Ann Arbor Wastewater Treatment staff evaluating operations after 10K gallon raw sewage spill
Courtesy of the city of Ann Arbor
Though plant managers attribute the staff’s quick response to preventing a larger spill, an estimated 10,000 gallons of sewage spilled over a 10-minute period beginning about 5:20 p.m. June 27. The June 27 thunderstorm quickly flooded Ann Arbor streets and knocked out power.
Ann Arbor's wastewater plant is at 49 S. Dixboro Road in Ann Arbor Township and treats an average of 18 million gallons of wastewater per day from the city of Ann Arbor and Pittsfield, Scio and Ann Arbor townships. It has an East Plant, built between1977 and 1981, and an older West Plant, which has been taken offline.
At the East Plant, staff noticed the flow rate of wastewater coming into the plant triple in half an hour. They responded by turning on additional pumps to handle the increase in flow, said Keith Sanders, assistant manager at the plant.
By the time the pumps were turned on, workers noticed the sewage spill, Sanders said.
“They saw (the flow rate) rising, they started doing what they needed to do and then they never saw it come back up,” Sanders said. “No one saw it overflow.”
A massive construction project financed by a $120 million bond is underway to replace the West Plant and upgrade the East Plant. Sanders said the construction was not a contributing factor to the spill.
The spill occurred at the point where wastewater flows into the headworks building of the treatment plant.
All of the wastewater flows through a grate and into a concrete trough before entering the plant. The trough is open at the top and is covered by a metal grate.
The raw sewage overflowed the trough and onto the ground, traveling about 100 feet to draining in through storm sewer grate by a roadway, Sanders said.
The storm sewer empties directly into the Huron River upstream of the plant’s outflow pipe, which is at the southeast corner of the treatment plant complex, Sanders said.
Sanders said there was standing water around the plant because of the rainstorm at the time of the spill.
“It’s rare that you see your plant flow triple in half of an hour,” Sanders said. “If you get a storm out of the blue you respond as quickly as you can.”
The wastewater treatment plant is designed to treat 29.8 million gallons per day. On average, Sanders said the plant treats 18 million gallons per day.
The plant is designed to be able to handle short peaks in flow rate that, if extrapolated to a daily rate, would be the equivalent of 66 million gallons in a day.
At the time of the spill, 10,000 gallons were estimated to have spilled in 10 minutes and 350,000 gallons of wastewater were processed.
Officials at the wastewater treatment plant stated the sewage was significantly and immediately diluted once it hit the river swollen by rain. The flow rate in the Huron River in the 10 minutes the spill was estimated to have happened was about 11 million gallons.
Health officials caution individuals to stay out of the Huron River during a major rain event and for the first 48 hours afterward to avoid coming into contact with heightened levels of bacteria and other risk-posing debris in runoff that flows to the river.
Sanders said the staff is evaluating what happened during the incident to see if it could have been prevented. No conclusions have been reached at this time.
“We responded pretty quickly,” Sanders said.
The Michigan Department of Environmental Quality and Washtenaw County Public Health were notified of the spill.