Problems preceded fatal sewage tank collapse
Newly released city documents show that workers at Spokane’s wastewater treatment plant were having trouble processing sewage sludge in the weeks leading up to a catastrophic tank failure that killed one worker and seriously injured two others.
The documents confirm earlier employee reports that pumps, heaters and a sludge-thickening system were not working properly in the weeks prior to the May 10 explosion of digester tank No. 3.
One of the operators who had been working on the problem said in an interview Wednesday he believes the problems were fixed by May 6. As a result, acting assistant plant superintendent Mike Coster said it is not clear what role, if any, the sludge processing problems played in causing the accident.
“I’ll defer to the experts,” Coster said
Top city officials have scheduled a press conference for this morning to update the status of accident investigations as well as the condition of the plant and its mechanical components.
“While the investigation is not complete, the city believes it’s important to keep Spokane citizens informed about the investigation,” Marlene Feist, city public affairs officer, said in a press release Wednesday.
Waves of sludge spilled from the upper walls of the tank when its roof separated and collapsed into more than 2 million gallons of sludge. Maintenance mechanic Mike Cmos Jr. was killed after he went up on the tank roof to contain the slurrylike substance draining from roof vents prior to the explosion.
Workers reported that there had been a rapid rise in the tank’s sludge level shortly before the accident. Cmos and the injured workers were trying to stop the leaking sludge from running into the Spokane River.
The city has hired a nationally recognized consulting firm to determine what caused the accident. Its report is due later this summer. The state Department of Labor and Industries is conducting its own investigation, and officials from that agency have said they will issue a full report, probably later this year.
Documents obtained by The Spokesman-Review through the state’s public records act show that supervisors meeting on April 15 knew about the problems in sludge digesting. Notes from the meeting indicate that Superintendent Tim Pelton wanted to stop odors coming from the digesters, and he asked for a reduction in polymers being fed into a gravity belt thickening system.
The gravity belt thickener was installed more than a year ago as part of a wide ranging upgrade of the 1977 wastewater facility, which serves utility customers in the city, unincorporated county and the city of Spokane Valley.
Higher pressure in lines coming off the gravity belt thickener was creating resistance in recirculation lines coming off sludge heaters. That prevented heaters from warming sludge to the optimum processing temperature of about 100 degrees, so workers were making adjustments and apparently had found a solution just days before the accident, Coster explained on Wednesday.
Documents back that up.
On April 19, three weeks before the accident, one worker in an e-mail asked, “Are we still having problems with the bypass pump system?”
Another worker responded that bypassing was halted the previous weekend “because of Dig #3 (digester No. 3) heating concerns.”
On April 26, one supervisor at the plant reported an inconsistency between the amount of sludge that operators believed had been pumped into a tank and the corresponding level reported inside the tank, according to notes from a meeting that day.
On May 6, another supervisor reported that methane gas from the digesters was being burned to keep down gas pressures inside the tank. The same supervisor also commented on sludge “foam problems in vents of digesters,” according to meeting minutes.
Each tank has two gas vents about one-third of the way up their domelike roofs. Methane gas is a byproduct of anaerobic bacterial digestion of sludge. Most of the gas is captured and used as a source of energy for heating sludge and other plant operations, but gas can also be released from the tanks and burned in the air, plant operators have explained.
Employees previously reported that sludge was leaking from gas vents in tank No. 3 when the accident occurred.
Also on May 6, Coster wrote in an e-mail to another plant employee that he believed the processing problems had been solved by installation of mechanical separations between lines coming from the gravity belt thickeners and sludge heaters. “Digester acids and temps appear to be coming around,” he reported optimistically in the memo.
On the day of the accident, May 10, supervisors reported during their meeting that they were reducing the volume in digester No. 2 to 20 feet, and then planned to mix two feet of sludge from digester No. 1 into No. 2.
Employees previously reported that digester tank No. 2 had stopped its bacterial processing of sludge, and workers were seeking to get it restarted by pumping sludge from No. 2 into digester No. 3 when the failure occurred. Sludge levels rose unexpectedly fast in tank No. 3, they said.
Top city officials last week said the investigation was slowed because it took longer than anticipated to remove remaining sludge and clean the damaged tank.
Coster said salvage workers have now removed about three-fourths of the reinforced concrete roof at the bottom of tank No. 3. Some of the smaller pieces of concrete from the roof were being removed by hand shoveling, he said.
Investigators from Exponent Failure Analysis Associates of Menlo Park, Calif., have taken concrete samples for testing. In addition to determining the cause of the accident, the firm is expected to make recommendations on whether to demolish or reconstruct tank No. 3.
The state Department of Labor and Industries is conducting a separate investigation. City police and fire investigators have assisted both investigations.
“The people down here want to find out what happened as much as the citizens or anyone else,” Coster said.