A sewage-pipe break that spewed 57,107 gallons of raw effluent into the Redwood Shores lagoons in August was likely caused by damaging pressure fluctuations that occurred after a pump control system failed, according to Redwood City.
The Aug. 25 break caused a spill totaled 94,907 gallons of sewage, 37,800 gallons of which was captured by cleanup teams before it entered the lagoons, according to the city and public records held by the state Water Resources Control Board.
In the face of a continuing investigation by the state board that could lead to fines, the Redwood City City Council voted recently to hire an outside law firm specializing in water issues for $150,000 to represent it during the process.
“It’s to the city’s benefit and to protect the city that we bring on somebody with a special expertise in that area to make sure that the city complies with these very specialized laws,” said City Attorney Pamela Thompson.
The spill shut down the lagoons and required a cleanup that Department of Public Works Superintendent Marilyn Harang said cost about $250,000, an effort that was complicated when a small plane crashed into the contaminated waters Sept. 2. Health officials finally gave the all-clear Sept. 7.
The city submitted the results of its own investigation into the spill to the state water board on Dec. 16.
Harang said the city’s investigation, which was done by an independent consultant, determined that a likely cause was the failure of the pump station’s variable frequency drive, a system that moderates the speed of the pump to prevent “hard starts” that could cause pipe fatigue.
As a result, the pipe had “frequent pressure transients” that “exceeded the design operating pressure of the pipe,” according to public records.
“It just wore it to a crack,” she said, noting that the report found no corrosion on the pipe.
The city shut off the flow of sewage from the roughly 40-year-old pipe at Pump Station 10 on Davit Lane on Aug. 26 about 30 hours after it started, according to a summary of the city’s investigation into the spill released by the state water board.
Harang said the city is hiring an outside engineering firm to investigate why the system was malfunctioning and prevent future breaks.
The state board is continuing to investigate the spill to determine potential fines, a process that can take up to a year after the spill, said spokesman David Clegern. He said aging pipes are a problem throughout California.
“This is infrastructure that wears out just like highways but you can’t see it,” he said.