An RO system under sink forces cold water through a semipermeable membrane that rejects dissolved solids, sending the concentrated leftover down the drain and the treated water to its own small faucet. It is the most thorough drinking-water treatment you can fit in a kitchen cabinet, and it asks for two things in return: a drain connection and a cartridge schedule you actually keep.
What happens inside the cabinet
Water passes a sediment prefilter, then a carbon stage that strips chlorine before it can damage the membrane, then the membrane itself where dissolved solids are rejected. A final polishing carbon stage tidies the taste on the way to the faucet. Because membranes work slowly, traditional systems store treated water in a small pressurized tank; tankless designs use a pump to produce water on demand instead, trading the tank's cabinet space for a powered unit.
Tank or tankless, and the drain question
Tank systems are simple and unpowered but claim more cabinet room and can taste stale if the tank sits unused. Tankless units free the cabinet and deliver fresher water but need an outlet and cost more up front. Both must connect to the drain for the reject stream, usually through a clamp-on drain saddle above the trap. If your sink already hosts a disposal and dishwasher connection, check the layout before buying; makers such as iSpring publish full installation diagrams for their under-sink RO range.
The maintenance that keeps it working
Prefilters and postfilters change on a routine schedule, and the membrane itself lasts several years when the prefilters are kept fresh. Skip the prefilter changes and chlorine or sediment quietly destroys the membrane, and the system keeps dispensing water that is no longer treated. A cheap TDS meter is the honest check: measure the tap water and the RO water occasionally, and a shrinking difference tells you the membrane is due. Our best RO system under sink roundup notes which models make the changes tool-free.
Questions people ask about ro system under sink
Does an under-sink RO system need electricity?
Tank-based systems run on water pressure alone. Tankless systems and any unit with a booster or permeate pump need an outlet under the sink.
Why does RO send water down the drain?
The membrane rejects dissolved solids into a concentrate stream that must be flushed away. That reject flow is what carries the removed material out of the system.
Can I run the RO line to my fridge as well?
Usually yes. Most systems support a tee to the refrigerator ice maker line, though long runs may need a pump to keep pressure up. Check the maker's guidance first.