Fremont Siphon Replacement Project

Just south of where NW 36th Street bends and turns into Leary Way NW there is a large construction site. It is difficult to see what is going on there because most of the activity is occurring underground but we do know that two new 500-foot long microtunnels have recently been installed to replace the existing siphon that runs under the Ship Canal between the Fremont and Queen Anne neighborhoods.

The existing siphon is almost 100 years old and at the end of its service life. It’s amazing to realize that the two large cast iron pipes that make up the siphon can carry up to 220 million gallons of sewage and stormwater per day from a 100-square-mile service area covering a large portion of north Seattle and even parts of north King and Snohomish counties. From the Queen Anne side of the siphon, a single large pipe delivers the charge to the West Point Treatment Plant in Discovery Park.

The remaining work now consists of connecting the existing sewer line to the new siphons via a 9-foot diameter pipe and the construction of an odor-control facility.