer to lead, nitrate, sodium and stubborn taste complaints. It also fills slowly, sends water to the drain while it works and needs servicing on a schedule, so the honest case for buying one is narrower than the marketing suggests.
What reverse osmosis is genuinely good at
The membrane rejects things a carbon cartridge cannot touch: dissolved salts, nitrate, fluoride, arsenic in its common form, and much of what makes water taste mineral or metallic. That breadth is the whole argument for it. If your test came back with a dissolved contaminant rather than a taste or odor problem, reverse osmosis is usually the only point-of-use technology that will move the number, and vendors publish stage counts and rejection claims per model so you can compare like for like.
The costs that are not printed on the box
Every unit sends some water to the drain to carry rejected minerals away, so a household on a metered supply pays for water it never drinks. Production is slow, which is why there is a pressure tank taking up half the cupboard under the sink. Low feed pressure cripples output. Pre-filters and post-filters get changed on a schedule and the membrane itself is replaced far less often but costs more, and the finished water tastes flat to people used to mineral water.
When a cheaper filter is the better buy
If the complaint is chlorine taste, a swimming-pool smell or general staleness, a good carbon block at the sink does the same job for the drinker at a fraction of the running cost and none of the drain water. If the complaint is scale on the kettle and spotting on glassware, no reverse osmosis unit under the sink fixes the rest of the house; softening does. Read the test result first, then choose, and see our whole house reverse osmosis system page before scaling this technology up to the main line.
Questions people ask about reverse osmosis pros and cons
Does reverse osmosis remove useful minerals as well?
Yes. The membrane does not distinguish between calcium and contaminants, which is why the water tastes flat and why some systems offer a remineralizing cartridge on the outlet.
Can I fit reverse osmosis on the whole house?
You can, but whole-house units are far larger, need storage and repressurization, and are usually reserved for supplies with a dissolved contaminant that nothing else will remove.
How often does the membrane need changing?
Far less often than the pre-filters, but the interval depends on feed water and use. Follow the maker's schedule, and change the sediment and carbon pre-filters on time to protect the membrane.