Combined sewer systems are the oldest type in Seattle (70 to 100 years old). Wastewater and stormwater travel together to treatment plants through the same sewer pipe. Read more.

In separated sewer systems, sewer pipes convey wastewater from homes and businesses to treatment plants. Different, separated pipes carry stormwater to drainage outlets. Read more.

Partially separated sewers convey wastewater and stormwater from roof gutters and downspouts to treatment plants; all other stormwater is conveyed in separate drainage pipes. Read more.