The cost of building a new wastewater treatment plant in Oak Harbor could top $100 million, according to the city’s hired engineering firm.

That was one of the numbers thrown around during a special meeting Wednesday with the city council and Brian Matson, a partner with Carollo. The national engineering firm was hired in August 2010 at a cost of $1.09 million to create a preliminary engineering and facilities plan for the proposed treatment plant.

Oak Harbor’s comprehensive plan calls for the facility to be built and running by 2017. It will take over for the city’s two existing plants at Windjammer Park and at Crescent Harbor on the Seaplane Base, both of which have been deemed in need of replacement.

The meeting with Matson was largely to provide the council with an update on the firm’s work, particularly efforts to narrow down the number of possible sites from five to three. However, the cost of the new facility was also discussed.

In 2011 dollars, the city can expect to pay between $85 to $100 million. Although Matson warned that the accuracy of the range could vary widely, from half as much to 30 percent more, the estimate naturally prompted discussion among council members.

“That’s a $15 million difference,” Councilman Bob Severns said. “That’s a lot of millions.”

Cost hinges on siting

Matson explained that the range in cost was largely a reflection of the facility’s location and the type of technology used. For example, the five sites being considered are located throughout the area but each would discharge into Oak Harbor bay. That means the cost of infrastructure needed for some would be far greater than for others.

The five sites include: Windjammer Park, the Oak Harbor Marina, the old city shops at the northern end of City Beach Street, Beachview Farm, which is more commonly known as Fakkema Farm and located just outside the city’s western boundary, and Navy property located behind the existing lagoon site on Crescent Harbor.

According to Matson, infrastructure costs could swing as much as $8 million, with the Windjammer Park site being the least expensive and Beachview Farm being the costliest due to its distance from the bay.

Is the plant still affordable?

The type of technology used will also vary the final cost. Under consideration are membrane bioreactor and activated sludge facilities. Membrane bioreactor is the newer of the two. It takes up about half as much space, is more effective but comes at greater cost.

The conversation also prompted questions about whether the city’s plan to pay for the facility was still viable as Matson’s estimate was at least $15 million more than the $70 million that has been quoted by city officials.

Councilman Rick Almberg asked if the recently approved utility increase, which will hike sewer rates aggressively between 2011 and 2018, would still provide enough funds to cover any future bond payment. Matson said the firm was not far enough along in its work to answer that question quite yet but that it would be addressed in time.

“I’ll be looking for that,” Almberg said, good naturedly.

In a later interview, Matson said the cost range may increase due to inflation. The average rate is about 3 to 4 percent a year, though the past few years have stayed largely flat, so it’s difficult to know what it will be in 2016 when the project goes out to bid.

“There’s no question this will be a very expensive project,” Matson said.

But it’s also too soon for sticker shock. These are estimates based on a lot of moving parts and there is still a lot left to do before an exact cost is nailed down, he said.

The other big news that came from the meeting was Matson’s recommendation that membrane bioreactor technology be used.

Carollo conducted a phone survey, a six-week online suvey, and held two public forums. Although more expensive, 80 percent of respondents placed public health and protecting the environment ahead of increased cost. Just over 30 percent said cost was the most important factor.

According to Matson, the results make membrane bioreactor technology a clear choice.

Matson is also reccommending that only three sites be considered for further study.

Each of the original five locations was subjected to a range of criteria, from environmental and technical needs to financial and social considerations. Stacked up side-by-side, Crescent Harbor, the old city shops and Windjammer Park emerged as the best choices, he said.

The next step in the process is to hold the third of a series of public forums Aug. 24 and then come back to the council on Sept. 20 with a recommendation to proceed with three or fewer sites. A final location is hoped to be determined late this year or in early 2012.