Fitting reverse osmosis for sink use is mostly a question of what your cabinet and countertop will allow. The system itself is a manifold, a membrane housing and a small storage tank, and the work is in three connections: a feed from the cold stop valve, a drain saddle on the waste pipe, and a dedicated faucet through the countertop.
Measure the cabinet before you buy
The manifold needs wall or side-panel space with clearance below it for cartridges to drop out, and the storage tank needs floor space standing upright, ideally beside the trap rather than in front of it. A full tank is heavy, so it stands on the cabinet base or a bracket, never hanging from tubing. If the cabinet holds a garbage disposal, a pull-out bin and a water filter already, measure honestly: an alternative is mounting the tank in an adjacent cabinet and running tubing through a drilled partition.
The three connections
The feed comes off the cold stop valve using an adapter tee, which is also the point where a seized old valve turns a two-hour job into a plumber's visit. The drain saddle clamps onto the waste pipe above the trap and on the opposite side from the disposal outlet, drilled to the size the kit specifies. The faucet goes through a spare hole in the sink deck, a blanked sprayer or soap dispenser hole, or a hole drilled in the countertop. Stone and quartz need a proper diamond bit and a steady hand.
Commissioning it properly
New systems are flushed before use: the first tank or two are drained to rinse manufacturing residue out of the cartridges and to wet the membrane. Check every push-fit connection under pressure with a dry paper towel before closing the cabinet, since a slow leak inside a cabinet is discovered by the smell of the chipboard. Set the tank's air charge with the tank empty, note the date on the cartridges, and leave the tank valve fully open, because a half-open valve is the most common post-install complaint.
Sink-mounted reverse osmosis has limits
It treats one faucet, so the shower, the water heater and the washing machine still get untreated water, and hardness keeps scaling them. If scale is your complaint, a softener at the point of entry is the system you actually need, with the reverse osmosis unit behind it for drinking water. Softened feed water also protects the membrane. If cabinet space simply is not there, a countertop unit does the same separation on the worktop. Our best under-sink reverse osmosis guide compares the systems that fit these constraints.
Questions people ask about reverse osmosis for sink
Do I need to drill the countertop for an under-sink reverse osmosis system?
Only if there is no spare hole. Blanked sprayer or soap dispenser holes often take the filtered faucet, which avoids drilling stone entirely.
Can the system feed the refrigerator ice maker too?
Yes, with a tee off the tank line and a check valve. Expect slower ice production, since the membrane refills the tank at a modest rate.
Should the reverse osmosis unit go before or after a softener?
After. Softened feed water protects the membrane from scaling, and the softener handles the whole house while the reverse osmosis unit handles the drinking tap.