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New Technology Saves a Sewer Interceptor—and $1.5 Million!
The city of Westlake, OH, got an interesting wake-up call in 2006 when a sanitary sewer interceptor line in the nearby city of Lorain failed dramatically.
“It forced the evacuation of several dozen homes for months,” says Westlake Director of Engineering Robert P. Kelly, P.E.
“We realized we'd better take a closer look at our system, because we have a similar interceptor arrangement in Westlake, and our pipe is about as old as theirs.”
The interceptor in question was built in the 1960s and runs from west to east along Westlake’s northern limits, increasing in size from 36 to 60-inches. Dozens of 18 to 24-inch trunk lines, serving the entire city, drain northerly into the interceptor. The arrangement is efficient and has many advantages. But there are a few weaknesses as well; for one thing, a failed interceptor could affect all of Westlake. “There are about 70,000 people depending on our system,” says Kelly. “We simply could not allow a problem to develop.”
Interceptors are also particularly prone to microbiologically induced corrosion (MIC), an insidious plague of concrete sewer systems. In Westlake, the south-north trunk lines dump directly into the interceptor and are constructed with 24-inch drops to keep flows moving quickly, which unfortunately creates a great deal of turbulence. Turbulence, combined with organic waste, warm temperatures and oxygen, leads to high concentrations of hydrogen sulfide gas. The gas feeds colonies of Thiobacillus bacteria, which excrete sulfuric acid -- some strains of Thiobacillus can thrive in acid concentrations as high as 7 percent. The acid attacks concrete directly, turning it into crumbly calcium sulfate (gypsum). Once established, MIC works from the inside out to destroy concrete integrity in just a few years.
The interceptor had been cleaned and thoroughly inspected at the 40-year mark. But five years later, the Lorain interceptor failed and Westlake retained URS Corporation to perform video inspection, do detailed analysis and prepare an emergency evacuation plan in the event of failure.
What URS found was disturbing.
“When we inspected at year 40,” says Westlake Field Engineer James J. Smolik, P.E., “we televised the lines -- everything was in good shape.” However, five years later, URS found significant MIC damage.
“We could walk inside the interceptor and chip away concrete,” Kelly adds. “It was a real problem.” MIC had eaten away 1/2-inch to 1 1/2-inches of the concrete pipe’s inner surface.
“And, since the pipe thickness is only six-inches, we knew we had a problem,” says Smolik.