Li, Yitao

Yitao Li is affiliated with Virginia Tech and is engaged in research related to water and environmental engineering.

Titles from this speaker

Description: Catalytic Defluorination of PFAS in Municipal Wastewater, Sludge, and Dewatering...

Catalytic Defluorination of PFAS in Municipal Wastewater, Sludge, and Dewatering Filtrate at Mild Temperatures (≤ 80C)

Abstract

INTRODUCTION
Per- and polyfluoroalkyl substances (PFAS) are persistent, bioaccumulative 'forever chemicals' that pose significant environmental and public health challenges due to their resistance to traditional wastewater treatment processes. Current PFAS treatment technologies applied in full-scale water and wastewater treatment processes primarily rely on phase transfer methods, such as ion exchange, adsorption, membrane filtration, and foam fractionation [1, 2], or high-temperature (350-1000°C) destruction processes like incineration, pyrolysis, and supercritical water oxidation [3-5]. However, these approaches are limited by inherent drawbacks, such as high energy consumption, high capital investment and operating cost, and operational complexity, making full-scale, in situ implementation by utilities challenging.

This study for the first time introduces a novel, bio-inspired, transition metal oxide-based catalyst for PFAS destruction at a mildly heated temperature of 80°C, offering a potential energy-efficient alternative. Attributed to the catalyst's low-temperature nature, the process can be integrated into existing heated solids treatment processes, such as thermophilic anaerobic digestion, thermal hydrolysis pretreatment (THP), or pre-pasteurization. The objective of this study is to assess the effectiveness of this catalyst on the destruction of PFAS compounds within the PFAS-accumulating matrices, including municipal wastewater, sludge, and sludge dewatering filtrate. The goal of the current experimental phase is to provide proof of concept and to understand the extent of PFAS destruction in such complex matrices.

Methods and Materials
Samples of treated wastewater effluent, pre-dewatered raw sludge (THP feed sludge), and dewatering filtrate were collected from WSSC Water's Piscataway Bioenergy Facility, a centralized biosolids processing facility that is designed to receive biosolids from six regional water resource recovery facilities (WRRFs), representing a potential full-scale application site for this technology for destroying concentrated PFAS compounds. The experimental design, including the setup of different sample matrices, spiked PFAS concentrations, catalyst doses, reaction temperatures, and reaction time, is shown in Table 1. Briefly, the experiments (Samples 1-36) involved dosing wastewater and sludge samples with synthesized catalyst and performing reactions at 80°C for 24 hours. Additional experiments (Samples 37-42) utilized a pressure vessel (Parr Instrument Company, IL, USA) to simulate full-scale THP conditions at 165°C for 30 minutes. Post-reaction liquid and solid phases were separated and analyzed for PFAS concentrations by following EPA Method 1633.

Results and Discussion Preliminary results of this study, Samples 1-18, are shown in Figures 1 and 2. As can be seen in Figure 1, approximately 57.3% to 69.4% of the total PFAS spiked before the reaction was destroyed. The destruction can be corroborated by the defluorination during the reaction shown Figure 2. It is interesting to observe that the amount of fluoride ion released from the reactions for both types of samples were approximately more than 10 times the stoichiometric release of fluoride (Figure 2). It should be noted that rather than the target concentration of 500 ppb PFOS as shown in Table 1, the actual PFAS concentration spiked into the shown groups before reaction turned out to be approximately 1,100 ppb PFOS with a significant amount of impurities accounting for about 10% of the added PFOS. The average concentration of PFOA+PFOS after the treatment was surprisingly reduced to only about 8.3 ppb, marking a PFOA+PFOS destruction efficiency of over 99% ± 0.03% on average. Furthermore, approximately 10.1% to 19.0% of the total PFAS spiked was adsorbed onto the catalyst. In the liquid phase, approximately 17.2% to 26.5% of the total PFAS were either transformed to other PFAS species identified by EPA 1633 or remained undegraded. It is noteworthy that the PFAS destruction across the sample types were insignificantly different (T-test, p > 0.05), indicating that defluorination in WRRF effluent samples was not affected by the more complex characteristics.

These preliminary findings demonstrate the potential of the studied catalyst for efficient PFAS destruction in wastewater matrices and further provide the confidence for testing samples with more complex constituents. Results from more complex samples, including sludge dewatering filtrate and THP feed sludge (Samples 19-42), will be made available in early 2025 and incorporated in the manuscript.

This paper was presented at WEFTEC 2025, held September 27-October 1, 2025 in Chicago, Illinois.

Presentation time

13:30:00

13:45:00

Session time

13:30:00

15:00:00

SessionRemoval, Fate, and Sequestration of PFAS in Biosolids

Session locationMcCormick Place, Chicago, Illinois, USA

TopicContaminants of Emerging Concern & Trace Organics

Author(s)

Li, Yitao, Leary, Rodney, Shi, Jiaming, Taylor, Malcolm, Wang, Zhiwu, Zhao, Dongye (Don), Fan, Dimin, Zhou, Chao

Author(s)Y. Li1, R. Leary2, J. Shi3, M. Taylor4, Z. Wang1, D. Zhao2, D. Fan3, C. Zhou3

Author affiliation(s)Virginia Tech1, San Diego State University2, Geosyntec Consultants3, WSSC Water4

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2025

DOI10.2175/193864718825160122

Volume / Issue

Content sourceWEFTEC

Word count17

Description: Effect of Thermal Hydrolysis Pretreatment on the Friability of Thermally-Dried...

Effect of Thermal Hydrolysis Pretreatment on the Friability of Thermally-Dried Digested Biosolid Pellets

Abstract

Louisville MSD plans to implement a new biosolids processing system consisting of a thermal hydrolysis pretreatment (THP)-enhanced anaerobic digestion (AD) process and rotary drum dryers at its 120-MGD Morris Forman Water Quality Treatment Center (MFWQTC), the largest wastewater treatment facility in Kentucky, USA. The new biosolids processing system will receive thickened primary sludge (TPS) and thickened waste activated sludge (TWAS) from the MFWQTC, as well as imported wasted activated sludge (IWAS) cake from other regional treatment facilities. Friability refers to the tendency of a material to fracture and breaking into smaller fragments under external pressure. In the case of biosolids, such external pressure could result from agitations during rotary drum drying, conveying during processes, and transportation, which ultimately leads to dryer inefficiency, dust generation, and poor product quality for land application. It has been reported that some full-scale installations experienced challenges with drying THP-AD sludge due to the high friability and dust issues within the dryer (Spalding & Smoot, 2019). However, the friability of dried biosolids and the causes have never been quantitatively measured and investigated. Therefore, this study aims to experimentally evaluate the impact of THP on the sludge drying property (in terms of friability) following anaerobic digestion of TPS, TWAS, IWAS, and blended sludge. To understand the THP effect, the experiment was executed in two parallel batches with Batch 1 as a control with AD only and Batch 2 as an experiment by adding a THP upstream of the AD (Figure 1). Samples of TPS and TWAS collected from MFWQTC were pre-dewatered using a pilot-scale screw press thickening unit and then blended with samples of dewatered IWAS collected from another Louisville MSD-operated WQTC at a dry mass ratio of TPS, TWAS, and IWAS was 2.4 : 2.2 : 1. The pre-dewatered sludge were diluted to 16% total solids (TS) followed by processing through a pilot-scale Cambi THP unit operated at 165 SMƒ for 30 minutes. The pretreated sludge was further diluted to 10% TS and then inoculated with AD sludge collected from DC Water Blue Plains WWTP followed by anaerobic incubation at 36.5 SMƒ until the cumulative biogas production reached a plateau. The digested sludge dewatering method was modified from the lab-scale dewatering protocol developed by Bucknell University detailed in a previous study (Higgins et al., 2014). To the best of our knowledge, there is no established protocol to evaluate the friability of dried biosolids. Thus, a lab-scale vortex mixer-based friability test method was developed by VT-CAWRI for this study. The dewatered sludge samples were uniformly pelletized using a 1-cm semi-sphere silicone mold. The wet pellets were then dried using a moisture balance (Figure 2) at 105 SMƒ to dryness of 90%, 95% (for blended groups only), and 100%, respectively. Four dried biosolids pellets were accurately weighed and placed in a 50 mL centrifuge tube. The tube containing the pellets was then constantly agitated by a digital vortex mixer at 3000 RPM for 10 minutes (Figure 3). After the test, any particles retained on a 1-millimeter (mm) stainless steel mesh sieve screen, i.e., particles with diameters greater than 1 mm, were weighed again, and the friability of the pellets was quantified as the percent weight loss through the sieve as defined in Eq. 1. In summary, three variables were investigated in this study: sludge types (i.e., TPS, TWAS, IWAS, and blended sludge), with and without THP before AD, and pellet drying dryness (90%, 95%, and 100%). The results in Figure 4 showed that the pellets produced from THP-AD sludge generally exhibit higher friability than those without THP. It was observed that the friability of digested TPS pellets was the lowest and remained essentially unchanged after applying THP. However, WAS-containing pellets tended to have the higher friability regardless of the use of THP. Moreover, pellet friability always increased as the pellet dryness increased from 90% to 100% for each sludge type (Figure 4). Statistical analysis showed that such increases in friability were significant (Student's t-test, p < 0.05), indicating over-drying of biosolids could lead to higher friability. Results in Figures 5a and 5b showed that friability tended to monotonously decrease with cake dryness and increase with CST. This observation suggests that the pellet friability is inversely related to the sludge dewaterability. It is our hypothesis that biosolids with poor dewaterability tended to trap more water and thus formed more porous structures in heat-dried pellets after water evaporated. It was this porous structure that led to a higher tendency to crumble under mechanical agitation. In summary, over-drying led to increased friability of biosolids pellets, and both types of WAS pellets became more prone to exhibiting friability at higher levels of dryness when THP was introduced, thereby potentially compromising the efficiency of dryers due to continuous recycling of undersized pellets or dust. In full-scale operation, our data suggest that these two obstacles may be addressed by drying the biosolids to lower dryness levels (~90-92%) and blending WAS with more PS prior to THP (if applicable) or AD.

Few full-scale installations experienced challenges with drying THP-AD sludge due to the high friability and dust issues within the dryer. However, the friability of dried biosolids and the causes have never been quantitatively measured and investigated. Therefore, this study aims to experimentally evaluate the impact of THP on the sludge drying property (in terms of friability) following anaerobic digestion of TPS, TWAS, IWAS, and blended sludge.

SpeakerLi, Yitao

Presentation time

14:30:00

14:50:00

Session time

13:30:00

15:00:00

SessionTHP: Downstream Process and Maintenance Challenges

Session number201

Session locationRoom 240

TopicBiosolids and Residuals

Author(s)

Li, Yitao, Zhang, Dian, Iboleon, Rafael, Burch, Robin, Novak, Alex Eugene, Wang, Zhiwu

Author(s)Y. Li1, D. Zhang2, R. Iboleon3, R. Burch4, A.E. Novak5, Z. Wang1

Author affiliation(s)1Virginia Tech, VA, 2Stantec, VA, 3, VA, 4Louisville & Jefferson County MSD, KY, 5Retired, KY

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159639

Volume / Issue

Content sourceWEFTEC

Word count14

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

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160518

Volume / Issue

Content sourceWEFTEC

Word count8

Filtrate rDON and Ortho-P Control Through Coagulant Addition During Dewatering of Thermal Hydrolysis Pretreatment-Enhanced Anaerobic Digester Sludge

Abstract

In 2024, the Washington Suburban Sanitary Commission (WSSC Water), the largest water/wastewater utility in Maryland, will commission a centralized biosolids processing facility (Bioenergy Facility) utilizing a thermal hydrolysis pretreatment (THP)-enhanced anaerobic digester (AD) process. The Bioenergy Facility will receive 100 MGD worth of biosolids from six Water Resource Recovery Facilities (WRRFs). The filtrate from the Bioenergy Facility's dewatering process will be treated using the ANITATM MOX process and then discharged to the headworks of the 30 MGD Piscataway WRRF. Prior evaluations estimated that the total nitrogen (TN) in Piscataway's effluent could increase by as much as 1 mg/L due to the production of recalcitrant dissolved organic nitrogen (rDON) via the Maillard reaction resulting from the THP process as detailed in our previous study (Zhang, et al., 2021). The anticipated increase in rDON loading will place the Piscataway WRRF at risk of exceeding the 3.25 mg/L TN effluent limit. In addition, due to operational changes in coagulant dosing and phosphorous removal strategies at the six raw biosolids source plants, higher ortho-phosphorus (ortho-P) presence is expected in the Bioenergy Facility's AD effluent and dewatering filtrate, potentially causing struvite formation in downstream equipment. To address these issues, a temporary, easy-to-implement approach was tested to control rDON and ortho-P that can be applied at this and other WRRFs operating the THP-AD process. Therefore, this study aims to assess the impact of dosing Al3+ versus Fe3+ in AD effluent prior to dewatering on rDON and ortho-P levels in filtrate. Samples of dewatered mixed sludge were collected from WSSC Water's four largest WRRFs (Piscataway, Western Branch, Seneca, and Parkway) with a blending ratio (wet mass) of 4.3 : 3.3 : 2.4 : 1 (Figure 1). The blended cake samples were diluted to 16% TS followed by processing through a pilot-scale Cambi THP unit (Cambi, Asker, Norway) operated at 165 SMƒ for 30 minutes. The pretreated sludge was then mixed with AD inoculum collected from DC Water Blue Plains WWTP and anaerobically incubated at 36.5 SMƒ until the cumulative biogas production reached a plateau. Three variables were investigated in this study: coagulant type and dose, and the dewatering polymer dose. Briefly, various doses of polyaluminum chloride (PACl) or ferric chloride (FeCl3) solutions were added to the digested sludge followed by dosing polymer at either optimal polymer dose (OPD) or half OPD as described in Table 1. The lab-scale dewatering protocol includes shearing of the sludge-polymer-coagulant mixture and piston compression of the solids. Samples of dewatering filtrate were collected for characterization and testing for recalcitrant species. The recalcitrant species test consisted of ammonia (TAN) stripping of the dewatering filtrate to minimize background noise when calculating DON = sTN - NOx-N - TAN and subsequent aerobic incubation to eliminate biodegradable DON, with the remainder treated as rDON. Effect on DON and rDON removal Preliminary results from this study revealed that adding medium (5.5 % Al3+ or Fe3+) and high (8.2%) doses of PACl or FeCl3 along with the optimal dose of polymer during dewatering could reduce > 60% DON (Figure 2a). When the polymer dose was reduced by half but the coagulant doses were maintained at medium (5.5%), the effect of DON removal was only slightly diminished, increasing the DON level in the filtrate to approximately 45% remaining (Figure 2a). This slight decrease in DON removal efficiency at half OPD indicates that it was the coagulants rather than the polymer playing a dominant role in DON removal. rDON test is in progress. Effect on ortho-P removal Without any coagulant dosing, only 5 and 11% of ortho-P can be removed from the liquid phase at half and full OPD of polymer during dewatering, respectively (Figure 2b). However, it is interesting to observe that even adding low doses (2.8%) of PACl or FeCl3 was able to remove nearly 100% of ortho-P in the filtrate, eliminating the risk of struvite formation. Effect on filtrate pH The results showed that increasing the PACl and FeCl3 doses prior to dewatering significantly dropped the pH almost linearly to as low as 6.1 (Figure 2c). This can be explained by the hydrogen ion release from the formation of metal-hydroxide complexes after metallic coagulant dosing (Persson, 2018). Therefore, to maintain the dewatering filtrate within the suitable pH range of 6.5 to 7.5 for ANITATM MOX's ammonia-oxidizing bacteria and anammox bacteria activities (Hu, et al., 2023) without pre-pH adjustment, adding low or medium doses of coagulants would be preferred. Effect on cake dryness and cation overdosing Figure 2d showed that adding a low dose (2.8%) of coagulants during dewatering with OPD substantially aggravated cake dryness from 27.2% to less than 25% due to cation overdosing. With the coagulant doses increasing to medium (5.5%) and high (8.2%) doses, the cake dryness at OPD further deteriorated to as low as 23% and 22.4% TS, respectively, which corroborated the belief that overdosing has occurred as a result of coagulant addition. However, reducing 50% polymer dose while maintaining a low (2.8%) PACl and FeCl3 doses improved cake dryness to 27.5% and 27.2% TS. However, adding more coagulants beyond 2.8% with ½ OPD deteriorated cake dryness again due to cation overdosing.

Prior evaluations estimated that the TN in Piscataway's effluent could increase by up to 1 mg/L due to the production of recalcitrant dissolved organic nitrogen (rDON) during the THP process, which puts this 30-MGD facility at risk of exceeding the 3.25 mg/L TN permissible limit. This pilot study assessed a temporary, easy-to-implement approach to control rDON and ortho-P in THP-AD dewatering filtrate by adding different doses of coagulant during sludge post-dewatering.

SpeakerLi, Yitao

Presentation time

14:00:00

14:20:00

Session time

13:30:00

15:00:00

SessionTHP: Downstream Process and Maintenance Challenges

Session number201

Session locationRoom 240

TopicBiosolids and Residuals

Author(s)

Li, Yitao, Taylor, Malcolm, Nguyen, Caroline, Novak, John, Wang, Zhiwu

Author(s)Y. Li1, M. Taylor2, C. Nguyen3, J.T. Novak4, Z. Wang1

Author affiliation(s)1Virginia Tech, VA, 2Washington Suburban Sanitary Commision, MD, 3, CA, 4Virginia Tech

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159611

Volume / Issue

Content sourceWEFTEC

Word count18

Description: Full-scale implementation of coagulant dosing for recalcitrant nitrogen and...

Full-scale implementation of coagulant dosing for recalcitrant nitrogen and orthophosphate control during dewatering of thermal hydrolysis pretreatment-enhanced anaerobic digester sludge

Abstract

Introduction In 2024, WSSC Water, the largest water/wastewater utility in Maryland, commissioned a centralized biosolids processing facility (Bioenergy Facility), which utilizes a thermal hydrolysis pretreatment (THP)-enhanced anaerobic digester (AD) process. The Bioenergy Facility will receive biosolids from six regional Water Resource Recovery Facilities (WRRFs), which have a combined treatment capacity of 100 MGD. The filtrate from the Bioenergy Facility's dewatering process is treated using the AnitaMox deammonification process before being returned to the headworks of the 30 MGD Piscataway WRRF. Bench-scale evaluations estimated that the total nitrogen (TN) in Piscataway's effluent could increase by up to 1 mg/L due to the production of recalcitrant dissolved organic nitrogen (rDON) via the Maillard reaction resulting from the THP process as detailed in our previous study [1]. The anticipated increase in rDON loading will place the Piscataway WRRF at risk of exceeding the stringent TN effluent limit of 3.25 mg/L. In addition, operational changes at two of the biosolids source WRRFs, which plan to shift to biological phosphorous removal, are expected to result in higher orthophosphate (ortho-P) presence is in the Bioenergy Facility's AD effluent and dewatering filtrate, potentially causing struvite formation in downstream equipment. To address these issues, a temporary, easy-to-implement approach, that can be applied at this and other WRRFs operating a THP-AD process, was tested in pilot-scale to control rDON and ortho-P. A previous study reported that rDON mainly contains negatively-charged molecules, which have a remarkable coagulation capacity with many metallic ions, such as ferric chloride (FeCl3) [2, 3]. Phase 1 of this study evaluated the influence of FeCl3 addition during dewatering of AD effluent on filtrate rDON and ortho-P concentrations at lab scale. Results from these lab-scale experiments reported herein demonstrate the strong potential of FeCl3 for reducing rDON and ortho-P concentrations in AD filtrate. Phase 2 of the study will focus on validating these lab-scale results by conducting a second round of testing at full-scale. A comparison of the lab-scale and full-scale results will be reported, alongside an assessment of the practicality and effectiveness of FeCl3 addition for rDON and ortho-P control at this centralized biosolids processing facility. Materials and Methods The process flow diagram of the study is shown in Figure 1. Samples of dewatered mixed sludge cake with an approximately 22% total solids (TS) content were collected from WSSC Water's four largest WRRFs (Piscataway, Western Branch, Seneca, and Parkway) with a blending ratio (wet mass) of 4.3 : 3.3 : 2.4 : 1 that is based on an estimated future biosolids production. The blended cake samples were diluted to around 16% TS followed by processing through a pilot-scale Cambi THP unit (Cambi, Asker, Norway) operated at 6 bar steam pressure (equivalent to 165 °C) for 30 minutes. The pretreated sludge was then mixed with AD inoculum collected from a full-scale THP-AD system at a local WRRF and anaerobically incubated at 36.5 °C until the cumulative biogas production reached a plateau. After digestion, the optimal polymer dose (OPD) of the digested sludge was determined based on capillary suction time (CST) [4]. The lab-scale dewatering protocol, developed by VT-CAWRI, includes shearing of the sludge-polymer mixture and piston compression of the solids, which has been calibrated to a full-scale belt filter press installation according to methods detailed in a previous study [5]. Two variables were investigated in this study: FeCl3 dose and polymer dose. Briefly, FeCl3 solutions were added to the digested sludge during dewatering at doses of 0.5%, 1.0%, 2.0%, 2.7%, and 5.5% Fe (g Fe3+/g DS) along with polymer dosing at either OPD or 1/2 OPD. The intent for varying polymer dose is to understand whether FeCl3 can compensate part of the role played by polymer during dewatering, ultimately saving the cost of polymer consumption. Samples of dewatering filtrate were collected for characterization and recalcitrant species tests. The recalcitrant species test, developed by VT-CAWRI, consisted of ammonia (TAN) stripping to minimize background noise when calculating DON = sTN - NOx - TAN and subsequent aerobic incubation to eliminate biodegradable DON, with the remainder treated as rDON which ultimately contributes to Piscataway WRRF's effluent. Results and Discussion Effects on rDON removal Without FeCl3 addition, around 24.1% and 9.4% of rDON were removed as part of the biosolids during dewatering with OPD and 1/2 OPD after accounting for polymer dilution of the sludge, leaving nearly 76% and 91% of rDON in the filtrate, respectively (Figure 2a). As the FeCl3 dose increases, a linear reduction in filtrate rDON was observed, with a nearly 60% removal at a 5.5% FeCl3 dose alongside 1/2 OPD (Figure 2b). Previous studies have reported that rDON produced from the THP process mainly consists of melanoidin molecules, which are negatively charged nitrogen-containing byproducts of the Maillard reaction that have shown remarkable binding capacity with metallic ions [6]. Therefore, the removal mechanism of rDON in this tested approach was believed to be precipitation or coagulation by ferric ions and cationic polymer. Although both polymer and FeCl3 demonstrated rDON removal capability according to Figure 2a, the contribution from FeCl3 was greater since polymer alone could only achieve a maximum removal of 24.1% at the OPD. Effect on ortho-P removal Unlike rDON removal, the removal of ortho-P without any FeCl3 addition was minimal. Only less than 5% of ortho-P was removed from the liquid phase at OPD, and even no change was observed with 1/2 OPD of polymer during dewatering (Figure 3a). However, it is notable that even the lowest dose of FeCl3 (i.e., 0.5% Fe) combined with 1/2 OPD of polymer achieved an over 85% removal of ortho-P from the filtrate, while dosing 2% Fe resulted in nearly complete elimination (Figure 3a). This exponential reduction of ortho-P in Figure 3b indicates that FeCl3 dosing during dewatering is more efficient for ortho-P removal than for rDON removal, making rDON levels in the filtrate likely the determining factor for appropriate FeCl3 dose selection. Moreover, the effective removal of ortho-P from the dewatering filtrate essentially eliminated the risk of struvite formation in downstream equipment. Effect on sludge dewatering and cation overdosing It is our hypothesis that the addition of metallic coagulants during dewatering along with the optimal dose of polymer may lead to cation overdosing, which disrupts charge neutralization and ultimately results in worse cake dryness. To test this, cake dryness at full and half OPD, with and without coagulant addition, was analyzed. Without any FeCl3 addition, results in Figure 4 showed exceptional cake dryness of 33.1% TS at OPD and a deteriorated cake dryness of 25.3% TS at 1/2 OPD due to underdosing of cations. Adding 5.5% Fe on top of OPD substantially aggravated cake dryness to less than 25% TS (Figure 4). However, in addition to 1/2 OPD, dosing 1% Fe could recover the cake dryness to as high as 31.6% TS from the 25.3% TS at 1/2 OPD only (Figure 4). The results suggest that the deterioration in cake dryness as a result of overdosing of cations during dewatering can be solved by cutting back polymer usage while not sacrificing the superiority in the removal of rDON and ortho-P by FeCl3. Effect on pH and alkalinity It is our concern that the drop in pH and alkalinity at high FeCl3 doses may negatively impact the anammox-based sidestream deammonification process in full-scale operation. As shown in Figure 5, consistent decreases in filtrate pH and alkalinity as a result of FeCl3 dosing during dewatering was observed due to hydrogen ion release from the formation of metal-hydroxide complexes [7]. Without extra alkalinity supplementation, the tradeoff between effective rDON removal and decreases in pH and alkalinity becomes a crucial consideration for selecting FeCl3 doses in full-scale operations. It is important to ensure a sufficient alkalinity for deammonification and a suitable pH range of 6.5 to 7.5 for AOB and anammox bacterial activities [8, 9]. Conclusions and Ongoing Work The results from this pilot study revealed that FeCl3 dosing along with polymer during digestate dewatering can (1) reduce rDON by nearly 60% at 5.5% Fe, (2) achieve nearly 100% ortho-P at 2% Fe, and (3) maintain excellent cake dryness while reducing polymer dosing by 50%. These promising findings provide strong proof of concept, which is currently being validated in full-scale operations at the Bioenergy Facility through early 2025. Potential side effects, such as decreased pH and alkalinity will also be verified and addressed.

This paper was presented at the WEF Residuals & Biosolids and Innovations in Treatment Technology Joint Conference, May 6-9, 2025.

SpeakerLi, Yitao

Presentation time

11:25:00

11:45:00

Session time

08:30:00

11:45:00

SessionInnovations in Sludge Management: Enhancing Anaerobic Digestion and Phosphorus Control

Session number10

Session locationBaltimore Convention Center, Baltimore, Maryland, USA

TopicAerobic Digestion, Anaerobic Digestion, Biogas Utilization, Biosolids, Biosolids treatment, Class A, Dewaterability, Phosphorus recovery, struvite, electrochemical, Pilot Scale, Process Intensification, Resource Recovery, Solids Pre-Treatment, Sustainability, thermophilic, THP, CAMBI

Author(s)

Li, Yitao, Taylor, Malcolm, Nguyen, Caroline, Novak, John, Wang, Zhiwu

Author(s)Y. Li1, M. Taylor2, C. Nguyen2, J. Novak1, Z. Wang1

Author affiliation(s)Virginia Tech, 1Washington Suburban Sanitary Commission, 2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2025

DOI10.2175/193864718825159762

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count21

Is A Second Thermal Hydrolysis Energy-positive? A Net Energy Analysis of Dual-THP Configuration

Abstract

1. Introduction Thermal hydrolysis pretreatment (THP) has been commercially applied in water resource recovery facilities (WRRFs) for over 20 years to intensify anaerobic digestion (AD) [1]. Reported advantages of THP include improved sludge digestibility and dewaterability, increased biogas production, and reduced pathogen, odor, and foaming [1-3]. Among these benefits, enhanced biogas production has received a lot attention because methane in biogas can offset energy demand and be a revenue source for WRRFs. Prior studies have reported that THP not only accelerated municipal sludge biodegradation but also increased its bioavailability during AD. Building on this established knowledge, it is our hypothesis that applying a second THP to THP-AD reactor digestate may release or create new methane potential that can be utilized by methanogens, which would otherwise remain non-bioavailable. To test this hypothesis, an on-site pilot study was conducted in collaboration with Arlington County Water Pollution Control Plant (Arlington WPCP) to investigate: (i) whether applying a second THP on the THP-AD reactor effluent can release or create additional methane potential; and (ii) the practicality of integrating a second THP into the THP-AD process from a net energy output perspective 2. Materials and Methods A blend (3:2 v/v) of primary sludge and waste activated sludge dewatered to 30% total solids (TS) was collected from Arlington WPCP, Virginia. The dewatered cake was diluted to 16% TS and then pretreated in a pilot-scale THP system (Cambi, Asker, Norway) at 165 â„ƒ (equivalent to 6 bars) for 30 minutes. The THP-treated sludge was then fed to three semi-continuous mesophilic AD reactors operated at 10-, 12.5-, and 15-day solids retention time. Digestate collected from the three AD reactors was first tested for their residual methane potential via a biological methane potential (BMP) test using a 16-channel anaerobic respirometer system (AER-800, Challenge Technology, Springdale, AR, USA) with a triplicate setup. At the end of the BMP test, all bioavailable organics in the THP-treated sludge were hypothesized to be experimentally depleted, and the remaining organics were regarded as non-bioavailable or non-biodegradable by AD microbes. Then, the non-bioavailable sludge from the BMP test was processed through a second THP process followed by a second BMP test to experimentally evaluate how much new methane potential can be created by the second THP from the BMP-exhausted sludge that essentially has no bioavailable organics. Based on the new BMP created by the second THP, an energy balance analysis was conducted to evaluate the impact of the second THP integration on the overall net energy output of the entire solids treatment process by following the approach used by Chen, et al. [4]. 3. Results and Discussion New BMP created by a second THP As shown in Figure 1, on top of the CH4 yields of 0.292, 0.304, and 0.321 L CH4/g volatile solids (VS) fed obtained from the THP-AD reactors operated at the 10-, 12.5-, and 15-day SRTs, the residual CH4 yields of the reactor effluent were 0.104, 0.094, and 0.082 L CH4/g VS fed, respectively. To this end, the total methane potential of the THP-treated sludge was around 0.399 L CH4/g VS fed (Figure 1). More importantly, the new BMP in terms of CH4 yield created by the secondary THP ranged from 0.099 to 0.121 L CH4/g VS fed within only 6 days of incubation time, which surprisingly marks an additional up to 30.1% increase in CH4 yield compared to the total methane potential of the sludge treated by the first THP. The creation of the new methane potential from the BMP-exhausted THP-AD reactor effluent indicates that THP can enhance sludge bioavailability by converting previously non-bioavailable organics into methanogen-accessible substrates. It should be emphasized that if the residual BMP in the THP-AD reactor effluent and the new BMP created by the second TH can both be recovered, their combined contribution would increase the total methane production by 56.2% to 77.1% relative to the CH4 yield achieved in the THP-AD reactors alone. Figure 2 shows the COD turnover in terms of soluble COD (sCOD) and particulate COD (pCOD) throughout the treatment process. The unchanged sCOD fraction throughout the first BMP test suggests that the sCOD in the AD reactor effluent, regardless of their SRTs, are completely recalcitrant under anaerobic conditions (Figure 2). Following that, the second THP hydrolyzed 18%-22% tCOD in the form of pCOD into sCOD (Figure 2). Interestingly, the BMP test of the second THP effluent (2nd BMP) only reduced sCOD and converted it to methane, leaving pCOD untouched (Figure 2), implying that the fraction of pCOD that has survived the second THP was completely recalcitrant. Process concept for dual-THP implementation There are two key findings that are valuable to potential full-scale applications for integrating a second THP process. First, the two BMP tests indicate that, if both the residual BMP in the THP-AD reactor effluent and the new BMP created by the second THP are recovered, total methane production could increase by up to 77.1% relative to the methane production achieved in the THP-AD reactors alone (Figure 1). Second, the sCOD in the THP-AD reactor effluent and the pCOD in the second THP effluent were completely recalcitrant as they did not contribute to the methane production during the two BMP tests regardless of the reactor SRTs (Figure 2). By using these three findings, a THP-AD-THP process with centrate recirculation, or a dual-THP process, that combines both the pre-AD THP and post-AD THP setups is proposed in Figure 3. Based on the two BMP tests results and COD turnover discussed above, the THP-AD reactor effluent can be post-thickened to ~16% TS, allowing only the thickened solids to undergo a second THP to convert residual hardly microbial-hydrolyzable particulate organics into methanogen-accessible soluble organics. This configuration may enable the second THP to both release the residual BMP remaining in the reactor effluent particulate fraction and create additional BMP as observed in the second BMP test (Figure 3). The post-thickening centrate, mainly comprising recalcitrant sCOD, can be discharged to mainstream treatment without energy loss. Excluding this centrate reduces the required secondary THP capacity to approximately half that of the primary THP. Following the second THP, newly generated BMP resides almost entirely in the soluble phase as supported by the results above, enabling post-dewatering separation of recalcitrant solids for disposal and recirculation of methane potential-rich centrate to the AD reactor. It should be noted that the centrate flow is sufficient to dilute the primary THP effluent without potable or reclaimed water addition, which avoids increases in digester volume (Figure 3). How much net energy output can the second THP bring? Although coupling a second THP with a conventional THP-AD process can potentially increase the methane production by up to 77.1% (Figure 1), the practical value of this intensification ultimately depends on whether the additional methane energy recovered substantially outweighs the operational energy consumption introduced by the second THP and associated unit operations. As can be seen from Figure 3, the second THP process and the additional post-thickening process are the two processes that demand extra energy input compared to a typical THP-AD configuration. The analysis results reveal that integrating a second THP yields 3,080-3,797 megajoules (MJ)/ton-TS of additional recoverable energy (heat + electricity), which substantially exceeds the added operational energy consumption (1,894-2,126 MJ/ton-TS) for the second THP and post-thickening. In terms of net energy output, as the SRT of AD reactors increases from 10 to 12.5 and 15 days, the net energy output of the entire dual-THP configuration increases from 5,168 to 5,310 and 54,04 MJ/ton-TS, respectively, marking 47.8%, 39.7%, and 28.1% increase in net energy output compared to the baseline THP-AD configuration at the corresponding SRTs. These results suggest that the dual-THP configuration brings the most energy recovery benefits for utilities constrained to short SRT operation (e.g., high-flow facilities with limited footprint). 4. Conclusions The following major remarks can be drawn from this study: 1) A second THP performed on THP-AD digestate can release 77.1% more methane potential compared to that of THP-AD reactor operated at 10-day SRT. 2) The sCOD in the THP-AD reactor effluent and the pCOD in the second THP effluent were completely recalcitrant and did not contribute to the methane production during the two BMP tests. 3) Implementing a second THP with centrate recirculation to AD can increase the net energy output as heat and electricity by 47.8% at 10-day AD SRT without increasing AD volume.

This paper was presented at the WEF Residuals, Biosolids, and Treatment Technology Conference in Kansas City, MO, May 11-14, 2026.

Presentation time

14:00:00

14:15:00

Session time

13:30:00

15:00:00

SessionTargeting Net-Zero With THP: Innovative Energy Plays

Session locationKansas City Convention Center

TopicResource & Energy Recovery

Author(s)

Li, Yitao, Luo, Hao, Haile, Fasil, Strawn, Mary, Wang, Zhiwu

Author(s)Y. Li1, H. Luo1, F. Haile2, M. Strawn2, Z. Wang1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2026

DOI10.2175/193864718825160253

Volume / Issue

Content sourceResiduals, Biosolids and Treatment Technology Conference

Word count14

Description: Pushing Feed Rates Beyond "Limits" to Accelerate THP Digestion...

Pushing Feed Rates Beyond "Limits" to Accelerate THP Digestion Startup: Insights from an In-Situ Pilot Study

Abstract

Introduction
Thermal hydrolysis pretreatment (THP) has been gaining popularity for intensifying the anaerobic digestion (AD) of municipal wastewater sludge by decreasing sludge viscosity, enhancing solids reduction, and boosting biogas production [1, 2]. The digested sludge also gains significant improvement in dewaterability, and the dewatered Class A biosolids are qualified for direct land application [3-5]. However, the startup phase for THP-enhanced digesters remains risky and time-intensive due to improved hydrolysis and faster acid production, as well as longer time needed to achieve shorter solids retention times (SRT). Current industry norm for digester feed rates during startup, typically ranging from 0.10 to 0.25 lb COD/lb VS/day, often lead to a prolonged timeframe to achieve full digester throughput capacity. To address this limitation, this study explored the possibility to further increase maximum feed rates to a level that are at least twice the industry norm, aiming to accelerate the startup of THP-enhanced digesters. A total of three maximum feed rates, namely 0.31, 0.33, and 0.38 lb COD feed/lb VS digester/day, were tested with the hope to investigate the upper boundary of startup pace achievable with a commonly adopted full-scale startup strategy for THP-enhanced digesters.
Materials and Methods
Samples of thickened primary sludge (TPS) and waste activated sludge (WAS) were mixed at a target dry mass ratio of 3:2 and then pre-dewatered to a TS content of 16% total solids (TS) with a cationic polymer using a pilot-scale screw press thickener. The pre-dewatered cake samples were processed through a pilot-scale THP unit (Cambi, Asker, Norway) operated at 6-bar steam pressure (equivalent to 165 C) for 30 minutes. The pretreated sludge was diluted to 9% TS and fed daily into three pilot-scale mesophilic (36.5 C) digesters with a 10 L working volume and a target SRT of 15-day.
To simulate the future startup protocol for full-scale operations, each pilot digesters was initially filled to 30% full (v/v, 3 L) which is just enough for initiating the mixing, with 20% (v/v, 2 L) filled with tap water and the other 10% (v/v, 1 L) seeded with full-scale THP-enhanced mesophilic AD sludge freshly collected from a local plant. The headspace in the digesters were purged with nitrogen gas before feeding started.
Results and Discussion
The startup plan illustrated in Figure 1a consisted of three phases. In Phase 1, the goal was to acclimate the seed to the new feed sludge produced at the Arlington WPCP. The initial feed rate on Day 1 used in all three digesters was 50% of the steady-state feed rate of the seed digester, or around 0.11 lb COD feed/lb VS digester/day. The feed rates were ramped up at 8% per day until reaching the maximum feed rates designed for this study, namely 0.31, 0.33, and 0.38 lb COD feed/lb digester VS/day (Table 1). As can be seen in Figure 2a, the actual feed rate aligned well with the plan from Day 0 to Day 17. Digestate wasting did not start until the digester volume was filled up as scheduled in Figure 1b.
During Phase 2, the three digesters maintained their respective maximum feed rates until reaching the designed daily feed volume, that is 6.7% of the digester working volume which is equivalent to a 15-day SRT. However, the actual feed rates started to be approximately 5 to 10% lower than the planned values due to a reduction in the VS content in the THP-processed sludge. This lower VS content in this batch of sludge was attributed to a lower TPS:WAS blending ratio produced by Arlington WPCP. The drop in actual feed rates between Day 59 and 76 was another example of changes in feed sludge characteristics (Figure 2a). After Day 76, the actual feed rate once again aligned well with the designed plan. As shown in Figure 2b, the digesters with designed maximum feed rates of 0.31, 0.33, and 0.38 lb COD/lb VS/day successfully achieved the 15-day SRT on Days 34, 53, and 80, respectively, without any delay. The microbial concentrations in the digesters indicated by digester VS contents showed steady buildup and well alignment with the designed plan in general (Figure 2c).
During Phase 3, all three digesters maintained the 15-day SRT until reaching steady state. The steady state was defined as the timeframe during which VSR, daily biogas production rate, and VFA concentration did not show statistical differences based on T-test results. Throughout, key digester stability indicators, including TS, VS, pH, VFA, alkalinity, biogas production rate, and biogas composition were closely monitored.
Conclusions
By increasing the maximum feed rate from 0.31 to 0.33 and 0.38 lb COD/lb VS/day, the time required to reach the target15-day SRT was significantly reduced from 80 to 53 and 34 days, respectively. Throughout the startup phases, digester stability indicators suggested that VFA accumulation has occurred during the first 16 days of the startup process; however, the digesters quickly acclimated and stabilized without any rescue or corrective measures, such as feeding pauses or alkalinity supplementation, even at the more aggressive feed rate. These findings suggest that THP-enhanced digesters can successfully withstand higher loading rates during startup, offering a practical strategy to expedite achieving full operational capacity in AD processes.

This paper was presented at WEFTEC 2025, held September 27-October 1, 2025 in Chicago, Illinois.

Presentation time

09:30:00

09:45:00

Session time

08:30:00

10:00:00

SessionInnovative THP Startup and Biosolids Finishing

Session locationMcCormick Place, Chicago, Illinois, USA

TopicBiosolids & Residuals

Author(s)

Li, Yitao, Strawn, Mary, Racey, Lisa, Haile, Fasil, Balchunas, Brian, Moline, Chris, Higgins, Matthew, Novak, John, Wang, Zhiwu

Author(s)Y. Li1, M. Strawn2, L. Racey2, F. Haile2, B. Balchunas3, C. Moline3, M. Higgins4, J. Novak1, Z. Wang1

Author affiliation(s)Virginia Tech1, Arlington County Water Pollution Control Bureau2, HDR Engineering Inc.3, Bucknell University4

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2025

DOI10.2175/193864718825160166

Volume / Issue

Content sourceWEFTEC

Word count17

Description: The Impact of Solids Retention Time Reduction on the Thermal Hydrolysis...

The Impact of Solids Retention Time Reduction on the Thermal Hydrolysis Pretreatment-Enhanced Mesophilic Anaerobic Digestion

Abstract

INTRODUCTION Thermal hydrolysis pretreatment (THP) holds promise to intensify the solid handling capacity of mesophilic anaerobic digesters (AD) by improving the sludge biodegradability, pumpability, dewaterability, and biosafety. Typically, the digester feedstock total solids concentration (TS) can be doubled from 5 to 10%, and the solids retention time (SRT) can be halved from 30 to 15 days after THP is installed, which directly enables existing AD to quadruple the loading rate capacity (Higgins et al., 2017 and Walley, 2007). Although the feedstock TS is hard to further increase due to its viscosity limit and the potential to cause free ammonia inhibition, the digester SRT has been believed to have room for further intensification (Zhang et al., 2022 and Xue et al., 2015). However, to what extent the SRTs can be further shortened without deteriorating the expected digester performance needs to be explored. To this end, a pilot study was conducted in Arlington Water Pollution Control Plant (AWPCP) to understand the impact of further reducing SRT from 15 to 12.5 days and then to 10 days on AD. This study is anticipated to provide engineering guidance for other utilities that consider pursuing process intensification of THP-AD systems. METHODS AND MATERIALS As shown in the process flow diagram in Figure 1, a pilot-scale CAMBITM THP unit shown in Figure 2 was operated at a steam pressure of 6 Barr (equivalent to 165 oC) for 30 minutes to pretreat municipal sludge in preparation for subsequent ADs. Three 10-liter, mechanically mixed ADs shown in Figure 3a were operated in parallel with temperature maintained at 36.5 ± 0.3 oC and SRTs controlled at 10, 12.5, and 15 days, respectively, by manipulating the daily sludge exchange volume. The system temperature control configuration is illustrated in Figure 3b. All ADs were inoculated with the effluent from a full-scale THP-AD system in DCWater. AWPCP cake consists of primary sludge (PS) and wasted activated sludge (WAS) blended at a dry mass ratio of 3:2. The cake was diluted with distilled water or THP steam in each step of the flow diagram according to the TS levels shown in Figure 1. This AD system was continuously operated for 273 days from 6/3/2021 to 3/2/2022. RESULTS AND DISCUSSION AD Startup All three ADs were started with the same SRTs of 30 days (Figure 4). Their SRTs were then step-wise decreased by 0.5 days per day until reaching a 16-day SRT at which all three reactors were stabilized for 1.5 months to prevent reactor failure due to high volatile fatty acid (VFA) concentration. After that, the SRTs were further reduced to 15 days and stabilized for another 1.5 months. In the end, the SRTs of two ADs were finally reduced to 12.5 and 10 days, respectively, at a pace of 0.5 days every other day, and all three ADs were operated without any SRT change thereafter for the remaining 152 days of operation. Effect of SRTs on solids reduction and methane yield As shown in Figure 5a, the steady-state volatile solids reductions (VSRs) were measured to be almost linearly correlated to the SRTs, indicating that shortening SRTs played a major impact on the sludge hydrolysis rate. This is because shortening SRTs from 15 days to 12.5 days and then to 10 days has reduced VSR from 60.7% to 57.5% and then to 53.6%, respectively. In contrast, the methane yield did not show a clear dependence on the SRTs even though the AD with 15-day SRT produced 7% more methane than those with 10-day and 12.5-day SRTs (Figure 5b). This asynchronization between VSR and methane yield suggests that it was the methanogenesis but not hydrolysis limiting the methane production (Figure 5a and 5b) probably due to the THP enhancement on hydrolysis. This can be seen from the substantial reduction of sludge particle size before and after THP as well as AD as shown in Figure 5c. It is noteworthy that SRTs did not cause any significant impact on the particle size distribution of digested sludge (Figure 5c). Effect of SRTs on COD, nitrogen, and phosphorus turnover The total chemical oxygen demand (tCOD) was slightly decreased after THP probably due to the loss of some volatile organics during sludge transfer to AD (Figure 6a). A linear SRT effect on tCOD can be seen in Figure 6a, which is consistent with the trend of VSR in Figure 4a. In contrast, total kjeldahl nitrogen (TKN) in Figure 6b remained unchanged during THP but slightly decreased in ADs with increasing SRT probably due to the volatilization of ammonia nitrogen at a higher bulk concentration as shown in Figure 6d regarding the total ammonia nitrogen (TAN). The soluble COD (sCOD), on the other hand, was significantly increased after THP, evidencing the effectiveness of thermal hydrolysis (Figure 6c). Figure 6c showed that about 60% of sCOD was removed in the form of methane in ADs. The levels of residual sCOD, especially the 1.5 µm filtered, appeared to be inversely related to the SRTs, corroborating the rate-limiting step is methanogenesis inferred from Figure 5a (Figure 6c). It is interesting to see that the orthophosphate (OP) released into the bulk solution was mainly affected by THP but not by AD SRTs probably due to the convolution between hydrolysis and precipitation in ADs (Figure 6e). Effect of SRTs on sludge rheology and dewaterability The sludge rheology was mainly influenced by the THP instead of SRTs. This can be seen from the dramatic drop of apparent viscosity from 3.6*105 millipascal-second (mPas) to 2.8*105 mPas measured at a shear rate of 0.021 s-1 after THP, while only slight changes were observed at different SRTs (Figure 7a). Nevertheless, all of the sludge showed non-Newtonian, shear thinning behavior which is in consensus with the literature (Higgins et al., 2017). The sludge dewaterability as indicated by the capillary suction time (CST) showed that the sludge dewaterability became worse after AD which is expected based on the literature (Novak et al., 2003). However, SRTs did not show any significant impact on the CST, suggesting that the sludge dewaterability should remain the same when SRTs were reduced from 15 days to 10 days. CONCLUSIONS This study revealed that shortening SRTs from 15 to 10 days reduced VSR from 60.7% to 53.6%; however, other key AD performance parameters such as methane yield and sludge properties in terms of rheology and dewaterability largely remained unchanged.

This paper was presented at the WEF/IWA Residuals and Biosolids Conference, May 16-19, 2023.

SpeakerLi, Yitao

Presentation time

15:45:00

16:15:00

Session time

13:30:00

16:45:00

SessionSession 07: Intensifying Digestion

Session number07

Session locationCharlotte Convention Center, Charlotte, North Carolina, USA

TopicDigestion & Stabilization

Author(s)

Y. Li

Author(s)Y. Li1, H. Luo2, M. Strawn3, L. Racey4, F. Haile5, B. Balchunas6, C. Moline7, L. Hentz8, Z. Wang9,

Author affiliation(s)Virginia Tech1; Arlington County Water Pollution Control Bureau2; HDR3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2023

DOI10.2175/193864718825158860

Volume / Issue

Content sourceResiduals and Biosolids

Word count16

Description: To What Extent Can Solids Retention Time Be Reduced Without Deteriorating the...

To What Extent Can Solids Retention Time Be Reduced Without Deteriorating the Performance of the Thermal Hydrolysis Pretreatment-enhanced Mesophilic Anaerobic Digestion

Abstract

The solids retention time of anaerobic digesters is one of the most important parameters for operational and design purposes. It has been believed to have the potential for process intensification with thermal hydrolysis pretreatment upstream. To assess the extent and impact of digester solids retention time reduction, the digestibility and dewaterability of three pilot-scale, continuous flow, mesophilic anaerobic digesters operating at 15-, 12.5-, and 10-day SRT were evaluated. These 10-liter digesters were fed with 9% total solids CAMBITM THP-processed primary and wasted activated sludge every day for nearly a year. The study revealed that shortening the solids retention time from 15 to 10 days linearly decreased volatile solids reduction from 60.7% to 53.6%, highlighting the reduction in hydrolysis rate as the major drawback. However, other digester performance parameters such as methane yield, rheology, and dewaterability largely remained unchanged.

The solids retention time in anaerobic digesters has long been considered as a potential parameter for process intensification, particularly with the installation of thermal hydrolysis pretreatment upstream. This pilot study aims to assess the impact of further reducing digester SRT from 15 to 12.5 days and then to 10 days on the digestibility and dewaterability. It is anticipated to provide engineering guidance for utilities that consider pursuing process intensification of THP-AD systems.

SpeakerLi, Yitao

Presentation time

09:00:00

09:20:00

Session time

08:30:00

10:00:00

SessionIntensifying and Optimizing Anaerobic Digestion

Session locationRoom S503 - Level 5

TopicIntermediate Level

Author(s)

Li, Yitao

Author(s)Y. Li 1; H. Luo 1 ; M. Strawn 2; L. Racey 2; F. Haile 3; B. Balchunas 4; C. Moline 4; L. Hentz 4; Z. Wang 1; Y. Li 1;

Author affiliation(s)Department of Civil and Environmental Engineering, Virginia Tech 1; Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, Virginia 24061, United States 1 ; Arlington County Water Pollution Control Bureau 2; Arlington County Water Pollution Control Bureau 2; Arlington County Water Pollution Control Bureau 3; HDR 4; HDR Engineering, Inc. 4; HDR Engineering 4; Department of Biological Systems Engineering, Virginia Tech 1; 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2023

DOI10.2175/193864718825159151

Volume / Issue

Content sourceWEFTEC

Word count22