Record-breaking rainfall like last weekend’s overwhelm Chicago’s sewer system. That forces a mix of raw sewage and storm water to flow into Lake Michigan. But officials say improvements in the works will make these discharges much less frequent.

Metropolitan Water Reclamation District Director David St. Pierre thinks the addition of 3 .5 billion gallons in overflow capacity coming by year’s end will greatly reduce backups into the lake.

“I expect us to have zero overflows with the exception of events like Saturday’s,” St. Pierre said.

The National Weather Service says that day was was the rainiest October day in Chicago history.