A reverse osmosis under sink system treats just the drinking water tap by forcing water through a membrane fine enough to reject most dissolved contaminants, with carbon stages before and after it. It lives in the sink cabinet, feeds its own dedicated faucet, and sends its reject water to the drain. It is the standard answer when the problem is dissolved: lead, nitrate, arsenic, PFAS reduction and flat-out better-tasting water.
The stages between the valve and the faucet
Water enters through a sediment prefilter that protects everything downstream, then a carbon block that removes chlorine, which would otherwise destroy the membrane. The membrane itself does the real separation, splitting the stream into treated water and a concentrate stream that carries the rejected solids to the drain. A final carbon polish removes any taste picked up in storage, and the treated water exits at a dedicated faucet on the counter. Vendors such as iSpring publish the stage count and membrane ratings for each model, which is how systems are compared like for like.
Tank or tankless: the cabinet decision
Traditional systems fill a pressurized storage tank so the faucet pours instantly; the cost is cabinet space and a tank that must hold air pressure to push water out. Tankless systems make water on demand through a larger membrane and a pump: they free the cabinet and never store water, but draw power and pour at the speed of production. Households that fill stock pots want the tank's instant volume; tight cabinets and minimal-maintenance preferences favor tankless. Both designs need their filters changed on schedule, and both send concentrate to the drain while running.
What the installation actually requires
The system needs a cold-water feed valve under the sink, a hole in the counter or sink deck for the dedicated faucet (many sinks have a knockout or a spare sprayer hole), and a drain connection for the concentrate line, made with an air-gap faucet or a drain saddle above the trap. Tankless models also need an outlet in the cabinet. None of this is beyond a patient DIYer with the included fittings, but counter drilling on stone is a job for a pro. Once running, verify the membrane with a TDS meter, and see our best under sink reverse osmosis guide to compare current systems before buying.
Questions people ask about reverse osmosis under sink
Does under sink reverse osmosis need its own faucet?
Yes, in almost all setups. The system produces treated water slowly, so it feeds a dedicated drinking faucet, and some codes require an air-gap faucet for the drain connection.
Does reverse osmosis waste water?
Every RO system sends a concentrate stream carrying the rejected solids to the drain while producing treated water. Ratings vary by model, so check the maker's published figures.
How do I know the RO membrane is still working?
Compare a TDS reading of the tap water with one from the RO faucet. A treated reading that climbs toward the tap reading over time means the membrane or prefilters are due.