Where plumbing code requires an air gap on a reverse osmosis drain connection, the familiar answer is an air gap faucet with three tubes and a large deck hole. It is not the only answer. An approved in line air gap device fitted under the sink satisfies the same requirement while leaving you free to choose a plain faucet, which is worth knowing before drilling a worktop.
What the requirement is protecting against
The system's drain line connects to the waste pipe, and without a physical break in that path a blockage or pressure event downstream could push waste water back towards the system and, from there, towards drinking water. An air gap is an open vertical break that makes backflow impossible, the same principle that puts a tap spout above the rim of a basin. Codes vary by jurisdiction, so confirm what applies where you are rather than assuming either way.
How an in line device works
It sits under the sink in the drain line rather than in the faucet. The reject line enters, discharges across an open gap inside the device, and the water then falls to the drain saddle. Functionally identical to the faucet version, with the break moved out of sight. The advantages are a plain faucet with a single tube, a smaller deck hole, and no gurgling at the tap. It has to be an approved device rather than an improvised arrangement for it to satisfy an inspector.
What the drain side needs either way
The tube leaving the gap flows by gravity, so it must run continuously downhill with no dips, kinks or long horizontal runs, and it should be the diameter specified rather than something smaller. Connect above the trap, never below it, so the water seal keeps sewer gas out of the line. Align the saddle's opening with the hole drilled in the waste pipe, which sounds obvious and is a frequent installation error. And keep the run short.
Choosing between them
If the deck hole is already large or you are drilling anyway, the faucet version keeps the parts count down. If the worktop is stone, if you want a particular tap, or if the gurgling would bother you, the in line device is the better route. Either way, the fault that produces water on the worktop is a restricted gravity line below, not the gap itself, so routing that tube properly matters more than which device you chose. Our whole house RO guide covers drain arrangements at larger scale.
Questions people ask about reverse osmosis air gap
Must I use an air gap faucet?
No. An approved in line air gap device in the drain line satisfies the same requirement with a plain faucet and a smaller deck hole.
Is an in line device as good?
Functionally identical: it provides the same open break, just under the sink rather than in the tap.
Where should the drain connect?
Above the trap, never below it, so the water seal continues to keep sewer gas out of the reject line.
What causes water on the worktop?
A restricted gravity line below the gap: a sag, a kink, a narrow tube or debris at the saddle. Not the gap itself.