LEWISTON – A plan to divert downtown storm water runoff into massive underground tanks moved forward Monday.

The project received approval from the City Planning Board, the last official step for the project. The city began advertising for bids on the project Wednesday.

Councilors purchased two parcels between Lincoln Street and the Androscoggin River earlier this month. The city plans to install a series of five 350-foot-long concrete culverts underground there.

It’s part of federally-mandated work to create a separate storm sewer system. Combined sewer overflow systems like the one Lewiston has now link storm drains with sanitary sewers from homes and businesses. The street runoff can overwhelm the treatment plant during heavy storms, forcing a combination of storm water and sewage into the river.

Water from downtown storm drains will flow into the tanks during heavy storms, once the work is finished. That water would be pumped back through the treatment plant when the storm had passed and the runoff had subsided.

The project will cost $5 million, compared to the $21 million for a combined sewer overflow project. That kind of overflow project would involve digging up most of the downtown streets over the next several years.

The city will take bids on the project through May 29 and has scheduled a meeting to discuss the work on May 14. Preliminary work should begin this fall and wrap up next summer.

Lewiston has spent $12.1 million since 1997 on sewer overflow projects, installing about 58,000 feet of sewer drains. Those projects, part of a 15-year program to meet federal water quality standards, involved digging up streets to build a duplicate drainage system for rain and snow runoff.