Ann Arbor officials are investigating the discharge of more than 1 million gallons of partially treated wastewater into the Huron River earlier this week from the city's wastewater plant after maintenance work shut down a component in the treatment process.

The spill began around 11 a.m. on Tuesday and continued for about three hours, resulting in a total discharge of around 1.375 million gallons of wastewater into the river. The wastewater had undergone all treatments except for ultraviolet disinfection, which inactivates microorganisms in the wastewater, said plant manager Keith Sanders of Ann Arbor Wastewater Treatment.

Maintenance work caused wastewater levels in the disinfection channels to drop, which turned off the UV bulbs that treat the wastewater before it is discharged into the river, Sanders said. The plant typically treats 14 to 18 million gallons of wastewater every day before discharging it into the Huron River.

"It (wastewater) flowed through the UV system, the UV system was off," he said. "We were working on a gate and we had to pump around the gate and during that process, at some point, the water got low and the UV shut itself off to protect the bulbs."

Once they realized the UV disinfection system had shut down, staff closed the outflow of wastewater to the river and restored water levels in the disinfection channels.

Plant staff are investigating the incident and adding alarms to notify them as soon as the UV disinfection system shuts down, Sanders said.

The city provided a public notice about the incident and notified the Washtenaw County Health Department and the Michigan Department of Environment, Great Lakes and Energy.

The health department has not issued any health advisories related to the spill but additional testing is required after this type of incident and more information may be released, department spokeswoman Susan Ringler Cerniglia told The News in an email.

EGLE staff have been in contact with the plant to monitor the situation and the plant must submit a report documenting the partially treated discharge and steps to prevent it from happening again.

"As the discharged water has been diluted into the flow of the river, and no further partially treated discharge has taken place since Tuesday afternoon, EGLE does not anticipate any impact on public health or the environment," EGLE spokesman Jeff Johnston told The News in an email.

This is the most recent in a string of potentially hazardous spills into the Huron River this year.

Officials were notified in August about the potential leak of hexavalent chromium, a chemical linked to cancer, into the Huron River after a chemical spill at Tribar Manufacturing in Wixom. An underground oil storage tank was responsible for an earlier chemical leak into a Huron River tributary in Flat Rock last February.