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APPROVE MOTION TO AUTHORIZE CONTRACT AMENDMENT NO.1 WITH BROWN AND CALDWELL FOR THE PRESSURE-BASED DIGESTER GAS CONTROL SYSTEM FOR THE RWCF COGENERATION PLANT PROJECT
recommended action
RECOMMENDATION
It is recommended that the City Council approve a motion authorizing Contract Amendment No. 1 with Brown and Caldwell of Rancho Cordova, CA, for the Pressure-Based Digester Gas Control System for the Regional Wastewater Control Facility (RWCF) Cogeneration Plant Project. If approved, Amendment No. 1 will add $120,573 to the contract and extend its term by 90 days, bringing the total contract amount to $198,047.
It is further recommended that the City Manager be authorized to take appropriate and necessary actions to carry out the purpose and intent of this motion.
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Summary
The Municipal Utilities Department (MUD) is comprised of three enterprise funds including Water, Wastewater, and Stormwater Utilities. The Wastewater Utility includes a sanitary sewer collection system of approximately 900 miles of gravity sewer main, 554 miles of lower lateral pipe, 27 pump stations, and 30 miles of pressurized force main that route sewage to the Regional Wastewater Control Facility (RWCF) for treatment.
The RWCF uses a process called anaerobic digestion that prepares wastewater sludge for disposal. This process produces biogas, a mix of methane, carbon dioxide, and trace gases, where the methane is used as fuel to run large cogeneration engines that generate electricity and heat for RWCF processes. The methane is collected from the digesters and stored in a gas holding tank prior to use. A malfunctioning relief valve caused the holding tank to be taken offline until repairs could be completed. Included in the repairs will be the completion of the design for a pressure-based engine fuel control system that can bypass the holding tank and transfer the gas from the digesters to the cogeneration engines.
If approved by the City Council, Contract Amendment No. 1 (Attachment A) will add $120,573 to the original contract with Brown and Caldwell, and extend the contract term through May 1, 2026, for a total amount not-to-exceed $198,047.
DISCUSSION
Background
On June 21, 2024, MUD operators and plant mechanics detected irregularities in the volume level readings from the digester gas holder. Upon inspecting the unit, staff determined that a faulty relief valve caused the gas system to over-pressurize and fail.
The gas holding tank was taken offline, which caused MUD operators to monitor gas pressure and manually adjust the cogeneration system. Under these conditions, natural gas and digester gas cannot blend effectively, which can reduce system efficiency and reliability, affect fuel quality, and damage equipment. Staff recommended implementing an alternative digester gas control system. This would involve developing a new pressure-based control strategy to restore automated cogeneration and fuel-blending operations.
On September 30, 2024, MUD advertised a Request for Proposals (RFP) to design a pressure-based cogeneration engine fuel control system to bypass the gas holding tank and transfer the gas directly from the digesters to the cogeneration engines. Brown and Caldwell was the only firm to submit a proposal that was determined by the selection committee to be acceptable and meet the project specifications.
On March 3, 2025, the City of Stockton awarded a $77,474 contract to Brown and Caldwell for the Pressure-Based Digester Gas Control System for the RWCF Cogeneration Plant Project.
Present Situation
Brown and Caldwell prepared and submitted a review of the current volume-based controls of the digester gas unit and outlined how to implement the desired pressure-based controls utilizing the existing equipment and systems. The firm also recommends additional system performance improvements, including mechanical modifications to the blowers and emergency flare. Additional engineering services are necessary to design these improvements and support startup and commissioning of the new pressure-based cogeneration control system.
If approved by the City Council, Amendment No. 1 will add $120,573 to the contract and extend its term by 90 days, bringing the total contract amount to $198,047.
FINANCIAL SUMMARY
There are sufficient funds available in account number 6518-000-800007-610-612-00-65-000-000, CIP Pressure-Based Digester Gas Control System for the Regional Wastewater Control Facility Cogeneration Plant, Project No. UW24013, for the proposed Amendment No. 1.
This action has no impact on the General Fund or any other unrestricted funds.
Attachment A - Amendment No. 1