An under sink reverse osmosis system is a small treatment plant in a cabinet: prefilters, a membrane, a storage tank or pump, a dedicated faucet and a drain connection, each with a job. Understanding the anatomy is what lets you choose a configuration deliberately, because the differences between systems are almost entirely which of these parts they include and how.
The parts in the cabinet, one by one
Water enters through a tee on the cold line and meets the prefilters: sediment to protect the plumbing, carbon to strip the chlorine that would otherwise degrade the membrane. The membrane then splits flow into treated water and a concentrated reject stream. Treated water fills a pressurized storage tank so a useful amount is ready on demand, passes a final polishing postfilter on its way up, and pours from a dedicated faucet on the sink deck. The reject stream leaves through a saddle fitting clamped to the drain pipe. Every RO install is these six or so elements arranged in a cabinet.
The three install points that decide the work
Fitting the system is really three small plumbing jobs. The feed connection tees off the cold supply valve, usually with an included adapter. The faucet needs a hole in the sink deck or countertop, either an existing sprayer knockout or one you drill, and this is the step that gives most DIYers pause. The drain saddle clamps to the tailpiece under the basin, above the trap, to carry the reject stream away. None of the three is beyond a patient amateur with the manual open, and all three are routine for a plumber if drilling stone counters is not your idea of a weekend.
Tank or tankless, and the space arithmetic
The classic configuration stores treated water in a pressurized tank roughly the size of a small propane cylinder, which claims real cabinet space alongside the filter manifold. Tankless variants swap the tank for an electric pump and filter on demand, freeing the cabinet but requiring an outlet below the sink. Measure the cabinet with the disposal, trap and existing valves in view before choosing: vendors such as Aquasana and iSpring publish full dimensions for each configuration, and cabinet fit disqualifies more systems than any performance difference does.
Choosing stages and extras with a straight face
Stage count is marketing shorthand, not a quality score: what matters is that sediment and carbon stand ahead of the membrane and a polishing stage after it. Worthwhile extras are concrete: a remineralization stage if you find RO water flat-tasting, a leak-stop valve if the cabinet holds anything you love, and quick-change cartridge heads if you know honestly that fiddly maintenance will get skipped. Beyond that, more stages mostly add cartridge cost. Our best reverse osmosis system under sink roundup compares configurations on exactly these terms.
Questions people ask about under sink reverse osmosis system
Does an under sink RO system need its own faucet?
Conventional systems do, because RO output is a separate low-flow supply. Some tankless models offer integrated or existing-faucet options; check the specific unit's install design.
Can I install an under sink RO system myself?
A patient DIYer can: the feed tee and drain saddle are simple, and the faucet hole is the only intimidating step. Stone countertops without an existing knockout are the usual reason to call a plumber.
How much cabinet space does the system take?
A tanked system claims a corner for the storage tank plus wall space for the filter manifold. Tankless units shrink to a single slim housing but need an outlet. Measure before ordering; dimensions are published.
Why does the system connect to the drain?
The membrane rejects a concentrated stream that has to go somewhere, so a small saddle fitting on the sink drain carries it away. It is a normal, sanctioned connection, installed above the trap.