Note in the picture above that the tubes are of different diameters. The tube on the left in the picture delivers the “permeate” or purified water to the user via the tall spout when the faucet’s handle is pressed. Reverse osmosis units produce waste water when they are in operation. The drain water flows along the open trough until it falls through a hole into the top of the larger tube on the right in the picture. Water falls by force of gravity down the large tube and into the undersink drain pipe.The hole in the faucet body serves as an overflow drain if the black tube is obstructed.
The drain hole, indicated with the arrow in the top picture, is an escape hole so that drain water can have a way out if the drain line stops up. Or, the obstruction may be in the drain saddle or the drain pipe itself. Since air gap faucets are often unpopular with homeowners because of noise and messes on the sink when the drain line blocks, some manufacturers provide reverse osmosis with standard faucets. Plumbing codes differ from place to place, but most still require the air gap faucet. They are never used with filters because the air gap is not needed.