State officials have identified a substance that shut down part of the Mendenhall Wastewater Treatment Plant this month as aromatic mineral spirits, but they aren't sure where it came from.
The city estimated 50 gallons of the material were illegally dumped into Juneau's sewer system on the evening of Dec. 10.
The solvent may have been used as an automobile degreaser, paint thinner or drying agent, according to Lester Leatherberry, a responder with the state Department of Environmental Conservation's Southeast Alaska Spill Response Team.
"We sent it to the DEC lab in the Valley, where they're really familiar with trying to fingerprint material and trying to figure out what it is," he said.
The next step is to start looking for the source of the spill, Leatherberry said. The material most likely came from a business, not a private residence, he said.
At the Mendenhall Wastewater Treatment Plant, the mineral spirits killed sewage-eating microorganisms in one of eight treatment basins entirely. Plant workers pulled the basin out of service and processed the sludge inside before bringing it back online, plant supervisor Roger Hulse said.
"We reacted rapidly. We called ADEC to analyze and collect samples and we captured as much as possible," he said. "We captured most of it in the facility. Very little went into the river."
The plant was never out of compliance because of the December incident, Hulse said.
"The plant was doing well and was well within its parameters the day after," he said.
The person or business responsible for the spill may face state and city penalties. A state penalty would depend on the situation and the circumstances, Leatherberry said. Disposal of solvents and similar materials into the sewer system is a misdemeanor under city ordinance.
Each basin at the Mendenhall plant can hold 350,000 gallons of sewage. The fact that the solvent killed an entire tank is unusual and an indication of how powerful it was, Hulse said.
"None of the equipment was damaged, but it is bad for the environment, which is why we get concerned about it and so does the ADEC," he said. "It can impact the environment, so we take it pretty serious."
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