Developing a Digital Protocol to Optimize Biological Nutrient Removal Performance

Introduction
Recent advancements in sensor technology, automation, and the decreasing costs of digital technologies have significantly accelerated the development of digital solutions in wastewater treatment. When implemented effectively, these solutions prioritize insightful analysis over sheer data volume, concentrating on the importance of measuring data at the most strategic locations. This paper presents a distinctive combination of operational expertise and theoretical foundation behind a novel digital optimization control strategy for Biological Nutrient Removal (BNR). This strategy is informed by over 35 years of operational experience at full-scale BNR facilities, with specific proof-of-concept trials carried out at the Westside Regional BNR facility.

In 2019, WERF published the report 'Optimization and Design of a Side-Stream EBPR (S2EPR) Process.' A survey of six full-scale facilities found the Westside Regional BNR Facility achieved the lowest and most stable secondary effluent orthophosphorus concentrations among all plants studied.

Building on the findings of the WERF study and their evaluation of the Westside Regional WRF facility, an assessment of the facility's operational parameters was carried out to pinpoint the control strategies responsible for its exceptional performance. A standard operating procedure (SOP) was created and stress tested to evaluate how specific setpoints influenced overall effluent quality and interplay between key parameters. The developed SOP can be incorporated into a digital optimization strategy available for those utilities looking to improve BNR performance.

Approach
Over the course of a year, an optimized BNR control philosophy was developed by examining the interactions among several key operating parameters. The resulting control strategy achieved stable effluent PO4 concentrations below 0.1 mg/L without chemical use by establishing interdependent controls for the following:
1. Automated ammonia based SRT control
2. Automated ammonia based aeration control (ABAC)
3. RAS control for sludge blanket and underflow PO4/NO3 refinement
4. Automated carbon diversion for nutrient and energy optimization
5. Automated control of anaerobic zone HRT based on PO4 release

Discussion
Most treatment facilities in the Okanogan region must meet an effluent total nitrogen < 6 mg/L and total phosphorus < 0.25 mg/L. As a result, maintaining low effluent ammonia is an indirect requirement to achieving the total nitrogen limit. Therefore, the purpose of the automated ammonia SRT controller was to provide sufficient removal of ammonia, without an excessive aerobic SRT which in-turn leads to foaming, endogenous release of ammonia, and other unfavorable conditions. The automated ammonia SRT controller was set-up and based on real time effluent ammonia results. Setpoints were such that if the daily average ammonia concentration increased above 1 mg/L, the controller would increase SRT by 1.0% each day. If the daily average ammonia concentration dropped below 0.5 mg/L, the SRT controller would decrease SRT by 1.0% each day. The gradual increase and decrease of SRT, controlled by effluent ammonia, led to a high-rate process, which in turn facilitated an efficient subsequent step for phosphorus removal.

In BNR facilities, controlling the secondary clarifier sludge blanket level is crucial to prevent the contamination of RAS nitrates from returning to the anaerobic zone. Clarifier sludge blankets are capable of reducing nitrates by more than 5 mg/L depending on operation. However, too much NO3 removal can result in excessive PO4 release in the blanket and an overall decline in effluent quality. Full scale trials that will be presented in the full paper indicated that balancing NO3 reduction with PO4 release in the sludge blanket is an important control parameter. These full scale trials indicated that the optimum PO4 release in the blanket was in the 1-2 mg/L range. An indicative scan is shown in Figure 1. The concentration of NO3/PO4 in the sludge blanket was used as a control strategy for the RAS rate. When RAS PO4 release was greater than 2 mg/L the RAS rate was increased, and when less than 1 mg/L the rate decreased.

A control algorithm was developed to ensure consistent PO4 release in the anaerobic zone. Operational experience revealed that when PO4 release in the anaerobic zone exceeded 25 mg/L, it became challenging to effectively uptake such a high concentration of PO4 in the downstream aeration zones. Excessive release was an indication that carbon/VFA's would be more beneficially used for denitrification, and triggered diversion of high VFA fermentate to the downstream anoxic zone. The result was an optimized use of carbon for both phosphorus removal and total nitrogen removal.

Effluent ammonia was integrated into ammonia-based aeration control to complement the automated SRT controller. When effluent ammonia fell below 0.7 mg/L, the final cell's aeration D.O. setpoint was reduced to 0.5 mg/L. If ammonia rose to 0.8 mg/L, the D.O. setpoint increased to 1.0 mg/L.

Finally, controlling anaerobic zone HRT was critical, as excessive HRT not only released PO4 but also ammonia. This ammonia release ultimately necessitates oxidation to nitrate and subsequent denitrification, lowering plant efficiency. Further details will be provided in the full paper due to space constraints.