State Point in Action: Evaluating Assumptions and Making Operational Decisions

INTRODUCTION The process engineering group of Clean Water Services (CWS) is working side by side with Operations to develop and validate tools that can help in making day-to-day decisions to operate the treatment facilities, and to plan for future conditions. This work requires an understanding of the level of risk compounded in process assumptions. State Point Analysis (SPA) is a simple but well-loved tool that is used across the industry to evaluate secondary clarification capacity. CWS has historically used SPA to establish secondary expansion timelines, and more recently, to routinely evaluate clarifier operation. With active validation of assumptions and consistency of use, this tool has been useful for Operations to create protocols and assess risks, despite its simplicity and limitations. CWS's process engineers work to make sure the tool can actually indicate risk under conditions that are changing such as flows, settling characteristics, nitrification requirements and underflow pumping capacity.

OVERVIEW The Rock Creek Water Resources Recovery Facility (WRRF) is one of four CWS treatment facilities. It is a 37 MGD plant operating six secondary trains rated for 125 MGD peak day flow, with an option to divert to high-rate clarification. In recent years, managing the clarifiers has become critical as flows increased and settling worsened. Filamentous outbreaks occur more often in winter and spring than 5+ years ago, and a high mixed liquor suspended solids (MLSS) inventory (~3000 mg/L) is maintained year-round to ensure nitrification to meet effluent ammonia limits. Return activated sludge (RAS) flow requirements have increased to prevent blanket rise under current process conditions. Compared to design assumptions, the current conditions have higher influent flows, higher MLSS inventories and, at times, poorer settling. The RAS system was designed for a RAS ratio of 30%, but operations now require a RAS ratio of 60% for typical operation. This deviation between operational need and design assumptions has caused the RAS pumping system to be operated without redundancy. This creates an operational risk that must be actively managed while the RAS system capacity is expanded. Asset failure during these periods of risk would reduce secondary treatment capacity, impacting downstream units and compliance. It is even more critical for Operations to have a tool to test changes to flow distribution and basin configurations before challenging events, to establish operating procedures and contingency plans.

STATE POINT TOOL DESCRIPTION AND USES The SPA used by CWS is a typical graphic representation of a solids balance around the clarifier developed by (Daigger, 1995). It plots the solids flux to the clarifier against the solids concentration. The flux curve is constructed using a relationship of Sludge Volume Index (SVI) to Vesilind parameters that have been measured in-house by conducting settling column tests at the Rock Creek WRRF. To expand the SPA tool's functionalities, an MLSS calculator was added to test aeration basin configurations and solids inventory impacts. Figure 1 shows the spreadsheet interface. Operators can simulate MLSS dilution by changing aeration basin configurations, and visualize clarification impacts in the SPA tool. SPA tool uses at CWS's facilities:
• Annual review of secondary expansion timelines: o Assess clarification performance for future plant capacity evaluations using projected MLSS inventory and peak flows. o Evaluate changes in Vesilind parameters or SVI and their effect on timelines.
• Day-to-day operations:
o Adjust flow distribution when SVI or MLSS differ between basins or RAS pumping is limited.
o Prepare for high flows, which can rise very quickly with large rain events, by simulating flow distributions and basin configurations (e.g., step-feed, contact stabilization) to manage solids and mitigate risk of lack of RAS pump redundancy.

VALIDATION OF VESILIND PARAMETERS The usability of SPA depends on representation of full-scale settling through the solids flux curve, which is impacted by measurements of SVI and settling column Vesilind parameters. SVIs are easy to measure daily but may be less accurate than settling column tests, which are more difficult to conduct routinely. The relationship between these can also change over time. More than 30 settling columns tests have been conducted in 2024 and 2025 to obtain a new empirical relationship of Vesilind parameters to SVI. As shown in Figure 2, this is compared to a 2014 relationship that was used for all previous planning and operational analysis. The 2014 relationship is not conservative and does not indicate appropriate levels of risk under more recent operational conditions (Figure 3). The risk of using these relationships to build the solids flux curve is daylighted in Figure 4, where different events of clarification/thickening failure are represented in SPA.

TAKEAWAYS
• Because of their simplicity, tools such as SPA are readily available for operators to simulate clarification scenarios and develop procedures, while being a learning resource for newer generations.
• The limitations of SPA and risk factors can be understood with consistency of use and ability to compare results to full-scale operation.
• Recurring validation of key assumptions (i.e. Vesilind parameters) is recommended to be confident in the tool's ability to predict issues.