Sewer lake-line blockage forced emergency response, city says sewage reached the lake

Mercer Island officials told the City Council on March 3 that crews cleared a blockage in a main lake-conveyance sewer line after alarms indicated a pump-station failure and a subsequent partial discharge to the lake.

"This started last Monday when our system sent off alarms saying that pump station 10 on the north end of the island had a problem," City Manager Jesse Bond said. "Unfortunately, the backup resulted in some discharge of sewage into the lake. Your staff team notified King County Public Health, the Department of Ecology, and residents that were impacted along the shoreline."

Bond described a 36-hour response that used a sewer-jet truck and required ingenuity to access the line. Staff credited maintenance, engineering and emergency-management teams for restoring the line to operation by Tuesday evening and said they would insert a camera between pump stations 10 and 11 to document conditions and connections.

"We're meeting this week and next to debrief the event, review our actions and see if there's any opportunities for improvement," Bond said. He told council members staff will work on a contingency permitting and design package to speed future emergency access, noting that lack of access can delay repairs from days to weeks or longer.

The city said it notified public-health authorities and affected shoreline residents and thanked residents for temporarily limiting wastewater discharges during the emergency.

What's next: staff will inspect the affected pipe segment with a camera, continue after-action review and incorporate lessons into maintenance and contingency design planning. The city identified lake-line vulnerability and signalled a need for continued capital planning for aging infrastructure.

Sources: City Manager Jesse Bond; city public-works staff.