Monthly bills for the average residential customer could go up nearly $4, from $36.10 to $39.75.

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Sewer rates in King County would increase 10.4 percent over the next two years under a proposal by King County Executive Dow Constantine.

Monthly bills for the average residential customer would go up nearly $4, from $36.10 to $39.75.

The charge for new sewer hookups would increase by 3 percent, to $53.50 per month. Hookup charges are usually paid over 15 years.

With the increase, rates would be 3 cents below the county’s projection, Constantine noted.

A large part of the proposal is dedicated to repaying money borrowed in the past decade to fund projects, including Brightwater, the $1.8 billion treatment plant in Woodinville that opened last year.

The increase also would be spent on combined sewer-overflow projects in Seattle, which would keep sewage from flowing into local waters during heavy rainfall. Another big cost is replacing a 77-year-old wooden sewage pipe that runs under the Ship Canal between Ballard and Interbay.

Any increase for next year must be approved by the Metropolitan King County Council before June 30. Constantine sent his proposal to the council Thursday.

The actual amount ratepayers see on their monthly bills is set by the 34 different cities and sewer districts that buy treatment services from the county.

Monthly rates in King County have more than doubled since 1999, when they averaged $19.10.

According to a study of the 50 largest cities in the U.S. by consulting firm Black and Veatch, the average annual increase in sewer rates between 2001 and 2009 was 5.5 percent. King County averaged 5.6 annual increases during that period.

A county analysis of 25 large wastewater agencies around the country found that only five had higher rates than King County: Atlanta, Cincinnati, Honolulu, Portland and San Francisco. Among the cities with lower rates: Austin, Boston, Kansas City, Nashville and Philadelphia.

Constantine’s rate proposal would’ve been higher if the county had not identified $1.9 million in new operating efficiencies. The county’s ratepayer report says increases in labor costs account for $1.8 million of the total $38.5 million projected revenue increase.

There’s some relief in sight for ratepayers, according to the report. With Brightwater completed and debt payments stabilized, the county is projecting annual increases of less than 1 percent for 2016 to 2020.

Bob Young: 206-464-2174 or byoung@seattletimes.com