Computer error was factor in tank failure

A computer malfunction at Spokane’s wastewater treatment plant played a critical role in last May’s sewer tank eruption that claimed the life of one worker and left two others with serious injuries.

Results of an independent investigation showed that the domed lid of “digester” tank No. 3 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, the lead investigator said on Friday.

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

“What has really happened is really quite straightforward,” said Piotr D. Moncarz, of Exponent, the California firm hired to conduct the investigation.

“Simply there was too much sludge pumped into the dome,” he said.

Moncarz said the dome lifted suddenly, sending its pressurized contents spewing out. The dome then fell back into the tank and settled to the bottom. There was not an explosion, he said.

Prior to the rupture, maintenance mechanic Mike Cmos Jr. had gone onto the roof of the tank in an attempt to contain sludge leaking from the 2 million-gallon vessel. Witnesses said he was trapped there when the rupture occurred. His body was found near the bottom two days later after most of the remaining contents had been removed.

Two other workers suffered serious injuries in the May 10 accident, and a quantity of sludge spilled into the Spokane River.

Moncarz said overflow valves originally placed on the tank had been closed in a previous renovation. City officials said the work was done in the early 1980s to reduce a risk of a methane gas explosion. As a result, excess sludge had no way of escaping and began draining into an overhead gas collection pipe when the tank erupted.

For 90 minutes before the accident, workers sought to reduce the level of sludge, but were confronted with a confusing array of pipes and valves that were poorly marked. As a result, emergency efforts to lower the sludge level were ineffective, Moncarz said.

When the sludge levels reached the top of the roof, one eyewitness reported hearing it drain from a pipe. Investigators later discovered the sludge had filled into the gas collection pipe about 2 feet above the top center of the dome, which was 20 feet higher than the recommended maximum safe level.

“They (workers) did not realize how serious the condition was because they had the wrong information …,” Moncarz said.

Since the accident, the city has had new emergency overflow systems installed in the plant’s two undamaged tanks. Also, exterior see-through polycarbonate tubes, called “sight glasses,” have been installed so that operators can now visually verify the levels of sludge. Those tubes were recently installed at a cost of less than $30,000, said Roger Flint, director of public works and utilities.

The sight glasses “probably would have prevented an accident,” Moncarz said.

Additional upgrades to the remaining tanks No. 1 and No. 2 are being planned. Repairs are also planned for hairline cracks on the domed roofs, although the cracks are not a danger, Moncarz said. Cracks in the domed roof of tank No. 3 did not contribute to the accident, he said. He said the tanks are structurally sound.

The damaged tank No. 3 will be used for limited storage during winter months. Two new digester tanks are being planned at a cost of $25 million.

To date, the city has spent $820,000 as a result of the accident, including $443,000 on the Exponent contract.

The city has filed a $12 million partial damage claim with its insurance carrier to cover its losses, plus costs of retrofitting existing tanks.

A mayor’s citizen oversight committee is expected to review the accident and investigations, including a state Department of Labor and Industry probe that found 16 violations, 12 of which carried fines totaling $66,600.

The digesters take concentrated sewage and use anaerobic bacteria to remove pathogens and volatile acids. After excess water is removed, the sludge becomes a fertilizer product.

Trouble at the plant had been reported periodically following installation of new processing equipment. In March, a sludge-foam mixture had overflowed in tank No. 3, but workers then made no effort to verify that sensors were working properly, the investigative report said. Also, operators failed to reconcile flow meters with sensor levels when there were discrepancies prior to the accident, according to the report.

City officials said corrective actions are being taken on those and other findings of both the Exponent and state investigations.