The rain that falls on your roof and runs down the street in front of your house goes to one of the following places.
1. Fully combined sewer and stormwater pipes where household wastewater meets up with stormwater and most of the time they flow together through the same pipes to the West Point Sewage Treatment Plant in Magnolia (via the Ballard Siphon.)
2. Partially separated sewer and stormwater pipes where stormwater runoff from streets is handled through stormwater pipes. Stormwater runoff from roofs usually goes into the combined sewer system. The separated stormwater generally flows to the receiving water body, currently without treatment.
3. Fully separated sewer and stormwater pipes. The separated system generally is north of 85th street in Seattle. In this case, stormwater runoff flows into stormwater pipes and discharges (untreated) into the receiving water. Sewage flows through separate pipes into King County’s pipes and is routed to Waste Point Sewage Treatment Plant.
4. On the land and other permeable surfaces. Some of the rain lands in yards, parks, forests, beaches and other natural surfaces. It infiltrates the ground until it gets saturated, at which point it gets soggy and may start to follow the closest pathway to the water (e.g. tumbling down the hillside at Golden Gardens, joining the creek, racing towards Puget Sound.)
Where it goes depends on where you live. If you live east of 32nd NW, between NW 65th and NW 85th, you are in a fully combined area. If you live south of NW 65th, you are in a partially separated area. If you live north of NW 85th you are in a fully separated area.
Seattle’s combined, partially separated and fully separated sewer systems feed into King County’s Wastewater Treatment System. The Ballard basin flows are pumped and otherwise carried through pipes under NW 34th Street and then along Shilshole Ave NW until they meet up with King County pipes and are pumped through the Ballard Siphon, under Salmon Bay, to King County pipes under W. Commodore Way (in Magnolia) and eventually flow into the West Point Sewage Treatment Plan in Magnolia. Map of Sewer Pipes
The western part of Ballard, from approximately 33rd Avenue NW to Puget Sound, is part of CSO Basin 010-126. It contains three Combined Sewer Overflow Outfalls that are considered to be controlled. Outfalls 056 and 057 have not overflowed in the past three years. Outfall 059 overflowed once in the past three years, spilling 915 gallons one day in 2011 following 54.6 hours of rain. Map of basin_010-026
In some areas of Ballard the City of Seattle is responsible for managing the combined sewer pipes and outfalls; in other areas King County is responsible for managing the combined sewer pipes and outfalls. Both governments have been working for more than 20 years to reduce the number and volume of combined sewer overflow (CSO) spills of raw sewage into receiving waters when stormwater overwhelms the pipes.
Links for more information: