A “grit buildup” in a South-end sewage line caused a few thousand gallons of sewage to spill into Lake Washington two weekends ago. It took the city three days to unclog the pipeline successfully.

Although such incidents are rare, according to City Maintenance Director Glenn Boettcher, they are a tremendous hassle for city workers when they do occur. And the best prevention is public awareness.

A big contributor of sewage-pipe backup is when residents dump cooking fats, oils and greases down the drain. Indeed, this was part of the cause in the Nov. 14 South-end overflow, according to Utilities Operation Manager Terry Smith.

“As soon as hot water with fats, oils and grease hits cold water, it coagulates. The grease adheres to other grease and gets bigger until it obstructs the pipe — just like a clogged artery,” he explained.

Instead of disposing of fats, oils and grease down the drain, Smith asked residents to scrape or pour them into a can and drop it off at a local biodiesel facility, or throw it out with the trash.

In order to avoid sewer buildup, the city flushes all of Mercer Island’s underground pipes systematically throughout the year — an average of three or four times over 12 months, said Smith. This keeps the system running smoothly. That is, until a grit buildup like last week’s creates sewage backup. In such a case, there are two scenarios — neither of which is pretty.

“When a pipe is blocked, the sewage will surcharge back up into other pipes and sometimes into homes,” Smith said; unless the city goes in and flushes the waste forward into Lake Washington, like it did on Nov. 14.

Around the perimeter of Mercer Island is the sewer lake line. On that sewer lake line are 18 pump stations that transfer the sewage from station to station. The sewage is then pumped off the Island into a King County facility.

There are also overflow pipes at each of the Island’s pump stations. When backup occurs, the sewage discharge floods the overflow pipes and spills into the lake.

“This overflow is what keeps the sewage from going back up,” Smith said, adding that he encourages all waterfront homeowners to purchase backwater valves to prevent this undesirable scenario. “We strongly recommend existing residences to install [backwater valves] if they don’t have them already.”

Additionally, Smith recommends that Islanders notify the city before calling private plumbing companies to fix household problems.

“One thing that occurs commonly is if a resident has a sewer problem, they have a company come clean out the roots and push them into our sewer main. Often they don’t collect it, so somebody else’s roots clog our system and creates damage elsewhere,” Smith said.

The result of which — like earlier this month — can become a three-day struggle to flush and clear a backed-up city sewer pipe. An unenviable job for any city worker.

For more information on city sewage, call Terry Smith at 275-7812.