Energy recovery and sustainability projects at water resource recovery facilities (WRRF) are typically discretionary and require an attractive return on investment to justify the capital expense. The Inflation Reduction Act (IRA), which provides federal funding for projects that produce renewable energy, dramatically changed financial considerations for energy projects. The certainty of the IRA to provide up to a 50% rebate on qualified capital expenditures means WRRF bioenergy projects that otherwise would not have been attractive have moved forward across the United States. To qualify, construction had to begin before December 31, 2024, creating increased urgency to accelerate project schedules that is novel to our industry. Many WRRFs pursued eligibility by implementing biogas fueled cogeneration (also known as combined heat and power, or CHP) systems to produce renewable electricity and heat on-site. With greater urgency in project schedule comes greater potential for overlooking big picture and fine details that are critical to a successful cogeneration system. Even with IRA rebate, cogeneration can be a big capital investment that hinges on the upside of its O&M benefit, so when they are not designed properly, the consequences can be severe for the owner. This is key to our industry's future, because interest in beneficial use of biogas is surging nationally as more utilities and municipalities implement voluntary climate action plans to reduce near-term greenhouse gas (GHG) emissions. We will see more facilities pursuing energy recovery technologies like cogeneration. OBJECTIVE This manuscript will summarize: 1.The cogeneration system sizing approach recently taken by a large WRRF that all stakeholders can learn from, including: owners, consultants, and plant staff. 2.Evidence for why it's important to size cogeneration systems properly. 3.The array of site-specific factors that drive the sizing and configuration of a cogeneration system. The intent is to give the audience an idea of what questions one should ask if interested in adding cogeneration to their facility. The session will use an evaluation done for the City of Columbus, Ohio (City) Southerly WWTP (SWWTP) Bioenergy Project that includes: 6 MG of digestion, fats oils grease (FOG) receiving facility for co-digestion, and 6 MW cogeneration system, as a case study showcasing the influence of IRA funding on cogeneration system alternatives development. Drivers that are client site-specific include: 1.The City has a climate action plan with aggressive goals to slash GHGs. 2.City staff have familiarity owning and operating a cogeneration system because their sister plant recently installed engines. 3.Life cycle economics are significantly better over the status quo. 4.Adding cogeneration to the Bioenergy Project makes it eligible for IRA Credit. To determine how to best leverage IRA funding, the City considered biogas conditioning and biogas upgrading paired with internal combustion engine or turbine technology (also referred to as 'prime mover'). Although very uncommon, pairing biogas upgrading to renewable natural gas quality with cogeneration was investigated to evaluate the reduced O&M requirements of the engines or turbines and simplification of fuel blending. SIZING APPROACH Goals and objectives must be established, and understanding how they differ helps to manage expectations and decision making as the design progresses. Goals serve as long term aspirations while objectives are requirements that must be met for the system to be deemed functional. Objectives SHOULD be weighted heavier, and oftentimes these aren't identified until the project team drills down into what's required to achieve goals. For the City it was more important that the system had high uptime, but it wasn't a necessary to keep the prime mover loaded 100% 24/7. The City wanted turndown for startup through year 20 biogas production which includes ramping up the FOG program - but they could tolerate some flaring. The heat recovery performance was weighted heavy because this will have a large impact on reducing the natural gas bill and meeting City climate action plan goals. The manuscript and presentation will do a deep dive into the details of matching the cogeneration system's size to the facility's biogas production and heat demands, which include: - Know Your Gas: having a firm handle on biogas production is going to bracket the cogeneration system's operation. The design biogas flow is estimated to be nearly 2,000 SCFM, while the startup minimum flow could be as low as 650 SCFM. - Utilize Your Gas, Big Picture: establish the universe of prime mover size alternatives that could fit the amount of biogas produced (Figure 1). Smaller sizes will be fully loaded early in the design life but may be overtaken by peak biogas flows in future years. Larger sizes can utilize all biogas flows through the design year, but the impacts of turn-down limitations need to be carefully considered. - Dynamic Conditions: understand the distribution, or bell curve, of biogas production, so that we can be confident of how much and how frequently the cogeneration system is loaded. When biogas production falls within the 'gaps' of the cogeneration system's operating bands, this biogas may need to be flared (Figure 2). - Natural Gas Blending: natural gas can be blended with biogas to keep the prime mover loaded in its acceptable operating range, but its not necessarily a 'silver bullet'. Owners that seek to reduce GHGs may not tolerate elevated natural gas consumption. - Heat Recovery: consider how the prime mover heat recovery fits the needs of the facility. Primer mover heat recovery performance can fluctuate throughout the year due to changes in climatic conditions impacting efficiency. EXPORT POWER? Cogeneration systems cannot be sized solely based on estimate biogas production. It needs to be coupled with the facility's electrical demand to capture the full picture of possible restrictions on the cogeneration system's daily operation and overall output. Depending on the frequency and magnitude of electrical demand fluctuations, there can be periods where surplus power is produced, therefore its critical to engage the power utility very early in the design to understand the implications of sending power back to the utility grid. Power utilities may require electromechanical safeguards that increase project cost and complexity. Permits required to export power can be logistically burdensome on the owner and are known to take years for approval. CONCLUSION AND KEY TAKEAWAYS The manuscript and presentation will tell the story of Columbus's cogeneration system sizing approach. Facilities of this size implementing cogeneration are limited, consequently there are invaluable lessons learned and considerations that all stakeholders can learn from. Carefully consider how your digester gas production lines up with the turn-down of the prime mover: this is going to drive operating costs and its especially important if utilizing all produced biogas is a must. The manuscript will include simulated 20-year biogas variability using daily biogas flow data from the SWWTP and similar size facilities to reflect the dynamic operating conditions and turn-down requirements of the cogeneration system. For some owners the recoverable heat can be more valuable than the electricity. For ALL owners, meeting the heat demands of your process IS the first priority. The manuscript will include an analysis that illustrates how SWWTP's heat demands vary season to season and compare prime mover technology heat recovery performance. For the plant-wide electrical system: the devil is in the details. The manuscript will include a deep dive into SWWTP's electrical data (Figure 3) and highlight key considerations of the plant-wide electrical infrastructure that are often overlooked during design. The ultimate outcome is confidence the cogeneration system design provides the City the maximum benefit. For the audience, an inside look at a sizing approach framework that can help our industry pursue successful energy recovery projects.

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

LEARNING OBJECTIVE: Pima County's Tres Rios Water Reclamation Facility (TRWRF) recently evaluated process benefits and cost effectiveness of including PONDUS into their process, ultimately deciding to proceed with the design and construction based on several advantages. Participants of this presentation will learn the findings of the evaluation including present-value and life-cycle costs of implementing the new process, positive impacts to digestion capacity, biogas production and sales, and solids loading rates at dewatering and the future Class A biosolids solar dryer system. INTRODUCTION | BACKGROUND The TRWRF, owned by The Pima County Regional Wastewater Reclamation Department (County), serves as the centralized solids handling and treatment system for the entire county, offering a treatment capacity of 50 MGD. The TRWRF currently produces approximately 180 wet tons per day (WTPD) of Class B biosolids at approximately 18-20% total solids content. With the goals to reduce TRWRF energy requirements, lower solids disposal costs, and address future biosolids disposal regulations the County is planning for a solar dryer to produce Class A biosolids. An opportunity to maximize these goals is to introduce a hydrolysis process that can further reduce biosolids volumes and reduce the size of the solar dryer. In addition, a hydrolysis process would create additional needed digester capacity, increase biogas production, and create a dryer cake going into the solar dryer amongst other potential TRWRF benefits. The thermochemical hydrolysis process PONDUS was specifically targeted based on limited additional operational requirements and small footprint. METHODOLOGY The PONDUS system is a thermal/alkaline hydrolysis process designed for breaking down bacteria cell walls from thickened waste activated sludge (TWAS). The cell membrane breakdown releases organic acids into the downstream anaerobic digester that ultimately feed methanogens to produce greater methane production. Note that this process is not designed to treat primary sludge (PS) that also goes into the anaerobic digester, as that material does not contain a high fraction of bacteria for lysing. The process combines the TWAS at around 6% TS with caustic soda (NaOH) to a pH of approximately 10. The TWAS is then heated to between 140o F and 160o F for 30 minutes for the reaction to take place. The system is offered from a single manufacturer Centrisys-CNP as a package which includes the chemical storage and delivery system, TWAS recirculation pumping and heat exchange, and instrumentation and controls. A preliminary mass balance and process flow diagram of the system is represented in Figure 1 below. Figure 1 PONDUS Mass Balance and Process Flow Diagram Potential Benefits The PONDUS system creates operational savings with increased biogas production and reduced total solids needing to be processed, which is the primary driver for using the system. Other benefits of PONDUS include the following: - Lower Viscosity: The hydrolyzed sludge reduces the digester feed sludge viscosity by over 50%. The biosolids can be pumped, mixed, and heated with lower energy requirements and allows for a higher TS loading rate into the digesters. A higher loading rate allows for reduced total digester volume required to meet minimum solids retention times (15 days), greater storage capacity, and allows greater flexibility for a digester to be taken offline. Additionally, the higher total solids feed rate to the digesters reduces the water heating requirement, and therefore reduces natural gas heating water needs on the digester boilers at an anticipated savings of approximately $80,000/year. The lower digester viscosity could also reduce energy demands on pumping and mixing systems. - Achieving Class A Biosolids: The PONDUS thermal/alkaline hydrolysis process will result in higher volatile destruction, resulting in more biogas and less sludge to manage. A secondary benefit to the volatile solids destruction is a further reduction in fecal coliform, salmonella, enteric viruses, and viable helminth ova in the biosolids exiting digestion. This further reduction will increase the ability for the Solar Dryer to achieve Class A biosolids under the 503 Regulations at a lower total solids concentration. The lower total solids exiting the future solar dryer will reduce dust production and create a better working environment. - Total Solids Cake Dryness: With a breakdown and reduction in volatile solids, a dryer dewatered cake total solids concentration can be assumed. Manufacturer data and an independent study of 17 different samples shows that the total solids of cake will increase between 3% and 6%. The independent study data from KBKopp Sludge Analytics is provided in Table 1. - Polymer Demand: Manufacturer data shows that a 20% decrease in polymer can be expected when using PONDUS. Table 1 implies that primary sludge subjected to PONDUS has a marginal polymer demand increase. Given the high WAS:PS fraction at TRWRF, and the nature of PONDUS breaking down volatile solids it can be assumed polymer demand will be reduced. - Dewatered/Dried Biosolids Odors: The PONDUS biosolids cake is reported to have a lower odor concentration than traditional anaerobically digested sludge. This is due to the greater volatile solids reduction. The benefits include reduced odor concentrations to treat, increased biofilter bed-life, and reduced risk of odor complaints. Operations and Maintenance Considerations The largest direct operational costs of the PONDUS system is caustic soda consumption. Daily caustic soda use is expected to be 155 gallons resulting as high as $280,000 yearly cost. Heating the TWAS in the PONDUS Lysing Reactor adds minimal cost from an overall perspective, as the heat energy required to heat the TWAS from 140o F to 160o F is eventually used in the downstream anaerobic digester process. Table 1 PONDUS Independent Study: Sludge Effect on PONDUS (Dr Julia Kopp, 2022) DISCUSSION Revised Mass Balance Estimate With a PONDUS system, the increase in biogas is expected to be 22% higher than the existing TRWRF anaerobic digester system. The daily biogas production would increase by 91 million BTU (MMBTU). The sell back value of those 91 MMBTU, with the EPA value of 11.7 Renewable Identification Numbers (RINs) per MMBTU and at $2.30 per RIN results in an additional $2,463 in biogas sales per day, or $899,000 in biogas sale per year. The expected increase in biogas production also reduces the total solids needing to be dewatered, dried through the solar dryer, and hauled away. Coupled with an expected dryer cake, the current 180 WTPD of solids through the existing plant processes ending at dewatering would be reduced to 124 WTPD of solids after dewatering. This is a 53 WTPD total solids reduction, or about two hauling trucks per day of reduction. At current costs per wet ton disposal of $27.20, this results in a potential savings of $526,000 per year. Implementation Risk The PONDUS process includes risks that must be recognized and managed to the extent possible. Key implementation risks include the following: - Full-Scale Implementation: The primary risk of the process is that PONDUS has limited installations to use as proofs and lessons learned. There are over 20 installations in Germany, with one fully operational facility in the US. While the thermal/alkaline process is well understood and proven, the unknowns of full-scale may reduce the estimated savings. To mitigate against this uncertainty, the County and its design team conduct a site visits and spend time at the existing US facility to help understand full-scale operations, maintenance, and data comprehension of the process. - The lysing process creates an opportunity for a lower centrate/pressate capture rate of solids downstream of dewatering due to the high volatile solids destruction rate. This can be addressed by using total solids metering on centrate/pressate to identify and fix under performance. - Increased nutrient ratios within the centrate/pressate from PONDUS is expected. Table 1 shows a 28% chemical oxygen demand (COD) increase, an 18% ammonia increase, and a 26% orthophosphate increase. Updated BIOWIN modeling may be necessary to to confirm impacts on centrate/pressate deammonifiaction and secondary treatment processes. RESULTS & STATUS Installation of PONDUS is anticipated to create an operational savings of $1,060,000 per year and a system payback in under 10-years. The County determined these savings opportunities along with added digester capacity and future solar dryer benefits justifies continuing with the project. The new PONDUS facility is now in design at TRWRF with a construction start date of 2025.

Energy consumption and greenhouse gas (GHG) emissions are the two important factors to consider when water utilities plan for sustainable capital upgrade and operational modifications of wastewater treatment plants (WWTPs). Researchers have reported the use of several methods, such as flux chambers, drone flux methods, optical gas imagers, etc. for monitoring the fugitive GHG emissions, mainly methane (CH4) and nitrous oxide (N2O), from WWTPs. However, limited by the short duration of most monitoring technologies, only 'snapshot' data can be obtained, necessitating extrapolation of the limited data for estimating the annual emissions. Extrapolation introduces substantial errors as it fails to account for the temporal variations of fugitive GHG emissions. This multi-year project, starting in June 2021, has performed a screening and evaluation of a variety of technologies, consisting of sampling- and non-sampling-based, point and long spatial-range, mobile and stationary methods. The technologies that are suitable for monitoring GHG from WWTPs have been selected to develop a tiered, cross-validating approach to cost-effectively characterize the temporal dynamics of both plant-wide and process-specific GHG emissions. This presentation will start with a brief introduction of how the team has combined satellite imagery, ground based open-path and closed-path optical sensing, optical gas imaging, flux chamber, dispersion modeling, and low-cost sensor network, for measuring fugitive GHG emissions from WWTPs, followed by a detailed case study of how a satellite imagery-based algorithm was developed for capturing the complex, temporally and spatially heterogeneous CH4 emissions associated with the biosolids treatment process at wastewater treatment plants. Satellite remote sensing has been widely used in finding and quantifying natural gas leaks in oil & gas facilities and methane emissions from landfills. However, to date, the use of satellite imagery for analyzing methane emissions from wastewater treatment has been extremely rare, if not none, for which the likely reason is that the low pixel resolution of earlier satellite images is not able to capture the relatively smaller plumes from wastewater treatment plants. Recent advances in satellite technology have made significant progress in methane monitoring that high-resolution tracking capability has become available. This allows for the development of image processing algorithms to capture the methane plumes from facilities of smaller footprints such as the WWTPs. This study has developed an algorithm for characterizing the long-term CH4 emissions from the biosolids handling process using the high spatial resolution Sentinel-2 data. Sentinel-2 satellite imaging, despite of its fewer spectral bands than hyperspectral satellites, is of a superior spatial resolution of 20 m x 20 m and a more frequent revisit intervals of 3 days in the study areas. This feature is especially beneficial for tracking time-varying fugitive emissions for sources with relatively lower intensity plumes such as the biosolids treatment process at WWTPs. Three representative WWTPs, with the following three different biosolids handling processes, have been evaluated in this study: Plant #1, conventional anaerobic digestion; Plant #2, intensified anaerobic digestion with thermal hydrolysis process; Plant #3, biosolids dewatering only with no anaerobic digestion. The short-wavelength infrared (SWIR) bands 11 (1560--1660 nm) and 12 (2090--2290 nm) of Sentinel-2 were used to retrieve methane column concentrations by analyzing reflectance differences between the spectral bands and across multiple satellite passes. Satellite images taken between 2019 and 2023 were processed to retrieve the CH4 column concentration distributions. Digital image processing techniques were developed and used for extracting the time- and space- varying features of CH4 emissions, revealing the daily, monthly, seasonal, and annual emission variations. Figure 1 summarizes the data processing procedure. Emission hotspots were identified and corroborated with ground-based measurement using an infrared optical gas imager. The findings reveal the 'ever-changing' dynamic nature of fugitive CH4 emissions from the biosolids handling units, as shown in Figure 2 and Figure 3, indicating the importance of performing continuous measurements for estimating the annual CH4 emissions. Emissions from the three biosolids processing facilities are also quantitatively evaluated for a comparison. This study demonstrates the feasibility of using satellite imagery to capture methane plumes from the biosolids processing area inside a WWTP, demonstrating the value of using satellite images for cost-effectively localizing emission sources and estimating plumes based on the column concentrations. On the other hand, the data also suggests that satellite imagery by itself is not a self-sufficient method. Hence, while satellite imagery serves to cost-effectively provide a 'big picture' view of the long-term emission dynamics as well as to localize emission hotspots, a full quantification of fugitive CH4 emissions requires incorporation of additional data such as meteorological information and ground-based measurements.

Aries Clean Technologies (Aries) is a gasification technology provider and patent holder that uses its gasification systems to dispose of waste. Aries retained Stantec to quantify annual Scope 1, 2, and 3 operational greenhouse gas (GHG) emissions for their existing and potential future (Generation 2) biosolids gasification facilities as a step to achieving their goal of a carbon neutral biosolids gasification facility. These facilities feature extensive heat recovery, electricity generation, and emissions control systems, necessitating the use of multiple calculation sources and a unique inventory approach to ensure accuracy and to avoid double-counting of emissions.

Introduction A commercial-scale short-term demonstration of biosolids pyrolysis will be conducted at the Synagro Drying Facility at the City of Baltimore Back River WWTP i, Maryland. The pyrolysis process is designed and supplied by CHAR Technologies, of Toronto, CA. This project aligns with current environmental priorities, particularly the need for PFAS (per- and polyfluoroalkyl substances) mitigation in waste streams. The proposed objectives for this temporary pyrolysis demonstration unit will be: a. Verify the PFAS (synthetic, per- and polyfluoroalkyl substances) characteristics of the biosolids, the Syngas and the Biochar confirming the anticipated PFAS destruction in the pyrolysis process. b. Demonstrate the viability of converting biosolids pellets into valuable by-products: syngas for potential energy recovery and biochar for agricultural or environmental applications.. c. Verify the performance of the pyrolysis process on biosolids continuously for at least six months. d. Verify the resultant Syngas characteristics, and the opportunities and requirements for reusing within the dryer system or upgrading to renewable energy. e. Verify the resultant Biochar characteristics, and suitability for its use as a soil amendment. This six-month demonstration, scheduled from January 2025 to July 2025, aims to provide valuable insights into the operational sustainability and environmental benefits of pyrolysis in biosolids management. Process Description The Pyrolysis Demonstration Unit from Char Technologies uses a pyrolysis process to thermally decompose biosolids pellets in the absence of oxygen; the absence of oxygen prevents incineration from occurring. This pyrolysis process occurs within a rotary kiln chamber with an external burner that after starting-up with propane, burns the Syngas generated during the pyrolysis process. The Char Technologies pyrolysis demonstration unit will process over 8 tons per day of dried biosolids pellets and is projected to produce approximately 143 standard cubic feet per minute of high heating value Syngas containing dust, steam, hydrogen, carbon monoxide, carbon dioxide and methane. The Pyrolysis Demonstration Unit includes Syngas scrubbers that convert the Syngas for internal use in the pyrolysis unit. The excess Syngas that is not used in the pyrolysis process will be combusted, in a thermal oxidizer unit before being discharged at the Demonstration Unit Stack. An isometric of the facility being constructed is presented in Figure 1. A photo of the pyrolysis kiln is provided in Figure 2. A process flow diagram is presented in Figure 3. Sampling and Analytical Methodology Comprehensive sampling and analysis will be performed throughout the six-month demonstration period, with multiple data points collected from key stages of the process. These sampling points are presented in red in Figure 3. Parameters measured include: - Feedstock and Biochar: Elemental and proximate analysis, metals, nutrients, PFAS - Wastewater Analysis: BOD/COD, nutrients, pH - Raw pyrogas, clean syngas, and exhaust gas analysis: hydrocarbons and VOCs, particulates and metals, acid gas, NOx and Sox, PFAS US EPA Method 1633 will be followed to test for all PFAS compounds. Sampling will be conducted at regular intervals to capture variability in process performance. Quality assurance measures, including duplicate and blank samples, will be implemented for data validation. The test methods used for analysis are outlined in Table 1. Results and Discussion We expect to have interim analytical results available by April 2025 for presentation at RBC. Primary discussion will include fate and transformation of PFAS compounds, but will also discuss biochar characteristics, syngas quality and lessons learned in operating and maintaining the pyrolysis unit.

Purpose: Struvite (MgNH4PO4 · 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.

Pyrolysis and gasification are experiencing a renewed interest within the wastewater industry as a method to process either the biosolids or the residuals. The reasons are a) mounting evidence that per and polyfluoroalkyl substances (PFAS) are removed from the resulting biochar, b) the volume of the final material may be reduced by 90%, c) the systems (in the US) are often easier to permit than incinerators and d) they allow for sequestration of carbon within the biochar thus reducing carbon footprint. In addition to removing PFAS from the biochar, removal of microplastics and pharmaceuticals has been demonstrated (Keller et al. 2024 and Liu et al. 2023). These reasons make a compelling case for thoroughly vetting these technologies for application in wastewater industry. In Germany, a number (6 -- 8) pyrolysis units have been operating on 100% biosolids for several years and have experienced difficulties due to siloxanes in the producer gas. In the United States two pyrolysis systems and one gasification system are operating and several others have been designed and are expected to be constructed or commissioned in 2025. However, there is limited information regarding the operational requirements and challenges of these technologies operating on a feedstock of 100% wastewater biosolids or residuals. Although these technologies are well established in other industries and their applications documented by a plethora of information when operating on feedstocks such as woody or herbaceous biomass and coal; there is limited information regarding operating on a feedstock of 100% sewage sludge biomass (SSB). The primary challenge of SSB is the a) high moisture content, b) relatively low heating value and c) presence of siloxanes. The focus of this presentation is the presence of siloxanes, which manifest in the producer gas, and plug the thermal oxidizer resulting in the system being offline as much as 30 -- 40% of the time. The presence of siloxanes in wastewater is well known. In one study measuring 17 siloxane compounds, the average influent loading of siloxanes was 15.1 kg/day (Bletsou et al., 2013). In another study, in which three siloxanes were measured, their presence constituted 39 mg/L of the total influent COD (Surita & Tensel, 2014). In addition, the fate of siloxanes in wastewater has been studied especially in relation to its presence in digester gas. However, the siloxanes remaining in the sludge after digestion create different challenges when the residuals are processed in thermochemical processes such as pyrolysis or gasification. Siloxanes are manmade organosilicon compounds consisting of an Si-O backbone with a methyl, ethyl, or other organic functional group. They exist either as linear chains, designated with an L or as cyclic rings, designated with a D (Dewil et al. 2006 and Appels et al. 2008). Two main classes of widely used siloxanes that enter wastewater treatment plants are polydimethylsiloxanes (PDMS) which are non-volatile and volatile methylsiloxanes (VMS) which are volatile organic compounds (VOCs) (Xu 1999 and Traina et al. 2002). Most (greater than 90%) of the PDMS are adsorbed to the extracellular polymeric substances (EPS) of the floc. Approximately, 50% of the VMS are either volatilized or air stripped in the aeration basin (Parker et al. 1999, Xu 1999 and Traina et al. 2002). The sludge sent to anaerobic digestion; therefore, contains almost all the PDMS and 50% of the VMS. During digestion much of the PDMS is hydrolyzed to smaller VMS compounds, some of which are removed with the digester gas. The remaining VMS and approximately 34% of total PDMS entering the plant, remain in the digested biosolids (Xu 1999, Parker et al. 1999, Traina et al. 2002 and Accettola et al. 2008). These remaining siloxanes, when sent to a thermochemical process such as pyrolysis or gasification manifest as a problem within the thermal oxidizer. The siloxanes remaining in the sewage sludge will volatize in the thermochemical process and exit with the producer gas. The problem manifests downstream, in the thermal oxidizer, which typically operates at 1,100 -- 1,200˚C. At these temperatures, siloxanes precipitate to silicon dioxide (SiO2) i.e., silica. The oxidative decomposition of siloxanes may begin at temperatures from 600 -- 850˚C depending on the local environment; with nearly complete oxidation occurring at temperatures ≥ 900˚C. The combustion equations for three common cyclic siloxanes are shown below (Tansel & Surita 2014): D3: C6Si3H18O3 + 12O2 → 3SiO2 + 6CO2 + 9H2O D4: C8Si4H24O4 + 16O4 → 4SiO2 + 8CO2 + 12H2O D5: C10Si5H30O5 + 20O5 → 5SiO2 + 10CO2 + 15H2O The equations indicate a) the production of SiO2, and b) the relatively high consumption of oxygen required. The latter may result in insufficient oxygen for the combustion of the gaseous fuels within the thermal oxidizer. Silica exists as either an amorphous or a crystalline (i.e., quartz, tridymite and cristobalite) structure. In the combustion of woody biomass amorphous silica may transform to cristobalite at 800˚C and cristobalite to tridymite at 1,000˚C (Yang et al.,2022). Therefore, the silica that forms in the thermal oxidizer maybe of multiple forms all of which pose a significant operational problem. The evaluation of several systems indicated the quantity of silica generated ranged from 3.25 -- 13.5 kg/day. See Figures 1 and 2. The result was that every 5 -- 7 days the systems had to be shutdown to remove the material. To do so required 1 day for cool down, ½ - 1 day for cleaning and 1 day for warmup/startup (i.e. 2½ to 3 days) or 30 -- 40% downtime. Typically scrubbing siloxanes from digester gas is accomplished in an adsorption process using granular activated carbon (GAC). However, this is not an option with producer gas because the high concentration of other energy rich VOCs that would be removed instead of combusted in the thermal oxidizer to provide the heat for the drying process. Removal of the siloxanes from the producer gas must be selective. An alternative, to removing them from the producer gas before precipitation is a thermal oxidizer designed around the precipitation of the silica, but without requiring shutdowns. Such a thermal oxidizer exists and is designed to effectively cause the siloxanes to precipitate to silica and automatically remove the material from the process. The system, shown in Figures 2 and 3, is a dual-chamber regenerative thermal oxidizer (RTO), utilizing a regenerative ceramic material that is automatically circulated and cleaned. The technology has been operating full scale at two facilities since 2012 and 2022 and will be piloted as part of a Department of Energy grant evaluating autothermal pyrolysis. Figure 5 shows the silica removed from the gas stream. Its use in the removal of siloxanes for the producer gas is a new and innovative approach to a recent problem arising from the combustion of producer gas generated from the pyrolysis or gasification of sewage sludge. This presentation will discuss the following: a) the forms of siloxanes in the wastewater and the operational problems resulting from their presence, b) a potential solution, using a technology already operating in industrial applications and c) the challenges in applying this technology to the removal of siloxanes in the producer gas.

This presentation will explore the status of gasification and pyrolysis system projects that are either in development or recently commissioned. With ever increasing restrictions on landfill and land application and a growing concern with emerging contaminants, specifically perfluorinated compounds (ie. PFAS), there is increasing interest in these types of thermal technologies. The state of the research on these technologies to deal with emerging contaminants will also be presented. The presentation will provide a brief background on biosolids process technologies, highlight current biosolids trends including the barriers impacting technology and resource recovery, and provide an update on gasification and pyrolysis technologies that are currently being implemented in the marketplace. The presentation will present a biosolids road map and risk triggers that utilities can use in planning (Figure 1). There are several biosolids gasification and pyrolysis systems and demonstrations that are being planned globally including several in North America (such as Bioforcetech, Ecoremedy, Earthcare, Harvest Technologies, Pyrocal, Aquagreen and Heartland Technologies). The presentation will present an update on the status of these projects in North America as well as a status of projects internationally including in Germany, Australia and Asia. As part of the presentation, the results of a technology screening evaluation conducted for a large regional gasification and pyrolysis facility in Australia will be discussed. As most gasification and pyrolysis processes require upstream drying (Figure 2), the evaluation considered various upstream processing alternatives including anaerobic digestion, solar drying and thermal drying. The screening evaluation only considered technologies that have full scale operating installations and technologies that produce a biochar for beneficial use. These gasification and pyrolysis technologies for sludge and biosolids have not had great success historically and many past attempts to commercialize these processes have not been successful. Lessons learned from early adopters and considerations for future planning will also be discussed. The presentation will provide an update on technologies that are being implemented in the marketplace.

INTRODUCTION/BACKGROUND Land application of biosolids provides benefits via recycling nutrients and carbon to soil. However, Per- and polyfluoroalkyl substances (PFAS) are becoming a main hindrance to land application of biosolids with increasingly stringent regulations (McNamara, 2023). PFAS are found ubiquitously in many industrial and consumer products due to their unique chemical properties. These same properties also make them highly persistent and potentially toxic to ecosystems and humans (Holmquist, 2020). New restrictions around land application coupled with a reduction in potential landfill sites for biosolids has led many WRRFs to investigate thermal processes for biosolids management (Winchell, 2022). Incineration provides the benefit of reducing residual solids loads to be managed. Pyrolysis and gasification are emerging thermal process alternatives that also reduce residual solids loads. Moreover, these two processes offer benefits of lower concerns for exhaust gas emissions and the ability to sequester carbon in the solid residuals (Winchell, 2022). Of particular interest now to WRRFs is that these thermal processes can remove and/or transform PFAS (Winchell, 2022; McNamara, 2023). While PFAS are part of the biosolids management decision tree, they are not the only important thing. Any process implemented today still needs to make sense in two decades even if PFAS are no longer a primary concern. Additionally, the relative importance of PFAS in light of other critical environmental concerns like global warming and eutrophication have not been widely studied. While facing a seemingly pivotal moment in biosolids management history, it is critical to understand management options and the relative importance of PFAS. For this research we combined PFAS fate data from our own experiments and from literature through thermal processes. These results were then included in the LCA as their own impact category and as an input to toxicity. To the knowledge of the authors, this is the first LCA on solids handling alternatives to date that has incorporated PFAS toxicity in this way and compared impacts to conventional LCA components. In short, LCA was used to compare thermal drying, incineration, pyrolysis, and gasification holistically with regards to their impacts on humans and the environment. APPROACH PFAS Fate Through Thermal Processes Lab-scale pyrolysis and gasification experiments were conducted as part of WRF Project 5211. Targeted analysis of PFAS was completed by Eurofins on influent biosolids and effluent solids (char), liquid (py-liquid), and gas (py-gas or gasification gas). These data were used to assess removal and release of PFAS in the effluent phases. Incineration data from Winchell, 2023 was used for incineration PFAS removal performance as a reference scenario. LCA Structure 1)Five biosolids treatment scenarios for a 'typical' WRRF were compared, distinguishing between different thermal processes: thermal drying only, pyrolysis at low (500 degrees C) and high temperature (800 degrees C), gasification, and incineration. See Table 1 for a description of each scenario's treatment train. 2)Mass and energy balances across the treatment trains for each scenario were conducted, as well as a global warming potential (GWP) evaluation, using the BEAM*2022 tool (NEBRA et al., 2022). 3)PFAS emissions were first characterized alone in an impact category called 'PFAS potential,' representing total mass of PFAS emitted in each scenario as fluoride equivalents. 4)PFAS emissions were then also characterized within the toxicity impact category using the modified USEtox PFAS framework from Holmquist et. al. 2020. 5)These results were than used as an input to the LCA to compare the five thermal process scenarios with five main impact categories: i) GWP, ii) eutrophication potential, iii) freshwater ecotoxicity potential, iv) human cancerous and noncancerous toxicity potential, v) and PFAS potential. RESULTS/DISCUSSION Global Warming Comparison The incineration scenario had the highest GWP, driven mainly by scope 1 emissions (Figure 1). The scenarios with a fertilizer offset credit applied had negative scope 3 emissions. This was particularly significant for thermal drying only, because the biosolids had the highest nitrogen content, which largely drove the GWP fertilizer credit. The pyrolysis scenarios were determined to have the lowest GWPs, largely driven by lower scope 1 emissions through carbon sequestration credits achieved by the stability of the biochar product. Eutrophication Comparison The thermal drying only scenario had the highest eutrophication impact, driven by the increased hauling of the wet biosolid compared to other scenarios (Figure 2). All land applied scenarios had a beneficial fertilizer offset credit as well. This credit was largely similar between these scenarios, as the phosphorus content of the solid drove eutrophication impact for fertilizer credit and was assumed to be similar between scenarios. The lowest eutrophication potential was in the pyrolysis scenario. Toxicity Comparison Little difference was observed in freshwater ecotoxicity and human cancerous toxicity, largely due to the dominance of heavy metals, making up at least 99.5% and 96.2% in these two categories across scenarios. Heavy metal loads in the end product were assumed to be similar across all thermal processes, as only a small portion of the heavy metals would volatize, and the majority would remain in the solid residuals. In terms of PFAS potential, pyrolysis did not appear to remove much PFAS, but instead transform them into different PFAS species. Incineration and gasification were the lowest in PFAS potential, due to their high PFAS removal performance. This led also to lower human noncancerous toxicity potentials (Figure 3). Overall, heavy metals had the highest impact in human noncancerous toxicity for each scenario, but PFAS were still impactful. It should be noted that two effect factors were presented for noncancerous toxicity in Holmquist's modified USETOX model (based on epidemiological vs. rodent data). To be conservative, the higher of the two was used. Relative Impacts of Inputs Figure 4 shows the relative contribution of each input to the relevant impact categories, focusing on the thermal drying only scenario for simplicity. Most notably, it can be seen that the impact of heavy metals makes up between 93.2-99.5% of each toxicity potential category for thermal drying only. PFAS emissions comparatively only made up to 6.7% of the toxicity emissions (for human noncancerous toxicity). Eutrophication is mainly impacted by electricity use, while GWP is mainly impacted by scope 1 emissions. The PFAS potential impact category was not shown in the figure. This is because there is only one input to the PFAS potential broken up by the effluent phase the PFAS are emitted to in each scenario. CONCLUSION/SIGNIFICANCE This analysis highlights the importance of PFAS when taken into context with other known and regulated toxicants in biosolids such as heavy metals. Heavy metals made up at least 99.5% and 96.2% of freshwater ecotoxicity and human cancerous toxicity impacts across all scenarios. Even when a conservative assumption for PFAS toxicity was used for human noncancerous toxicity, heavy metals made up the largest portion of this impact as well. This analysis also highlights two biosolids alternatives, gasification and pyrolysis. Gasification was observed to be a relatively low impact alternative in most of the LCA categories that also safeguards against uncertainty around PFAS toxicity and future regulations in biosolids. This option provides not just transformation, but potentially good removal of PFAS species. Pyrolysis however was observed to be the best option in this analysis when considering the relatively low impact of PFAS on the toxicity potential impact categories, and that pyrolysis had the lowest impact in the other critical LCA impart categories. The key impact of this work is the reframing of PFAS as not the only consideration, but one of many aspects to be included for holistic biosolids management.

Introduction Metro Water Recovery, based in Denver, Colorado, serves 2.2 million population equivalents through the Robert W. Hite Treatment Facility (RWHTF) and the Northern Treatment Plant. RWHTF has a design capacity of 220 MGD and treats an average daily flow of 130 MGD. The facility's solids handling complex includes gravity thickening, grease processing, dissolved air flotation thickening of waste activated sludge, two-phase mesophilic anaerobic digestion, centrifuge dewatering, phosphorus recovery, and biosolids management for storage, hauling, and land application. In 2016, Metro piloted the MagPrex post-digestion phosphorus recovery system (PREX) to address challenges posed by enhanced biological phosphorus removal (EBPR). EBPR results in high concentrations of soluble phosphorus in the digesters, which combines with magnesium and ammonia to form nuisance struvite, causing scaling in equipment, lowering dewatering efficiency, and upcycling phosphorus through the treatment process. In response to stricter effluent TP limits imposed by the Colorado Department of Public Health and Environment (CDPHE), Metro commissioned the world's largest PREX reactor in October 2020. This system allows RWHTF to reliably meet a TP target of 0.7 mg/L, with fewer operational disruptions associated with nuisance struvite formation. Phosphorus Recovery System Overview The PREX reactor is a completely mixed system receiving 600-800 GPM of digester effluent with orthophosphate (OP) concentrations between 350-450 mg/L. PREX quickly transforms OP into struvite by adding magnesium chloride and raising pH through air stripping via coarse air bubble diffusers . The reactor can harvest struvite as a reusable product, with large crystals settling in the reactor cone for separation, Figure 1. Since startup, the reactor has achieved >90% OP conversion, enabling Metro to maintain effluent TP concentrations below 0.4 mg/L. Optimization Objectives While the PREX system has successfully achieved the required OP conversion, opportunities for further optimization quickly became apparent. Initially, only 2% of the struvite formed in the PREX reactor was captured for reuse, and maintenance costs to address nuisance struvite downstream remain high (Figure 2), estimated at over $9,000 per month in labor alone. This study aims to enhance the system design and assess the downstream impacts on centrifuge dewatering, struvite scaling in PREX effluent, and centrate piping. The optimization efforts are guided by three key objectives: 1. Maximize Struvite Recovery: Increase recovery beyond the initial average of 300 lbs/day to meet the reactor's estimated potential of 2,000 lbs/day. This effort investigates changes to process operation and configuration to increase struvite recovery. 2. Minimize Downstream Struvite Accumulation: Address scaling in the PREX effluent and centrate system. Although anerobic digestors have observed a reduction of nuisance struvite, portions of PREX effluent system require a high pressure cleaning every three weeks to remove struvite accumulation. 3. Optimize Residual Magnesium for Dewatering: Evaluation of residual magnesium from the PREX reactor to improve dewatering efficiency by reducing polymer demand and lowering chemical costs. Methods The PREX reactor was operated under various conditions, including adjustments to hydraulic retention time (HRT), pH, and Mg:OP ratios. A 3D multi-phase turbulent model was developed to refine operational settings and predict particle settling efficiency. To assess struvite scaling, ion mass balances were performed on centrate samples collected at two points: upstream at the centrifuge discharge and downstream at the outlet to the sidestream deammonification reactor. These samples were analyzed for OP, TP, magnesium (Mg), and pH. Additionally, soluble and total magnesium concentrations were measured in the PREX effluent and centrate. The relationship between these concentrations, polymer demand, and dewatering performance was explored. Conductivity and zeta potential measurements were collected as potential proxies for cation concentrations, enabling real-time monitoring of dewatering performance. Results 1.Struvite Recovery Optimization The 3D CFD model revealed that only 10% of struvite crystals under 250 μm in size could be captured under typical airflow conditions (Figure 3). Particles with a diameter >700 um had an average recovery rate of 80%. However, over 50% of the struvite produced by PREX was less than 250 μm, significantly limiting recovery. Improvements included adjusting classifier operation such as classifier bed height , introducing dilution water, and recycling classifier overflow. These changes increased struvite recovery from 300 to 1,200 lbs/day. 2.Minimizing Downstream Struvite Accumulation While the PREX system significantly reduced nuisance struvite formation in the digesters, accumulation persisted in the PREX effluent piping due to the presence of small struvite crystals and turbulent flow, which caused localized pH shifts and the precipitation of residual OP, Mg, and NH4. Polyvinylidene fluoride (PVDF) liners are being installed in the effluent lines to reduce turbulence and mitigate scaling. Ongoing ion mass balance sampling will provide further insights, with results to be presented at the conference. 3.Residual Magnesium and Dewatering Performance Maintaining residual magnesium concentrations of 50-60 mg/L reduced normalized dewatering polymer usage per flow by 39%, resulting in chemical cost savings of $800,000 annually (from $2.9 million to $2.1 million) (Figure 4). Conductivity and zeta potential measurements strongly correlated with cation concentrations, indicating their potential use for real-time monitoring and future optimization. Conclusions The optimization of Metro's PREX system demonstrates the potential for substantial improvements in phosphorus recovery and operational reliability. Struvite recovery increased fourfold, highlighting the importance of fine-tuning operational parameters and integrating recycling strategies. Addressing scaling in effluent lines through material upgrades and improved chemistry management will further enhance long-term system reliability. Optimizing magnesium levels improved dewatering performance and delivered substantial chemical cost savings. Potential real-time monitoring using conductivity and zeta potential as proxies for cation concentrations offers an innovative approach to operational control and performance optimization. As phosphorus regulations become more stringent, these advancements position Metro as a leader in sustainable resource recovery. These findings provide valuable insights for other utilities aiming to enhance phosphorus recovery, minimize operational disruptions, and reduce chemical usage, advancing the next generation of phosphorus management for water resource recovery facilities.

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