Do storm drains and sanitary sewers connect?

The storm drain system carries rain and runoff from streets, parking lots, rooftops and other outdoor areas. Chandler's stormwater page says that system directs excess water through storm drains, pipes, ditches and retention basins to nearby rivers, lakes or treatment areas, and that storm systems generally do not treat the water before it is discharged.

The sanitary sewer is a separate underground piping network. Chandler describes it as carrying wastewater from homes, businesses and industrial facilities to a wastewater treatment plant. That wastewater comes from sinks, toilets, showers, washing machines and other fixtures.

Mesa's sewer code uses the same split in its definitions. It defines a sanitary sewer as one that carries wastewater and to which storm water, surface water and groundwater are not intentionally admitted. It defines a storm sewer as one that carries storm and surface waters and drainage but does not include wastewater containing pollutants.

Put simply, the two systems are separate on purpose. Water that has been through a fixture goes to the plant. Water that fell from the sky goes to the basin.

The 2024 International Plumbing Code (IPC) keeps the two apart with three short rules.

• Section 1101.3: Storm water shall not be drained into sewers intended for sewage only.
• Section 1104.2: Floor drains shall not be connected to a storm drain.
• Section 1101.2: Storm water from paved areas, yards and courts shall drain to an approved place of disposal.

The IPC does allow for one exception in principle. Its definitions include a "combined" building drain and building sewer, which convey both sewage and storm water. Where the public sewer is a combined system, the code says the storm sewer shall be connected independently to the public sewer. That language is for cities that built combined public sewers. The Valley cities cited here describe separate storm and sanitary systems in their own materials, so on a Valley site you should expect two separate networks. Ask your city if you are unsure.

### What else may go into a storm drain

Rain is not the only thing the codes tolerate in a storm system. Phoenix's code lists a short set of allowed non-stormwater releases, provided they are not a significant source of pollutants and do not create a public nuisance. They include water line flushing, landscape irrigation, air conditioning condensation, foundation and footing drains, individual residential car washing, dechlorinated swimming pool discharges and street wash water. Releases under a state or federal discharge permit are also allowed. Commercial wash water and process water are not on the list.

That list is narrow on purpose, and other cities write their own. If you want to send anything other than rain to a storm drain, ask first. Roof water is the ordinary case. The IPC defines a conductor as a pipe inside the building that conveys storm water from the roof to a storm or combined building drain, so roof drainage needs the approved storm or combined-system disposal arrangement; it must not be sent to a sewage-only sewer contrary to the applicable rules.

### Cleanouts and backwater valves

Both systems need to be serviceable. IPC Section 1101.8 requires cleanouts in the storm drainage system that meet the code's rules for sanitary drainage cleanouts, and Section 1101.9 requires backwater valves on storm drainage systems as required for sanitary systems. These matter when you trace a line, since a cleanout is often the easiest place to send a camera or add dye.

A link is usually a mistake or a leftover from old work, not a plan. Ways one can exist include:

• A floor drain plumbed to the wrong pipe. A drain in a shop or dock is tied to a storm line because it was close by during construction.
• An area drain tied to a building drain. A roof or patio drain is connected to a sanitary line to save trenching.
• Old work that was never mapped. Additions, tenant changes and repairs over decades leave pipes that no plan shows.

Phoenix's code has a term for the dangerous version. It defines an illicit connection as any man-made conveyance or drainage system, pipeline, conduit, inlet or outlet through which the discharge of any pollutant to the public storm drain system occurs or may occur. It excludes legal nonconforming connections. A floor drain tied to storm fits that description.

### What goes wrong when they connect

There are two directions, and they cause different problems.

Sewage or wash water reaches storm. Pollutants flow to a retention basin, drywell or wash. A drywell sends water into the ground instead of to a treatment plant, so what enters it matters. Chandler notes that pollutants like grease and oil can stick to storm pipes, and that debris can clog the drain system and cause flooding. Under Phoenix's code, the owner of record of a property where a violation occurs is presumed to have control of it, so a hidden connection can land on the owner. See can you hose a parking lot into the storm drain for that rule in Phoenix.

Rain reaches the sanitary sewer. This adds flow the sewer was not sized to handle. Mesa's code bars storm water, surface water, groundwater and roof runoff from the sanitary sewer unless the city manager or water resources director approves it. During monsoon storms, extra flow is one reason drains inside a building can back up. See why drains back up during monsoon.

### How to find out if yours are linked

EPA's fact sheet for motor freight facilities lists the fixes for improper connections to the storm sewer. They read like a checklist for any site:

1. 1Plug floor drains connected to sanitary or storm sewer, or where the connection is unknown. Alternatively, install a sump that is pumped regularly.
2. 2Perform smoke or dye testing to determine whether interconnections exist between the sanitary and storm systems.
3. 3Update facility schematics so they show every plumbing connection accurately.
4. 4Install a safeguard against vehicle washwater entering the storm sewer unless permitted.

A licensed plumber can run a dye test or a camera inspection and mark the results on a plan. Do that before a sale or a lease change, and again if a site has had repeated backups or a spill.

For a dock, the walk-through in where does a loading dock drain go applies. For the names of the drains you find, use floor drain vs trench drain vs area drain.

The default assumption should be two separate systems, and every drain should be one or the other. If a drain cannot be traced, treat it as unknown and keep wash water and spills away from it. Map the site, test the doubtful spots, and keep the results in your drainage records. If a test finds a link, fix it with a licensed plumber and talk to your city before the next storm season.