TASK ORDER NO. 4
CITY OF PACIFICA
AND
DHI WATER & ENVIRONMENT, INC.
SECTION 1 – PURPOSE
The purpose of this Task Order is to authorize and direct DHI WATER & ENVIRONMENT, INC. to proceed with the work specified in SECTION 2 below, in accordance with the provisions of the Master Agreement for Consultant Services between the City of Pacifica (“CITY”) and DHI WATER & ENVIRONMENT, INC. (“CONSULTANT”) hereto dated JANUARY 24, 2025 (“MASTER AGREEMENT”)
SECTION 2 – SCOPE OF WORK
The items authorized by this Task Order are presented in Attachment “A” – Scope of Services, which is attached hereto and incorporated by this reference.
SECTION 3 – COMPENSATION AND PAYMENT
Compensation shall be as provided in the MASTER AGREEMENT. Compensation for services as set forth in SECTION 2 shall be as set forth in Attachment “B” – Compensation, which is attached hereto and incorporated by this reference. Total compensation for all services provided under this Task Order shall not exceed Twenty-Four Thousand Eight-Hundred Dollars ($24,800.00) (“Cost Ceiling”).
SECTION 4 – TIME OF PERFORMANCE
The services described in SECTION 2 of this Task Order shall be completed by December 31, 2027 or as extended in writing by the City Manager.
SECTION 5 – ITEMS AND CONDITIONS
All terms and conditions contained in the MASTER AGREEMENT are incorporated by reference and remain in full force and effect.
Approved this ___24th__ day of _January_, __2025_.
[Signatures on Next Page]
OAK #4818-2196-0582 v1
CITY OF PACIFICA
By: Date
Kevin Woodhouse, City Manager
Approved As To Form
By: Date
Michelle Marchetta Kenyon, City Attorney
CONSULTANT
DHI WATER & ENVIRONMENT, INC.
By: Date
Thomas L. Johnson, COO
1 September 2026
ATTACHMENT “A”
SCOPE OF SERVICES
4 TO4: Advanced SBR Process Modeling
Objective: Develop a robust, dynamic, customized SBR model to validate initial aeration optimization simulation findings with special sampling data and support ongoing modeling initiatives for the CCWRP.
The SBR model supporting TO2 and TO3 was limited by the lack of data required to characterize influent diurnal flows, SBR cycle profile date, and the inputs for dynamically changing phase durations. The identified opportunities require validation and control strategy design to prepare for implementation and to inform sensor requirements.
For example, the SBR model was calibrated and used to identify where aeration could have been stopped without impacting effluent quality, indicating the 40% aeration reduction. However, the airflow could not be stopped at that point with the model. Customizing an SBR model to allow the proposed control while validating the calibration with special sampling data will allow for virtual testing of expected activated sludge impacts.
The TO4 scope is centered around developing a robust SBR model to simulate dynamic conditions and operating modes, addressing the limitations of the current SBR model. Further, the recommended special sampling regime will be used to validate the model results during each stage of SBR operation.
The following subtasks describe DHI’s proposed workflow for TO4.
Custom Model Design
DHI and CCWRP operations staff will collaborate to confirm understandings of the SBR operating cycles and the various operator inputs available. This will form the basis for the model parameters to be included in the custom model and how they impact the mass balance.
SBR operating cycles with annotation denoting recommended nitrogen sampling timing.
DHI will then collaborate internally with the WEST model development team to write a custom model script with relevant parameters, timers, level indicators, and mode selectors. This is intended to incorporate operational limits that are not currently feasible to simulate such as the top water level, air slope controller, and variable idle times.
Model Testing & Documentation
Any new model requires rigorous testing to assess if it responds as intended and evaluation of areas for defensive coding and potential errors (e.g., avoid the potential to divide by zero, or plan for proper 5 initialization). DHI will test the custom SBR under the conditions of the baseline model calibration from 2025-2026 DHI modeling exercises and document the custom model configuration for both internal reference and CCWRP process team review. Note that such an advancement in SBR modeling would likely be a candidate for presentation at various modeling and wastewater treatment conferences.
Validate with Nitrogen Sampling
DHI and CCWRP operations staff collaborated to develop a sampling plan that would support model validation for rates of nitrification and denitrification during various stages of the SBR operation. DHI will use these data to validate the updated SBR model calibration. The results may require several iterations of the model testing step, depending on the reliability of the dataset. This is a critical task to develop trust in the predictive capabilities of the SBR models for operational decision making and controller design.
Update Optimization Scenarios
DHI will update the optimization scenarios tested in the 2025-2026 optimization study. While the documented results indicated significant benefits for the CCWRP, the dynamic impacts of the optimization scenarios could not be fully evaluated given the SBR model limitations. The updated SBR model will be used to test:
• Different fill fractions with varying the number of SBRs in operation • Influent equalization via a 1 MG equalization tank to balance diurnal flows • Advanced aeration cycle length control methods using simulated ORP, pH, DO, and NH3 sensors.
The results will be documented as an addendum to the 2025-2026 technical memorandum describing similar scenarios assessed with the limited SBR model, highlighting any differences.
Example result from previous study showing the point (red x) where aeration could have been stopped during a cycle.
While this indicates potential for significant aeration energy savings, TO4 will assess the impact to activated sludge if aeration were repeatedly stopped at that point.
Ad-hoc Modeling Support
As the City of Pacifica continues to assess process requirements with other consultants, DHI will support other modeling efforts as needed. This ad-hoc support as an advisor for the City of Pacifica and peer reviewer for relevant modeling initiatives may include, for example, reviewing consultant modeling results and approaches, sharing input files, and collaborating regarding simulation findings and interpretations.
DHI will plan a 2-day site visit to collaborate with operations staff throughout TO4 and may tailor the site visit to support ongoing project efforts.
ATTACHMENT “B”
COMPENSATION
Project Cost
The estimated project costs, to be invoiced as a lump sum upon delivery of each Task Order, are tabulated below. The project terms and conditions are outlined in the Master Agreement for Consultant Services between DHI and the City of Pacifica dated January 24, 2025.
Advanced SBR Modeling and Controller Design
DHI Cost Estimate
TO4 Advanced SBR Process Modeling $ 24,800