Many causes likely in sewage tank explosion

Spokane wastewater employees reported operating problems at the city’s sewage treatment plant in the weeks preceding a catastrophic tank failure that killed one worker and left two others with serious injuries.

Six weeks after the accident, officials are in the middle of a series of investigations into the cause. But recent interviews with city employees familiar with the May 10 events offer an emerging picture of what happened that day.

Two workers were on top of a mammoth sewage “digester” trying to recapture sludge leaking from vents midway up the tank roof when it violently separated from the tank’s walls. The roof collapsed into the tank.

A wave of sludge estimated as high as 60 feet spewed from the upper walls of the tank as the fractured roof fell down into more than 2 million gallons of heated sludge. One eyewitness described the failure as like a volcano erupting.

Maintenance mechanic Mike Cmos Jr. was caught on the roof and swept into the sludge. His body was found two days later after much of the remaining sludge was removed.

Fellow mechanic Dan Evans was nearly killed when the eruption threw him from the edge of the tank roof onto the ground 30 feet below. His pelvis was broken. He is now recovering at home from multiple fractures and internal injuries.

Two other workers nearly drowned when they were submerged in the black slurry-like substance. Operator Larry Michaels suffered a serious knee injury and an infection to a gash in his shin. He is recovering at home with plans to return to work.

Plant Superintendent Tim Pelton, who grabbed a railing to keep from being swept away by the sludge, was transferred to the sewer maintenance office. He was assigned to work on the investigation and to continue coordinating ongoing construction upgrades at the plant, he said. He had minor injuries.

The state Department of Labor and Industries is expected to issue a detailed report on the accident, and has authority to issue fines for violations of worker safety, said Nadine Grady, industrial hygiene compliance supervisor for the agency.

Complete answers to what caused the accident may not be known until year’s end, said Deputy Mayor Jack Lynch. He declined to discuss specifics of the investigation, which will include a review by a nationally renowned engineering firm.

“We are spending a ton of money to get those answers,” Lynch said.

City Attorney Mike Connelly said more information about the plant may be released in coming weeks.

Officials said they are likely to discover that a series of circumstances and events played into the failure. As a precaution, they have reduced volumes and methane gas pressures in the plant’s two undamaged digesters. Workers are no longer allowed to go up on tank roofs without safety harnesses.

It is not known yet whether the damaged digester can be repaired. The City Council on Monday approved a $225,000 emergency expenditure, hiring Robert Goebel General Contractors of Spokane to remove debris and clean the tank as part of the investigation, and for an assessment of the crippled tank’s future.

Interviews with six city employees, including eyewitnesses and plant workers, along with a review of a limited number of public documents, provide new details of the catastrophic failure in digester tank No. 3.

Though top city officials would not discuss the employee accounts in detail, they acknowledged that have received reports from employees and are encouraging others to come forward if they can help the investigation.

The sources familiar with the plant said problems processing sludge date back more than a year, to the installation of a new component known as a gravity belt thickener, or GBT for short. The system was designed and installed to improve plant efficiency as part of a wider, ongoing upgrade intended to improve the quality of water effluent from the plant.

Treatment plant operators said they were having problems maintaining an optimum thickness of sewage sludge after the GBT was installed. The new component caused sludge to be thicker and heavier, making it more difficult to pump and to heat for processing, plant workers and others said.

Sludge is warmed to about 100 degrees and “digested” by anaerobic bacteria to render it safe for use as fertilizer. Processing gives off methane gas, which is captured and used as an energy source at the plant.

Roger Flint, director of public works and utilities, said issues involving plant operations and its components are part of the accident investigation. He declined to discuss specifics until a complete picture can be assembled. He said it was not clear what role processing problems might have played in the accident.

But four city employees said the problems preceded an unexpected and rapid rise in the level of digester No. 3 in the hours before the accident.

At 2:35 p.m., just 25 minutes before the rupture, one worker spotted sludge draining from two methane gas vents located about one-third up the domelike roof covering the tank. Methane is produced by the anaerobic decomposition of sludge.

Workers responding to the problem found sludge draining down the roof and spilling from rainwater drains onto the ground, a violation of pollution laws.

Some workers built soil berms to stop the sludge from going into the nearby Spokane River.

Cmos pulled out a hose that was to be used to clean the tank once the spill was stopped, city employees said.

Then, he was enlisted to help attach a semi-rigid three-inch hose to the drain spout so the spilling sludge would be rerouted back into the plant. He was up on the roof. Evans was along the edge of the roof. Pelton and Michaels were on a patio about 10 feet below the tank roof.

At least one worker reportedly refused to go up on the roof, Lynch acknowledged.

Other workers, including Pelton, said there was no reason to believe the concrete tank would fail.

The tank apparently ruptured at a tension ring connecting the domed roof to the tank’s reinforced concrete walls. Chunks of concrete were flying off the tank as sludge started to spill out.

Cmos disappeared when the roof collapsed into the sludge. Other employees and emergency crews searched the river for him, but his body was found inside the damaged tank two days later. Evans fell to the ground. Pelton and Michaels were submerged in the muck and could not breathe until the flow thinned out.

In the two weeks leading up to the accident, workers reported problems with a buildup of foam at the surface of the sludge inside the digester. The foam is a sticky combination of sludge and methane gas.

A request to the city by The Spokesman-Review for public sewer plant records has not been fulfilled, but City Attorney Connelly on Wednesday said some documents could soon be made available.

A May 28 letter to Connelly backs up employee reports the plant was having operational problems. The letter was written by the vice president of an expert engineering firm hired to investigate the accident.

Piotr D. Moncarz , of Exponent Failure Analysis Associates of Menlo Park, Calif., wrote that his firm would “evaluate waste flow into digester No. 3; evaluate foaming and other operational issues in the weeks and months prior to the incident; (and) evaluate characteristic waste flow into digesters at a point in time when there were no operational problems in the digesters.”

“The system wasn’t working too well,” said one experienced treatment plant operator, who was familiar with the problems. “We were having problems heating and moving product.”

On the morning of the accident, city workers said pressure readings within the tank were “unusual,” but the plant’s computerized monitoring system had been giving unusual readings in the previous weeks, so workers were not overly alarmed.

Digester No. 2 essentially had stopped its bacterial digestion process. As a result, plant workers were moving sludge from that tank into tank No. 3 in an effort to get the sludge in tank No. 2 processed.

But the level in tank No. 3 went up much faster than the corresponding drop in the level of tank No. 2, causing some concern among plant operators, because they couldn’t reconcile the differences in levels.

The tank itself is a subject of the investigation.

Experts are planning to take concrete core samples from the damaged digester No. 3, as well as two other digesters at the plant to study the structural integrity of the tanks.

Goebel has been hired to remove the tank roof and make pieces of the tank available for investigators to evaluate.

“There are a lot of unknowns,” Flint told the City Council on Monday.

He said the city hopes to use tank No. 3 for storage of sludge this winter when the plant is unable to dispose of processed sludge by spreading it on farm fields.

The cost of rebuilding the tank was estimated at $10 million. Part of the ongoing construction upgrades at the plant includes the addition of a fourth digester, he said, but that project is in its early design stages.

“We can’t evaluate No. 3 until we actually get in and look at the structure,” Flint said.