Operations at the Mendenhall sewage treatment plant are returning to normal today after an estimated 50 gallons of solvent were discharged illegally into the system Monday evening.
"Apparently, on the evening of the 10th, the plant got a hit of some kind of petroleum-based solvent or material," said city Public Works Director Ernie Mueller. He said it could have been diesel fuel or paint thinner.
Because the plant has eight tanks, which receive raw material in sequence, the solvent entered into only the first three tanks in the system. In the tank that received the brunt of the solvent, the microorganisms that help digest sewage were killed, Mueller said.
When staff arrived at the plant at 6 a.m. on Tuesday, they could smell a strong petroleum/hydrocarbon odor and immediately sensed an emergency. They shut down the three affected tanks and reported the incident to the Department of Environmental Conservation. DEC staff treated it like an oil spill, Mueller said, reporting it to the federal Environmental Protection Agency and the state Department of Fish and Game. DEC also scanned the nearby Mendenhall River, but found no sheen that would indicate pollution.
The two treatment tanks that were not as severely affected were left in service, but were topped off with activated sludge containing sewage-digesting bacteria from the five unaffected tanks to boost efficiency, Mueller said. The sludge from the first tank was removed and burned.
"It may be possible that a lot of this (contaminant), because of all the oxygen pumped in during the process, was stripped out and evaporated before the guys walked into the building," Mueller said.
Monday's incident was the worst the sewage treatment system has ever had, Mueller said. Staff narrowed down the source of the solvent to the area between Bartlett Regional Hospital and Brotherhood Bridge. Workers were dispatched to sniff at man holes in that area but found nothing.
Under city ordinances, disposal of solvents and similar materials directly into the sewer system is a misdemeanor.
The plant produced less-than-ideal effluent during the time sewage treatment was compromised, but whether that resulted in a violation of the plant's permit from EPA is "hard to tell," Mueller said.
A similar incident happened in February when a "bilious green" substance showed up in the Juneau-Douglas treatment plant at the Thane Road rock dump, Mueller said. That plant has only two treatment tanks and they cannot be isolated. It took a month before the plant was back to normal, he said, and the culprit was never found.
"If people want to get rid of hazardous waste, we will take care of it for them," Mueller said. "They should bring it out during collection events. Don't throw it down the sewer system. It's potentially dangerous because it could cause an explosion in someone's house."
The next hazardous waste collection event is scheduled for Jan. 26. Call (800) 433-5060 to preregister.
Ann Chandonnet can be reached at firstname.lastname@example.org.