Seal bursts at CdA plant, leaking sewer sludge

About 10,000 gallons of sewer sludge leaked onto the ground at Coeur d’Alene’s wastewater treatment plant Wednesday morning after a seal ruptured.

None of the sewage reached the Spokane River, and the spill was cleaned up by 9 a.m., according to wastewater Superintendent Sid Fredrickson.

The seal blew out around 1 a.m. after rags plugged the plant’s sludge digester.

The digester is a covered concrete tank where bacteria breaks down sewage during a 21-day process. Methane gas is released during the breakdown cycle.

The plugged pipe caused the digester to overflow, and pressure caused the rubber-like safety seal between the steel cover and wall of the concrete tank to blow out.

Fredrickson said the seal, which is designed to give way when pressure builds to a certain level, is a safety feature.

“It prevents an explosion and prevents the tank from collapsing,” Fredrickson said.

A May 2004 accident at the Spokane wastewater treatment plant killed one worker and left two others with serious injuries.

Fredrickson said that the Spokane plant didn’t have the safety seal.

Results of an independent investigation showed that the domed lid of the Spokane digester tank burst away from its anchors much like a cork popping from a bottle because the tank was so full of sewage there was nowhere else for it to go.

Plant operators was unaware of the peril because their computerized monitors had showed sludge levels that were considerably lower than the actual level in the tank, a malfunction apparently caused by a power-supply problem.

Fredrickson said the Coeur d’Alene crew used shovels, buckets and lime to clean up the spill. The city will continue to use the 244,000 gallon digester tank until a backup tank is put in service.

Crews will then repair the broken seal on the damaged tank.