PORTLAND — About a dozen businesses and residents spoke against a proposal to create a new storm-water fee to pay for about $170 million in impending work.

Residents and businesses alike asked the City Council tonight to give the city manager more leeway in crafting a proposal to fund the work being mandated by the federal government.

The proposed storm-water fee, which would take effect July 1, 2013, would fund the separation of about 133 miles of combined sewer and storm-water lines, or CSOs, which stands for combined sewer overflows. The city is borrowing the money for the work, but it is looking for ways to repay the debt.

The proposal would raise $85 million by increasing the city’s sewer rates and the remaining $85 million through a new storm-water fee that would be levied based on the amount of impervious surface – driveways, rooftops and parking lots – on a property.

The proposal would also direct the city manager to draft incentives for businesses and residents to reduce the amount of storm water they produce.

The city is under a federal mandate to separate its sewer and storm-water systems. More than $100 million has already been invested to separate 11 of the city’s 43 combined sewer overflows, but another $170 million is needed to fully comply with the Clean Water Act.

Normally, treatment plants can handle combined sewer overflow volumes. But when it rains, the treatment system becomes overwhelmed, and raw sewage and polluted runoff are discharged into the city’s waterways, including Back Cove and the Fore River.