Sewer System
Everyone in Seattle relies on the sewer system to move wastewater away from homes and businesses for treatment. SPU builds, manages and maintains the sewer system’s smaller pipes, pump stations and storage tanks while King County Wastewater Treatment Division manages larger pipes and treatment plants.
To see a map of where SPU wastewater assets are located in your area use the DSO Water & Sewer Map.
What is Wastewater?
Wastewater is the water—and everything in that water—that goes down the drain of your sink, shower, toilet, washing machine, and/or dishwasher. It is transported through the wastewater system to a wastewater treatment facility.
- Private side sewers: Side sewers are privately owned; they connect buildings to the public sewer system
- Maintenance holes: An opening for people to access underground pipes for maintenance, inspections and cleaning
- Pipes: Large concrete, tile, plastic or metal circular tubes that carry wastewater between two points
- Pump station: A facility that uses pumps to push wastewater through a force main from a lower elevation to a higher elevation or different location
- Force main: A pressurized pipe that carries wastewater from a pump station to another location.
- Storage facilities: An underground or above ground tank, pipe, or tunnel that temporarily stores wastewater
- Outfalls: Points where untreated wastewater exits the combined sewer system into a body of water during heavy rains
- Large pipes and wastewater treatment plants: King County Wastewater Treatment Division builds and manages the some of the larger pipes and wastewater treatment plants in Seattle
- Combined sewer system: wastewater and stormwater travel in the same pipes to treatment plants.
- Separated sewer system: wastewater and stormwater travel in separate pipes. Wastewater is conveyed from homes and businesses to treatment plants. Stormwater is conveyed through a separate drainage system to drainage outlets.
- Partially separated system: wastewater from inside homes and businesses, and stormwater from roof gutters and downspouts are conveyed in the same pipes to treatment plants, while stormwater from streets is conveyed through a separate drainage system to drainage outlets.
The wastewater system strives to keep people safe and healthy by preventing backups and overflows. Unfortunately overflows are more common in the Combined Sewer System because heavy rains overwhelm the pipes. Learn more about the condition of Seattle’s wastewater system in the Wastewater System Analysis.