MOUNT PLEASANT -- A sewer rate increase of up to 36 percent would be necessary to pay for $17.6 million in needed improvements at the aged Center Street wastewater treatment plant, officials said Wednesday.

To avoid such a steep increase, Waterworks plans to apply Feb. 1 for an $8.8 million federal grant, Waterworks Commission Chairman William Golightly said.

If the grant is approved, the increase would be 9 percent. The higher charges would be phased in over two years, Golightly said in a letter to Charleston County Council Chairman Teddie Pryor.

"It is vital to the future economic well-being of the Town of Pleasant that MPW find alternative methods to fund this critical project other than rate increases," Golightly said in his correspondence.

The possible sewer rate increases in Golightly's letter are preliminary numbers that have not been discussed in detail or approved by the commission, said Nicole Bates, Waterworks spokeswoman.

"They were very rough estimates to give us a basis for our pursuit of funding," Bates said in an e-mail. The rate increase could add $3.36 to a sewer bill if the federal grant for Center Street plant improvements is not received, she said.

In an interview, Golightly said the commission considered closing the Center Street plant and piping all of its sewage to the more modern plant on Rifle Range Road, which would be expanded to handle the additional wastewater. That would cost $100 million, he said.

"We really have no option except to re-do Center Street. It's something that we have to do," he said.

The 41-year-old Center Street plant serves 18,000 businesses and residents. It treats 2 million gallons of wastewater per day. Because of its age, there is an increased risk of plant failure, he said.

Improvements would enable the plant to process 3.7 million gallons of wastewater daily. Design of the new plant is under way. Construction could start in September of next year and finish in June 2014, Waterworks said.

The plant serves Coleman and Johnnie Dodds boulevards as well as Patriots Point. The wastewater processing improvements will help the town plan for revitalization of the boulevards. Redevelopment of Johnnie Dodds Boulevard is expected to bring an estimated 1,250 jobs during the next decade and a total of $500 million in private investment.

"These areas are critical to the future economic development east of the Cooper," Golightly said in his letter. In the correspondence, he asks for County Council support of the Waterworks application for $8.8 million in federal funding.

Waterworks customer bills went up twice in 2010. Water and sewer service was raised at least $3 per month to cover the $4.3 million tab to relocate Waterworks lines in the path of the U.S. Highway 17 widening project.

The "special assessment" became effective in July and is expected to last for three years, the projected time to complete widening of 10 miles of the highway to six lanes to relieve traffic congestion that starts at the Arthur Ravenel Jr. Bridge.

Last January, Waterworks raised basic water and sewer rates 9 percent, which added $4 to the bill of a typical residential customer. It cited as factors the economic downturn and resulting lower revenues because of empty foreclosed homes and nonexistent growth.

Reach Prentiss Findlay at 937-5711