Reverse osmosis at the kitchen sink is the most common way households get treated drinking water, and it is a plumbing job as much as a water treatment decision. Before choosing a system, work out what the cabinet, the worktop and the waste pipe will allow, because those four physical questions decide more about which unit fits than any specification sheet does.
The four things the kitchen has to provide
First, cabinet space: a conventional system needs room for the filter stack and a pressure tank the size of a small football, and the stack has to have clearance below it so cartridges can be pulled down and changed. Second, a cold feed you can tee into, usually at the shut off valve under the sink. Third, a drain connection above the trap for the reject line, made with a saddle or a purpose made fitting. Fourth, somewhere for the separate faucet: a spare knockout in the sink deck, or a hole drilled through the worktop, which is straightforward in stainless and a specialist job in stone.
Line pressure, and when a pump is needed
Reverse osmosis is driven by the pressure of the incoming supply pushing water against the membrane. Where household pressure is healthy, no pump is needed and the system runs silently on the line. Where pressure is low, production slows sharply, the tank never quite fills, and the water in the tank is drawn down faster than it is replaced, so a booster pump becomes part of the specification rather than an upgrade. Well systems on a low pressure switch setting are the common case. Check pressure at the sink before ordering rather than after.
Living with the tank and the separate faucet
The pressure tank is what makes an under sink system usable: the membrane produces slowly, the tank stores what it makes, and the faucet draws from the tank at a normal rate. That tank needs an air charge on its dry side, and a tank that has lost it delivers a trickle and then stops, which is the most common false alarm about a failing membrane. The separate faucet exists because RO water should not be run through the main mixer, which shares a path with unfiltered hot water. Some households add a fridge and ice maker feed from the same tank.
Whether the kitchen is the right place at all
Reverse osmosis at the sink treats drinking and cooking water only, which is where nearly all the health relevant consumption happens, and it does so far more cheaply than treating everything. If the complaint is scale on the kettle and dry skin from the shower, an under sink RO fixes none of it and a softener is the right buy. If the complaint is taste, or a specific contaminant found in a test, the kitchen sink is exactly the right place. Our under sink reverse osmosis guide compares the systems that fit a standard cabinet.
Questions people ask about reverse osmosis for kitchen sink
Does an under sink RO system need a separate faucet?
Yes, in practice. The treated water is kept on its own line to its own tap so it never shares a path with unfiltered hot water through the main mixer.
Will it fit in a cabinet that already has a waste disposer?
Often it will, but measure first. Compact and tankless systems exist precisely because disposers, deep sinks and shallow cabinets leave little usable room.
Can the fridge and ice maker be fed from the same system?
Yes, with a tee off the treated line, though a long run to the fridge and the extra draw may call for a larger tank or a permeate pump.
Why is the flow from the RO faucet slow?
Usually the storage tank rather than the membrane: a tank that has lost its air charge delivers a trickle. Check the tank pressure before condemning the membrane.