Li, Cyrus

Cyrus Li is a Ph.D. student in Biological Systems Engineering at Virginia Tech researching municipal wastewater process intensification. His research focuses on utilizing onsite pilot systems to evaluate densified activated sludge, low dissolved oxygen operation, and shortcut nitrogen removal as strategies for expanding treatment capacity of secondary processes at WRFs. Outside the lab, Cyrus enjoys the outdoors, photography, and travel.

Titles from this speaker

Description: Arrested Anaerobic Digestion Digestates Uncouple Denitratation from Denitritation...

Arrested Anaerobic Digestion Digestates Uncouple Denitratation from Denitritation During Shift from External to Intracellular Storage Carbon Sources

Abstract

Introduction In recent years, partial denitrification anammox (PdNA) process has been developed in mainstream wastewater treatment as a low-carbon approach for biological nitrogen removal(Du et al., 2019). A prerequisite for effective PdNA process is the stable and reliable production of nitrite through partial denitrification (PdN) (Zhang et al., 2020), which is governed by the balance between denitratation (NO3- → NO2-) and denitritation (NO2- → N2). Volatile fatty acids (VFAs) have been widely exploited as effective electron donors to promote PdN and achieve nitrite accumulation. VFAs are commonly produced from organic waste through arrested anaerobic digestion (aAD) (Giduthuri & Ahring, 2023). Beyond serving as external substrates, VFAs can also be stored as intracellular storage carbon (ISC) polymers (Oehmen et al., 2007), primarily polyhydroxyalkanoates (PHA), with concurrent glycogen turnover. When external carbon is depleted, denitrification may continue by utilizing ISC (PHA or glycogen), potentially reshaping the competition between denitratation and denitritation. While partial denitrification under external carbon addition, especially VFAs, has been extensively studied, the competition between denitratation and denitritation during the ISC-driven phase remains poorly understood. Therefore, the objective of this study was to elucidate how denitratation and denitritation compete across the transition from external carbon-driven to ISC-driven denitrification with VFAs-containing aAD digestates as initial carbon sources. Two types of arrested anaerobic digestion (aAD) digestate, one was predominated by VFAs and the other containing a mixture of lactic acid and VFAs with a COD ratio of 1:0.62, were fed as carbon sources to activated sludge for denitrification dynamic studies. Meanwhile, as a widely used carbon source for PdN as well as the most common VFA component (Ji et al., 2025), pure acetate, was used as a positive control. Moreover, pure lactic acid was used in this experiment to provide counter evidence in support of VFA's role. Methodology Seed sludge was collected from a local municipal wastewater treatment plant. Before adding sludge to batch reactors, it was aerated for 1.5 h to deplete initial ISC. All batch tests were conducted at an initial COD/N ratio of 4, with a fixed initial COD of 50 mg/L. Buffer chemicals and trace elements were dosed identically across all groups before carbon and nitrate addition. Each reactor was purged with pure nitrogen gas for 2 min to establish anoxic conditions before experiment started. The initial mixed liquor volatile suspended solid (MLVSS) was controlled at 250.7 ± 15.2 mg/L. Samples were withdrawn hourly using sterile syringes and needles. Results Along with the denitrification timeline, the peak of nitrite accumulation synchronized with the peak of internal carbon storage until the 4th hour, both occurred when majority of the external carbon was utilized (Figure 1). Before external carbons were depleted, all four groups achieved substantial nitrite accumulation, achieving partial denitrification (PdN) efficiencies of 73.3±2.3%, 42.7±9.1%, 72.1±5.9% and 18.4±11.0% for VFA only digestate, VFAs + lactic acid digestate, pure acetate, and pure lactic acid, respectively. As shown in Figure 2, the concurrent PHA synthesis and phosphate release also observed along the denitratation suggest ISC formation driven by denitrifying polyphosphate-accumulating organisms (dPAOs). The corresponding specific PHA synthesis amounts were 110.8 ± 2.9, 52.5 ± 2.5, 108.3 ± 25.7, and 27.5 ± 2.4 mg PHA/g VSS, with phosphate release of 4.04 ± 0.94, 2.54 ± 0.47, 12.33 ± 3.92, and 1.68 ± 0.20 mg P/g VSS for the aforementioned four types of external carbon sources, respectively. Although there was no external carbon available after the 4th hour, the denitratation and denitritation continued but the rates of the former were only 78%, 86%, 81%, and 89% of the latter. These results suggested that ISC-driven denitrification prefers denitritation over denitratation. The fact that dPAOs lack the respiratory nitrate reductase enzyme (nar) and cannot effectively use nitrate as electron acceptor was regarded as the mechanism responsible for such an uncoupled rates between denitratation and denitritation when denitrifying glycogen-accumulating organisms (dGAOs) are presence (Rubio-Rincon et al., 2017). Thereby, this study determined the different competition performance between denitratation and denitritation under external carbon stage and ISC stage when VFAs and lactic acid are provided as initial carbon sources. Conclusions This study showed a clear shift in denitrification behavior when carbon transitioned from external VFAs to ISC (primarily PHA). During the external carbon phase (0-4 h), VFAs-rich aAD digestate and acetate achieved higher nitrite accumulation and PdN efficiency, while the presence of pure lactic acid impaired its PdN performance. After biodegradable COD was depleted, denitrification continued using ISC, but denitritation exceeded denitratation, indicating that ISC-driven denitrification favors nitrite consumption rather than nitrite production. These results suggest that maintaining stable nitrite supply for PdNA will require strategies to sustain external carbon availability or limit ISC-driven denitritation when utilizing VFA-containing aAD digestates.

This paper was presented at WEFTEC 2026 in New Orleans, Louisiana.

Presentation time

14:30:00

15:00:00

Session time

13:30:00

15:00:00

SessionBalancing Nutrients, Energy, and Emissions in BNR

Session locationErnest N. Morial Convention Center

TopicFacility Operations and Maintenance, Municipal Wastewater Treatment Design, Nutrients

Author(s)

Liao, Zhangtong, Sun, Yewei, Wang, Mingxi, Li, Cyrus, Li, Yebo, Wang, Zhiwu

Author(s)Z. Liao1, Y. Sun2, M. Wang1, C. Li1, Y. Li3, Z. Wang1

Author affiliation(s)Virginia Polytechnic Institute and State University, 1Hazen and Sawyer, 2Virginia Tech, 1Virginia Tech, 1Quasar Energy Group, 3Virginia Tech, 1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160475

Volume / Issue

Content sourceWEFTEC

Word count18

Evaluating Synergistic Intensification Strategies Under Mainstream Conditions

Abstract

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.

This paper was presented at WEFTEC 2026 in New Orleans, Louisiana.

SpeakerLi, Cyrus

Presentation time

11:30:00

11:45:00

Session time

10:30:00

12:00:00

SessionSupercharging Next Gen BNR: Intensification through Smarter Strategic Approaches

Session locationErnest N. Morial Convention Center

TopicFacility Operations and Maintenance, Nutrients, Research and Innovation

Author(s)

Li, Cyrus, Sun, Yewei, Khunjar, Wendell, Rivera, Sofia, Platt, Kathryn, Schlosser, Erick, Hague, Douglas, Steglitz, Brian, Duan, Yufei, Liao, Zhangtong, Sun, Runze, Li, Yitao, Wang, Zhiwu

Author(s)C. Li1, Y. Sun2, W. Khunjar2, S. Rivera2, K. Platt3, E. Schlosser3, D. Hague3, B. Steglitz3, Y. Duan4, Z. Liao1, R. Sun1, Y. Li1, Z. Wang1

Author affiliation(s)Virginia Tech, 1Hazen and Sawyer, 2Hazen and Sawyer, 2Hazen and Sawyer, 2Upper Occoquan Service Authority, 3Upper Occoquan Service Authority, 3Upper Occoquan Service Authority, 3Upper Occoquan Service Authority, 3John Brown University, 4Virginia Polytechnic Institute and State University, 1Virginia Tech, 1Virginia Tech, 1Virginia Tech, 1