Optimizing Final Clarifiers: From Concept To Demonstrated Performance

This paper will provide an in-depth analysis of the steps required to maximize the performance of existing 145-foot square (squircle) secondary clarifiers utilizing computational fluid dynamics (CFD), field verification testing, operational strategies, and equipment optimization. Monroe County owns and operates the Frank E. Van Lare Water Resource Recovery Facility (FEV) in Rochester, New York which is permitted for 135-million gallons per day (MGD) through high rate secondary treatment with a peak flow through biological treatment of 200-mgd (1,600 gpd/sf). Effluent quality requirements are secondary treatment with an effluent TP limit of 1.0 mg/l. The activated sludge is typically operated at a solids retention time of <4 days. The clarifier equipment has surpassed its useful life and requires replacement, however, FEV has also experienced performance challenges that have made it difficult to meet their New York State Pollutant Discharge Elimination System permit requirements. Monroe County was faced with a choice between constructing additional secondary clarifiers to reduce the surface overflow rate of the existing units or implementing improvements to maximize the performance of the existing secondary clarifiers. MCDES is under an order on consent with New York State Department of Environmental Conservation which includes a compliance schedule requiring improvements to the aeration system and the secondary clarifiers be completed by December 31, 2026. Monroe County elected to proceed with improvements to the existing secondary clarifiers with the following Project goals: Improve permit compliance; Maximize wet weather flow through the activated sludge process while maintaining permit compliance; Increase underflow concentrations; and Improve solids capture from the clarifiers. The project was implemented in two phases. Phase 1 included a trial of the improvements in one clarifier (the Test Clarifier) early in the project so that key features could be tested and optimized at full scale before being implemented in the remaining five clarifiers. This phased approach quickly and cost effectively yielded optimized modifications in the Test Clarifier that could be confidently repeated in Phase 2, reducing the risk of costly adjustments to enhance performance being required in six clarifiers compared to only the Test Clarifier. An existing secondary clarifier at FEV is shown in Figure 1. Phase 1 began with the development of a CFD sector model to represent a typical existing secondary clarifier at the FEV. Results of historical field testing performed in October 2016 were used for model validation based on velocity estimates, drogue results, vertical solids profiles, and influent and effluent solids concentrations. Based on the results of initial CFD simulations, small changes to model parameters that control turbulence levels and solids settlement were made to refine the model results. This calibration improved the model's ability to predict patterns resulting from design changes. The results of the sector model showed that it was able to accurately depict existing conditions within the secondary clarifiers and, therefore, could be used to simulate the ability of proposed clarifier improvements to improve performance as compared to the existing conditions. The following potential improvements were then evaluated: Energy Dissipating Inlet (EDI); Cylindrical Baffle; and Effluent Trough and Weirs– Placement and spacing variability. The design components with the best performance based on the results of the CFD modeling are summarized in Table 1. CFD modeling of the corner effluent troughs and Stamford baffles did not indicate any measurable improvement in effluent performance and therefore these components were not included in the Test Clarifier design. Figure 2 and Figure 3 show project CFD modeling results. Following initial operation, adjustments were made in the Test Clarifier which included removing the bottom 1-foot of the cylindrical baffle to provide a 1-foot floor gap and increasing the distance between the bottom of the draft tubes and floor. While the increased distance between the draft tubes and floor improved the maximum RAS flow from the clarifier, the 1-foot floor gap had no measurable improvement. Following the modifications, field verification testing performed on both the Test Clarifier and an existing secondary clarifier confirmed that both performed well under all flow conditions, with the Test Clarifier showing better solids retention performance at high flow rates due to having 2X lower density currents than the remaining clarifiers. However, the squircle corners of the Test Clarifier showed a loss of solids at low and high flows. A drone was used to collect visual observations during the testing, as shown in Figure 4. Table 2 summarizes the effluent TSS results of the first round of Field Verification Testing, Figure 5 includes the dye curve results, and Figure 6 and Figure 7 show the drogue measurements from existing secondary clarifier and Test Clarifier, respectively. Based on these results, additional Test Clarifier optimization improvements were developed which included adjusting the plow blade configuration, increasing the draft tube size, and changing the draft tube location. The Test Clarifier was modified to incorporate these improvements and field testing was repeated. While the performance improved, the solids loss near the corners was still a concern. Therefore, Stamford baffles were installed along the sidewalls of the clarifier and lattice baffling was installed tangential to the scum baffle in the corners of the clarifier. Figure 8 shows the Test Clarifier following installation of the Stamford and lattice baffles. Additional testing following the installation indicated a consistent improvement in effluent quality of the Test Clarifier compared to an existing secondary clarifier. The testing results are shown in Table 3 and Figure 9. Phase 1 of the Project has been completed, and Phase 2 is in design with an anticipated Fall 2022 start of construction. The overall success of this project is due to the lessons learned in Phase 1, which had a significant positive impact on Phase 2 in the selection of final design components and by avoiding unnecessary expenses. By implementing the Test Clarifier and limiting the optimization modifications to only one of the six existing clarifiers, an additional expense to the project of approximately $4 million was avoided. This $4 million in avoided costs would have equaled an approximately 36% increase to the original total project budget. Additionally, while it was time consuming, performing the modifications incrementally with field verification testing between each modification was essential in understanding which modifications yielded positive results suitable of being incorporated into the subsequent clarifiers.