Advancing PdNA: Development of Sensors and Control for Next-Gen Nutrient Removal

Introduction: Mainstream shortcut nitrogen (N) removal is a game changer not only for lowering aeration energy, and carbon dosing but also increasing treatment plant capacity and carbon capture efficiency in Water Resource Recovery Facilities (WRRFs). Partial denitrification–anammox (PdNA) offers shortcut N removal including reduced aeration requirements for nitrification, less supplemental carbon for denitrification, decreased alkalinity consumption, lower biomass production, and a reduced carbon-to-nitrogen (C/N) ratio. Mainstream anammox systems can substantially reduce aerobic solids retention time (SRT) because only a fraction of incoming ammonia requires aerobic oxidation. Presence of significant organic carbon in wastewater influent and successful low DO operation at WSSC Water's Seneca WRRF is the basis for analysis of the benefit of shortcut nitrogen removal from this study. This study aims to establish design criteria for both pre- and post-anoxic PdNA configurations and to define optimal control and operation strategies, enabling utilities to efficiently test this innovative treatment technology. Methodology: WSSC Water's Seneca WRRF – a 26 MGD, low DO WRRF – is testing PdNA approaches through pilot testing with the goal of reducing aeration by 50% and external carbon dosing by 80%. The pilot kicked off in November 2025 and operated with anammox media beginning in January 2026. During six months of piloting with anammox media, testing schedule was classified into three phases including startup, post-anoxic and pre-anoxic mixed liquor (ML) feeding from full scale aeration basin. Startup phase focused on selecting the feed point, hydraulic testing, and verifying sensors. The post‑anoxic phase centered on operating the pilot under low and high ammonia residuals with different aeration strategies. During the pre‑anoxic phase, the pilot ran PdNA‑only while shifting the feed point to various pre‑anoxic zones. The pilot consists of three ~80-gallon tanks for aeration and PdNA zones along with the capability to dose external carbon, ammonia and nitrate as additional levers to test nutrient removal capacity and efficiency with a flowrate of 3.3 gpm (Figure 1). The pilot reactor is continuously fed with ML with the capability to pull from various points within the mainstream BNR reactors (AT2) to mimic a pre-anoxic or post-anoxic configuration (Figure 2). The PdNA tank was seeded with anammox-populated plastic media from the HRSD's James River Treatment Plant. Real-time sensors (flow, DO, ammonium, nitrate, dissolved nitrous oxide, temperature) control and monitor system performance across a matrix of loading rates, aeration strategies, and carbon doses. The pilot applies advanced aeration control strategies (e.g., ABAC, AvN, DO setpoint) to identify practical approaches that optimize energy for aeration and chemical consumption. Experimental plans are designed based on constant and diurnal feed. Nutrient profiles (ammonia, nitrate, nitrite) were evaluated for the pilot to explore the removal of N by PdNA. Continuous monitoring of influent, intermediate, and effluent NH₄, NO₂, NO₃, DO, temperature, and N₂O fluxes is paired with high throughput sequencing of biomass. Moreover, the partial denitrification activity was analyzed to quantify the biomass retention and distribution on the media. The grab sample analysis quantifies internally stored carbon, soluble COD providing a comprehensive dataset to evaluate nitrogen removal efficiencies and greenhouse gas impacts. Results and discussion: Prior to PdNA pilot startup, ammonia and nitrate profiles were measured throughout the full‑scale Seneca BNR tank (AT2), and location OX‑2 was selected as the mixed‑liquor source based on ammonia‑to‑nitrate ratio of 3:1 (Figure 3) for baseline period. Throughout pilot operation, sensor readings were continuously verified against laboratory analyses, with weekly single‑point calibrations performed as needed. Strong agreement between lab and sensor data (Figure 4) confirms that sensors provide reliable measurements to support real‑time control of pilot operations. This consistency enabling the control strategy to adjust operational parameters readily based on sensor readings to optimize performance and ensure stable pilot operation. Figure 5 showing progressive transformation of ammonia as the wastewater moves through the treatment train. Ammonia concentrations are highest in the upstream tanks (AT2 and HT), indicating high loading to the aerobic zone. Substantial ammonia removal occurs in AER1, followed by near-complete oxidation in AER2, where nitrate peaks as the primary nitrification product. This nitrogen profile highlights the need to adjust influent ammonia availability to the PdNA zone. In the next testing phase, with media addition, aeration controls modified to maintain a higher ammonia load entering the PdNA reactor. These adjusted conditions allow the pilot to evaluate and demonstrate the activity of the anammox bacteria across different operational scenarios, supporting optimization of the PdNA process Conclusion:This research effort will conclude with a comprehensive design summary to empower stakeholders with the guidance necessary to effectively integrate PdNA into both existing and new facility designs. This study serves as part of WRF 5183, building upon its outreach initiatives and collaborative information sharing to further enhance the dissemination and technology of PdNA.