About $3.5 billion in sewer construction projects planned over the next 10 years will help the city comply with a consent decree to settle years of litigation alleging deficiencies in the city’s sewage and waste-water treatment systems, officials said.

The amount includes projects already under way and new construction required by the settlement reached between the city, U.S. Environmental Protection Agency, state Department of Health and three environmental groups.

Under the agreement the city has 10 years to renovate and improve sewers, and until 2024 and 2035 to refit its two sewage plants—at Honouliuli and Sand Island, respectively—to meet secondary-treatment standards required by the federal Clean Water Act. Those two plant upgrades are estimated to cost about $1.2 billion.

Mayor Mufi Hannemann said his administration would seek to minimize the cost to ratepayers by seeking state and federal assistance where possible.

"Yes, there’ll be costs, but rest assured we will look to identify ways to do it," Hannemann said at a news conference yesterday.

Projects to bring the city into compliance also will lead to increases in sewer fees for Oahu residents, estimated at about 3 percent to 5 percent each year for about 25 years, he said.

On average a single-family house on Oahu pays about $91 a month in sewer fees.

The City Council gave unanimous approval to the settlement on Wednesday.

It now goes back to the U.S. Department of Justice and federal court for a 30-day public comment period.

If approved by the federal court, the consent decree would settle a 2004 lawsuit filed against the city by the Sierra Club and other environmental groups, while also addressing various issues raised by the state and EPA.

Robert Harris, director of the Hawaii chapter of the Sierra Club, said the group was generally pleased with the settlement but was hopeful that some of the sewer work could be completed sooner.