The estimated price tag of well over $90 million for Oak Harbor’s proposed sewage treatment plant is so massive that the Navy may not partner with the city on the project, according to Mayor Scott Dudley.

For ratepayers, the lack of Navy involvement will likely mean higher sewer bills for city ratepayers.

The mayor said he sent a city team to speak with officials from the Navy and its consulting firm Tuesday, but talks remain at an impasse.

“Could the Navy still come on board? Yes,” Dudley said, “but based on what we’re heard, we’re pretty far apart.”

Dudley said the city gave the Navy until Oct. 31 to make a decision, but he sees no reason for optimism.

But several council members said they believe Dudley is playing politics and spinning the issue.

Councilwoman Beth Munns said she is still optimistic about the prospects of a partnership, though she acknowledges obstacles. At the Navy’s request, Munns — the wife of a former base commander — has been involved in meetings between base officials and the city over the issue.

“This is a work in process,” she said. “No decisions have been made.

THE CITY is in the midst of planning for a new wastewater treatment plant, which the city’s engineering consulting firm estimates will cost as much as $96.7 million — in current dollars —for the full buildout by 2030. The project, however, will be phased so the full cost will be spread out over years.

The City Council is scheduled to choose the specific site for the treatment plant next month. The council previously decided the plant would be built in the “vicinity” of Windjammer Park downtown and officials are looking at a handful of commercial sites on Pioneer Way.

THE GIANT price tag assumes that Whidbey Island Naval Air Station will continue to partner with the city in treating sewage and share the costs. Without the Navy participation, the plant can be smaller and the estimated costs falls to $79 million, but the rates would be higher for residents.

The Navy hired the Seattle firm CDM Smith to create an “Alternatives Study for Sanitary Sewer Treatment Plant at Seaplane Base NAS Whidbey Island.”

The Whidbey News-Times obtained a copy of the draft study. Munns said council members haven’t seen the report yet.

City Engineer Joe Stowell emphasizes that the draft is only “50 percent complete” and that there’s still a lot of work left to do. He cautions against drawing conclusions from the incomplete study.

DUDLEY SAID the draft confirms his concerns about the costs and Carollo, the engineering firm contracted by the city.

Munns, however, defends the work by Carollo and points out that the company earned nothing but praise for similar projects in other cities, as well as from city staff. She said the Navy’s consultant may not have all the information yet and may be using different formulas. She said the two firms are going to get together and try to work out the differences.

CDM Smith analyzed the facilities plan created by Carollo Engineering and identified concerns with cost estimates. It estimates that Phase 1 of the sewage treatment plant should cost $57 million, while Carollo estimates $84 million; that’s a variance of 50 percent.

“Carollo’s cost methodology does not inform reviewers as to how they actually developed their cost other than the (Wastewater Treatment Plant) Cost Curve,” the draft states. “The project’s highly specific design constraints do not make this a reliable estimate methodology.”

THE DRAFT study states that Carollo’s estimates appear inflated because of “unnecessary redundancy in bioreactor tankage” as well as the urban siting of the plant.

“Selection of the Windjammer site has caused the project costs to be very high due to a need for expensive buildings and structures,” the draft states.

The document points out that the option of an “activated sludge” plant at a more rural site north of Crescent Harbor Road would have been less expensive and still would have met “the effluent goals of the project.”

The draft study also considers the costs if the Navy takes over operation of the sewage treatment lagoons on the Seaplane Base. The city currently manages the facility that’s located on Navy property.

The study concludes that the option could be much less expensive than partnering with the city, depending on the variables, but that there are environmental concerns.

CITY LEADERS chose a “membrane bioreactor” technology for the new plant because it does a better job of cleaning the effluent before it’s discharged into Puget Sound. They were concerned about meeting increased pollution standards in the future.

In 2012, an engineering consultant projected what sewer service rates will be in 2020, based on several variables associated with the new sewage treatment plant.

The rates were the highest without the Navy involvement; last year’s rate of $36.52 a month would more than double to $104 in 2020 if the city receives a favorable rate on a 20-year bond, the consultant said.

COUNCILMAN RICK Almberg said he hasn’t read the draft report from the Navy yet, but said the two engineering firms may be comparing “apples and oranges.” He admits that Carollo’s estimates are probably high and speculated the real costs will be much less.

“Carollo is doing the right thing by having high estimates — what I think are high estimates — and we can work backwards from there.”

Almberg said any certainty about the real costs are impossible at this point.

Almberg is among the council members who voted to build the plant at the Crescent Harbor North site; he was concerned about building on hydraulic fill that makes up the Windjammer sites and the possibility of inadvertent discoveries of Native American remains. Dudley broke a council tie in favor of the Windjammer site.

DUDLEY MAINTAINS that the draft study is further evidence that the project is too costly. He said city officials visited several wastewater treatment plants in other communities recently and found that they cost significantly less that the estimates for Oak Harbor’s plant.

He doesn’t think the problem is just overestimation, but that Carollo’s costs are out of the ordinary. He complained that the facilities plan was flawed and costly.

“They charged us $1.2 million to tell us where the treatment plant should be located,” he said.

THE MAYOR made an effort to replace Carollo. Dudley said he wanted the city to put out “a request for qualifications” to find the best possible engineering firm — with an eye on value — but council members said they were pleased with Carollo and didn’t want to delay the project or spend extra money in the RFQ process.

Munns emphasized that she still believes Carollo is doing outstanding work.

She agreed that it didn’t make sense to go through the RFQ process again, especially in the likely event that Carollo would be chosen once more.

Dudley suggested that the city doesn’t need to build the “Rolls Royce” of facilities and should consider other options besides the expensive “membrane bioreactor” technology.

AS THE Navy’s draft study states, only a membrane bioreactor plant would work at a Windjammer site because of the size limitations.

Dudley was the one who ultimately chose the Windjammer site, and all the council members were in favor of the more advanced technology.

For now, Dudley said he looks forward to the results of a value engineering study in which independent experts are analyzing the project and will suggest ways to save money.

Dudley said there will be a meeting on the analysis later this month.

“I will be there with bells on,” he said, “because I think we should seriously consider every one of the proposals.