Seattle residents can expect to see their bimonthly utility bills rise more than $90 in the next six years after the City Council on Monday approved rate increases for residential garbage, water, sewage and stormwater collection.

The average bimonthly household bill will increase about $15 each year, going from $323 in 2015 to $414 in 2020. That’s an average 4.6 percent each year.

“That number sounds huge. That’s a scary number, but it’s a lower annual rate increase than we’ve seen over the last seven years,” said Councilmember Sally Bagshaw, chairwoman of the utilities committee. She noted that the previous average increase was 7 percent a year, from 2008 to 2014.

Bagshaw said the six-year plan will give ratepayers predictability and accountability. Previously the utility submitted rate increases for each line of business to the City Council separately.

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Some rates, such as for wastewater drainage, will rise as much as 10 percent in 2015 and 2016 because of the costs of addressing sewage and wastewater overflow into surrounding waters, including Lake Washington and Puget Sound.

The city is under a consent decree from the U.S. Environmental Protection Agency and state Department of Ecology to reduce overflow, typically a problem during heavy rainstorms.

Projects to add holding capacity to the city drainage system and treat the wastewater before releasing it are estimated to cost $500 million through 2025.

Councilmember Kshama Sawant called the rate increases unacceptable for working people, but said the overall spending plan for the utility was driven in large part by federal mandates for water quality and sewage overflow.

She also noted that because the city makes money on its services, its revenues decrease as customers conserve water and throw out less garbage.

Sawant said that rather than rate increases, the city needs to find more progressive solutions to rising costs, such as a tax on the superwealthy. She ultimately joined with the other five council members present to approve the rate increase.

The council also called for doubling the number of customers enrolled in the low-income utility-discount program by 2018, which council staff estimated would cost other residential users about $16 a year to make up the difference in revenue.

Currently, 14,650 customers are enrolled in the program, but the city estimates that is fewer than 20 percent of those eligible. Mayor Ed Murray is expected to announce a plan in the coming weeks to increase participation in all city programs, including utility discounts.

Seattle Public Utilities (SPU) did manage to shave about $1.5 million a year off its projected expenses over the coming years by renegotiating lower interest rates on bond payments. Director Ray Hoffman said the utility has identified other efficiencies, including worker productivity and technology investments, that will mean long-range savings.

After the utility announced plans to raise rates about 30 percent through 2020, it held 13 public meetings in February and March. Hoffman said customers said they want the utility to operate efficiently and be thrifty, and to be able see the value of its investments.

Noel Miller, who chaired a nine-member customer review panel that included business and residential customers, said the group recommended the rate increases be approved.

“There are compelling needs that SPU faces right now, particularly controlling where sewage goes. We’re adding holding capacity to the system now along Lake Washington. We’re going to have to do the same thing along Lake Union and the Ship Canal,” said Miller, a former public-works director for Edmonds.

Other big-ticket expenses facing the utility include reducing chronic flooding in South Park and Broadview and replacing or repairing miles of aging sewer lines, some as old as 110 years.

Lynn Thompson: or 206-464-8305 On Twitter @lthompsontimes