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UPDATE | Low-flow alarm did not activate during Eagle Harbor sewage spill

Last week's sewage spill into Eagle Harbor wasn't big enough to trigger a low-flow alarm at the city of Bainbridge Island's wastewater treatment plant, according to an incident report released Tuesday on the sewer pipe failure.

The spill, which was discovered by a nearby resident before 5 p.m. Thursday, Jan. 23, occurred when a 40-year-old force main broke open just 100 yards from the last line failure in late August, officials said.

The failure was similar to other problems that have cropped up on the pipe in recent months.

The pipe failed, according to the city's report, because an 18-inch long section along the bottom of the pipe had corroded, causing it to break under pressure and spill approximately 145,000 gallons of raw sewage into Eagle Harbor over two days.

According to city documents, the sewage flows did not drop below theĀ  point that would set off an alarm.

On average, the pipe has a daily flow of 435,000 gallons of sewage. On the first day of the spill, 105,000 gallons of sewage was released into Eagle Harbor, not enough to trigger the low-flow alarms.

Crews mobilized by 5:30 p.m. Thursday, however, to investigate the break, stage equipment and establish a temporary bypass at the Highway 305 pump station.

Two 200-gallon vacuum trucks began the bypass of sewage away from the broken line that evening.

By 6 a.m. Friday, four 4,000-gallon vacuum trucks took over the operation.

Before the restoration was completed, an additional 40,000 gallons of sewage was released into the harbor. That sewage came from draining the lines for repair, and from the city's Lower Hawley lift station, which continued to send sewage into the broken line because additional vacuum trucks were not available to collect the sewage.

The main was tested and placed back into normal service at 6 p.m. the same day.

The city's public works department will continue to monitor flows at the main. In the meantime, residents are being warned to avoid contact with the waters of Eagle Harbor until further notice.

Officials said the sewer line is scheduled for replacement later this year.

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