Gonzalez, Dana

Dana Gonzalez is a speaker at WEFTEC 2026.

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: Improving Biosolids Management Resiliency Through Regional Collaboration Among 15...

Improving Biosolids Management Resiliency Through Regional Collaboration Among 15 Agencies

Abstract

[b]APPLICABILITY[/b] Over the past several years, wastewater treatment plants across the country have experienced restrictions and disruptions to biosolids end uses driven by legislative and regulatory actions. Among these challenges are limitations on land application and landfill availability, legislative threats, increasing costs, and consolidation of providers. Collectively, these factors have reduced agency resiliency. Other agencies possess greater levels of resiliency but seek to reduce costs and risk or further diversify their portfolios for long-term reliability. Within this landscape, 15 Southern California (SoCal) agencies, Participating Agencies (PAs), joined to form the Southern California Regional Biosolids Coalition (SoCal RBC) and complete a study to determine the feasibility of establishing a regional, publicly owned and operated biosolids management facility. This facility would be one of the largest of its kind and would treat the biosolids generated by a significant portion of SoCal's population. The study was split into phases, with Phase 1 completed. This phase sets the basis for facility sizing, siting, viable technologies, and considerations related to governance and public engagement. [b]RESULTS [/b] [b]Methodology[/b] The methodology of the feasibility study focused on five elements: •[i]Current practices:[/i] A survey was developed which covered each PA's project drivers, biosolids management strategies, biosolids characteristics, and biosolids contributions to the proposed facility. •[i]Regulations and market conditions:[/i] The regulatory and market considerations in SoCal for building a biosolids facility and distributing biosolids for beneficial use were analyzed. Other regional efforts across the US were also reviewed and lessons learned were incorporated into our guidance. •[i]Alternatives development:[/i] The universe of potential sites and technologies were evaluated to develop a shortlist of three sites and treatment options. •[i]Governance:[/i] Governance models were reviewed, and three governance options identified. •[i]Strategic public engagement:[/i] Key messaging themes for public engagement were developed and a plan for public outreach was established. [b]Results[/b] [i]Current Practices[/i] Figure 1 shows the current biosolids end uses and backup management options for each of the PAs. This study determined that a regional facility would be initially sized to accommodate a low-capacity 2030 estimate with room to expand up to a high-capacity 2060 estimate. [i]Regulations and Market Conditions[/i] Many large agricultural counties in SoCal have severe restrictions on land application, which increases hauling distances for beneficial use. Additionally, while per- and polyfluoroalkyl substances (PFAS) regulations are not yet implemented in California, concern about these contaminants is mounting and considering the potential for future PFAS regulations is prudent. Additionally, three air districts were considered relative to siting a regional facility. The most restrictive air district would likely require a Title V permit and offsets. A market assessment was also conducted and concluded that the SoCal RBC should focus on producing a Class A product because it is the most resilient, understanding that it is unlikely to generate reliable or significant revenue. [i]Alternatives Development[/i] Figure 2 shows the universe of site and technology alternatives considered. Due to the evaluation criteria chosen by the PAs, advanced thermal technologies were eliminated during screening. However, to address PFAS concerns, the future addition of such technology was considered when assessing footprint. Three sites and biosolids treatment options were ultimately chosen for more detailed analysis, shown in Figure 3. Class 5 capital cost estimates and site layouts were developed, and 20-Year Net Present Worth costs were calculated. Figure 4 shows the non-economic evaluation conducted and Figure 5 shows the cost comparison. As shown in Figure 5, composting has the lowest capital cost but highest operations and maintenance costs. Of the two dryer alternatives, total costs are lower for rotary drum drying. While operated at a lower temperature than drum drying, belt drying has a greater natural gas (NG) cost to handle higher exhaust volume due to the increased NG requirements for RTO operation which is required for the SoCal air districts. [i]Governance[/i] This study also evaluated possible governance structures for the SoCal RBC. The three viable governance structures identified are shown in Figure 6. [i]Strategic Public Engagement[/i] This study also identified key messaging themes for future public engagement, shown in Figure 7. From there, a public engagement plan for implementation in future project phases was developed and stakeholders identified. [b]Conclusions[/b] This collaboration among 15 PAs focuses on advancing a resilient biosolids management solution and represents one of the largest collaborations of its kind. Subsequent phases of this project will advance one of the short-listed options through preliminary design and possibly through to final implementation. The approach taken and key findings from this work, including siting, technologies, governance, and public engagement may be useful for other regions and groups considering options to increase their biosolids management resilience

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

SpeakerCharbonnet, Elizabeth

Presentation time

13:50:00

14:00:00

Session time

13:30:00

15:00:00

SessionAdvancing Biosolids Planning Through Optimization and Collaboration

Session locationErnest N. Morial Convention Center

TopicBiosolids and Residuals, Circular Water Economy, Public Communication and Outreach, Utility Management and Leadership

Author(s)

Charbonnet, Elizabeth, Stewart, Kevin, AghaGholizadeh, Erica, Berninghaus, Ashley, Chapman, Thomas, Chavan, Seema, Gonzalez, Dana, Guevarra, Karla, Gupta, Rashi, Judy, Caroline, Ma, Persephone, Rossetto, Sarah, Sierra, Natalie, Young, Michelle

Author(s)E. Charbonnet1, K. Stewart2, E. AghaGholizadeh2, A. Berninghaus1, T. Chapman2, S. Chavan2, D. Gonzalez1, K. Guevarra3, R. Gupta1, C. Judy4, P. Ma2, S. Rossetto6, N. Sierra2, M. Young1

Author affiliation(s)Carollo Engineers, 1Brown and Caldwell, 2Carollo Engineers, 1Brown and Caldwell, 2Brown and Caldwell, 2Carollo Engineers, 1Pierce County, WA, 3Carollo Engineers, 1Project Finance Advisory Limited, 4Brown and Caldwell, 2Brown and Caldwell, 2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160466

Volume / Issue

Content sourceWEFTEC

Word count11

Interpreting PFAS Fate Through Thermal Hydrolysis To Support Biosolids Management Decision-Making

Abstract

APPLICABILITY Growing regulatory scrutiny of per- and polyfluoroalkyl substances (PFAS) is increasing pressure on utilities to reassess biosolids management strategies in the absence of finalized regulatory thresholds or widely deployable treatment solutions. Evolving state requirements, uncertain federal timelines, reduced land application options, and incomplete understanding of PFAS fate through solids treatment processes complicate long-term planning for biosolids treatment, reuse, and disposal. In this environment, utilities are increasingly required to make consequential, risk-informed decisions using incomplete data, evolving analytical methods, and treatment configurations that were not originally designed with PFAS considerations in mind. Thermal hydrolysis has emerged as a key solids treatment technology for capacity expansion and process intensification, yet questions remain regarding how PFAS behavior through thermal hydrolysis and downstream anaerobic digestion should be interpreted and incorporated into broader biosolids management strategies.

DEMONSTRATED RESULTS AND OUTCOMES Methodology A full-scale evaluation of PFAS fate through solids treatment was conducted at Hampton Roads Sanitation District (HRSD) at the Atlantic Treatment Plant (ATP). The evaluation spanned an operating period characterized by conventional anaerobic digestion, followed by a post-consolidation period after implementation of thermal hydrolysis to provide additional solids treatment capacity required to support consolidation of wastewater flows at ATP following closure of another major treatment plant in the service area. The ATP-specific evaluation focused on changes in PFAS class distributions through solids treatment under differing operating conditions. In parallel, HRSD conducted broader biosolids PFAS characterization across additional treatment facilities to assess spatial variability and system-level implications for biosolids management. Together, these datasets were used to contextualize site-specific findings within a utility-wide decision-making framework under regulatory uncertainty. Results and Interpretation At ATP, anaerobic digestion during the pre-thermal hydrolysis operating period was associated with reductions in PFAS precursors and redistribution toward terminal PFAAs. Following implementation of thermal hydrolysis, observed PFAS class distributions in treated solids differed from those observed under digestion-only conditions (Figure 1). These differences reflected not only the change in treatment configuration, but also shifts in influent composition, internal recycle streams, and upstream flow routing associated with regional consolidation. Across operating periods, total fluorine concentrations exceeded fluorine attributable to quantified PFAS, highlighting the presence of uncharacterized organofluorine and reinforcing the limitations of PFAS-targeted analytical approaches when used in isolation. System-wide biosolids characterization further demonstrated substantial variability in PFAS concentrations among facilities within the same regional program (Figure 2), underscoring that PFAS-related risk is not uniformly distributed across a biosolids management system. Rather than indicating a single treatment outcome or technology effect, the results emphasize the importance of interpreting PFAS fate data within the broader operational and system context in which the data were generated. Conclusions/Outcomes Insights from the full-scale thermal hydrolysis evaluation, when considered alongside system-wide PFAS characterization and evolving regulatory context, were used to inform adjustments to biosolids management strategy while maintaining operational flexibility. Following release of the U.S. EPA's Draft Sewage Sludge Risk Assessment for PFOA and PFOS, biosolids routing and management approaches were revisited to balance PFAS-related risk considerations with capacity, reliability, and long-term planning needs (Figure 3). This work demonstrates that PFAS fate data are most valuable when interpreted cautiously, integrated across facilities, and viewed as one component of a broader decision-making framework. The presentation highlights how utilities can apply full-scale operational data to support adaptive biosolids management strategies without over-interpreting incomplete science or waiting for definitive regulatory endpoints.

RELEVANCE TO AUDIENCE This presentation will benefit utility managers, biosolids professionals, process engineers, planners, and regulators involved in solids treatment and biosolids management. The discussion is particularly relevant for organizations evaluating thermal hydrolysis or other solids process changes while navigating PFAS-related uncertainty. Rather than promoting a specific technology or prescriptive outcome, the presentation focuses on how PFAS fate data from full-scale operations can be responsibly interpreted, contextualized, and incorporated into practical biosolids management decisions under real-world constraints. The lessons presented are broadly applicable to utilities facing similar challenges associated with influent variability, evolving regulations, and uncertainty surrounding PFAS behavior in solids treatment systems.

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

Presentation time

13:30:00

14:00:00

Session time

13:30:00

15:00:00

SessionThermal Treatment of PFAS in Biosolids

Session locationKansas City Convention Center

TopicContaminants of Concern (PFAS, Inhibitory Compunds)

Author(s)

Gonzalez, Dana, Ward, Barbara, Matel, Holly Anne, Alukkal, Caroline, Lee, Linda, Wilson, Christopher, Bott, Charles

Author(s)D. Gonzalez1, B. Ward2, H. Matel2, C. Alukkal3, L. Lee3, C. Wilson2, C. Bott2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2026

DOI10.2175/193864718825160234

Volume / Issue

Content sourceResiduals, Biosolids and Treatment Technology Conference

Word count12

Interpreting PFAS Fate to Inform Biosolids Management

Abstract

APPLICABILITY Growing regulatory scrutiny of per- and polyfluoroalkyl substances (PFAS) is increasing pressure on utilities to reassess biosolids management strategies. Evolving state requirements, uncertain federal timelines, reduced land application options, and the absence of widely deployable PFAS destruction technologies complicate long-term planning for solids treatment and reuse. In this environment, utilities must make risk-informed decisions using incomplete and evolving data. This work describes how PFAS fate and transport data generated through full-scale solids treatment evaluations were interpreted and applied to biosolids management decisions under regulatory uncertainty. The effort draws on a detailed evaluation of PFAS behavior through anaerobic digestion and thermal hydrolysis conducted at Hampton Roads Sanitation District's (HRSD's) Atlantic Treatment Plant (ATP), followed by broader, system-wide PFAS characterization across HRSD's other treatment facilities. Together, these efforts illustrate how site-specific treatment data, when considered within a regional context, can support adaptive biosolids management decisions. DEMONSTRATED RESULTS AND OUTCOMES Methodology A focused, full-scale evaluation of PFAS fate through solids treatment was conducted at ATP during two distinct operating periods. The first occurred in 2019 and was characterized by conventional anaerobic digestion. The second followed implementation of thermal hydrolysis to provide the additional solids treatment capacity required to support consolidation of wastewater flows at ATP after closure of another major treatment plant in the service area. This ATP-specific work examined changes in PFAS concentrations and class distributions through treatment under different operating conditions. Analytical targets for this evaluation included PFAS precursors, intermediate compounds, terminal PFAS (the perfluoroalkyl acids - PFAAs), and total fluorine to assess both quantified and unquantified organofluorine contributions. In parallel with and subsequent to this work, HRSD conducted PFAS characterization of biosolids produced at its other treatment facilities to document PFAS concentrations across the system and compare PFAS levels among facilities. Results At ATP, anaerobic digestion during the 2019 operating period was associated with reductions in PFAS precursors and redistribution toward terminal PFAAs (Figure 1). Following implementation of thermal hydrolysis to support increased wastewater flows at ATP, influent PFAS concentrations and class distributions entering the solids train changed as a result of upstream flow consolidation and routing adjustments. Post-treatment PFAS class distributions observed after thermal hydrolysis and downstream anaerobic digestion differed from those observed during the earlier anaerobic digestion-only period (Figure 1). These differences reflect the combined influence of influent composition, treatment configuration, and matrix effects. Evaluation of PFAS at ATP also indicated that total fluorine concentrations exceeded fluorine attributable to quantified PFAS, highlighting the presence of uncharacterized organofluorine and limitations associated with PFAS-targeted analytical approaches. Separately, PFAS characterization across HRSD's other treatment facilities demonstrated variability in PFOS concentrations among biosolids produced across the region (Figure 2). These results indicate that PFAS-related risk is not uniformly distributed across the regional biosolids program and underscore the importance of facility-specific data when evaluating management options. Conclusions/Outcomes Insights from the ATP solids treatment evaluation, when considered alongside system-wide PFAS characterization and evolving regulatory context, informed adjustments to HRSD's biosolids management strategy (Figure 3). Following release of the U.S. EPA's Draft Sewage Sludge Risk Assessment for PFOA and PFOS in early 2025, biosolids routing was adjusted to maintain operational flexibility while addressing PFAS-related risk considerations. In parallel, HRSD initiated additional efforts to better understand PFAS loadings within the treatment process, including characterization of internal recycle streams and evaluation of post-processing steps such as curing and drying. Together, these efforts reflect an adaptive approach to biosolids management that integrates facility-specific data, system-wide characterization, and cautious interpretation of PFAS fate under conditions of regulatory and scientific uncertainty. RELEVANCE TO AUDIENCE This presentation will benefit utility managers, biosolids professionals, process engineers, and regulators involved in solids treatment, biosolids reuse planning, and PFAS risk management. The work is particularly relevant for utilities navigating PFAS-related decision-making in the absence of finalized regulatory thresholds or widely deployable treatment solutions. Rather than proposing a single technology or prescriptive approach, the presentation emphasizes how PFAS fate data from full-scale operations can be interpreted cautiously and applied alongside system-wide characterization to support practical biosolids management decisions. The lessons presented are broadly applicable to utilities facing similar challenges associated with influent variability, evolving regulations, and uncertainty surrounding PFAS behavior in solids treatment processes.

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

Presentation time

09:30:00

09:45:00

Session time

8:30:00

10:00:00

SessionPFAS in Biosolids: Fate, Transport, and Land Application

Session locationErnest N. Morial Convention Center

TopicBiosolids and Residuals, Disinfection and Public Health, PFAS, Research and Innovation

Author(s)

Gonzalez, Dana, Ward, Barbara, Matel, Holly Anne, Alukkal, Caroline, Lee, Linda, Wilson, Christopher, Bott, Charles

Author(s)D. Gonzalez1, B. Ward2, H. Matel2, C. Alukkal3, L. Lee3, C. Wilson2, C. Bott2

Author affiliation(s)Carollo Engineers, 1Hampton Roads Sanitation District (HRSD), 2Hampton Roads Sanitation District (HRSD), 2Purdue University, 3Purdue University, 3Hampton Roads Sanitation District (HRSD), 2Hampton Roads Sanitation District (HRSD), 2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160404

Volume / Issue

Content sourceWEFTEC

Word count8

Optimizing Class A Biosolids: THP Startup, Troubleshooting, and Long-Term Benefits

Abstract

APPLICABILITY
Hampton Roads Sanitation District (HRSD) serves 1.9 million residents across southeastern Virginia and transitioned to Class A biosolids production at the Atlantic Treatment Plant (ATP) by implementing a Cambi Mark II B6-4 Thermal Hydrolysis Process (THP) in 2020. Initially, HRSD focused on digester startup and stabilization, monitoring critical health parameters such as volatile fatty acids, pH, alkalinity, and methane production. Once the digesters achieved stability, attention shifted to ensuring consistent Class A compliance in line with HRSD's permit requirements.

The startup process involved a rigorous, phased approach. Digesters were disinfected, and then filled with 800,000 gallons of disinfected non-potable water prior to seeding with 190,000 gallons of digestate from DC Water. Feed rates of thermally hydrolyzed sludge (THS) were ramped up methodically, starting at 50% of expected loading rates to allow operators to closely monitor critical health metrics. This controlled approach allowed HRSD to respond quickly to upsets.

This presentation highlights ATP's approach to overcoming operational challenges, including troubleshooting cross-connections, addressing rapid rise events, mitigating pinhole leaks, and managing solids to enable the shutdown of incineration at the Army Base (AB) facility. These insights provide a roadmap for utilities transitioning to Class A biosolids production.

DEMONSTRATED RESULTS AND OUTCOMES

Methodology
HRSD implemented improvements and troubleshooting measures to ensure Class A compliance:

Early Startup Troubleshooting: Cross-Connection Identification and Resolution
- Cross-Connection Troubleshooting: During startup, HRSD observed intermittent fecal coliform (FC) detections that threatened Class A compliance. A detailed review of design drawings revealed three cross-connections where raw solids lines were inadvertently tied into THP cooling loops for Digesters 1, 2, and 3. Maintenance teams systematically blinded off these connections, beginning with Digesters 1 and 2. Immediate FC reductions were observed (Figure 1).

Longer-Term Lessons Learned and Benefits
- Rapid Rise Events: A rapid rise event caused by mixing loss and intermittent gas release prompted an in-depth analysis of SCADA trends (Figure 2). Indicators such as pumped flow reductions, cover rise rates, and gas spikes were identified as early warnings. Operator alerts and real-time response protocols were implemented to mitigate recurrence.
- Pinhole Leaks: After approximately 4.5 years of operation, pinhole leaks developed in the flash tank-to-pulper line. The likely cause was buildup that narrowed the pipe diameter, increasing fluid velocity and accelerating wear, particularly at bends. In response, a routine cleaning protocol was implemented to prevent future leaks and extend the asset's lifespan.
- Shutdown of AB Incinerator: ATP's capacity improvements enabled HRSD to accept thickened waste activated sludge (TWAS) from the Army Base facility. This allowed HRSD to shut down the AB incinerator in Fall of 2024, reducing costs and increasing biogas production.

#Result
Blinding the cross-connections resulted in significant FC reductions in Digesters 1 and 2 (Figure 1), while Digester 3 stabilized after an additional cross-connection was addressed. Routine monitoring was implemented to ensure the consistent production of Class A cake.

#Analysis of a rapid rise event revealed SCADA patterns that served as critical early warnings, leading to improved monitoring and response. Addressing pinhole leaks through routine cleaning successfully mitigated further issues, preventing costly failures. Finally, the shutdown of the AB incinerator validated ATP's ability to manage additional solids sustainably while enhancing biogas production and reducing environmental impacts.

Conclusions/Outcomes
HRSD's efforts resolved key operational challenges, ensuring Class A biosolids production through systematic troubleshooting and long-term improvements:
- Early Startup Fixes: Blinding cross-connections and implementing a monitoring program now ensures Class A quality of biosolids.
- Operational Improvements: Addressing rapid rise events and pinhole leaks improved monitoring, resilience, and infrastructure integrity.
- Regional Benefits: The shutdown of AB's incinerator reduced costs and increased biogas production, demonstrating ATP's capacity for sustainable biosolids management. These outcomes offer a framework for utilities transitioning to or optimizing Class A biosolids production.

RELEVANCE TO AUDIENCE

HRSD's experience at ATP delivers actionable insights for engineers, operators, and managers implementing advanced biosolids treatment. Attendees will gain knowledge on:
- Systematic Troubleshooting: Resolving contamination sources like cross-connections and improving pathogen removal.
- Proactive Maintenance: Mitigating pinhole leaks with routine cleaning protocols to extend asset life.
- Operational Resilience: Addressing rapid rise events through real-time monitoring and improved response strategies.
- Regional Optimization: Leveraging TWAS acceptance to enable incinerator shutdown while enhancing energy recovery.

This presentation provides practical solutions for achieving Class A biosolids compliance, addressing operational challenges, and improving plant performance.

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

Presentation time

08:30:00

08:45:00

Session time

08:30:00

10:00:00

SessionInnovative THP Startup and Biosolids Finishing

Session locationMcCormick Place, Chicago, Illinois, USA

TopicBiosolids & Residuals

Author(s)

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 Engineers1, Hampton Roads Sanitation District (HRSD)2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2025

DOI10.2175/193864718825159978

Volume / Issue

Content sourceWEFTEC

Word count11

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)