The Metropolitan Water Reclamation District of Greater Chicago has reached a Clean Water Act settlement with the federal government to upgrade its combined sewer system.

Under the consent decree, announced on Dec. 14, the MWRD will complete a tunnel and reservoir plan to increase its capacity to handle stormwater and address combined sewer overflows.

The project, which involves building two reservoirs with a combined storage capacity of 18 billion gallons, will be completed in stages in 2015, 2017 and 2029.

According to the MWRD, the district already has spent about $3.1 billion on upgrades to its combined sewer system through its Tunnel and Reservoir Plan, which was originally conceived in the early 1970s.

MWRD says it has approximately $185 million of work currently under construction, with another $215 million in planned construction costs.

The settlement also calls for the MWRD to use green infrastructure to reduce stormwater runoff. The MWRD estimates the green infrastructure projects, the first of which must be completed by the end of next year, will produce at least 10 million gallons of new runoff retention during the life of the settlement.

The consent decree between the U.S. Dept. of Justice, the Environmental Protection Agency and the MWRD was filed with the U.S. District Court for the Northern District of Illinois.