City Commissioners approved a $569,263 contract to HDR Engineering Inc. during their Aug. 18 meeting for the Wastewater Treatment Plant primary clarifier rehabilitation project.

The city’s wastewater treatment plant operates two primary clarification basins, each more than 60 years old, that are essential to remove solids, grit and a substantial portion of biochemical oxygen demand before secondary treatment.

In June, staff from the city and Veolia, the city’s contractor operating the plant, found a failure in one of the basins, which was taken out of service and is no longer being used for normal operations, according to a city staff report.

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Staff found a drive assembly failure that was caused the rake mechanism to be misaligned and grind into the basin floor.

Staff are concerned that the second clarifier may fail similarly due to its age and identical design.

The second clarifier is currently operational, “but the risk of a future failure remains significant,” according to the city staff report.

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During the Aug. 18 meeting, Calob Marquis, the city’s water infrastructure manager, told commissioners that Veolia was talking to their counterparts nationwide for contingency plans.

The project is an “urgent priority” for the sanitary sewer system, he said, and staff is prepared to defer other projects due to budget constraints.

Commissioner Rick Tryon moved to approve the contract and said his “thoughts and prayers are with Clarifier 1.”

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Marquis said it will likely be early 2028 before construction actually begins on the project based on the design process and lead times on necessary components.

The contract commissioners approved on Aug. 18 is for engineering assessment, design and construction support services to rehabilitate both clarifiers.

Since the failed clarifier has been removed from service, all primary clarifier responsibilities are on the remaining clarifier, and reliance on a single basin increases operational risk, according to city public works.

If the remaining clarifier were to fail before the failed equipment is repaired or replaced, the city’s wastewater treatment plant would have to operate without any primary clarifiers, which is possible, but would have “significant negative impacts,” according to city and Veolia staff, including:

• higher solids and BOD loading on secondary treatment processes
• increased grit accumulation leading to accelerated wear on pumps and piping
• reduced ability to use gravity basins for sludge settling
• increased load on flotation thickeners and potential impacts to digesters
• elevated odor generation and increased likelihood of odor complaints, and
• long-term degradation of downstream process effectiveness and reliability

Short-term operations without primary clarifiers is allowable within regulatory compliance, the long-term consequences could cause system damage, increased maintenance costs, reduced treatment performance and increased operational uncertainty, according to city staff.

Under the contract, HDR Engineering will complete project design, develop plans, assemble bid packages, assist with bidding, assist with construction management oversight, assist with construction inspection, and complete as-built drawings for the project, according to city staff.

The project is being funded through the sanitary sewer fund, which is an enterprise fund largely comprised of user fees for sewer services.