Evaluating Synergistic Intensification Strategies Under Mainstream Conditions

Introduction Water resource recovery facilities (WRRFs) are challenged by stringent effluent nutrient limits, rising energy demands, and increased flows driven by population growth. In recent years, process intensification methods such as sludge densification, low dissolved oxygen (DO) operation, and mainstream deammonification have drawn interest due to their ability to increase treatment capacity while reducing chemical usage, energy inputs, and footprint (An et al., 2021; Jimenez et al., 2025; Wang et al., 2024). Although these individual intensification technologies have demonstrated promising benefits, they are typically evaluated in isolation, and limited research has examined the synergistic integration of multiple intensification approaches within a single treatment process under mainstream conditions. To address this gap, a flexible pilot-scale experimental platform was developed to evaluate the combined benefits of densified activated sludge (DAS), low DO operation, and mainstream deammonification. The main objectives of this study are: 1) biological densification driven by feast-famine selection with or without external physical selectors; 2) incorporation of anammox for mainstream shortcut nitrogen removal; and 3) low DO operation to enable simultaneous nitrification–denitrification and internal stored carbon (ISC) for energy efficient BNR. Outcomes from this work will help define key operational requirements, biological selection pressures, and process control strategies to support full-scale implementation at WRRFs. Methods Reactor Configuration: The pilot consists of a 100-gallon train comprising ten continuous stirred-tank reactors coupled with a 50-gallon clarifier and is operated on-site with primary effluent from a 54-MGD WRRF (Figure 1). This configuration enables flexible redox alternation to simulate varying processes, including AO, AOA, MLE, A2O, and 4- and 5-stage bardenpho. It also approximates plug-flow behavior and provides feast–famine cycling to promote biological selection for densification (Sun et al., 2019; Sun et al., 2021) (Figure 2). An optional upflow selector will be included to simulate a hydrocyclone and provide SRT decoupling, enabling further benefits for nitrification and anammox retention. System Automation: A ChemScan wet chemistry analyzer and Hach probes were integrated to support high-resolution nutrient monitoring across the treatment process and for automation-based process control, including nitrate-based nitrate recycle (NRCY) control and ammonia-based aeration control (ABAC). Low DO operation was automated using DO probes and air control valves. Current Results Densification: The pilot was operated for 80 days under an anaerobic/oxic (A/O) configuration without the upflow selector, during which densification was achieved solely through biological selection. Operating with the same wastewater characteristics and temperature as the full-scale facility, the pilot maintained a sludge volume index (SVI) of 80-90 mL/g, comparable to full-scale operation with hydrocyclone. In contrast, the full-scale train without hydrocyclone saw a rapid deterioration in settleability over the same period, with SVI increasing to over 250 mL/g (Figure 3). Pilot aerobic SRT was maintained at ~8 days, consistent with full-scale operation, resulting in similar MLSS concentrations between 3,000 and 4,500 mg/L (Figure 4). Analysis of sludge particle size distribution revealed >20% of the pilot sludge can be classified as granules exceeding 212 μm in diameter (Figure 5). This along with particle strength and extracellular polymeric substances (EPS) fractionation demonstrated that the pilot sludge consistently fell within the characteristic range of DAS (Figure 6). Treatment Performance: The system achieved 76% soluble COD and 82% phosphorus reduction (Figure 7), showing effective effluent carbon removal and enhanced biological phosphorus removal (EBPR). Complete ammonia removal was also maintained (Figure 8). Under the current A/O configuration, nitrate recycled by the RAS stream consistently achieved ~7 mg/L total inorganic nitrogen (TIN) removal, leading to system effluent TIN ranging from 20 to 30 mg/L N. Endogenous Denitrification: A TIN decrease exceeding 10 mg/L was observed across the pilot's last aerobic cell and RAS with minimal ammonia increase (Figure 9). This indicates endogenous denitrification driven by ISC has occurred in the sludge blanket despite the high DO (>2 mg/L) maintained in aerobic cells. Nitrite Accumulation: While NO3- comprised the majority of the NOx at the system effluent with minimal NO2- present, a stable concentration of nitrite exceeding 5 mg/L was observed mid-train during nitrification (e.g., Cell 7) (Figure 9). This can be attributed to the plug-flow nature of the system (Liu et al., 2024) and insufficient alkalinity (~ 225 mg CaCO3/L at PE). However, the ~1:0.86 NH3:NO2- ratio observed is notably a favorable environment for anammox growth, therefore presenting unique opportunities for mainstream partial nitritation anammox (PNA). Next Steps and Ongoing Work Ongoing work focuses on integrating mainstream anammox using different forms of carriers, transitioning to low DO operation, and implementing NRCY and ABAC control using real-time wet chemistry analyses. This work advances the mechanistic understanding of synergistic interactions in intensification strategies and informs practical pathways for full-scale implementation.