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Weekend rains could dampen improvements at Mill Creek treatment plant

Bill Rinehart
Mill Creek Wastwater Treatment Plant pump station.

The Mill Creek Wastewater Treatment Plant is running at near-normal capacity after a "catastrophic" power failure earlier this month sent untreated and partially treated water into the Mill Creek and Ohio River for a few days.

Metropolitan Sewer District Director Diana Christy briefed Hamilton County commissioners on the status of the plant Thursday.

"We're at capacity in terms of the ability to pump into the treatment plant and provide primary treatment and disinfection," Christy says.

That means there have been no bypasses in at least a week, meaning no untreated water going into local waterways.

So is it safe to resume recreation?

"I believe there is still generally a caution in terms of being on the on the creek, because that type of analysis is being conducted by third parties," Christy said. "It's really not our area of expertise to make those recommendations, so I don't want to say it's safe at this time. But the condition has definitely improved."

Christy says thanks to temporary generators installed about 48 hours after the power failure, the plant is now pumping up to 130 million gallons per day, which is the standard capacity during dry weather. That wastewater is going through the first phase of treatment as well as disinfection.

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The part that's still a work in progress is the secondary treatment phase, in which microorganisms break down any remaining solids. Christy says they lost this "biology" during the power outage and it takes time to "re-seed" the tanks. The plant is back to a capacity of 120 million gallons per day, a bit short of the permitted 130 million.

The capacity for wet weather is typically 430 million gallons per day. Christy says they hope to expand from 120 million gallons of fully treated water to 200 million by Friday.

"We're really focused on doing everything we can so that during the weekend rain, we can take the maximum possible through secondary treatment and then we will not have any issues with that," Christy said. "But that is all to be determined at present."

Another function has been on hold as well, although it doesn't affect wastewater treatment: the plant incinerates solids removed from wastewater during treatment, but for the past few weeks those solids have been placed in temporary storage instead.

"With the power outage for a few days, the incineration temperature dropped below the temperature that it can continue to be operated or continue to be on hold, essentially, and then it locks itself out," Christy said. "Even though we had restored power, it takes quite some time to bring those back up to be able to incinerate."

Christy says they expect that part of operations to be back online by the end of the day Thursday.

What caused the power failure?

The Mill Creek plant has its own power substation with a transformer. MSD was in the process of switching from two old transformers to two new ones when the outage happened.

The plant only needs one transformer to operate at full capacity, but the second is a backup in case of failure.

One of the old units has already been decommissioned and was replaced by a brand new transformer in early February. The other old unit was turned off Feb. 28 so that it could also be upgraded to a new transformer.

On March 5, the brand new transformer malfunctioned and and failed. Because the other unit had been disconnected, there was no backup system to take over.

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"While the specific failure of the new transformer is unknown, we consider it at this time a warranty claim — it seems to be an internal transformer failure," Christy said.

MSD brought in portable generators to take over temporarily, and re-connected the old transformer.

What happens next?

The temporary generators will be the backup for the old transformer, which is currently operating the plant.

A second new transformer was supposed to be delivered soon, but that's on hold until the failure investigation is complete.

"We do know that there's an extremely long lead time on this type of custom high-voltage electrical equipment," Christy said. "It is something that has to be designed and fabricated and tested and could take easily more than a year if we have to switch to a different transformer than the one that is already built and ready to be delivered."

Local Government Reporter with a particular focus on Cincinnati; experienced journalist in public radio and television throughout the Midwest. Enthusiastic about: civic engagement, public libraries, and urban planning.