Harris, Caitlyn

Caitlyn Harris achieved her Bachelors degree in environmental engineering at the University of Texas at Austin, and her masters degree in...

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About Harris, Caitlyn

Caitlyn Harris achieved her Bachelors degree in environmental engineering at the University of Texas at Austin, and her masters degree in environmental engineering at Virginia Tech.

Harris, Caitlyn

Caitlyn Harris achieved her Bachelors degree in environmental engineering at the University of Texas at Austin, and her masters degree in environmental engineering at Virginia Tech.

Titles from this speaker

Description: Comparing Modeling Tools Visual MINTEQ and OLI Studio to Evaluate Scaling Tendency...

Comparing Modeling Tools Visual MINTEQ and OLI Studio to Evaluate Scaling Tendency of Aerated Anaerobically Digested Solids: A Pilot Study

Abstract

Purpose: Common phosphate minerals in wastewater treatment plants, such as struvite/magnesium ammonium phosphate (MAP, MgNH4PO4- 6H2O), calcium phosphates such as amorphous calcium phosphate (ACP, Ca3(PO4)2), and iron salt precipitates such as vivianite (Fe3(PO4)2 8H2O), are known for their tendency to scale and deposit within anaerobic digesters, pipe bends, and dewatering equipment downstream of anaerobic digesters. These deposits pose maintenance and operational challenges. Precipitation of these minerals occurs when the concentrations of ionic constituents exceed the solubility product (Ksp) of the solid, measured as scaling tendency (ST) or saturation index (SI). The solubility of these precipitates depends on factors such as pH, temperature, competing ions, and nucleation sites. Chemical thermodynamic modeling can evaluate the parameters contributing to scaling and, thereby, safeguard process equipment from scaling-induced damage, improve treatment efficiency, and regulate phosphorus sequestration to produce nutrient-rich biosolids. This modeling work could benefit the wastewater treatment industry by providing rapid optioneering for solutions limiting and targeting scaling mineral deposition and assessing various process optimizations for new pilot technologies. In this study, two chemical thermodynamic models were evaluated: (i) Visual MINTEQ Version 4.0 and (ii) OLI Studio. Model inputs were gathered from a pilot-scale experimental setup at Hampton Roads Sanitation District's (HRSD) Atlantic Treatment Plant (ATP), in Virginia Beach, VA. The pilot evaluated the impact of aeration, mixing, and chemical addition on the scaling tendency of thermally hydrolyzed, pretreated, anaerobically digested solids. Critical parameters such as pH, temperature, alkalinity, orthophosphate (OP), ammonia (NH3), and major cations and anions concentrations were measured as model inputs. The solids formed during these evaluations were characterized by: X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM)/Energy Dispersive X-Ray Spectroscopy (EDS), and Raman Spectroscopy analysis to validate predictions of the two models. Methods: The pilot setup at ATP consisted of four tanks (Figure 1), each 11 feet tall and capable of holding about 45 gallons of digestate. These tanks operated as daily batch fed continuously stirred tank reactors, maintained by pump recirculation at a 3-day solids retention time (SRT). Aeration was provided using fine bubble diffuser membranes, operated either at a constant airflow rate or via a dissolved oxygen (DO) set point. The effects of chemical injection of magnesium hydroxide [Mg(OH)2] and calcium hydroxide [Ca(OH)2] were also studied at various Ca+Mg:OP ratios of 0 (control), 0.25:1, 0.5:1, and 1:1. The modeling process was based on pilot operational conditions: anaerobic digestate (pilot influent), aerated anaerobic digestate constantly at 5 LPM, and aerated anaerobic digestate on a DO setpoint of 0.2 mg/L. Inputs for these operational conditions were collected during pilot tests and averaged for model insertion. For each model run, pH and temperature sensitivity analyses were conducted from pH 7-9, and temperatures 20 - 40°C to determine effects on precipitate formation according to Visual MINTEQ and OLI Studio. Experiments with Mg(OH)2 and Ca(OH)2 were conducted in both models for the aerated cases to simulate chemical injection and evaluate the effectiveness of each chemical in OP removal, as well as the accuracy of each model in predicting mineral formation. Results: Both Visual MINTEQ and OLI Studio effectively predicted mineral formation in anaerobic digestate under varying conditions. Initially, both models predicted the formation of metal phosphates (Mg, Ca, Fe) in anaerobic digestate. Upon aeration at 5 LPM, both models identified struvite as the dominant mineral formed in the pH sensitivity analysis, though they differed in predicting other compounds. Visual MINTEQ predicted the formation of calcite and vivianite, while OLI Studio predicted tricalcium phosphates, calcite, and siderite. In the temperature sensitivity analysis, both models yielded consistent results for calcite and struvite formation, suggesting that temperature has a less significant impact on mineral precipitation than pH. In the aerated run targeting a DO setpoint of 0.2 mg/L with Mg(OH)2, both models accurately predicted struvite as the dominant mineral, with its formation increasing alongside the chemical dose, as depicted in Figures 2 and 3. Table 1 compares the OP removal predicted by each model to the measured pilot results for the same test. Both models closely aligned with the measured OP removal via struvite formation, and solid-phase characterization confirmed struvite as the major compound, as indicated by XRD pattern peaks. The presentation will showcase the outcomes derived from each model, offering insights into their respective abilities to predict the potential for the precipitation of controlling solids. The precipitate characterization analysis will also be compared to determine the validity of results and to further optimize both modeling software. The results of this study will enhance our understanding regarding scaling minerals likely to form within anaerobic digestion pretreated with a thermal hydrolysis process, post anaerobic digestion, and on downstream equipment, as well as in a potential post aerobic digestion setup. A validated model could then be used to predict scaling potential and characterize minerals formed at other WRRFs to better treat these nuisance scaling minerals and determine their potential for harvesting/sequestration as a nutrient rich product.

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

SpeakerHarris, Caitlyn

Presentation time

15:45:00

16:05:00

Session time

15:45:00

16:45:00

SessionAdvances in Process Modeling: Aeration, Scaling, and Anaerobic Digestion Dynamics

Session number20

Session locationBaltimore Convention Center, Baltimore, Maryland, USA

TopicChemical Treatment, Modeling, Phosphorus Removal and Recovery, Specific Methanogenic Activity Test

Author(s)

Harris, Caitlyn, Oza, Shubhashini, Nicholson, Jeffrey, Ward, BJ, Matel, Holly Anne, Bott, Charles, Muller, Christopher, Wilson, Christopher

Author(s)C. Harris1, S. Oza1, J. Nicholson2, B. Ward2, H. Matel2, C. Bott2, C. Muller1, C. Wilson2

Author affiliation(s)Brown and Caldwell, 1Brown and Caldwell, 1Hampton Roads Sanitation District (HRSD), 2Hampton Roads Sanitation Department, 2Hampton Roads Sanitation District (HRSD), 2Hampton Roads Sanitation District (HRSD), 2Brown and Caldwell, 1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2025

DOI10.2175/193864718825159765

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count21

Comparing Modeling Tools Visual MINTEQ and OLI Studio to Evaluate Scaling Tendency of Aerated Anaerobically Digested Solids: A Pilot Study

Abstract

Common phosphate minerals in wastewater treatment plants such as struvite (MAP, MgNH4PO4*6H2O), calcium phosphates such as amorphous calcium phosphate (ACP, Ca3(PO4)2), and iron salt precipitates such as vivianite (Fe3(PO4)2*8H2O) have been recognized as a maintenance and operational nuisance for their tendency to scale and deposit within anaerobic digesters, in pipe bends, and on dewatering equipment downstream of anaerobic digesters. Precipitation of these minerals occur if the concentrations of the ionic constituents exceed the solubility product (Ksp) of the solid which is measured as scaling tendency (ST) or scaling index (SI). The solubility of these precipitates is dependent on factors such as pH, temperature, other competing ions, and nucleation sites. Chemical thermodynamic modeling could be used in evaluating the parameters that contribute to scaling and, thereby, safeguard process equipment from scaling-induced damage, improve treatment efficiency, and regulate phosphorus sequestration to produce nutrient rich biosolids. In this study two commercially available models were evaluated: (i) Visual MINTEQ Version 4.0 and (ii) OLI Studio. The model inputs were gathered at a pilot-scale experimental setup at Hampton Roads Sanitation District's (HRSD) Atlantic Treatment Plant (ATP), Virginia Beach, VA. The pilot evaluates the impact of aeration, mixing, and chemical addition of thermally hydrolyzed pretreated, anaerobically digested solids on scaling tendency. Critical parameters such as pH, temperature, alkalinity, orthophosphate (OP), ammonia (NH3), and major cations and anions concentrations were measured as model inputs. The solids formed during these evaluations were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM)/Energy Dispersive X-Ray Spectroscopy (EDS), and Raman Spectroscopy analysis to validate prediction by the two models. The pilot set up at ATP consists of four tanks (Figure 1). Each tank is 11 feet tall and can hold up to about 63 gallons of digestate. The tanks are operated as daily batch fed continuously stirred tank reactors maintained by pump recirculation at a 3-day solids retention time (SRT). The tanks are aerated with fine bubble diffuser membranes which can be operated at constant or intermittent air flow rates, via dissolved oxygen (DO) or pH set points. Chemical injection lines will be added to measure the effects of calcium hydroxide [Ca(OH)2] and magnesium hydroxide [Mg(OH)2] at varying dosages. The modelling process was based on the pilot influent solution compositions depicted in Table 1 of anaerobic digestate. In both modeling programs, anaerobically digested solids at a temperature of 30 degrees C, a pH of 7.4, and initial parameters from Table 1 were analyzed to determine the pilot influent potential precipitate formation. Figure 2 represents the fractions of solids predicted to form at equilibrium in Visual MINTEQ and Figure 3 displays similar results produced from OLI, both in mg/L. The two model predictions are closely related, listing siderite, struvite, and calcite as the top three controlling precipitates, respectively. This is a prediction of what is forming currently in the digesters, prior to manipulation in the pilot from aeration or chemical addition. A pH sweep from 7 to 8.5 as well as a temperature sweep from 20 degrees C to 40 degrees C were also analyzed in both modeling programs to understand the effects of aeration within the pilot on pH, biological activity, and how the solubility of scaling precipitates were affected. These results, as well as future work that includes modeling the effects of chemical addition in the pilot, will be presented. Precipitate Characterization was conducted in a contract laboratory for XRD, SEM/EDS, and Raman Spectroscopy to determine precipitate identification. The major peaks in XRD patterns produced likely matched with struvite. Further testing will be completed and presented to validate the solids predicted to form by the models. The presentation will showcase the outcomes derived from each model, offering insights into their respective abilities to predict the potential precipitation of controlling solids. The precipitate characterization analysis will also be compared to determine the validity of results and to further optimize both modeling software.

Two models were evaluated: (i) Visual MINTEQ and (ii) OLI Studio. A pilot study assessed the impact of aeration and mixing of thermally hydrolyzed pretreated, anaerobically digested solids on scaling tendency and measured pH, temperature, orthophosphate, ammonia, and major cations and anions concentrations as model inputs. Sensitivity analyses for pH and temperature were conducted for both unaerated and aerated anaerobic digestate in each model.

SpeakerHarris, Caitlyn

Presentation time

13:30:00

14:00:00

Session time

13:30:00

15:00:00

SessionInnovations in Phosphorus Management: From Models to Solutions

Session number603

Session locationRoom 340

TopicFacility Operations and Maintenance, Intermediate Level, Municipal Wastewater Treatment Design, Nutrients, Research and Innovation

Author(s)

Harris, Caitlyn, Oza, Shubhashini, Nicholson, Jeffrey, Matel, Holly Anne, Ward, Barbara, Bell, Katherine, Muller, Christopher, Wilson, Christopher, Bott, Charles

Author(s)C.A. Harris1, S. Oza2, J.D. Nicholson3, H. Matel3, B.J. Ward3, K.Y. Bell4, C. Muller5, C. Wilson1, C.B. Bott3

Author affiliation(s)1HRSD, VA, 2Brown and Caldwell, NC, 3Hampton Roads Sanitation District (HRSD), VA, 4Brown and Caldwell, CA, 5Brown and Caldwell, MA

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159528

Volume / Issue

Content sourceWEFTEC

Word count21

Description: Phosphorus Sequestration in Biosolids, Nuisance Struvite Control via PAD and...

Phosphorus Sequestration in Biosolids, Nuisance Struvite Control via PAD and Chemical Addition to TH-AD Digestate, and Downstream Effects

Abstract

Purpose: Struvite and other common precipitates have long posed persistent maintenance and operational challenges at wastewater treatment plants. These precipitates can be enhanced at plants that utilize biological phosphorus removal and can precipitate within anaerobic digesters and on final dewatering equipment which causes damage and decreases treatment efficiency. Struvite can be sequestered as nutrient rich biosolids if properly controlled to remain in the cake. In a digested solids storage tank (DSST) that is in between anaerobic digestion and final dewatering, precipitation can be controlled by manipulating variables such as mixing, aeration of digestate to achieve a shorter solids retention time (SRT) version of post aerobic digestion (PAD) and by chemical addition. Optimizing this via batch and pilot testing can control nuisance struvite and other crystal formation, enhance downstream effects such as increasing phosphorus in the cake, improve biosolids odor profile and cake dryness, reducing polymer demand, and achieve additional volatile solids reduction (VSR).
This research will review findings from batch and pilot scale tests and discuss the nature of these precipitates. Objectives include determining the mechanism that provides optimal phosphorus removal, odor control, cake characteristics, and analyzing the precipitated solids of interest. The presentation will discuss results across batch, pilot, and full-scales and examine the lessons learned of each.

Motivation and Background: The Atlantic Treatment Plant (ATP) in Virginia Beach, VA, operated by the Hampton Roads Sanitation District's (HRSD) is a high-rate facility with an A/O process configuration. In solids handling, a Cambi Thermal Hydrolysis Process (THP) is utilized prior to anaerobic digestion, leading to the formation of scaling precipitates and nuisance struvite. ATP is actively engaged in efforts to control these issues for optimal operational efficiency.
Precipitate formation is determined predominantly by the solids solubility product (Ksp) and the saturation in solution and can be controlled by pH [1]. Manipulation of pH via CO2 stripping from agitation and aeration has successfully been shown to control solubility reactions [2, 3]. Increasing the saturation index via chemical addition can also control precipitation reactions, decrease polymer demand, and improve cake dryness with an SRT of a few hours [4]. Likewise, PAD has been shown to achieve ammonia removal, additional VSR, and break down odor compounds formed in anaerobic digestion at a 5-6 day SRT [5]. Based on these findings, a pilot scale set up (Figure 1) will run at an SRT of 3 days to provide benefits from PAD, but with a longer reaction time that may be more beneficial for phosphorus sequestration. Various mixing, aeration rates, and chemical addition will be manipulated to determine the optimal operation conditions on phosphorus sequestration, cake characteristics, VSR and odors.

Pilot Design and Preliminary Operation: The pilot set up at ATP consists of four tanks. Each tank is 11 feet tall with a volume of about 63 gallons. Simulating the DSST, the tanks are operated as daily batch fed continuously stirred tank reactors (CSTRs) maintained by pump recirculation with a 3-day SRT. The tanks are aerated with fine bubble diffuser membranes which can be operated at constant or intermittent air flow rates, or via Dissolved Oxygen (DO) or pH set points. Preliminary operation has included mixing and aerating tank 1 at a constant airflow rate of 5 L/min, while recording online measurements of temperature, DO, and pH (Figure 2). At this operation, about 50% OP removal across the pilot has been measured with minimal NH3 removal due to lack of pH increase at this airflow rate. Future plans include tracking COD across the pilot, increasing aeration, and the addition of Ca(OH)2 or Mg(OH)2 at various dosages. Batch Testing Results: Prior to running pilot scale tests, some objectives can be mechanistically answered by preliminary batch testing. Chemical Addition: The following test was conducted on a Phipps and Bird PB-900 series Jar Tester at a continuous mixing speed of 300 rpm. Five jars of digestate consisting of a control with no chemical addition, two jars dosed with a carbide lime slurry at Ca2+ + Mg2+/P ratios of 0.5:1 and 1:1, and two jars dosed with thioguard, similarly, at Ca2+ + Mg2+/P ratios of 0.5:1 and 1:1. Chemical dose was injected as a spike at time zero of the test. Analytical measurement of OP-P and pH were determined daily via spectrophotometry (Phosphorus HACH TNTplus Vial Test UHR 2-20 mg-PO4-P/L) and pH bench meter, respectively) (Figure 3).
The pH of the digestate sample was about 7.4 and increased up to a maximum of 8.7 within 24 hours likely due to CO2 stripping from the mixing intensity. OP removal in the control condition with no chemical addition was measured as 68%. The addition of chemical helped to increase OP removal as high as 95%. The digestate pH increasing towards alkaline within the first test day showed the greatest impact on OP removal rate. The higher Ca2+ + Mg2+/P molar ratio tested of 1:1 for both the carbide lime slurry and thioguard had higher OP removal rates than those dosed at lower ratios of 0.5:1. Thioguard compared to the carbide lime slurry overall had a slightly more significant effect on OP removal. The pH decrease that is measured after 24 hours may be due to struvite's production of hydrogen ions in the formation reaction, however, future plans of repeat experiments as well as X-Ray Diffraction for precipitate analysis will further determine this.

Cake Characteristics: A separate test was completed to look at aeration effects on cake characteristics. A sample from the DSST was conditioned, dewatered, and analyzed to evaluate polymer demand and cake solids. Polymer was tested on two buckets of digestate - a gently mixed control bucket and a bucket aerated at about 5 L/min. After 72 hours, polymer was used to condition each digestate. After polymer was added to the solids, it was mixed, and a capillary suction time (CST) instrument was used to find the optimal dose of polymer. The solutions with the lowest CSTs were identified to be the optimal dose of polymer needed and were then dewatered by the centrifuge cup method and measured for cake solids and solids capture, per the Higgins method [6]. In Table 1, the Aerated bucket had a higher polymer optimal dose, TS simulated cake value, and TSS simulated filtrate value compared to the Control bucket while the Control bucket had a higher VS simulated cake value. From these results, the aerated digestate had poorer solids capture when dewatered with the polymer compared to the Control bucket. In addition to cake dryness, improving cake odors will be researched in the future by the characterization of odor profiles and gas sampling the reactor head space using Chromatography Mass Spectrometry solid phase microextraction and Gastec Tubes.

Conclusion: This paper will present the challenges that ATP faces as well as batch and pilot research conducted to combat these issues. Optimization learned on this scale will advance the discussion on similar issues shared by wastewater treatment plants across the country. Further advancements have the possibility to improve plant efficiency and save plant capital costs.

This paper was presented at the WEF Residuals and Biosolids Conference, June 18-21, 2024.

SpeakerHarris, Caitlyn

Presentation time

13:30:00

14:00:00

Session time

13:30:00

15:00:00

SessionNutrient Considerations and Digestion

Session number16

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicAnaerobic Digestion, Pilot Scale, Struvite

Author(s)

Harris, Caitlyn

Author(s)C. Harris1, M. Garcia1, D. Gonzalez2, A. Williamson1, B. Ward3, J. Nicholson1, H. Matel1, C. Bott1, C. Wilson1

Author affiliation(s)HRSD 1; HRSD 1; Carollo Engineers 2; HRSD 1; HDR 3; Hampton Roads Sanitation District 1; Hampton Roads Sanitation District (HRSD) 1; HRSD 1; HRSD 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Jun 2024

DOI10.2175/193864718825159414

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count19

Phosphorus Sequestration in Biosolids, Nuisance Struvite Control via PAD and Chemical Addition to TH-AD Digestate

Abstract

Purpose: Struvite (MgNH4PO4 Â· 6H2O) and other common precipitates have long posed persistent maintenance and operational challenges at wastewater treatment plants. These precipitates can be enhanced at plants that utilize biological phosphorus removal and can precipitate within anaerobic digesters and on final dewatering equipment which causes damage and decreases treatment efficiency. Struvite can be sequestered as nutrient rich biosolids if properly controlled to remain in the cake. In a digested solids storage tank (DSST) that is in between anaerobic digestion and final dewatering, precipitation can be controlled by manipulating variables such as mixing, aeration of digestate to achieve a low solids retention time (SRT) post aerobic digestion (PAD) and by chemical addition. Optimizing this via pilot testing can control nuisance struvite formation, increase phosphorus in the cake and achieve additional volatile solids reduction (VSR). This research will review findings from pilot scale tests and discuss the nature of these precipitates. Objectives include determining the mechanisms that provide optimal phosphorus removal, ammonia removal, solids removal, and analyzing the precipitated solids of interest. The presentation will discuss results and lessons learned at the pilot scale and how they might translate to full scale plant upgrades. Motivation and Background: The Atlantic Treatment Plant (ATP) in Virginia Beach, VA, operated by Hampton Roads Sanitation District (HRSD) is a high-rate facility with an A/O process configuration. In solids handling, a Thermal Hydrolysis Process (THP) is utilized prior to anaerobic digestion. ATP is actively engaged in efforts to control formation of scaling precipitates and nuisance struvite for optimal operational efficiency. Precipitate formation is determined predominantly by the solids solubility product (Ksp) and the saturation in solution and can be controlled by pH [1]. Manipulation of pH via CO2 stripping from agitation and aeration has successfully been shown to control solubility reactions [2, 3]. Increasing the saturation index via chemical addition can also control precipitation reactions and improve cake dryness with an SRT of a few hours [4]. Likewise, PAD has been shown to achieve ammonia removal and additional VSR at a 5-6 day SRT [5]. Based on these findings, a pilot scale set up (Figure 1) was operated at an SRT of 3 days to provide some benefits from PAD such as ammonia removal, but with a longer reaction time that may be more beneficial for phosphorus mineral sequestration. Mixing, aeration rates, and chemical addition were manipulated to determine the optimal operation conditions on phosphorus sequestration in the Class A biosolids. Pilot Design and Operation: The pilot setup at ATP consisted of four tanks. Each tank had a volume of about 45 gallons. Simulating the DSST, the tanks were operated as daily batch fed continuously stirred tank reactors maintained by pump recirculation with a 3-day SRT. The tanks were aerated with fine bubble diffuser membranes which were operated at either constant air flow rates, or via Dissolved Oxygen (DO) or pH set points. Results: Six pilot campaigns were conducted to test the effects of aeration at a constant flow rate of 5 LPM, at a DO setpoint of 0.2 mg/L, and at pH setpoints of 7.5, 8, 8.5, and 9. Mg(OH)2 and Ca(OH)2 were injected at various Ca2++Mg2+:P ratios of 0 (control), 0.25:1, 0.5:1, 1:1, 1.3:1 and 1.6:1, and the effects of mixing was studied. Pilot results indicated aeration on a DO setpoint provided more stability to pH and DO measurements than constant aeration, which had a direct improvement on OP-P removal, NH3-N removal, the formation of precipitates, and alkalinity removal. Both Mg(OH)2 and Ca(OH)2 addition effectively removed OP-P, with improved efficiency as the Ca2++Mg2+:P ratio increased. At the higher 1:1 dosage, the performance of these two chemicals and aeration settings converged, showing no statistically significant difference in their phosphorus removal efficiency. The maximum OP-P removal achieved was 97% with aeration on a DO setpoint and Mg(OH)2 addition at a ratio Ca2++Mg2+:P of 1.3:1 (Figure 2). NH3-N removal averaged around 10-20%, with more efficient removal measured in the tests aerated at a DO setpoint. Ammonia-N removal was assumed to be due mostly to precipitation as struvite, with moderate ammonia stripping available at the pH values measured in these tests. No nitrification was observed in PAD most likely due to free ammonia (FA) inhibition. Alkalinity removal in each test surpassed what was calculated as the alkalinity consumption due to the predicted struvite formation, indicating that further coprecipitation reactions with struvite could be occurring. Finally, implementing aeration and mixing on TH-AD solids consistently achieved about 20% total COD removal across the pilot. However, when the influence of mixing was eliminated, total COD removal decreased by nearly half, suggesting that physical shear from the mixing pumps played a significant role in breaking down colloidal COD into soluble forms, which were more readily biodegradable. Persistent pilot research can lead to striking the balance between chemical and aeration costs, phosphorus removal benefits, struvite control to protect downstream equipment and treatment efficiency, as well as additional VSR achieved from PAD. This research will influence the DSST design modifications and operation in a future plant upgrade at ATP as well as provide key takeaways for plants with similar issues with scaling precipitates altogether.

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

SpeakerHarris, Caitlyn

Presentation time

11:05:00

11:25:00

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)

Harris, Caitlyn, Gonzalez, Dana, Williamson, Arba, Nicholson, Jeffrey, Ward, BJ, Matel, Holly Anne, Bott, Charles, Wilson, Christopher

Author(s)C. Harris1, D. Gonzalez2, A. Williamson3, J. Nicholson3, B. Ward3, H. Matel3, C. Bott3, C. Wilson3

Author affiliation(s)Brown and Caldwell, 1Carollo Engineers, 2Hampton Roads Sanitation District (HRSD), 3Hampton Roads Sanitation District (HRSD), 3Hampton Roads Sanitation Department, 3Hampton Roads Sanitation District (HRSD), 3Hampton Roads Sanitation District (HRSD), 3

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2025

DOI10.2175/193864718825159755

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count16

Phosphorus Sequestration in Biosolids, Nuisance Struvite Control via PAD and Chemical Addition to TH-AD Digestate

Abstract

Purpose: Struvite (MgNH4PO4*6H2O) and other precipitates have long posed persistent maintenance and operational challenges at wastewater treatment plants. These precipitates can be enhanced at plants that utilize biological phosphorus removal and can precipitate within anaerobic digesters and on final dewatering equipment which causes damage and decreases efficiency. Struvite can be sequestered as nutrient rich biosolids if properly controlled to remain in the cake. In a digested solids storage tank (DSST) that is in between anaerobic digestion and final dewatering, precipitation can be controlled by manipulating variables such as mixing, aeration of digestate to achieve a shorter solids retention time (SRT) version of post aerobic digestion (PAD) and by chemical addition. Optimizing this via batch and pilot testing can control nuisance struvite formation, increase phosphorus in the cake and achieve additional volatile solids reduction (VSR). This research will review findings from batch and pilot scale tests and discuss the nature of these precipitates. Objectives include determining the mechanism that provides optimal phosphorus removal, ammonia removal, and analyzing the precipitated solids of interest. The presentation will discuss results across batch, pilot, and full-scales and the lessons learned of each. Motivation and Background: The Atlantic Treatment Plant (ATP) in Virginia Beach, VA, operated by the Hampton Roads Sanitation District's (HRSD) is a high-rate facility with an A/O process configuration. In solids handling, a Cambi Thermal Hydrolysis Process (THP) is utilized prior to anaerobic digestion. ATP is actively engaged in efforts to control formation of scaling precipitates and nuisance struvite for optimal operational efficiency. Precipitate formation is determined predominantly by the solids solubility product (Ksp) and the saturation in solution and can be controlled by pH [1]. Manipulation of pH via CO2 stripping from aeration has successfully been shown to control solubility reactions [2, 3]. Increasing the saturation index via chemical addition can also control precipitation reactions and improve cake dryness with an SRT of a few hours [4]. Likewise, PAD has been shown to achieve ammonia removal and additional VSR at a 5-6 day SRT [5]. Based on these findings, a pilot scale set up (Figure 1) will run at an SRT of 3 days to provide benefits from PAD, but with a longer reaction time that may be more beneficial for phosphorus sequestration. Various mixing, aeration rates, and chemical addition will be manipulated to determine the optimal operation conditions on phosphorus sequestration in the class A biosolids. Pilot Design and Operation: The pilot set up at ATP consists of four tanks. Each tank has a volume of about 63 gallons. Simulating the DSST, the tanks are operated as daily batch fed continuously stirred tank reactors maintained by pump recirculation with a 3-day SRT. The tanks are aerated with fine bubble diffuser membranes which can be operated at constant or intermittent air flow rates, or via Dissolved Oxygen (DO) or pH set points. Preliminary operation has included mixing and aerating the four tanks at a constant airflow rate of 5 LPM, while recording online measurements of DO, pH (Figure 2), and temperature, which has been maintained between 22 and 38 degrees C. At this operation, about 50% OP removal and 10% COD degradation across the pilot has been measured with minimal NH3 removal. Future plans include operating at higher aeration rates, DO control setpoints, the potential establishment of partial nitrification/nitritation, and the addition of Ca(OH)2 or Mg(OH)2 at various dosages. Preliminary mixing operations conducted under suboxic conditions without the use of chemical addition results in a sustained OP removal by about half across the pilot. This suggests value in this continued effort. Batch Testing: The following test was conducted on a Phipps and Bird Jar Tester at a continuous mixing speed of 300 rpm. Five jars of digestate consisting of a control with no chemical addition, two jars dosed with a carbide lime slurry at Ca2+ + Mg2+/P ratios of 0.5:1 and 1:1, and two jars dosed with thioguard, at the same Ca2+ + Mg2+/P ratios. Chemical dose was injected as a spike at time zero of the test. Analytical measurement of OP-P and pH were determined daily via spectrophotometry (HACH TNTplus UHR mg-PO4-P/L and pH meter) (Figure 3). The pH of the digestate sample was about 7.4 and increased up to a maximum of 8.7 within 24 hours likely due to CO2 stripping from the mixing intensity. OP removal in the control condition was measured as 68%. The addition of chemical helped to increase OP removal as high as 95%. The digestate pH increasing towards alkaline within the first test day showed the greatest impact on OP removal rate. The higher Ca2+ + Mg2+/P molar ratio tested of 1:1 for both the chemicals had higher OP removal rates than those dosed at lower ratios of 0.5:1. Thioguard compared to the carbide lime slurry overall had a slightly more significant effect on OP removal. The pH decrease measured after 24 hours may be due to struvite's production of hydrogen ions in the formation reaction, however, future plans of repeat experiments and X-Ray Diffraction for precipitate analysis will further determine this.

A pilot setup evaluated the effects of mixing, aeration, and chemical addition on thermally hydrolyzed anaerobic digestate. Optimizing pilot operation can manage the formation of nuisance struvite and achieve a shorter solids retention time (SRT) version of post aerobic digestion (PAD). This pilot work will evaluate the benefits and limitations of a 3-day PAD process with chemical addition on OP-P removal, ammonia removal, alkalinity consumption, and COD removal.

SpeakerHarris, Caitlyn

Presentation time

09:00:00

09:20:00

Session time

08:30:00

10:00:00

SessionEnhancing Digestion to Improve Operations and Resource Recovery: Capture of Control

Session number501

Session locationRoom 256

TopicBiosolids and Residuals, Energy Production, Conservation, and Management, Facility Operations and Maintenance, Intermediate Level

Author(s)

Harris, Caitlyn, Nicholson, Jeffrey, Williamson, Arba, Matel, Holly Anne, Ward, BJ, Gonzalez, Dana, Bott, Charles, Wilson, Christopher

Author(s)C.A. Harris1, J.D. Nicholson2, A. Williamson2, H. Matel2, B. Ward3, D. Gonzalez4, C.B. Bott2, C. Wilson1

Author affiliation(s)1HRSD, VA, 2Hampton Roads Sanitation District (HRSD), VA, 3Hampton Roads Sanitation Department, VA, 4Carollo Engineers, CA

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159527

Volume / Issue

Content sourceWEFTEC

Word count16

Base text size -

Harris, Caitlyn

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