ATTLEBORO - City sewer users, already reeling from big rate increases to pay for critically needed renovations to the wastewater treatment plant, could soon be in for more hikes unless the city can convince state and federal environmental officials to delay the start of tough new discharge standards.

A draft discharge permit, received by the city recently, imposed strict new limits on the discharge of nitrogen and would require more alterations to the wastewater treatment plant that could add millions of dollars to the project that's already costing rate payers $30 million.

Mayor Kevin Dumas said the exact future cost is unknown, but it could be anywhere from $1 million to $10 million.

The permit, now only a draft, can be appealed and the city aims to do just that, Dumas said Friday.

He wants to convince environmental officials to allow the city to delay implementation and put the new requirement on line gradually. Sooner or later, however, the city will be required to build additional facilities to enable the plant to meet the standards.

"Eventually we'll have to do it, but we want to phase it in over a series of years," Dumas said.

As a precaution, the city has gotten authorization from the state to borrow as much as $7 million from the state revolving fund, which helps cities and towns finance wastewater facilities.

No money has been borrowed, nor will it be until the city is able to determine from the state's Department of Environmental Protection what is required, when it's required and gets an idea from engineers how much it will cost.

"If we're forced to do something now, we wanted to be in a position to act," Dumas said. "The SRF has a much better interest rate than private lenders."

An expected surplus of $500,000 from the current project and wastewater department earnings could help to cut the eventual bill, he said.

Rate payers have seen the fees go from $2.72 to $6.59 per 100 cubic feet over three years to pay for the $30 million in renovations at the Pond Street plant that has exceeded its 20-year life expectancy.

GEORGE W. RHODES can be reached at 508-236-0432 or at grhodes@thesunchronicle.com.