A Seattle Public Utilities plan to keep sewage and stormwater from flowing into Lake Washington and Puget Sound, combined with other costs and King County's adopted 2011 rate increase, would raise sewer rates and drainage fees for the average household by $12.27 a month over two years.

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Seattle Public Utilities announced Monday a 15-year, $500 million project to reduce the amount of untreated sewage that washes into local waterways during storms — a venture that will contribute to a projected $8 increase in combined sewage and drainage rates over two years.

Adding in King County’s adopted 2011 rate increase to help pay for the Brightwater treatment plant and other capital projects, the typical household’s monthly cost for sewage handling would rise by $12.27 from the current $63.87 to $76.14 by 2012, officials will tell the City Council’s Seattle Public Utilities (SPU) and Neighborhoods Committee on Tuesday.

The Seattle project is intended to comply with orders from the U.S. Environmental Protection Agency and the state Department of Ecology to cut the city’s combined sewage and stormwater system overflows into Lake Washington and Puget Sound to no more than once a year at each outfall.

Seattle agrees with that goal, SPU Director Ray Hoffman said in a statement. “We believe it’s the right thing to do because it enables us to better preserve the region’s environment and natural resources for future generations.”

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The city’s plan is subject to approval by the federal and state agencies.

Since the 1960s, the city and county have reduced the volume of such overflows to one-twentieth of what they previously were, SPU’s combined-sewer overflows project manager, Andrew Lee, said.

King County spent $360 million on overflow-control projects between 1988 and 2005 and plans to spend an additional $388 million by 2030, said county spokeswoman Annie Kolb-Nelson.

SPU plans to install large underground storage tanks in some parts of the city and make small neighborhood improvements over the next five years. The new reservoirs will be in the Windermere, Genesee and South Henderson basins.

In an area between Northwest 65th Street and Northwest 85th Street, the city has started building “roadside raingardens” — planted areas to capture street runoff — and is offering to pay homeowners most of the cost of installing raingardens or cisterns that keep water out of the sewer system.

City Councilmember Mike O’Brien, chair of the SPU and Neighborhoods Committee, said the proposed improvements are “consistent with what I think people in Seattle want to see. We know people want to do all the things they can to clean up Lake Washington and Puget Sound.”

He said smaller projects such as raingardens could reduce the overall cost. “I’m still hopeful maybe we can figure out a way to do this for $100 million less, which would help rates in the long run,” O’Brien said.

Seattle bills customers for the combined cost of sewage collection by the city and treatment by the county. Rate increases are expected — but have not been calculated — beyond 2012.

Ratepayers don’t see the full cost of sewage collection and disposal on monthly utility bills because part of the cost is paid through a drainage fee added to property-tax bills. Sewage bills depend on a household’s monthly water use.

Keith Ervin: 206-464-2105 or kervin@seattletimes.com

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