If Lawrence wants to add another 20,000 residents at some point in the future, city commissioners first must become comfortable with another number: $45 million.

Commissioners at their meeting tonight are expected to approve a $45.2 million bid by Garney Construction to build a sewage treatment plant south of the Wakarusa River. When completed in 2018, the project will give the city the capacity to serve an additional 20,000 people or significant numbers of new industrial customers with large wastewater treatment needs.

“And it can easily be expanded if someone comes knocking on the door,” said Dave Wagner, the city’s director of utilities. “We feel like it is a long-term solution.”

It also is one that fits into the city’s budget, although that required some shoehorning. The bid from Kansas City-based Garney actually came in at $47.15 million. That amount, however, would have required city officials to ask for a sewer rate increase over and above the increases city commissioners already have committed to.

Instead of seeking the additional rate increase, engineers cut about $2 million worth of items out of the project. Among the cuts were: $753,000 for a maintenance and vehicle storage building and $224,000 to eliminate some redundant processing equipment.

Wagner said the plant was designed to have redundant pieces of key equipment to reduce the amount of down time during repairs. Wagner said the buildings will still be constructed to include space for the redundant equipment, but the city will seek to purchase that equipment later.

The city operates another sewage treatment plant in East Lawrence that is set up to serve the entire city.

The new plant will be built south of the Wakarusa River near where O’Connell Road dead ends with the river. If the bid is approved tonight, construction work could begin in January. Construction is expected to last until early 2018.

Commissioners have long been planning for the new plant. Original plans called for construction to begin several years ago, but the commission in 2007 put the project on hold because it was uncertain that population growth totals warranted the expansion.

But now city officials have said population growth will require the new plant, and the city could face additional regulatory requirements from federal officials if the new plant is not built. Currently the city’s East Lawrence treatment plant operates beyond its designed capacity during heavy rainstorms that flood the sewer system.

The new plant also will remove additional nutrients — mainly ammonia and phosphorus — that aren’t currently removed by the city’s existing plant.

The city already has spent a little more than $20 million on the project, including land acquisition, roads, piping, design work and other miscellaneous functions. The previous commission approved a multi-year plan that increases sewer rates about 6 percent a year through 2017 to help pay for the project.

Commissioners meet at 5:45 p.m. today at City Hall.