Addressing Controllability Challenges in Aeration Systems: Insights from Blue Plains

Introduction
Advanced aeration control strategies, including DO control, ABAC, and AvN control, are critical for optimizing biological nutrient removal systems1. Despite their potential, confidence in these systems is often lost due to limitations and complexity2. Nearly 50% of wastewater facilities still operate in manual mode, bypassing basic PID control loops3. This highlights the need for online evaluation tools to identify issues related to tuning, probe maintenance, or control logic performance. This study evaluates DO control, ABAC with black-box and mechanistic feedforward approaches, and introduces Nitrate-Based Aeration Control (NBAC), aiming to establish a framework for real-time assessment and adjustment of aeration controls.

Methodology
Blue Plains operates 12 nitrification reactors with a daily flow of 300 MGD. Odd-side reactors use DO control, while Even-side reactors use ABAC (Fig.1). ABAC and DO control were evaluated over 267 days and 153 days, respectively. Weekly sampling assessed probe reliability and maintenance demands. Control performance was evaluated using setpoint accuracy, air valve limitations, and laboratory corrections. Setpoint accuracy measured the percentage of time setpoints were met within error thresholds. Physical limitations were identified by valve positions outside controllable ranges, and probe offsets were analyzed through laboratory correction. An integrated scoring metric reflected the percentage of time each criterion was met.

Results and Discussion
1.Overall Performance of the Control Systems
ABAC, operating with a setpoint of 2.0 mgN/L for NH4, demonstrated better performance compared to DO control, maintaining error margins within 30% thresholds (Fig.2). These margins were calculated as probe readings within 30% of the setpoint, detailed in Fig.2. In contrast, DO control systems, with setpoints ranging from 1.5 to 2.5 mgO2</Sub>/L, exhibited variability across zones, except for Zone 2B (Fig.2). Although NH4 concentrations remained below 0.5 mgN/L following DO control, the observed inconsistency suggests reliance on airflow and air valve control may not achieve precise control under dynamic conditions. Notably, Zone 2B displayed enhanced controllability following air valve maintenance in mid-Sep., highlighting the role of equipment upkeep in ensuring system performance.

2.Control System Performance Within Operational Limits
Physical limitations play a crucial role in achieving system performance. Positions below the controllable range are uncontrollable due to insufficient airflow, while those above the inflection point yield diminishing returns, reducing control precision. The DO control system demonstrated a lower percentage of time within the controllable range compared to ABAC, with Zones 1A and 2A exceeding the upper limits 77.8% and 63.2% of the time, indicating a reliance on fully open valves to meet airflow demands (Table1). In contrast, Zone 2B in the DO control system achieved setpoints more effectively when valve positions remained within the controllable range, as evidenced by the green data points in Fig.2D. The ABAC system outperformed DO control overall (Table1); however, instances where valve positions exceeded the upper limit (Fig.2A) hindered ABAC's ability to meet NH4 setpoints.

3.Control performance Considering Lab Correction
The reliability and maintenance demands of NH4 probes are critical for achieving setpoint accuracy in ABAC. Over the 267-day, ABAC required significant maintenance, including 14 matrix adjustments and electrode replacements (Fig.3A). While ABAC generally outperformed DO control in meeting setpoints, lab corrections revealed fluctuations due to probe inaccuracies (Fig.2A&3B). Scoring metrics showed 74% accuracy within a 10% error margin, rising to 82% within controllable airflow limits but falling to 29% with probe offsets (Fig.4). In contrast, DO control showed lower setpoint achievement due to physical limitations (Fig.4). Despite these constraints, DO probes demonstrated higher accuracy compared to NH4probes, with potential accuracy improvements from 24% to 72% within a 10% margin if physical limitations are resolved. Integrated scoring tools highlighted discrepancies between hourly and daily ABAC data, mainly driven by secondary effluent flow rates and air valve control (Fig.4&5). Fig. 5 illustrates Black Box ABAC control performance at Blue Plains, where valve positions were well-controlled between 0 to 11 AM, adjusting to NH4 loading. However, during high flows, the system often exceeded its capacity to open valves further, resulting in compromised setpoints. Comparisons between hourly and daily results in ABAC indicate that hourly real-time data reveal the need for improvements to handle flow rate dynamics (Fig.4A&B).

To address these challenges, novel aeration controls have been proposed. Mechanistic Feedforward ABAC integrates predictive modeling with feedback controls to anticipate system behaviors and adjust aeration proactively, reducing reliance on error-prone probe feedback5. Additionally, due to the unreliability of NH4 probes, a novel NBAC has been developed, incorporating a high-accuracy Wet Chemical Analyzer for NH4 feedforward signals and a UV NO3probe for feedback.

Conclusion
This study evaluates controllability to identify key constraints in aeration control and proposes strategies and solutions. The full paper will present proof-of-principle NBAC.