Unexpected discoveries will delay completion of the county’s $2.5 million sewage treatment plant expansion project.

Bill Oakes, director of Island County Public Works, told the board of commissioners last week the project won’t be completed until early December, about two months beyond the expected completion date.

The commissioners agreed to increase the amount paid to the contractor, PACE Engineers, Inc., by $49,000.

While excavating, workers discovered air-supply piping to the existing digesters are severely corroded and need to be replaced.

After excavation went farther than expected, two groundwater monitoring wells were exposed and had to be re-cased.

Additional inspections and testing took place at the lagoon that were not included in the original contract’s scope of work. And because of the location of the existing alarm panels, more electrical work had to be done to connect them to the control buildings for the new digester systems.

The county is in the processes of doubling the capacity of the plant due to increased use. The treatment plant, located behind the county transfer station, accepts septage from companies that pump septic tanks on the island as well as effluent from the Town of Coupeville’s sewage plant and the Penn Cove and Holmes Harbor sewer districts.

During the winter months the capacity of the plant was being pushed to its limits, Oakes said. The addition of two more anaerobic digesters to the existing two should increase the life of the plant another 15 to 20 years.

By carefully timing treatment and construction, the plant hasn’t had to close throughout the expansion process, said Oakes.

“They’ve done a great job keeping the plant up and receiving through this,” he said. The bulk of the expansion is finished and the facility shouldn’t close during the rest of construction.