Brightwater, the cheerfully named $1.8 billion sewage-treatment plant north of Woodinville, opens Saturday with a fanfare of speeches, music and 3,000 free tacos provided by project contractors.

The largest public-works project in King County history also opens with a big question hanging over it.

Brightwater was built to meet projected population growth. And local officials struck a deal that ground couldn't be broken for the plant unless "growth pays for growth" — meaning new customers throughout the county, not existing ones, would pay for expanding the sewage system. But the county isn't sticking to that principle, Seattle officials have argued.

A City Council analysis contends that by 2030 "existing ratepayers in Seattle will pay $320 million more than they should" for sewage-system improvements. That amounts to an added cost of about $900 for the average Seattle customer, the city auditor reported this year.

Brightwater is just a part of that total, which comes from an array of county sewer projects that will benefit Seattle and its suburbs.

But concern about the plant's costs "put an exclamation point on the discussion and gave it a new focus," says Martin Baker, deputy director of Seattle Public Utilities.

County officials have a different interpretation of their financial formulas. They say the charge Seattle wants on new customers — about $16,000 for a residential hookup — would be too high. And it might impede growth.

County Councilmember Larry Phillips says Seattle has been "barking" at the county mainly to shift attention away from the city's own high water rates.

The debate between the county and city has recently mellowed, according to key participants. But the current negotiations still reveal concerns about an issue that hits the wallet of everyone who flushes a toilet in the county: How much can we charge for growth, and how much must we subsidize it?

Existing customers can only hope the growth comes soon, because having built Brightwater for newcomers, we now depend financially on their arrival.

Increasing capacity

The city-county debate goes back to the late 1990s, when the building industry, labor unions and environmentalists pressured former King County Executive Ron Sims to increase the county's capacity to treat and dispose of sewage.

Builders and developers "were scared to death" because state regulators had threatened to impose a moratorium on new construction, recalled Chuck Clarke, EPA regional administrator at the time and later head of Seattle Public Utilities.

Sims (who did not respond to requests for comment) and the County Council decided a new plant was needed to serve North King and southern Snohomish counties.

Before Sims embarked on a controversial search for a site, though, representatives from the county and local cities met at a Bellevue conference center, the Robinswood House.

That summit produced the Robinswood Agreement, which struck the 1998 deal that "growth pays for growth." It called for new customers to pay for "new capacity."

Existing customers were defined as those whose homes or businesses were hooked up to sewers before 2003; new customers were those who connected afterward. These financial policies would apply until 2030.

Seattle officials say the county has departed from the agreement by not charging enough for new customers. Connections charges are now $50 a month. But they should be more than $90 monthly, the city contends, or about $16,000 over the typical 15-year billing cycle for hookups.

Phillips maintains that current charges are fair: "We have consistently reviewed those financial plans over time and Seattle is the only voice that says we need to do something differently."

It's true that no other jurisdiction has joined Seattle's cause.

But that's not to say no one agrees with its premise. "I do agree with Seattle that the methodology has flaws," says Ron Speer, district manager of the Soos Creek Water & Sewer District in Renton. The county's connection charge isn't "enough to cover debt service so somebody else has to pay."

"There's a political dynamic here," says Seattle City Council President Richard Conlin. Builders, developers and some suburbs want to keep the charges down so they don't inhibit growth, which means jobs.

Interpretations differ

Christie True, who oversees sewage treatment for the county, won't say the city's interpretation is flat wrong. There's just a "fundamental disagreement" with Seattle about what's equitable in cost-allocating formulas.

True takes the long, regional view.

"We agree that existing customers are subsidizing new ones. They have to. There's no way around it," she says. You have to build sewage-treatment facilities before the growth arrives, she explains. And in the early years of paying for Brightwater, existing customers must share some of the burden because there aren't enough new customers to carry the full load.

"At any point in time," she adds, "we are investing more money in one part of the system than another. That's the situation with a regional system. In the last decade we've been putting a lot of money into Brightwater. The next two decades our biggest investment will be in combined sewer overflows, which are all in the city of Seattle."

With any snapshot, True says, one customer group such as Seattle could claim they weren't getting their fair share, or were paying too much. But eventually, she says, more and more new customers will connect and end up paying 95 percent of Brightwater's costs.

Seattle officials, though, have not been convinced, as they watched Brightwater's cost more than double from 1999 estimates, while the recession has slowed growth.

Phillips notes that the city is not mentioning another part of the Robinswood Agreement, which spared Seattle from paying the entire bill to fix sewer-overflow problems that occur during heavy rain. Instead the cost will be spread around the county, saving Seattle approximately $200 million, according to county estimates.

Interestingly, city officials who have griped about county policies for several years now sound like diplomats.

Seattle Public Utilities chief Ray Hoffman issued a statement saying the city is now focused on "working in partnership" with the county. Mayor Mike McGinn did the same.

Conlin says relations with the county have greatly improved. Conlin attributes the detente to a "good working relationship" with King County Executive Dow Constantine. He said the county has shared more information, causing the city to reconsider some of its grievances.

"It gets nuanced," he explained, when debating, for example, who should pay for a new stretch of pipe. A particular part of the pipeline might be used by existing customers, or it might take pressure off the parts of the existing system. Thus, it's not unreasonable for existing customers to pay a share.

Phillips says the city and county are talking about new policies that might provide Seattle some things it wants. "What's going on now is a truce," he says. "Everyone agreed to stop rattling sabers."

A regional group is looking at improvements to the county's rate methodology, which is at the center of Seattle's complaints.

That group is expected to come up with recommendations soon, according to county economist Tom Lienesch. They'll be forwarded to a regional group of elected officials scheduled to act on the recommendations in December.

"It is a regional system," Conlin concludes, "so it's important everyone is satisfied. I don't know if (Brightwater costs) balance out in the long run. But we're just doing the best we can."

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