Williss Kohr, Zeena

Zeena Williss Kohr was a speaker at the WEFTEC 2025 conference held in Chicago, IL from September 27-October 1.

Titles from this speaker

Description: Innovative, Sustainable and Cost-Effective Nitrogen Removal Strategies to Meet New...

Innovative, Sustainable and Cost-Effective Nitrogen Removal Strategies to Meet New Colorado Regulations (TN < 2 mgN/L): Insights and Case Studies

Abstract

ABSTRACT: This study identifies cost-effective strategies for N removal to meet Colorado's regulation 31 of 2 mg N/L TN. Three levels of approaches are recommended. Case studies from operational plants validate the findings, guiding WRRFs in CO towards efficient regulatory compliance.

INTRODUCTION: Conventional BNR with advanced process control methods (Advanced BNR or ABNR) with AvN and ABAC have further enhanced efficiency and reduced costs. Building on these advancements, PdNA has emerged as a promising strategy, reducing aeration energy and carbon requirements. For meeting the most stringent TN limit of 2 mg/L, using PdNA as a polishing step provides opportunities for almost any activated sludge plant, instead of being constrained to a very limited set of process layouts. Additionally, sidestream PNA will aid the overall N removal performance. Cost remains a critical aspect for upgrades and meeting a stringent TN limit, necessitating thorough evaluation to ensure compliance and efficiency.

OBJECTIVES: This study evaluates advanced N removal strategies to meet Colorado's Regulation 31 (annual median TN of 2 mg N/L). By developing standard flow sheets and conceptual designs, the study compares air demands, chemical addition, sludge yield, and capital costs for both sidestream and mainstream.

METHODS: Three N removal strategies are evaluated. Table 1 shows the advantages and disadvantages of each improvement level.
Level 1: Conventional BNR: Achieves ammonia levels below 1 mg N/L using existing conventional BNR processes, serving as the baseline.
Level 2: ABNR with Densified Activated Sludge (DAS): Integrates advanced aeration control techniques and selective wasting to promote to enhance nutrient removal efficiency, increase capacity, and reduce energy consumption.
Level 3: ABNR/DAS with PdNA Polishing: Combines advanced BNR and DAS with PdNA polishing and sidestream PNA to improve N removal efficiency to drive sustainability and reduce operational costs.

While P removal is not discussed in this paper, there are synergies between N and P removal with the proposed improvements. Densification in Level 2 is expected to improve phosphorus removal efficiency by retaining PAOs. Implementing PdNA and sidestream deammonification will allow for reductions in carbon requirement and N load to the mainstream process, which can benefit EBPR process. For Level 3, reductions in operational SRT resulted in reductions in aeration demand and increased sludge production. For this study, PdNA efficiency of 65% was used.

CASE EXAMPLES
City of Boulder WRRF (BWRRF): BWRRF operates a 25 MGD facility with a 4-stage Bardenpho process with PAD for sidestream N removal. Future upgrades include a DAS system with a low DO Anaerobic/Oxic (A/O) configuration to promote SND using hydrocyclones. Three scenarios are evaluated using an existing calibrated BioWin model and performance data (Table 2). Flow sheets for each scenario are presented in Figure 1. Figure 2 compares monthly electricity usage for the scenarios under existing flow, 2040 Maximum Month flow, and build-out conditions. Implementing advanced aeration control results in approximately 30% energy savings. Comparing Scenario 1 with Scenario 3, nearly 40% energy savings are achieved by implementing PdNA in the Solids Contact Tanks. The majority of energy saving for Scenario 3 was attributed to sidestream deammonification. A comparison of sludge production as a result of SRT changes will be provided in the full paper.

Confidential Client: Client's secondary treatment system uses a two-pass, five-stage Bardenpho processes with step-feed, at current flow of 4.0 MGD. Three scenarios are evaluated using an existing calibrated BioWin model and performance data (Table 2 and Figure 1). In Scenario 3, existing tertiary filters are used for PdNA-based N removal. Table 3 compares the cost per unit of N for the three scenarios, including aeration and carbon costs for denitrification. Similar to Case Study 1, the major aeration saving was achieved from Scenario 1 to Scenario 2. Scenario 3 resulted in additional cost savings for aeration and carbon source. Net Present Worth comparisons for Scenarios 2 and 3 are presented on Figure 3. Scenario 3 enables the WRRF to meet TN of 2 mg/L at a lower cost compared with Scenario 2 at TN of 3 mg/L.

South Platte Renew: SPR is the third largest WRRF in CO, with a design capacity of 50 MGD. The treatment process includes trickling filter/solids contact, clarifiers, nitrifying trickling filters, and denitrification filters. Three scenarios are evaluated using an existing calibrated BioWin model and performance data (Table 2 and Figure 1). In Scenario 3, a portion of the solids contact tanks effluent is directed to denitrification filters to promote PdNA. Figure 4 shows that methanol usage decreases by about 50% in Scenario 3 (with PdNA and sidestream PNA) compared to Scenario 1, while achieving an effluent TN of 2 mg/L. With PdNA in denitrification filters, the N loading to the filters increases, as presented in Table 4. The changes in sludge production for this case is related to changes in methanol consuption only.

CONCLUSIONS: This study highlights the strategies for WRRFs to comply with stringent TN discharge limit of 2 mg N/L while maintaining operational and cost efficiency. Implementation of aeration control, densification, polishing PdNA, and sidestream PNA are proposed in three improvement levels.

This paper was presented at WEFTEC 2025, held September 27-October 1, 2025 in Chicago, Illinois.

Presentation time

15:30:00

15:45:00

Session time

15:30:00

17:00:00

SessionFull-Scale Shortcut Nitrogen Removal

Session locationMcCormick Place, Chicago, Illinois, USA

TopicLiquid Stream Treatment - Nutrient Removal and Recovery

Author(s)

Arabi, Sara, Regmi, Pusker, Loughran, Peter, Marks, Christopher, Sigmon, Cole, Williss Kohr, Zeena, Bilgin, Azra

Author(s)S. Arabi1, P. Regmi1, P. Loughran1, C. Marks2, C. Sigmon3, Z. Williss Kohr3, A. Bilgin4,

Author affiliation(s)Stantec Inc.1, City of Boulder2, South Platte Renew3, City of Longmont4

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2025

DOI10.2175/193864718825159904

Volume / Issue

Content sourceWEFTEC

Word count21

Description: OPERATIONS, PERFORMANCE, COSTS: OPTIMIZING CHEMICAL PHOSPHORUS REMOVAL THROUGH...

OPERATIONS, PERFORMANCE, COSTS: OPTIMIZING CHEMICAL PHOSPHORUS REMOVAL THROUGH COLLABORATIVE IMPLEMENTATION

Abstract

Background: South Platte Renew (SPR) is the third largest WRRF in Colorado, with a capacity of 50 MGD. SPR has implemented a chemical phosphorus (Chem-P) removal process to achieve effluent TP below 1.0 mg/L for regulatory compliance. The SPR Engineering and Operations teams took a holistic and collaborative approach from design to operation for optimum Chem-P implementation with the following core objectives:
1) Operations: Assess plant operation impacts and implement a treatment process with operation flexibility designed for optimization.
2) Performance: Evaluate and select chemical addition strategies for reliability and optimum performance.
3) Costs: Minimize chemical addition costs while achieving optimum performance.

Relevance and Status: This manuscript presents methodology and lessons learned that WRRFs can use to implement chemical P removal with flexibility for optimization targeting operations, performance, and costs. It also offers insight into potential optimization measures that WRRFs can implement on existing systems that rely on chemical P removal. The project is complete and full-scale results are presented.

Methodology: The collaborative implementation approach adopted by SPR spans from the pre-design to post-commissioning phases of the project, including the following:

Pre-design phase: The project team, including engineering, operations, maintenance, and laboratory staff, conducted a bench-scale testing campaign to define potential chemicals for design through Jar-Testing. SPR completed the Jar-Testing at multiple sampling locations using ferric sulfate (or ferric chloride) and alum.

Design phase: The next steps included process modeling and a full-scale pilot study. The team shortlisted three dosing points for consideration and finalized two chemicals. The process modeling effort evaluated these points, including one upstream of the primary clarifier (PC) at the headworks, and the other at the influent of the solids contact tanks (SCT). The goal was to define the optimal dosing strategy by adjusting dosage distributions between headworks and SCTs. The pilot study used existing odor control chemical storage and dosing facilities to validate the modeling effort and assess plant operational impacts using a single dosing point. The design adopted a dual point strategy and incorporated flexibility for dosing two chemicals in the two selected locations.

Operation phase: After commissioning, the team conducted a full-scale optimization study using dual dosing points for two scenarios. For scenario 1, a fixed minimum ferric sulfate dose of 13.5 mg/L was set at the headworks, and a variable alum dose at the SCTs. For scenario 2, the doses were varied in both points. The goal was to determine the optimum chemical combination, dosage, and dose location to meet the target effluent concentrations of 1.0 mg/L and 0.7 mg/L. In addition, the study investigated optimal removal, operation impacts, chemical volumes, delivery schedules, and costs for forecasting and budgeting.

Results: The key results of this project are summarized as follows:
Pre-design and Design Phases: The full-scale piloting demonstrated that a Ferric Sulfate dose of 35 mg/L and 40 mg/L at the headworks are required to reach effluent TP concentrations of 1.0 mg/L and 0.7 mg/L, respectively (Figure 1). However, process modeling indicated that applying different dosing combinations between the headworks and SCTs with a higher dose at the SCTs would be more effective. The advantages of dosing more to the SCTs include reducing overall chemical dosage requirement (Figure 2), less alkalinity consumption, less primary sludge production, and less solids loading to the digester.

#Operation Phase: The optimization study included a systematic approach to testing dosing strategies for target effluent TP of 0.7 and 1.0 mg/L. Figures 3 and 4 show the optimization study results for scenarios 1 and 2. For scenario 1, alum dosage to the SCT increased stepwise to 35 mg/L to achieve a TP level below 0.7 mg/L. For scenario 2, when ferric dosage was increased to 19 mg/L at the headworks and alum dosage was reduced to 15 mg/L at the SCTs, the effluent TP increased to above the target. Further stepwise reductions to both coagulants resulted in effluent TP concentrations higher than 1.0 mg/L. The results confirmed the design hypothesis that applying different dosing combinations between headworks and SCTs with a higher dose at the SCTs is more effective in terms of chemical use.

#Costs: A cost analysis was conducted primarily considering the cost of chemicals used. Table 1 shows the cost analysis and cost efficiency per unit mass of P removed for each P target and scenario. For the effluent TP target of 0.7 mg/L, a higher cost was observed for similar dosage of the chemicals for the dual point chemical dosing strategy (scenario 2) compared with higher coagulant dosage in SCT influent (scenario 1). On the contrary, for the effluent TP goal of 1.0 mg/L, the dual point dosing of similar dosage of coagulants (scenario 2) resulted in a lower cost than higher dosage at SCT influent (scenario 1). Overall, the dual dosing strategy resulted in 25% to 29% chemical costs savings in comparison with the single point dosing piloted during the design phase.

Summary: SPR successfully used a holistic and collaborative approach to design, implement, and optimize its chemical P removal process that showed important operational impacts.

This paper was presented at WEFTEC 2025, held September 27-October 1, 2025 in Chicago, Illinois.

Presentation time

13:30:00

14:00:00

Session time

13:30:00

15:00:00

SessionFundamentals of Physical and Chemical Phosphorus Removal

Session locationMcCormick Place, Chicago, Illinois, USA

TopicOptimization of Municipal Facility Operations

Author(s)

Williss Kohr, Zeena, Arabi, Sara, Machado, Chris, Trujillo, Shelley, Fang, Yuan

Author(s)Z. Williss Kohr1, S. Arabi2, C. Machado2, S. Trujillo2, Y. Fang2

Author affiliation(s)South Platte Water Renewal Partners1, Stantec Inc.2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2025

DOI10.2175/193864718825159973

Volume / Issue

Content sourceWEFTEC

Word count11

Description: Purpose Driven Work: Utilizing the motivation behind the new workforce to build a...

Purpose Driven Work: Utilizing the motivation behind the new workforce to build a successful internship program

Abstract

The intern program at South Platte Renew was formalized in 2019 to highlight the work of professional engineers in the water/wastewater field. Over the last four years, the program has evolved into a comprehensive experience for junior to senior level university and college engineering students to showcase the diverse engineering and problem-solving opportunities available within a professional setting. Experiences range from construction inspection, design and planning, budgeting, technical and process optimization, lab work, and piloting treatment technologies. In providing several resources and contacts, students can explore different facets of the wastewater field and experience the unique offerings from a public entity. Along with classic engineering technical skills, interns are shown the political drivers and challenges that must be considered when working as a government agency. Students are also given the opportunity to grow their professional networks, complete vital trainings, and build resumes with applicable experience. The main goal of the SPR Intern Program is to engage young engineers in the diverse work of wastewater in hopes to inspire them to join the workforce in the public sector. This presentation highlights the efforts and methods by SPR to build and maintain an intern program and how others can benefit from a university-to-organization pipeline. The framework and planning documents can be customized for the 3-month internship based on the organization's needs and focuses for that year.

This paper was presented at the WEF/AWWA Utility Management Conference, February 13-16, 2024.

SpeakerWilliss Kohr, Zeena

Presentation time

13:30:00

15:00:00

Session time

13:30:00

15:00:00

SessionWorkforce Development: A Focus on Operators

Session number05

Session locationOregon Convention Center, Portland, Oregon

TopicWorkforce Development / Succession Planning / Knowledge Management

Author(s)

Williss Kohr, Zeena

Author(s)Z. Williss Kohr1, A. Schroeder1

Author affiliation(s)South Platte Renew 1; South Platte Renew 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Feb 2024

DOI10.2175/193864718825159242

Volume / Issue

Content sourceUtility Management Conference

Word count17

Utilizing the Motivation Behind the New Workforce to Build a Successful Internship Program through Purpose Driven Work

Abstract

Over the past five years, the demand for engineers has surged across public and private sectors. Yet, recent graduates and early-career engineers often favor private roles, overlooking the contributions of public sector engineering. In response, the South Platte Renew (SPR) Engineering Division initiated an intern program to bridge this gap and highlight the rewards of public sector work. The SPR Engineering Intern Program aims to broaden budding engineers' horizons, providing hands-on experience in real-world problems within the public sector. The program emphasizes the Owner's viewpoint, addressing fiscal responsibility, direct impacts on waterways, political drivers, and purpose-driven leadership. Crafted over three to four months, the internship allows students to explore operations, facility maintenance, construction inspection, project management, and the multifaceted nature of engineering roles. The methodology involves insights from former interns, workshops aligning needs, and strategic scheduling connected to SPR's Vision, Mission, and Values. The internship includes technical projects, communication-focused activities, and emphasizes professionalism. Feedback sessions and dialogues with recent graduates contribute to continuous improvement, ensuring the program meets interns' expectations. To boost interns' profiles, the program includes resume-building opportunities. This not only enhances immediate job prospects but empowers interns to navigate future careers successfully. Training sessions cover treatment, communications, safety, and construction practices, aligning with SPR's specific needs. The program's success is evident in tangible job placements within the water renewal sector, addressing the need for new talent as professionals retire. The growth in interns and partnerships with universities signify the program's positive reputation. The internship's strategic alignment with SPR's mission ensures lasting contributions. Interns engage in tasks beyond the internship, such as asset management data analysis, construction inspection, and sustainability analyses, aligning with SPR's long-term goals. In the broader context, the SPR Engineering Division's internship program serves as a model for integrating academia and industry, addressing the growing demand for engineering talent in the public sector. The program's evolution reflects adaptability, ensuring continuous improvement in shaping the next generation of engineers. Looking ahead, the SPR team aims to refine and expand the internship program. By fostering partnerships, enhancing the curriculum, and incorporating feedback, SPR aims to sustain the positive impact of its internship initiative. The program not only contributes to individual engineers' growth but plays a pivotal role in advancing SPR's mission and vision. The SPR Engineering Intern Program serves as a beacon for integrating academia and industry, a catalyst for developing future engineers, and a testament to the enduring value of public sector contributions in engineering.

The South Platte Renew intern program highlights the work of professional engineers in the public wastewater field. The program evolved into a comprehensive experience for students, showcasing the different facets of engineering in the wastewater field, and the unique offerings as a public entity. Interns are shown the political drivers and government challenges; emphasizing the direct impact to the local community with the goal to inspire young engineers to enter the public sector.

SpeakerSchroeder, Anna

Presentation time

09:00:00

09:30:00

Session time

08:30:00

10:00:00

SessionBuilding the Workforce of Tomorrow

Session number513

Session locationRoom 238

TopicBusiness Organization and Technology Transformation, Intermediate Level, Utility Management and Leadership, Workforce Issues

Author(s)