A consent decree requiring renovations of the city’s aging sewer system and more than $1 billion in upgrades of Oahu’s two main sewage treatment plants is expected to raise sewer rates for Honolulu residents.

But City Council Chairman Todd Apo said the schedule set by the consent decree — made public yesterday — allows for smaller increases on a year-to-year basis.

The city has 10 years to complete a variety of projects for upgrading sewers and until 2024 to have the first of its two sewage treatment plants refitted to meet secondary treatment standards required by the federal Clean Water Act.

"That will give the city the time to plan that out and be able to account for that and fit that into financial models that can be spread out over time and not be forced to try to do some of these fixes really quickly, which would cost our taxpayers some immediate money right now," Apo said.

The City Council yesterday gave unanimous approval to the consent decree to settle years of litigation alleging deficiencies in the city’s wastewater treatment facilities.

Council members voted 9-0 to approve the settlement between the city; the U.S. Environmental Protection Agency; the state Department of Health; and local environmental groups Hawaii’s Thousand Friends, Our Children’s Earth Foundation and the Hawaii chapter of the Sierra Club.

The 94-page consent decree spells out specific upgrades to the sewage collection system that must be completed over the next 10 years.

Projects include construction of new force mains; inspection, repair and/or replacement of more than 1,000 miles of sewers; and specific repairs, storage projects and possible capacity upgrades at certain pump stations.

The city must prepare annual reports on progress and on sewage spills, with fines for not meeting compliance deadlines.

The consent decree provides a longer timetable for secondary treatment upgrades at sewage treatment plants at Sand Island and Honouliuli.

Honouliuli would have to meet secondary treatment standards by 2024. The Sand Island plant would have until Dec. 31, 2035, with the possibility of a three-year extension if the city can prove the earlier deadline would be technically impossible or cause undue financial hardship.

Mayor Mufi Hannemann had opposed the upgrades for years, at one point placing the cost at $1.2 billion. The city had previously received waivers exempting the treatment plants from having to perform secondary treatment, but the EPA denied an extension of the waivers.

The city and the EPA have disagreed over the need for secondary treatment, the process of removing organic matter from sewage, with both sides providing scientific analysis to back their claims.

The financial impact of the settlement likely will not be felt by the city, or ratepayers, this fiscal year, Apo said.

"The settlement documents say, ‘Here’s what you have to do,’" Apo said. "Now it’s up to the administration to figure out how that’s going to be budgeted year after year."

He said the timetable should allow for increases of no more than 2 percent to 4 percent on a year-to-year basis.