Li, Yitao

Yitao is a PhD student at Virginia Tech. As a biosolids researcher, he tackles applied-research problems related to thermal hydrolysis, anaerobic digestion, dewatering, thermal drying, and side-stream nutrient control.

Titles from this speaker

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

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

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