Construction design plans for a new main lift station and addition to a high rate treatment plant for the city of Terre Haute are under state review.

The design includes a new 13,124-square-foot building for a new city sewer main lift station and an 889-square-foot addition to an existing high rate treatment plant at 2500 Prairieton Road. The site is located behind a proposed area for a new county jail on the former International Paper property.

The project is estimated to cost about $60 million, with the city’s maximum bonding for the project at $70 million.

The city plans to bid out the project in December, Ennis said, with construction slated to start in spring 2019. The project will likely take 18 months to construct, the city engineer said.

The project is part of the city’s 20-year, $120 million combined sewer overflow, long-term control plan mandated by the federal government.

“This is phase two of our combined sewer overflow project. It [has] more capacity to handle more dirty water,” said City Engineer Chuck Ennis. “This is a considerably bigger lift station than the existing one, [designed in 1962], which is probably 3,000 square feet. It adds more capacity and has more options to where the flow goes. Currently, flow just goes to the wastewater treatment plant.”

Under the new main lift, “we can send flow to the high rate treatment facility or to the wastewater treatment plant. It adds capacity in dry weather and wet weather and gives options on how to handle that flow,” Ennis said.

The high rate treatment facility handles the city’s combined sewer overflow, which comes from sewer pipes that handle both stormwater and sewage, before dirty water, filtered from sewage, is discharged into the Wabash River, the city engineer said.

“You’re treating it at a faster rate than the normal process,” Ennis said of the high rate treatment facility.

“We use it just to handle the combined storm sewage during rain events. It is called the first flush, which is the normal sewage and the first part of a rainfall,” Ennis said. “It washes down to the wastewater treatment plant. After the first flush, then you have essentially dirty water and the sewage content is not that great and that is what is redirected to the high rate treatment facility.”

Excavation will be the biggest part of the project, as the new lift station will go down about 40 feet deep into the ground.

“It will be deep, deep foundations, which are always a challenge, especially close to the [Wabash] river, which adds another component of flooding,” Ennis said. That will require “coffer dams” to be added to keep water out of the perimeter, the city engineer said.

The main lift station, Ennis said, will look like an average one-story building, “but it has a really deep basement. It is four stories deep,” he said.

Reporter Howard Greninger can be reached 812-231-4204 or howard.greninger@tribstar.com. Follow on Twitter@TribStarHoward.