That's the equivalent of filling 8.5 million bathtubs at 50 gallons each.
The overflows occurred at retention basins where wastewater doesn't get fully treated.
West Michigan Environmental Action Council spokesman Dan Schoonmaker said it wasn't raw sewage. He said there were worse environmental threats during intense rain and floods.
The Grand River dropped below its flood stage in Grand Rapids on Thursday.
Meanwhile, the Saginaw Wastewater Treatment Plant has discharged about 1.5 billion gallons of treated wastewater into the Saginaw River. The area has also experienced heavy rains and flooding.