Officials say last week's heavy rains caused sewers in the Milwaukee area to spill an estimated 595 million gallons of untreated wastewater to local waterways.
The Milwaukee Metropolitan Sewerage District says to reduce the risk of sewage backups into basements, overflows to rivers and Lake Michigan began April 10. Those overflows ended Saturday.
On April 9, the district started pumping wastewater from the main tunnel directly to disinfection chambers at a sewage treatment plant, where it was mixed with chlorine before being discharged to the lake.
The Journal Sentinel reports the emergency measure was started in an unsuccessful attempt to provide more space in the tunnel and prevent overflows.
A state permit allows the district to divert up to 100 million gallons of wastewater a day during heavy storms.