PORTLAND — City officials and environmentalists marked a milestone Monday in Portland’s efforts to reduce the amount of raw sewage discharged into Back Cove.

Crews began installing one of two concrete conduits, each capable of storing one million gallons of sewer and storm water during significant rainstorms to prevent polluted overflow from entering the cove. 

Portland Mayor Michael Brennan, representatives from the Maine Department of Environmental Protection, the city’s Department of Public Services, Friends of Casco Bay, and the Casco Bay Estuary Partnerships were on hand to mark the milestone.

One tank is being installed under Baxter Boulevard and another is being placed beneath Payson Park. The tanks will store the sewer and storm water until it can be pumped the East End Treatment Plant to be treated.

The project is part of a multi-year plan ordered by the federal government to reduce the number of combined sewer overflows in the city, which discharge raw sewage into nearby waterways during rain storms.

From 1993 to 2010, the city has spent $100 million to reduce annual sewer overflow volumes by 42 percent from 1993 levels of 720 million gallons to 420 million gallons.

The city recently adopted a plan to invest $170 million to further reduce sewer overflow volumes to 87 million gallons annually. This plan is set to begin in 2014.