Gonzalez, Dana

Dana Gonzalez is a Principal Technologist with Carollo Engineers, based in California.

Titles from this speaker

Description: A Beautiful Biosolids Product in the Making: Dewatering Optimization and Aerobic...

A Beautiful Biosolids Product in the Making: Dewatering Optimization and Aerobic Curing of Thermally Hydrolyzed Solids at HRSD's Atlantic Treatment Plant

Abstract

Background In mid-2020 Hampton Roads Sanitation District (HRSD) transitioned to Class A biosolids production via thermal hydrolysis process (THP) using a Cambi Mark II B6-4 unit at the Atlantic Treatment Plant (ATP). THP increased solids capacity by allowing for higher digester loading, increased volatile solids reduction (VSR), and significantly improved final dewatering performance. This additional capacity will allow ATP to receive solids from other HRSD facilities, which will provide the opportunity to shut down multiple hearth incinerators at other plants. In addition, THP is near energy neutral and can be energy positive, especially with the addition of fats, oil, and grease (FOG) receiving at the ATP installation. The B6-4 system relies on steam injection within batch reactors to thermally hydrolyze solids prior to anaerobic digestion. Not only does this process kill pathogens within the solids, but it also reduces the viscosity of feed solids and allows for higher digester loading. Initial startup and seeding of the digesters with DC Water digestate occurred in May 2020; it took approximately six months to startup three anaerobic digesters after disinfection and seeding. Using the same dewatering centrifuges, final dewatering cake solids content increased from the baseline prior to THP (14-16% total solids (TS)) to a range of 20-25% TS with minimal dewatering optimization. Additional optimization at final dewatering resulted in reliable 30% TS final cake solids production without any change in polymer selection. Additional aerobic windrow turning of solids was shown to produce a low-odor product that was 55-60% TS. Purpose This presentation will focus on final dewatering centrifuge optimization that was conducted to reliably produce a 30% cake product. In addition, a series of curing pilots were conducted using the 30% cake and a windrow turner to determine the most efficient method for producing an aerobically cured product with low odor. Data from final dewatering optimization and the curing pilots were used to evaluate cost savings associated with each improvement. Final Dewatering Optimization Initial dewatering optimization focused on evaluating solids loading rates after transitioning to a thicker feed coming out of the THP digesters. After achieving optimal loading rates, staff focused efforts on polymer dosing locations and sludge-polymer mixing upstream of the final dewatering centrifuges. Adding 100% of the polymer upstream of the centrifuge at the sludge-polymer inline mechanical mixers allowed plant staff to minimize the mass of polymer required to get optimal capture. In addition, after testing various sludge-polymer mixing intensities, it was found that the optimal mixer speed was 900 rpm. This mixing intensity achieved the same or better recoveries with an even lower polymer usage rate (Figure 1). After capture optimization was completed, staff focused on cake dryness. To do this, the torque setpoint on the centrifuge was increased progressively and additional polymer was added to maintain equivalent capture. These data were used to compare the cost benefit of polymer usage vs. reduced hauling rates with increased cake dryness (Figure 2). Additional data will be presented to show the dewatering benefits of ferric addition upstream of the primary clarifiers. While plant staff initially added ferric to help minimize reduced sulfur compound odors in the final cake product, it was found that half of the ferric costs could be recouped with polymer savings in final dewatering. Curing Pilots Various iterations of curing pilots were executed to answer two main questions: (1) What are the characteristics of the final cured end-product? and (2) What is the most efficient way to achieve that end-product? Additionally, data collected during pilot studies were used to evaluate the business case for curing THP solids. End-Product Characteristics Figure 3 shows %TS and percent volatile solids (%VS) from several curing pilot experiments in which solids were aerated with a windrow turner at various frequencies, with and without the addition of cured cake to piles to 'seed' the process. When evaluated holistically, it appears that the final cured product tended to be 55-60% TS and 55-60% VS. Once the piles achieved those endpoints, plant staff found that the product was too dusty to continue turning. Seasonal VSR was evaluated using the Van Kleek method (Figure 4). During the winter, addtional VSR due to curing hovered around 10-15%, whereas it was seen as high as 40% in the spring and 45% in the summer. Production Efficiency Two main variables, seed addition and frequency of turning, were evaluated to determine if they impacted the speed at which cured cake could be produced. Seed addition consisted of adding variable amounts of already cured product to fresh cake by volume. Although this study did not determine the mechanism by which back mixing cured product with fresh product, two main explanations are likely: (1) Seed addition creates additional pore space for oxygen penetration into piles, similar to wood chip addition in the composting process, and (2) Seed addition kick starts the curing process by adding a large quantity of microorganisms adapted to aerobic curing. A variety of turning scenarios were employed within the five-day work week (10 times, 5 times, 3 times, and 2 times per week). Observations and data collected during these phases indicated that turning 5 times a week (once a day Monday to Friday) was sufficient to achieve the end-product. Seeding appeared to accelerate the curing process (Figure 5). While it took fresh cake-only piles approximately four weeks to reach the end point, the addition of 25% cured cake by volume reduced the curing time by at least one week. It should be acknowledged that using 25% of pad capacity for a 25% reduction in curing time is a breakeven point, however, qualitative observations during the process indicated that odors emanating from the curing piles were less offensive when 45% TS was achieved in the piles. Seed addition may therefore offer an additional benefit if that threshold is achieved faster in the process. Conclusions The combination of final dewatering centrifuge optimization and aerobic curing have been utilized at HRSD's Atlantic Treatment Plant to produce a low odor, high value soil amendment. Ultimately, dryer cake with lower odors will result in lower hauling costs and less truck traffic through neighborhoods surrounding the plant site.

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

SpeakerGonzalez, Dana

Presentation time

09:00:00

09:30:00

Session time

08:30:00

11:15:00

SessionTroubleshooting & Optimization

Session number26

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicClass A, Dewatering Optimization, Thermal Hydrolysis

Author(s)

Gonzalez, Dana

Author(s)D. Gonzalez1, J. Nicholson2, C. Wilson2, H. Matel2, C. Bott2

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

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Jun 2024

DOI10.2175/193864718825159435

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count22

Description: Enhancing Biosolids Management with THP: From Startup to Optimization and...

Enhancing Biosolids Management with THP: From Startup to Optimization and Troubleshooting at HRSD's Atlantic Treatment Plant

Abstract

APPLICABILITY Hampton Roads Sanitation District (HRSD) serves 1.7 million residents across southeastern Virginia. In 2020, HRSD installed a Cambi Mark II B6-4 Thermal Hydrolysis Process (THP) at the Atlantic Treatment Plant (ATP), transitioning from Class B to Class A biosolids. This upgrade has increased digester capacity, improved dewatering efficiency, and produced a marketable biosolids product. By combining THP with aerobic curing, ATP has improved biosolids quality while reducing environmental impacts. This presentation will explore operational lessons, including startup and commissioning, maintenance strategies, and solutions to challenges such as preventing rapid rise events. ATP's experience provides a roadmap for utilities to enhance biosolids management, reduce costs, and meet regulatory requirements. DEMONSTRATED RESULTS AND OUTCOMES Methodology: The THP system was installed at ATP to optimize biosolids management, focusing on increasing digester capacity, volatile solids reduction (VSR), gas production, and dewaterability. The startup involved seeding digesters with THP digestate from DC Water, followed by a gradual ramp-up of feed rates while closely monitoring health parameters. Early challenges, such as rising hydrogen sulfide (H2S) concentrations, were resolved with feed rate adjustments and liquid soda ash. Startup and Commissioning: Startup and commissioning of the THP system at ATP presented several unique challenges and opportunities for learning. To ensure a successful transition, the startup process began with seeding the first of three THP digesters using digestate trucked in from DC Water. Over 190,000 gallons of seed material were combined with approximately 800,000 gallons of filtered and disinfected non potable water. The goal was to establish a stable microbial population capable of processing thermally hydrolyzed sludge (THS). Initial seeding targeted a total solids (TS) concentration of 1% before introducing the THS feed, but practical limitations led to starting at 0.7% TS. The goal of the startup process was to begin at 50% of DC Water's loading rate and gradually ramping up over a 2-3 week period. During this time, key digester health parameters, such as volatile fatty acids (VFA), pH, alkalinity, and methane content, were closely monitored to identify any signs of process instability. Figures 1 and 2 show the trends in these parameters over the first 70 and 120 days of operation, respectively. Early challenges included an unanticipated rise in H2S concentrations in the digester headspace, attributed to residual sulfur compounds from a bioscrubber upgrade. This caused a temporary inhibition of methanogens, leading to increased VFA levels, reduced gas production, and lower pH starting around day 75, as observed in Figure 2. Through a combination of feed rate adjustments and the addition of liquid soda ash, digester health was restored. The experience highlighted the importance of routine, real-time measurements to identify and address upsets early in the process. By day 150, the digester had reached steady state, demonstrating resilience to variations in feed rates and loading (Figure 3). Retrospective Operational Analysis: Multiple retrospective analyses evaluated the performance of the THP system, focusing on gas production and VSR. A cyclical VSR pattern emerged, influenced by the primary sludge (PS) to waste activated sludge (WAS) ratio. Higher VSR rates were observed in winter due to a higher PS/WAS ratio in the feed, while lower VSR rates in summer were associated with reduced volatile solids content in WAS (Figure 4). This variation is driven by the digester's extended retention time, which allows it to consistently reach the same ending VS content in the finished digestate. Consequently, the VSR achieved is directly influenced by the initial VS content in the feed. Troubleshooting and Optimization: The integration of the THP system at ATP was not without its challenges. One of the most significant issues was a rapid rise event in one of the three THP digesters, primarily due to the loss of mixing (digesters are typically mixed using 3 perimeter and one central draft tube mixer). Figure 5 illustrates a plant SCADA trend leading up to and during the rapid rise event, which began on 6/4/24 and concluded on 6/7/24. Although digestate overflow did not occur until 6/7/24, a post-hoc evaluation revealed that the conditions leading to the overflow started as early as 6/4/24. The first indication of the event was the rate of rise in the digester cover level. As shown in Figure 5, the green line represents the cover level, and after the event began, the rate of rise increased despite no significant change in feed rates. Figure 6 further highlights this, showing the rate of cover rise exceeding 2 inches per minute once the rapid rise event started. On 6/5, pumped recirculation flows (purple line in Figure 5) began to intermittently drop despite constant pump speed, without any clear cause. The noise in the purple line from 6/1 to 6/3 is attributed to bubbles from foul gas condensate being added to the digester. However, during the flow drops on 6/5, no condensate was added to the digester. Additionally, these flow drops were immediately followed by a spike in digester gas flow rates (yellow line), suggesting that both issues were linked to intermittent gas release prior to the overflow event. This post-hoc analysis identified the need for a new alarm to monitor the rate of cover rise as an early warning for operators, potentially preventing future overflow events. Further optimization of the THP system focused on improving the efficiency of aerobic curing, where solids are formed into windrows and periodically turned with a windrow turner to produce a cured product that is drier and less odorous. By conducting a series of curing pilots, ATP determined that adding 25% cured product by volume accelerated the stabilization process, reducing curing time by one week. This approach not only improved the overall quality of the final product but also reduced the intensity of odors emanating from the curing piles. Figure 7 presents the seasonal variation in volatile solids reduction (VSR) during aerobic curing, with significant improvements observed in spring and summer. Operational Achievements and Financial Impacts: One of ATP's most significant achievements has been performing a rapid three-day annual THP O&M turnaround, eliminating the need for raw cake loadout, all without have a standby pulper or flash tank included in the design. Traditionally, THP systems require a one-week shutdown, leading to raw cake loadout and neighborhood odor complaints. ATP successfully completed maintenance without needing to haul unstabilized biosolids, saving the plant hundreds of thousands of dollars in hauling fees and preventing odor complaints. RELEVANCE TO AUDIENCE HRSD's experience at the Atlantic Treatment Plant offers critical insights for operators, design engineers, and managers looking to improve biosolids management practices through advanced technologies. The strategic combination of thermal hydrolysis, dewatering optimization, and innovative aerobic curing has resulted in sustainable, cost-effective operations that reduce environmental impact while meeting regulatory requirements. This presentation will highlight key takeaways such as the ability to rapidly perform a three-day O&M turnaround without raw cake loadout, providing both operational and financial benefits. Another key takeaway is the importance of monitoring and interpreting data to identify issues during start-up; the extensive monitoring has proven extremely useful in fine-tuning the THP digestion process. Attendees will gain practical knowledge on leveraging continuous monitoring, optimizing gas production, and enhancing volatile solids reduction, which can be applied to other facilities. The lessons learned at ATP present a valuable roadmap for utilities seeking to modernize their biosolids management practices, improve efficiency, and address the challenges of future regulatory demands.

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

SpeakerGonzalez, Dana

Presentation time

13:50:00

14:10:00

Session time

13:30:00

16:45:00

SessionCase Studies for Optimizing THP, Dewatering, and Digestion

Session number7

Session locationBaltimore Convention Center, Baltimore, Maryland, USA

TopicAnaerobic Digestion, Biosolids, Dewatering Optimization, Energy and Carbon Neutrality, fugitive methane emission, Full-scale Optimization, Greenhouse Gases, Mesophilic Anaerobic Digestion, Odor/Air Emissions, Operations And Maintenance, Process modeling, Reliability, Sludge Digestion, THP, THP, CAMBI

Author(s)

Gonzalez, Dana, Matel, Holly Anne, Ward, Barbara, Nicholson, Jeffrey, Wilson, Christopher, Bott, Charles

Author(s)D. Gonzalez1, H. Matel2, B. Ward2, J. Nicholson2, C. Wilson2, C. Bott2

Author affiliation(s)Carollo Engineers, 1Hampton Roads Sanitation District (HRSD), 2Hampton Roads Sanitation District (HRSD), 2Hampton Roads Sanitation District (HRSD), 2HRSD, 2Hampton Roads Sanitation District (HRSD), 2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2025

DOI10.2175/193864718825159764

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count17

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)