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

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.