A Phased Approach for Nutrient Compliance in the Bay Area - Alternatives Analysis to Construction

Presentation Description: In anticipation of nitrogen limits issued for the Bay Area, the Fairfield Suisun Sewer District initiated the “Aeration Basin Improvements Project for Nitrogen Removal” in 2023 to evaluate nitrogen removal alternatives for four of its existing aeration basins. The District had previously modified two aeration basins in 2006 to improve sludge settleability; the project also provided some nitrogen removal.

Hazen and Sawyer was contracted to help the District perform a process evaluation to identify alternatives for nutrient removal. During preliminary design, the District selected a preferred nutrient removal alternative (Modified-Ludzack Ettinger process) and decided to implement the project using a phased approach. Design for Phase 1 commenced in 2024, and the project was successfully bid in 2025. Construction of Phase 1 began in 2025 and is anticipated to be completed by 2027.

This presentation will demonstrate FSSD’s path from planning to construction to meet the nitrogen limits. Unique aspects of the project include:
•FSSD is able to meet the established effluent nitrogen limits with modifications to the existing aeration basins (no new tank volume).
•FSSD lab staff performed a detailed wastewater characterization during preliminary design that was used to validate the process modeling and design.
•The project will be completed in two phases. Phase 1 represents a multi-benefit project that provides nutrient removal and significant improvements to process performance and stability.
•The phased approach allows the District to “right size” Phase 2 to meet the ultimate nitrogen limits based on new scientific research in the San Francisco Bay and Suisun Marsh, or changes to the final nitrogen limits included in the next Nutrient Watershed Permit (expected in 2029). It also provides the District with additional time to evaluate other nutrient removal opportunities, such as recycled water diversions or digestate processing with Lystek.
•Construction on the basins is limited to the dry weather season in order to maintain plant operations during wet weather, which results in more complex construction sequencing and extended construction duration.
•The project also includes electrical upgrades and other design considerations that will simplify implementation of the future Phase 2 project and reduce the risk of project delays.