The drain connection is the least glamorous part of a reverse osmosis installation and the one that causes the most calls afterwards. The adapter itself is inexpensive; what matters is where it connects, how it is aligned, and whether the tube leaving it can actually flow, since the reject line runs by gravity in an air gap installation.
The saddle clamp
The traditional fitting: a two part clamp that grips the waste pipe with a rubber gasket between, and a hole drilled through the pipe wall to match the saddle's port. It works reliably when the hole is aligned with the port, which sounds obvious and is the most frequent installation error, and when the gasket seats cleanly on a pipe that is round and undamaged. Position it above the trap and, on a horizontal run, on the upper part of the pipe so the connection is above any standing water.
The disposer port
Many waste disposers have a moulded port intended for a dishwasher connection, and where it is unused it makes a neat drain connection for an RO system: no drilling, and a purpose made entry point. The knockout inside has to be removed first, which is a common oversight that produces a system with no drain flow at all. Do not connect into the disposer body where the port is already serving a dishwasher, and be aware that food waste passing through can find its way back towards a small tube.
Purpose made tailpiece fittings
A replacement tailpiece with a moulded connection built in is the tidiest and most reliable option, since there is no drilling and no gasket relying on a clamp. It means replacing a section of waste pipe under the sink, which is a straightforward job with a new washer, and it removes the alignment problem entirely. Where a sink is being replumbed anyway, this is the connection worth choosing. It also makes the installation easier to inspect later.
Above the trap, and the tube that follows
Whichever adapter is used, connect above the trap so the water seal continues to keep sewer gas out of the reject line. Then the tube: on an air gap installation it flows by gravity, so it must run continuously downhill, short, in the specified diameter and free of dips and kinks. Nearly every complaint about water at an air gap faucet traces to that tube rather than to the tap. Our inline filter guide covers the small components around a system.
Questions people ask about drain line adapter
Where should the drain adapter connect?
Above the trap, so the water seal keeps sewer gas out of the reject line, and above any standing water in the pipe.
What is the most common saddle clamp mistake?
Failing to align the drilled hole with the saddle's port, which restricts or blocks the connection entirely.
Can I use the disposer port?
Yes where it is unused, but the internal knockout must be removed first or the system will have no drain flow at all.
What is the tidiest option?
A replacement tailpiece with a moulded connection, which avoids drilling and removes the alignment problem.