Consultants recommend oxidation ditch, estimate $100–$160 million to replace Tulare domestic wastewater plant

Corolla Engineers told the Tulare Board of Public Utilities on Monday that the city’s domestic wastewater treatment plant is aging, increasingly outmatched by higher pollutant loadings, and should be replaced with a new oxidation-ditch process as the preferred path forward.

John Witter, project manager for Corolla Engineers, said the plant’s historical data and load projections support a planning design capacity of 7.1 million gallons per day (MGD) for full build-out, but that the existing plant’s effective biological capacity is closer to 4 MGD and is temperature-dependent. “With those increased loads that we’re seeing now, the actual plant capacity is closer to 4 MGD,” Witter said.

Why it matters: consultants noted most of the plant was built in the 1970s, with piecemeal work in later years, and several mechanical and structural assets are near or past their useful life. That combination, they said, makes the facility more prone to upsets and difficult to operate during storm-driven inflow events.

Corolla presented three secondary-treatment alternatives carried into detailed analysis: modified Ludzack–Ettinger (MLE, the plant’s current process), membrane bioreactor (MBR), and oxidation ditch. The firm reported life-cycle cost estimates showed less than a 10% cost difference among the three options at the facility-plan level of accuracy; operational and reliability factors drove the recommendation. “The oxidation ditch was the recommended alternative that we are looking to move forward,” Witter said, citing simpler operations, fewer controls and equipment, and larger basins that better handle influent variability.

The consultants recommended avoiding rehabilitation strategies that would retain the plant’s primary-treatment and anaerobic digestion systems, in part because thickeners and digesters have been out of service and gas-handling and permitting would be complex. Instead, they proposed routing headworks to a new secondary process so the project would not produce raw primary sludge requiring an active gas-handling system.

Cost and phasing: Corolla presented an order-of-magnitude cost range for the full build-out of roughly $100 million to $160 million (including common improvements such as a new administration and electrical building and solids-handling upgrades). The firm also described a reduced initial-scope phasing option—building a single oxidation ditch and later adding capacity—to lower near-term capital demands.

Next steps: the consultants said the draft facilities plan has been submitted to staff for comment, and recommended completing a rate study and predesign work to determine financing and final sizing. Tricia (city staff) confirmed the firm will present similar informational material to the City Council before returning to the Board for direction; no board action was requested at this meeting.

Board members pressed on staffing and rain impacts. Staff warned recent heavy rain events have caused process upsets and will require operators to reseed biological processes once flows stabilize. The consultants and staff emphasized tradeoffs between capital cost, operational complexity and future flexibility as the city refines funding options and the final project scope.

The board received the presentation and had no vote on item 6.1; consultants will return with financial and design details in future meetings.