Reverse osmosis water filtration under sink is the standard way a household gets genuinely purified drinking water without treating the whole building. The system lives in the cabinet, taps into the cold supply, sends concentrate to the drain line, stores treated water in a small tank and feeds a dedicated faucet on the counter. Understanding the four connections is most of what you need to judge whether it fits your kitchen.
The four connections in the cabinet
Every under-sink RO system needs the same four things: a cold water feed taken off the angle stop, a drain saddle or connection into the sink tailpiece for the concentrate, a dedicated faucet drilled through the countertop or fitted in the sprayer hole, and space for the storage tank. Tankless designs drop the tank in exchange for a pump and a power outlet. Refrigerator and ice maker lines can be teed off the treated side, though the extra run slows the fill noticeably.
How the stages divide the work
A sediment cartridge protects everything behind it, carbon cartridges strip the chlorine that would otherwise destroy the membrane, the membrane itself rejects dissolved contaminants, and a final polishing carbon cleans up storage taste before the water reaches the faucet. The pre-filters are the maintenance item and they are what determine how long the membrane lasts. Skipping a pre-filter change is the most common reason a membrane fails years early.
What it removes, and the mineral question
The membrane rejects dissolved ions broadly: lead, nitrate, fluoride, arsenic, sodium, sulfate and total dissolved solids, plus the microbiological hazards a membrane physically excludes. It also strips calcium and magnesium, which is why RO water tastes flat to some people and why remineralizing cartridges exist. The drinking water contribution of those minerals is small compared with food, so the choice is about taste rather than nutrition.
The real trade-offs before you commit
RO fills slowly, so a tank system that has just been drawn down needs time to recover, and a tank takes up most of the usable cabinet. Some water goes to the drain as concentrate, more on older units than on modern efficient ones. Line pressure matters: below a certain feed pressure the membrane underperforms and a booster pump becomes necessary, which is common on well systems. Our under-sink reverse osmosis guide compares the layouts side by side.
Questions people ask about reverse osmosis water filtration under sink
Do I have to drill a hole for the RO faucet?
Usually yes, unless your sink has an unused sprayer or soap dispenser hole. Some systems instead use a combination faucet that replaces the existing mixer.
Can an under-sink RO system feed my refrigerator?
Yes, by teeing the treated line to the fridge, though the longer run and the ice maker draw mean the storage tank recovers more slowly.
Why is my RO faucet slow?
Usually low tank air pressure, a nearly empty tank still refilling, clogged pre-filters or low feed pressure. Check the tank pressure with the tank isolated and drained.
Does under-sink RO treat the whole house?
No, it treats one tap. Showers, laundry and other fixtures stay on untreated water, which is why hardness or chlorine complaints need separate whole-house equipment.