UPC Section 611 covers drinking water treatment units, and Section 611.2 sets the drain rule for reverse osmosis systems. An RO unit's reject water must enter the drainage system through an air gap or an air-gap device that meets CSA B483.1 or NSF 58. That air gap is why RO systems use a special air-gap faucet.
The Uniform Plumbing Code is published and copyrighted by IAPMO. This page explains the section in our own words with a short excerpt only. Read the full official text at the source.
A reverse osmosis system makes clean drinking water, but it also makes waste. For every glass of filtered water, an RO unit pushes supply water across a reverse osmosis membrane and sends the rest down the drain to carry off the rejected minerals and contaminants. UPC Section 611 covers how these treatment units connect to your plumbing. Section 611.2 sets the one rule that matters most for that drain line, and it exists to keep dirty drain water from ever touching your clean supply.
What this section says
Section 611 governs drinking water treatment units, the family of gear that includes reverse osmosis systems. Section 611.2 speaks directly to RO. It controls how the unit's drain water, called the reject or concentrate, gets into the drainage system. The code states the rule plainly:
The discharge from a reverse osmosis drinking water treatment unit shall enter the drainage system through an air gap or an air gap device that meets the requirements of CSA B483.1 or NSF 58.
An air gap is a physical open space between the drain line and the drainpipe. Water falls across that gap. Because nothing connects the two sides, dirty water from the drain can never siphon back up into the RO unit or the potable supply. CSA B483.1 and NSF 58 are the product standards an air-gap device must meet to count. NSF 58 is the standard written specifically for reverse osmosis systems.
When this applies
This section applies to every RO install, at home or in a business. It is the reason a proper under-sink RO system comes with a special air-gap faucet. That faucet is not just a spout. It builds the required air gap right into the fixture on the countertop. You can often hear it: a faint trickle in the faucet body when the system flushes is the reject water crossing the air gap on its way to the drain.
Skipping the air gap is a real code violation, not a technicality. A no-air-gap faucet ties the RO drain straight into the sink waste line. If that line ever clogs and backs up, drain water can be pulled into the treatment unit. That is a cross-connection, and it is exactly what Section 611.2 is written to stop.
What this means for you
If you are buying or installing an RO system in Phoenix, make sure the drain connects through an air gap. Ask whether the faucet is an air-gap model and whether the drain saddle and tubing are set up for it. A drain line that runs straight into the waste arm with no gap is a red flag on an install.
To learn the concept behind the fixture, see the air gap glossary entry and what a dishwasher air gap does, which works on the same idea. The air gap is also a core cross-connection control method, and the RO reject is a form of indirect waste. For the full install, see what to expect from a reverse osmosis installation.
Full text and source
UPC Section 611 and 611.2 are part of the Uniform Plumbing Code. IAPMO publishes and holds the copyright on them. The excerpt above reflects the section as adopted. Phoenix enforces the 2024 UPC with local amendments as part of its Building Construction Code. Read the current text on UpCodes and confirm local amendments through the City of Phoenix Planning & Development Department at phoenix.gov/pdd.
Keep Reading
- UPC 1003: Which Fixture Traps Are Allowed (and Banned)
- UPC 1007: Trap Seal Protection for Floor Drains
- UPC Section 1101: Storm Drainage and Roof Runoff
- UPC 1208: Approved Fuel Gas Piping Materials
- What is involved in installing a reverse osmosis system?
- What is a dishwasher air gap and do I need one?
- Is a reverse osmosis system worth it?