The project has attracted criticism from environmental and tribal groups.
"We cannot continue to legitimize and provide cover for the MPCA's war on black and brown people," their resignation letter states.
High amounts of algae have been detected, which could include toxic blue-green algae.
They can now resume operations at their Minneapolis plant.
Minnesota environmental officials say cleanup efforts to cut phosphorous levels in the Minnesota River are making a difference, the Star Tribune reports. After testing this summer, the Pollution Control Agency found oxygen levels in one of the state's dirtiest waterways are better than expected. The agency credits a 2004 phosphorous reduction plan for sewage treatment plants
Cliffs Natural Resources has agreed to pay a $242,973 civil penalty to the Minnesota Pollution Control Agency and make improvements to the coal ash landfill at Northshore Mining plant in Silver Bay, the Duluth News Tribune reports. The company will also contribute $75,000 toward an expansion of the household hazardous waste reuse center in Duluth.