An upgrade to Springfield’s Wastewater Treatment Plant will cost about $58.5 million — believed to be the largest public works project in the city’s history — and will lead to higher utility bills.

The high-rate clarifier project will help control sewer overflows during heavy rainfalls and has been mandated by the Environmental Protection Agency. It will cost about $50.1 million. The city has already spent $4.1 million for design work, and will spend another $4.3 million on construction services.

“It’s a huge project,” Finance Director Mark Beckdahl said. “It’s by far the largest thing that we’ve done.”

The project will be funded by an increase in sewer rates — a 4-percent increase each year through 2014, which commissioners approved July 17 — and a new storm water utility, which began in January. Both utilities will be raised over time to help cover increasing costs for other combined sewer projects.

“We wanted to spread out the damage so people wouldn’t get hit too hard at any one time,” Mayor Warren Copeland said.

The city hopes to begin construction at the Wastewater Treatment Plant, 965 Dayton Ave., in September and will finish in 2014.

Service Director Chris Moore and Beckdahl both said it’s the largest project they’ve ever seen approved by city commissioners. Wastewater Treatment Plant Superintendent Bill Young said its the largest project he’s seen in his 32 years with the city.

A previous upgrade to the treatment plant in 1987 cost $22 million, and with inflation would cost approximately $44.4 million today. The city spent $5.7 million in 1979 to build City Hall, which would cost approximately $18 million today including inflation.

The Wastewater Treatment Plant currently treats 15 million gallons of sewage per day and has the capacity to treat 34 million gallons per day. However, when a large storm hits, the raw sewage overflows into Mad River. The clarifier will catch that overflow and treat it.

When construction is complete, the city will be able to treat 134 million gallons of sewage per day.

“It’s by far the biggest step toward a large amount of elimination (of sewer overflows) … All of that excess flow will go through a treatment process,” Moore said.

City commissioners agreed Tuesday to award a $50.1 million contract to Kokosing Corp. of Delaware to build the clarifier behind the treatment plant. It was the lowest of five bids received. They also approved approved an additional $4.3 million for Black and Veatch Corp for construction services for the project.

The city recently approved a $55 million loan through the Water Pollution Control Loan Fund, Beckdahl said, a state program that provides financial assistance to protect and improve the quality of Ohio’s water resources. The 20-year loan has an interest rate of 2.55 percent — the lowest rates the city has ever received, according to city Treasurer Bob Mauch.

Copeland said the city had no choice but to comply with the EPA’s Clean Water Act, which implemented a Combined Sewer Overflow Long-Term Control Plan, and weren’t offered any help in paying for it.

“We didn’t have any choice and we’re trying to figure out the best way to handle it,” Copeland said. “We can’t say no, we can only say, ‘How?’”

Moore said the EPA has been pleased with what the city has accomplished so far concerning sewer overflows. He said they’re still negotiating deadlines as far as completing additional phases of the project, such as building a new express sewer line on the northwest side, but estimated it could be completed by 2030.

The total price tag for all of the projects required to meet the EPA’s sewer overflow standards could cost an estimated $160 million.

“They know it can’t be completed overnight,” Moore said.

Historically it was common practice for sewer systems to be combined, handling both sanitary sewage and storm water. The Clean Water Act, though, required cities to stop polluting waterways with sewage.

Several other cities — including Columbus, Cincinnati and Akron — are also dealing with complying with the law.

Moore said Springfield has done projects over the years to whittle away at the problem, but the high-rate clarifier will have a huge impact on fixing the sewer overflow.

“It’s the most bang for the buck,” Moore said.