Unlocking Resource Intensification through Full-Scale Densification and PdNA

ABSTRACT
The Fairfax County Noman M. Cole Pollution Control Plant (NCPCP) is a 67 MGD facility using primary, secondary, and tertiary treatment to meet effluent limits of 3.0 mg N/L for total nitrogen (TN) and 0.18 mg P/L for total phosphorus (TP). Historically, nitrogen removal relied on secondary step-feed activated sludge tanks (ASTs) and a tertiary denitrifying moving bed biofilm reactor (MBBR) with methanol, operated in conventional modes with high resource demands for units, methanol, and energy. In the past three years, Fairfax County has implemented process intensification by introducing full-scale Densified Activated Sludge (DAS) in the secondary BNR system without external selectors like hydrocyclones and Partial Denitrification Anammox (PdNA) in the tertiary MBBR. DAS has lowered the Sludge Volume Index (SVI), reducing the need for ASTs and clarifiers while maintaining nitrification. PANDA has cut carbon and energy use without affecting effluent quality. These innovations have improved both efficiency and sustainability in the county's wastewater treatment operations.

INTRODUCTION | BACKGROUND
Densified Activated Sludge (DAS) and Partial Denitrification/Anammox (PdNA) are two examples of resource intensification processes have the potential to result in capacity, chemical and energy savings at water resource recovery facilities (WRRFs). In the DAS process, the ability to provide sufficient biological selection by operating at specific feast-to-famine (F/F) ratios that have been shown to be crucial for achieving sludge densification and aerobic granulation (Sun et al., 2021). The proper F/F ratio in the secondary is controlled via step-feed ratio and SRT control. The step-feed ratio is modified to increase flow to the upfront passes of the Activated Sludge Tanks (ASTs) to provide more feast condition and SRT/MLSS is maintained at reasonable levels for nitrification while preventing long-SRT forming filamentous microogranisms (Kent et al., 2018). Both operational parameters increase kinetic selective pressure and improve sludge volume index (SVI) in the ASTs. Successful Densification has been documented at full-scale. In the PdNA process, Partial ammonia oxidation is controlled to achieve a specific NO3/NH3 ratio (Wang et al., 2024). Partial denitrification driven by supplemental carbon addition (e.g., methanol) is then used to stably generate NO2--N. Anammox can then remove both nitrite and ammonia to achieve effluent NH3 and TN limits. Successful PdNA has been documented in pilot and full-scale.

METHODOLOY
Secondary AST Setup
The AST system (Figure 1) includes three large (~4.9 MG each) and six small ASTs (~1.7 MG each). The large ASTs operate in densification mode with a six-pass step-feed flow, where five passes receive primary effluent (PE) (Figure 2). Alternating anoxic and aerobic zones are equipped with mixers, diffusers, DO probes, and airflow control valves. PE flow, DO, and airflow are regulated by operator setpoints. Supplemental carbon is fed via flow-paced methanol dosing. SRT and MLSS are managed by adjusting the waste activated sludge (WAS) rate and basins in service. Densification was achieved through step-feed and wasting rate adjustments without hydrocyclones.
Tertiary MBBR Setup
The MBBR system (Figure 1) consists of six trains, five with fixed-film media. Each train has two anoxic denitrification cells (~2.4 MG total) and one re-aeration cell (~0.6 MG). Anoxic cells use mixers and flat screens; re-aeration cells have medium bubble diffusers and cylindrical screens. K1 media support anammox, denitrifying, and nitrifying bacteria. Methanol dosing is nitrate-paced, and influent NO3/NH3 ratios are maintained through secondary process adjustments, including step-feed, DO, airflow, load-paced equalization, and methanol feed adjustments.

RESULTS
Fairfax County has been deploying PdNA since Q3 of 2022 and coupled DAS with PdNA since Q2 2023. Results have indicated the following:
DAS in Secondary ASTs
- Historically, SVI fluctuated between 110—140 mL/g due to low kinetic selective pressure, which was addressed by adjusting step-feed distribution and reducing MLSS.
- Increasing step-feed to Passes A and C reduced SVI from over 100mL/g to ~80mL/g (Figure 3a). Lowering MLSS during stable summer flow further reduced SVI from ~110mL/g to 70mL/g (Figure 3b).
- Higher upfront step-feed and reduced MLSS raised the F/M ratio, enhancing feast-to-famine conditions and improving sludge settleability.
- SVI was negatively correlated with particles >212 µm (Figure 4), indicating that more granules under improved feast-to-famine conditions enhanced settleability.

PdNA in Tertiary MBBRs
- The facility consistently maintained effluent ammonia below 0.5 mg/L (Figure 5), with PdNA achieving anoxic ammonia removal of 1—2 mg/L (Figure 6).
- Monthly effluent TN ranged from 2—4 mg/L, averaging 2.8 mg/L over 28 months (08/22—12/24) (Figure 7).
- Optimized PdNA reduced methanol use by ~20% (Figure 8) and energy demand by ~5% compared to 2021—2022 operations.

Cumulatively, this work is the first to effectively demonstrate full-scale Densification with mainstream deammonification via PdNA while meeting stringent nutrient limits. This paper will discuss operational lessons learned and considerations to continuously support DAS/PdNA operation at NCPCP moving forward.