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

This 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: A drought in eastern Colorado in 2021 and 2022 spurred South Platte Renew (SPR) to reconfigure its data management practices for its Beneficial Use (BU) Program. Low SPR needed to become resilient to future scenarios that could prevent biosolids land application. Drought led to crop failure, which in turn led to high nitrate soil levels barring biosolid application. SPR needed an inventory to assess the lands that could be applied to, inform future biosolid management needs, and track environmental trends in application. A Power BI Dashboard would provide an efficient solution by centralizing current and historic data and enhancing communication across SPR. Design: To develop the site inventory, SPR redesigned its data management practices and developed a dashboard to assess each biosolid application site. SPR expanded its sampling plan while centralizing the existing data. SPR compiled application, soil, crop, and biosolid data. SPR increased its soil sampling frequency to pre-planting and post-harvest to monitor nitrate uptake. SPR also created a dynamic application site status, an interactive timeline for contingency planning, and historic graphs showing environmental parameters like weather conditions. The Power BI Dashboard consolidated data from multiple sources into a user-friendly, visual interface accessible to all teams. The dashboard allowed SPR staff to monitor and analyze this information in real-time, enabling informed decision-making and timely adjustments to the biosolids application process. Implementation: To address the diverse needs of stakeholders, the dashboard integrated data from agricultural systems, weather databases, and biosolids management software. It tracked each field's crop history, ensuring compliance biosolid application. The tool also facilitated scheduling and planning for soil sampling, aiding the biosolid application process. It also compiled information for each plot, summarizing metal and nutrients results from historic soil sampling events. Furthermore, it contains data from SPR's test plot that has crop data from the 1990s so SPR can review trends due to biosolid or fertilizer application. Project Results: By centralizing critical data and improving access to information, the Power BI Dashboard enhanced operational efficiency, reduced planning time, and promoted better communication. It enabled the program to respond to changing weather conditions and crop needs, ensuring the sustainable and responsible use of biosolids in agriculture. It has helped South Platte Renew be more resilient to climate change as land available for biosolid application and storage space can be predicted while using historic data to inform management practices.

The City of South Sioux City (SSC), Nebraska, faced a pivotal challenge in wastewater management when the Sioux City, IA Wastewater Treatment Plant (SCWWTP) announced its decision to cease acceptance of wastewater from local heavy industries (e.g., meat packing) . This was compounded by significant flooding in 2019 that damaged local infrastructure, prompted the city to accelerate plans for constructing its own wastewater treatment facility (WWTF). Anaerobic treatment is an ideal fit for high strength wastewaters, and with anaerobic systems comes the need for biogas handling. The City of SSC planned to utilize this resource beneficially, however sulfur quickly became a problem. The City of SSC embarked on constructing a covered anaerobic lagoon (CAL) system integrated with Nebraska's first Aerobic Granular Sludge (AGS) system to effectively manage its industrial wastewater. A site layout illustrated in (Figure 1). The lagoons where to provide pretreatment to the high strength wastewater. Lagoon discharge would then be collected in a wet well and pumped to an aerobic granular sludge system for final treatment, UV disinfection, then finally sent to the Missouri River. The anaerobic pretreatment would generate biogas that could be used beneficially in the WWTF boilers, with future plans to augment the City's natural gas by cleaning the biogas to a renewable natural gas level for injection. During the startup phase of the anaerobic lagoon, biogas was generated with unexpectedly high levels of hydrogen sulfide (H2S), reaching concentrations as high as 2.5% by volume, shown in (Figure 2). These elevated H2S levels overwhelmed the biogas H2S removal system, which utilized a proprietary activated carbon vessel designed to manage lower concentrations. The initial biogas management strategy intended for this gas was to supplement natural gas in a boiler system for heating the anaerobic lagoons, but the challenges presented by the high H2S levels made the biogas unusable in the boilers without a removal system capable of treatment. This led to the question of 'where is the sulfur coming from?'. The background water quality in the area was known to have a high sulfate concentration, and the industries were known to discharge elevated sulfur, however these two sources could only account for approximately 50% of the Sulfur coming into the facility. A sulfur balance was performed, and modeling was done to estimate the fate of the sulfur once it entered the WWTF. A simplified figure shows this balance in (Figure 3). The results lead to discussions with local industries, and the discovery of increased sulfuric acid usage at the industries on site pretreatment to meet the Cities new WWTF's influent FOG limits. HDR worked with the City, the industry, and the Nebraska Department of Environment and Energy to implement a pilot study at the industries pretreatment system. The intent was to reduce sulfuric acid usage to find a reasonable balance between acid usage and FOG discharge limits. Although the results of the pilot study succeeded in reducing sulfur to the WWTF it consequently generated colloidal none settling solids. These colloidal solids were not removed in either the anaerobic or AGS system and cause strain on both the WWTF's UV disinfection system, by reducing UVT, and effluent TSS limits. photos of the settleability testing and Reduced UVT effluent quality are shown in (Figure 4) and (Figure 5). The pilot study was immediately ceased, and the industry agreed to explore alternative acid sources in the future. In response to these challenges, SSC implemented immediate operational adjustments while exploring the economic feasibility of on-site sulfur management strategies. Both liquid phase and gas phase sulfur management strategies were evaluated, resulting in the decision to pursue installation of a more robust biogas chemical scrubber system. This planned upgrade aims not only to improve biogas quality but also to enhance the economic viability of the biogas produced. Future plans include integrating the upgraded biogas treatment system with a third-party renewable natural gas (RNG) facility, enabling further purification and injection into the natural gas grid. This strategic pivot highlights SSC's commitment to sustainability and innovation in its wastewater management practices. This presentation will delve into the technical specifications of the anaerobic lagoon design, the challenges encountered during the startup phase, and the innovative strategies employed to manage biogas quality effectively. It will also provide insights into the operational adjustments made to mitigate the high H2S levels. Additionally, facility operating data is being used to help develop and calibrate a desktop model of the overall sulfur balance around the facility including gas and liquid phase dynamics with variable loadings and operating conditions. This work will contribute to the broader discourse on effective sustainable biogas and sulfur management practices and innovative approaches in wastewater treatment facilities dealing with similar challenges. This study serves as a case study for other treatment facilities aiming to optimize their biogas management strategies while ensuring compliance with environmental standards and promoting sustainable practices in wastewater treatment.

Digester Microaeration: A Comprehensive Full-Scale Case Study 1. Introduction In-situ digester microaeration is an emerging solution for biogas hydrogen sulfide (H2S) control that offers several advantages including reduced chemical inputs and no need for additional treatment vessels. In this process, small quantities of oxygen are introduced directly into an anaerobic digester to facilitate conversion of hydrogen sulfide to elemental sulfur via sulfur oxidizing bacteria. The use of microaeration for hydrogen sulfide control has been described at bench scale (Azizi et al., 2023) and has been implemented to a limited extent in full-scale applications (Muller et al., 2022). However, very little information exists on the viability of using microaeration for hydrogen sulfide control in anaerobic digesters at municipal wastewater treatment facilities (Kraakman et al., 2023). 2. Objectives This study had several goals. A primary objective was to determine the optimal oxygen additional method and dose to facilitate in-situ biogas desulfurization via microaeration. This study was also designed to understand the system impacts of switching from ferric chloride addition to microaeration. This included looking at digestion impacts such as biogas quality and production, volatile solids destruction, and changes in sludge chemistry. Further, this study looked at other 'downstream' impacts including impacts to digested sludge thickening and filtrate processes, biosolids quality, biogas treatment, and corrosion concerns for the digester structure and appurtenances. 3. Study Setup This study involved microaerating a 1 million gallon mesophilic anaerobic digester treating municipal sewage sludges at the Nine Springs Wastewater Treatment Plant in Madison, WI. Microaeration was accomplished by addition of either compressed air or concentrated oxygen (~90% O2) using an oxygen concentrator. Oxygen was injected into the heated sludge recirculation line for the digester, along with ferric chloride. Oxygen and ferric chloride doses were adjusted over time to maintain biogas H2S concentrations similar to other digesters at the facility. The trial was conducted for 11 months. 4. Findings 4.1 Biogas Desulfurization and Biogas Quality Overall, results in Figures 1 and 2 show that microaeration is an effective method for reducing biogas H2S concentration. With air, biogas H2S was kept below 300 ppm while employing a ferric chloride dose 65% less than typical. However, with air, up to 330 cfh was needed to maintain H2S below 300 ppm. This resulted in a headspace oxygen content of 1.6-1.8% O2 and caused methane content to dip to 55% due to oxygen and nitrogen dilution. In contrast, when switching to concentrated oxygen, a gas flow of 55 cfh O2 was needed to maintain H2S below 200 ppm. At this dose the O2 content in the headspace was 0.6% or less and methane content was similar to the non-microaerated digesters (60% CH4). Overall, concentrated oxygen appears to be a more effective means of facilitating microaeration as desired results were achieved with lower oxygen dose, lower headspace oxygen content, and negligible dilution impact to the biogas. 4.2 Digestion Performance Microaeration did not result in identifiable negative impacts to digestion performance, and appears to have resulted in a slight improvement in volatile solids destruction. Digester pH and alkalinity increased slightly, ostensibly due to reducing in ferric chloride dose. Additionally, intermediate vs partial alkalinity ratio with microaeration remained similar to the ratio prior to microaeration. This demonstrates that the addition of oxygen directly to the sludge matrix did not have an inhibitory effect on methanogenesis. Figure 3 shows that volatile solids destruction in the test digester improved over time with microaeration. Prior to the study, the test digester had a lower volatile solids destruction that the average of the remaining digesters at the facility, whereas with microaeration the test digester showed a volatile solids destruction up to 4% higher than the remaining digesters. Biogas and methane yield also did not significantly change, further supporting the conclusion that injecting oxygen directly into the sludge matrix did not negatively impact methanogenesis. 4.3 Other Impacts of Interest One particular concern at the test facility is the formation of nuisance struvite due to high background magnesium and use of biological phosphorus removal. Dosing ferric chloride to the digesters is done to mitigate H2S, but this also helps bind phosphorus to reduce struvite formation potential in the digester. Over time the ortho-phosphate concentration in the sludge increased roughly 30%. However, there was no clear evidence of nuisance struvite formation in the digester as evidenced by lack of struvite accumulation in the sludge heat exchanger and associated piping as well as no discernable difference in material accumulation inside the digester during cleaning operations compared to other reactors. This observation, however, implies a greater side-stream phosphorus load back to the head of the facility. This load either needs to be dealt with in the main liquids treatment process, or could improve the viability of a phosphorus recovery process. 4.4 Microbial Corrosion Perhaps of greatest interest were findings during digester cleaning after 11 months of microaeration. As expected, the entire headspace of the reactor was coated in a yellow/white crust composed primarily of elemental sulfur. However, chemical analysis revealed that this crust also contained elevated concentrations of calcium, aluminum, iron, and magnesium, along with elevated sulfate. Presence of these constituents points to concrete degradation due to sulfuric acid production. Further, physical inspection of the concrete revealed surface softening, confirming that microbial corrosion of the concrete had occurred. While chemical analysis of the crust suggests that sulfide is primarily being converted to elemental sulfur, the level of concrete deterioration observed suggests long-term operation would be unwise without first treating the concrete to prevent corrosion. 5.0 Conclusions Results indicate that in-situ microaeration is an effective method to control biogas H2S. Use of concentrated oxygen appears to be more effective in terms of oxygen dose required and avoiding biogas dilution with nitrogen. Reduction in ferric chloride addition resulted in increased digestate pH and ortho-phosphate concentrations, but evidence of nuisance struvite formation was not identified. Evidence of sulfuric acid production and concrete deterioration were detected in the digester headspace, indicating that the use of microaeration is appropriate for applications where headspace construction is achieved with materials resistant to sulfuric acid attack.

Problem Statement: Wastewater treatment plants (WWTPs) are increasingly challenged to manage biosolids efficiently while meeting stringent regulations, especially for emerging contaminants like PFAS (per- and polyfluoroalkyl substances). Traditional biosolids disposal methods, such as landfill and land application, face restrictions due to environmental concerns. Rising energy costs further compel WWTPs to adopt treatment solutions that maximize energy recovery, reduce emissions, and align with regulatory standards. This study provides a mass and energy balance evaluation of six biosolids treatment technologies-drying, incineration, mesophilic anaerobic digestion (MAD) with post-digestion drying, thermal hydrolysis pretreatment (THP) followed by MAD and drying, and drying prior to pyrolysis or gasification-focusing on energy efficiency, life cycle costs, carbon sequestration, environmental impacts, and PFAS removal potential. Drying of biosolids from 20% to 90% total solids (TS) achieves a 90% reduction in mass, improving biosolids handling and transport for downstream processes. However, drying is energy-intensive, requiring approximately 1,000--1,200 BTU per pound of water removed, totaling around 1.76 MMBTU per ton of biosolids (Kreuger et al., 2020). Incineration does not require pre-drying and provides high energy recovery of around 6--8 MMBTU per ton of wet biosolids (Samolada & Zabaniotou, 2014). Operating at temperatures above 800°C, incineration shows promise for PFAS breakdown, though studies are ongoing regarding PFAS fate in ash and emissions. Incineration achieves 90% mass reduction with ash as the final residue. MAD operates with biosolids at 20--30% TS, producing approximately 7.9 MMBTU per ton in biogas energy (Zhou et al., 2019). Post-digestion drying requires an additional 1.76 MMBTU per ton, resulting in a net energy output of around 6.14 MMBTU per ton. While MAD is effective for moderate energy recovery and carbon sequestration, it has minimal impact on PFAS, making it less suited for facilities with strict PFAS requirements. THP + MAD enhances biogas production to approximately 9.0 MMBTU per ton by pre-treating biosolids with high-temperature thermal hydrolysis, which partially breaks down organic matter (Carrere et al., 2016). Combined, THP + MAD yields a net energy output of around 6.54 MMBTU per ton and reduces final digestate mass by 85%. Both pyrolysis and gasification require drying upto 90% TS. Pyrolysis decomposes biosolids into biochar, bio-oil, and syngas, providing stable carbon storage but resulting in a net negative energy balance due to drying (Winchell et al., 2022). Gasification, producing primarily syngas and ash, yields 0.8--1.0 MMBTU per ton and a 90% mass reduction. The ongoing studies need to be finalized to assess and understand the fate of PFAS byproducts in pyrolysis and gasification. Findings and Implications: MAD and THP+MAD are effective for facilities prioritizing moderate energy recovery and carbon sequestration. Incineration offers high energy output with significant emissions control costs. Pyrolysis and gasification, while energy-intensive, provide carbon sequestration and emerging PFAS removal capabilities. This analysis supports WWTPs in selecting biosolids treatment technologies aligned with regulatory, environmental, and energy objectives. Additional Insights on Life Cycle Costs, Environmental Impacts, PFAS Removal, and Carbon Sequestration Potential 1. PFAS Removal Potential - Incineration: Operating at temperatures above 800°C, incineration shows promise for PFAS breakdown. Studies are ongoing to assess the fate of PFAS and resulting compounds in ash and emissions (Samolada & Zabaniotou, 2014). - Pyrolysis and Gasification: High temperatures in pyrolysis and gasification may break down PFAS, but research is investigating the environmental fate of resulting byproducts in biochar (Winchell et al., 2022). 2. Life Cycle Cost Analysis - Drying: High operational cost due to drying energy demand (1.76 MMBTU/ton). Costs can be reduced with heat recovery systems (Kreuger et al., 2020). - Incineration: High initial capital and emissions control costs, but the significant energy output can provide a return on investment in high-throughput facilities (Samolada & Zabaniotou, 2014). - MAD + Drying: Moderate capital and operational costs; biogas revenue can offset drying energy costs (Zhou et al., 2019). - THP + MAD: Higher initial costs with THP, which improves biogas yield, making it favorable for facilities facing stricter regulations (Carrere et al., 2016). - Pyrolysis/Gasification: High operational costs for drying and emissions control, with net negative energy due to drying demands. Suited for facilities prioritizing PFAS destruction and carbon sequestration. 3. Environmental Considerations and Carbon Sequestration Potential - Carbon Sequestration: Pyrolysis produces biochar, offering stable carbon storage for soil. MAD and THP+MAD provide moderate carbon sequestration through digestate land application (Winchell et al., 2022). - Emissions Control: Incineration and gasification generate emissions requiring control systems, while gasification and pyrolysis emit lower greenhouse gases due to reduced oxidation (Samolada & Zabaniotou, 2014).

Abstract: Anaerobic digestion of sewage has been practiced over 150 years and results in the production of a methane enriched biogas which is considered a renewable energy source. After processing, biogas can be used directly to produce electricity via a co-generation plant, or following removal of the carbon dioxide within it, be used as a substitute to natural gas, or after compression, vehicle fuel. By displacing fossil-fuels, use of this biogas can help Water Utilities with their quest to become carbon-neutral. However, to make biogas, sludge needs to be anaerobically digested, the biogas cleaned, and the nutrients released to the digestate need to be destroyed. Furthermore, there are direct methane emissions from storage, leaks and incomplete processing. This paper looks to quantify these to determine the carbon impact of using biogas produced from municipal digestion of sewage sludge. KEYWORDS Anaerobic digestion; biogas; biomethane; carbon footprint INTRODUCTION There is increasing concern over the impacts of anthropogenic activities on the climate. This is leading to growing interest in renewable energy as an alternative to fossil-fuels. Biogas, a by-product generated from sewage treatment, is a valuable source of renewable energy. It can be used to generate electricity from engines which displaces energy made elsewhere at a power station resulting in a reduction in carbon footprint. However, to make biogas, sludge has to be anaerobically digested, and both the gas and liquid streams need to be processed (summarized in Figure 1). This processing generates a carbon impact in the form of electricity consumption (for example to mix digesters and to run conveying systems), chemical use (polymers for dewatering and carbon for nutrient removal) and direct methane losses (from storage, incomplete use or processing of the gas). However, the benefits of producing and using biogas are potentially becoming less clear. Globally, a shift towards cleaner forms of energy is making the displacement of energy using electricity derived from biogas less beneficial. Greater reductions of carbon were possible before when 'dirtier' forms of energy, such as coal were being displaced. Now, with cleaner energy mixes, these benefits are eroding. Despite this, direct losses of methane are still being seen. The wastewater sector is a major source of methane emission, contributing to 5--8% of global anthropogenic emissions (Ocko et al, 2021) and traditionally in the UK, open secondary digestion was common to provide additional destruction of pathogens from the wastewater. Recent measurements in the USA showing far greater losses of methane than previously believed (Vasquez, 2023). Methane has a global warming potential of 25 times that of carbon dioxide over a hundred-year period, but there has been emphasis in the industry to use a more realistic 20-year horizon, in which case it is 86 times worse. This means that leaks do not have to be large to offset the benefit of using the biogas in the first instance. For example, the digestion of sewage sludge can typically yield 350 m3 biogas/tonne solids fed to a digester. This is on average 65% methane, and based on its inherent energy, the biogas will have an energy content of 2,400 kWhr. After losses, a third of this energy could be used in an engine to offset fossil-fuel, i.e. 720 kWhr. This would displace 570 kg carbon equivalents of coal. A 1% loss of biogas, i.e. 3.5 m3 would contain 2.3 m3 of methane with a weight of approximately 4.6 kg. Using a 20-year horizon for methane, this has a carbon footprint of 396 kg. For a displacement of 570 kg carbon, therefore a loss of (576/396 =) 1.45% of the biogas would cancel the benefit of using the biogas. Figure 2 shows the contributing factors towards carbon footprint for the various stages on the carbon footprint of biogas used in co-generation, on the grid or as a displacement fuel for gasoline or diesel. This is only referring to the loss of methane. As mentioned, the sludge, biogas and liquid streams derived from biogas generation need processing and these also carry an environmental impact. This paper will go into detail into the processing impacts of biogas production and use, and compare that to the alternatives where digestion is absent, or organic materials are not digested but rather diverted to landfills, in order to provide a greater insight into the influence of anaerobic digestion on reducing carbon pollution. REFERENCES Ocko I. B.; Sun T.; Shindell D.; Oppenheimer M.; Hristov A. N.; Pacala S. W.; Mauzerall D. L.; Xu Y.; Hamburg S. P. 2021 Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming. Environmental Research Letters, 16 (5). Vasquez, K. (2023) Wastewater treatment releases nearly double the methane that was previously thought, Chemical & Engineering News.

Introduction Greenhouse gas (GHG), such as methane (CH4), nitrous oxide (N2O) and hydrofluorocarbons (HFCs), emissions are considered a major contributor to global climate change which has a substantial impact on the public and environmental health (Hopwood et al., 2008; Gautam et al., 2021). Multiple sources, including the Intergovernmental Panel on Climate Change (IPCC), highlighted that global temperatures have increased between 0.3 - 0.74 degrees Celsius in the past century due to the natural and anthropogenic sources including the GHG emissions (Solomon et al, 2010; Liu et al., 2019). Methane as a GHG has been reported to have 27-29 times higher global warming potential compared to carbon dioxide. According to EPA's 2022 report, the wastewater sector is the fifth largest methane source. For example, the methane emissions from domestic sewage in WWTPs can reach up to 25 Tg-CH4/yr (Wallington et al. 2019). Such estimations were developed based on top-down approaches where average emission factor is used to estimate such emissions. However, a major contributor to such emissions is direct process and fugitive methane emissions such as anaerobic digesters and waste gas burners. Such emissions may vary significantly according to operational conditions and infrastructure integrity. Moreover, there might be additional unexpected sources of GHG emissions that may contribute to the overall emissions from WWTPs. Therefore, the application of sensing technologies can aid in accurate quantification of the methane emissions from the WWTPs where these emissions can be correlated to specific treatment processes. Objectives In the current study, the main objectives are to: (1) utilize a multi-platform sensor approach to detect and quantify the fugitive methane emissions from a WWTP using drone and Optical Gas Imaging (OGI) sensing; (2) identify the hotspots of methane emissions within the wastewater treatment facility based on these different sensing technologies; and (3) determine the overall methane emissions from the treatment facility. Methods and Materials A drone and OGI sensing campaigns were carried out in a local WWTP in Ontario, Canada during the Summer of 2024. The WWTP contains the typical treatment processes including the primary treatment, secondary treatment, anaerobic digesters, biosolids loading facilities, and sludge settling tanks. Worth noting, this WWTP receives high organic loading rates which can significantly increase the methane emissions from the WWTP. During the campaign, the drone was flown to isolate each treatment process through creating flight paths upwind and downwind each process and building. These flight paths are known as the wind curtains where they are selected to be perpendicular on the wind direction at different altitudes around the potential process sources. To effectively capture the methane plumes from emission sources, the wind curtains were flown between 30 to 150 ft upwind and downwind the potential sources and treatment processes. The drone sensing approach generally features a quantification sensor (i.e., open-path laser spectrometer (OPLS)) that can detect methane levels as low as 10 parts per billion (ppb). In general, the flight conditions of the drone require a wind speed range of 4 and 20 mph while maintaining a consistent drone speed approximately 3.0 to 4.0 mph for optimal performance. In addition, an OGI camera was mounted on the drone for detecting the potential leakage within the treatment processes. A hand-held OGI camera comprising a cooled infrared sensor was used to access the areas that cannot be assessed using the drone including the indoor spaces and chambers. The OGI camera detects the methane using three factors: the infrared absorption of the gas aligning with the camera's spectral response, the temperature difference between the gas and background, and the gas concentration meeting the camera's sensitivity threshold. The OGI camera is connected with a gimbal which enables the OGI camera to rotate freely without restrictions. Results According to the outcomes of the drone and OGI campaigns, it was found that there are multiple hotspots for methane emissions within the WWTP. For example, the primary treatment process emitted approximately 8.0 kg-CH4/hr. This can be referred to the high organic loads received in such a treatment plant and mixing primary sludge with thickened waste activated sludge. In addition, the anaerobic digesters, gas burner facility, and sludge handling facilities yielded high emissions that ranged from 2.0 to 19.8 kg-CH4/hr. based on the sludge treatment stages and wind characteristics. For example, the anaerobic digesters had an emission rate of approximately 8.9 kg-CH4/hr. (Fig. 1a). Interestingly, considerable methane emissions were observed around the aeration tanks where the methane emission rate was approximately 3.8 kg-CH4/hr. (Fig. 1b). The activated sludge process in this plant does not entail any anoxic or anaerobic zones. Hence, the high emissions from the aeration basins can be attributed to unideal aeration and mixing conditions (coarse bubble aeration used for mixing and aeration), and/or the circulation of the centrate which is anticipated to be saturated with dissolved methane. A major methane emission spot was found to be the sludge holding tanks. Methane concentrations from such tanks reached more than 60 ppm, highlighting a major fugitive methane emission in the facility. However, there were some challenges in estimating the actual methane emission rates due to low wind speed at this area which ranged between 0.7 to 2.0 m/s that were lower than the recommended values of wind speed for accurate quantification of methane emissions using drone-based sensing techniques. In general, these hotspots were also surveilled using either the drone-based or hand-held OGI camera where the methane plumes were detected and recognized (Fig. 2). Conclusions and summary The current study aimed to accurately quantify the methane emissions from a WWTP using drone-based sensing approach to assist in calculating the methane emissions rates from various treatment processes. The methane emission quantification campaign at the WWTP demonstrated the efficacy of drone-based sensing combined with OGI cameras for detecting and measuring fugitive methane emissions where it provided important insights into the interplay between the treatment processes and methane emissions. The results showed significant methane hotspots near the primary tanks, aeration basins, sludge processing and anaerobic digestion units. This encompasses both anticipated and unexpected emissions sources which can enhance our understanding about the sources of fugitive methane emissions in WWTPs. The advantages of the aerial drone, coupled with the OGI camera, allowed for rapid and comprehensive coverage of the facility, providing accurate spatial emission data. Overall, the campaign underscored the potential of drone and OGI technology as a powerful tool for enhancing fugitive methane monitoring and supporting emission reduction strategies in wastewater operations.

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.

Cambi developed the Thermal Hydrolysis Process (THP) during the 1990's, based on ideas from Professor Halvard Ødegaard (Norwegian University of Science and Technology) and experiences reported from the Zimpro and Porteous processes developed decades prior. The Zimpro and the Porteous processes are sometimes referred to as Hydro-Thermal Carbonization (HTC) and were installed mainly to enhance dewatering. Unfortunately, these processes suffered from both design and operational challenges such as scaling in heat exchangers, odor release and reliability issues. Furthermore, the long holding times at high temperatures resulted in high concentrations of refractory compounds which are formed through caramelization of sugars and Maillard reactions. These reactions caused elevated concentrations of refractory Chemical Oxygen Demand (COD) and refractory Dissolved Organic Nitrogen (DON), and this became increasingly problematic with more stringent water emission requirements. Cambi developed its THP to operate at lower temperatures in order to form less refractory compounds. Most Cambi THP plants operate in the range from 140-165 °C and with 20 to 30 minutes retention time. HTC processes typically have somewhat longer retention times compared to THP and typically operate at or above 180 °C. The goal with the present work was to identify process conditions and develop a process design to improve dewatering properties while keeping formation or refractory compounds at low levels. All experiments carried out in the present work was on thermophilically digested mixed sludge which was treated in a lab-scale THP-pilot at temperatures between 140 and 200 °C for 30 or 60 minutes. Pre-AD-THP has now been widely adopted in several countries and many new plants have been successfully commissioned in North America. The main drivers are often intensification of the digestion process, improved dewaterability, increased biogas production and high-quality Class A biosolids with low odor. On average, the pre-AD-THP improves dewaterability by approximately 8 percentage points compared to conventional digestion. Full scale and laboratory studies shows that post-AD-THP improves dewaterability to a greater extent compared to pre-AD-THP. With post-AD-THP, the increased conversion of Volatile Solids (VS) into biogas and improved dewaterability can reduce the final cake volume by more than 60% compared to conventional digestion. Cambi´s post-AD-THP solution, Cambi SolidStream®, has been demonstrated both in lab scale and in full scale in Germany. The amounts of VS solubilized to soluble COD (sCOD) depends on the type of sludge and treatment conditions. In the present work, solubilization measurements, presented in Figure 2, shows that about 10 to 40 % of VS was solubilized. Previous dewatering experiments carried out by Cambi with a Bucher laboratory scale dewatering setup on sludge from the same plant shows that a final cake dryness in the range from 46 to 57 %DS was achieved after treatment at temperatures within the range from 140 to 212 °C and retention times between 30 and 120 minutes. The highest dryness levels were achieved with the most severe conditions. In SolidStream, most of the sCOD and some of the particulate COD (pCOD) will enter the centrate or filtrate which is returned to the anaerobic digester as shown in Figure 1. Refractory compounds formed through caramelization and Maillard reactions are difficult to degrade in biological processes and typically produce a yellow or brown color. The amounts of refractory compounds formed depends on the type of feedstock and increases with temperature and retention time. Some of the practical consequences for a WWTP are that refractory compounds are not converted into biogas, it may deteriorate the efficiency of UV-disinfection, reduce side stream nitrogen removal efficiency, and such compounds may end up in the final effluent if not removed. There is a link between the extent of formation of refractory compounds and color. In this work, color has been measured as APHA color of filtrate (0.45 µm) as shown in Figure 3. It should be highlighted that this work was carried out on digested sludge. Other work carried out on raw sludge typically shows a relatively larger increase in color already at lower temperatures. This could possibly be explained by the relatively higher concentrations of sugars and proteins in raw compared to digested sludges. While increased biogas production from centrate or filtrate returned to anaerobic digestion is a benefit with post-AD-THP, the main driver is typically improved dewaterability. Capillary Suction Time (CST) is a common measurement which to some extent can give indications on dewatering characteristics. Figure 4 shows the CST values measured on sludge treated at different process conditions. Measured CST decreased with more severe treatment conditions. As shown in Fig.3, color formation is somewhat limited at or below 165 °C and 30 minutes retention time, which are conditions typically used for THP while, it is more pronounced at typical HTC conditions which are at or above 180 °C and 30 min retention time. Furthermore, retention time has a somewhat limited influence on color formation below approximately 160 °C while the influence on CST, as shown in Fig.4, is large. This finding is relevant for the THP system shown in Figure 5. This system includes two energy recovery steps where temperature in the warmest pre-heating step and pressure reduction step is sufficiently high for desired reactions to take place. Cambi is currently delivering one post-AD-THP system in Norway which will be equipped with two energy recovery steps, and which is designed with flexibility for operating with reactor temperatures from about 140 up to 180 °C. Laboratory experiments on sludge from this plant shows that adding retention time in the warm pulper and flashtank helps to decrease polymer demand for final dewatering by around 70% compared to operation with only 40 minutes retention time at 180 °C and no holding time in pre-heating and pressure reduction steps. Results from this work shows that formation of colored compounds can be kept low at conditions typical for THP and that retention time has a large influence on dewatering properties when operating at low temperatures. A possible explanation for the relatively low color formation measured in the present work could be that sugar and protein concentrations are lower in digested compared to raw sludges. Cambi has developed a novel THP design which includes a cold pulper, hot pulper, reactors, hot flashtank and a cold flashtank. The additional retention time in the hot pulper and hot flashtank appears to provide substantial benefits with regards to dewaterability and polymer consumption while the impact on color formation appears to be limited in a post-AD-THP configuration.

The first North American installation of the Cambi B6 modular THP was installed at the HRSD Atlantic Treatment Plant (ATP) in 2019 and started up in March 2020. The drivers for installing THP at ATP were to achieve Class A biosolids, maximize existing digester capacity, improve final product dewaterability, stackability and minimize storage requirements, in addition to receiving FOG to improve biogas production. Each year, the THP pressure vessels (Pulper, four Reactors and Flash Tank) require inspection, resulting in a planned shutdown of this critical system and requiring storage of liquid and dewatered solids upstream of THP as well as an interruption of feed to the anaerobic digesters downstream of the THP. This has also required temporary hauling of unstabilized pre-dewatered cake, as the liquid storage capacity at ATP is insufficient to store for the typical shutdown duration. At the ATP, as with many THP installations worldwide, there is no redundancy of the single Pulper and Flash Tank on THP train, and therefore it has been critical for the plant to develop and optimize processes to perform the required cleaning, inspection, and maintenance activities while minimizing the shutdown duration and expedite the return to normal operations. Since commissioning, HRSD has conducted four planned 'turnarounds', as well as experienced a handful of unplanned shutdowns that required quick responsiveness for repairs. After each shutdown, the staff have reevaluated performance, documented lessons learned and revised processes to improve the next one. Through this process of refining and updating guidance documents, for the third shutdown in December 2022, ATP had optimized to a 5-day shutdown, from stopping feed to the THP to returning the solids processes to normal operations. While this duration was in line with the most efficient Cambi B6 shutdowns at other installations in the UK and elsewhere abroad, it still required the hauling of unstabilized solids during the shutdown, approximately 15 solids trucks per day. Hauling operations are not only costly, but also increase the instance of odor complaints from the nearby neighborhood and high school adjacent to the WRF. Following the third shutdown, two Maintenance Operators (MO - maintenance staff) identified some innovative ideas that with the potential to shorten the shutdown enough to completely eliminate the need for hauling of unstabilized pre-dewatered cake. These ideas stemmed from their intimate knowledge of the THP equipment and their collaboration with the operators in order to have a comprehensive understanding of the operating requirements of THP and the capabilities and constraints of the solids handling system. The new approach was discussed at length internal to the plant and then further with HRSD leadership, as it was not without risks and the potential for extra costs. With support from all levels of HRSD, the fourth annual THP was conducted in December 2023. Prior to ever shutting down the THP and interrupting operation, maintenance staff worked with operations to sequentially remove reactors and cooling heat exchangers from service, cleaned and inspected them, all while maintaining the THP in service. The intent was to do as much as possible while still processing solids and feeding the digesters. At the same time, operations took the lead on preparing for the shutdown, managing the solids inventory strategically to maximize the available storage volume for the liquid solids upstream of THP. The actual shutdown of the THP lasted just over 24 hours, rather than the previous best practice of five days. HRSD completed the shutdown and cleaned the Pulper and Flash Tank and other critical parts and completed all maintenance activities in one day -- which is the shortest outage ever recorded by the many dozen operating Cambi THP facilities. This shutdown did not require any hauling of solids, and was performed by HRSD's own staff, other than the contract high-pressure cleaning services by Jet Blast. By eliminating the need for contract hauling and contractor staff to assist the MOs, HRSD realized a nearly $80,000 savings for the shutdown (Table 1). Through this process collaborating between O&M staff, the ATP has learned that many of the maintenance activities can be completed while service to the plant remains in operation. This paper will focus on the innovative ideas that arose from the collaboration between the ATP O&M staff, without which the no-hauling shutdown would not have been possible. Additionally, this presentation will provide lessons learned from this and prior planned and unplanned shutdowns and the approach taken by ATP to efficiently perform the required tasks while minimizing downtime. This presentation will benefit all WRF personnel, regardless of if THP is a part of the solids handling process. These lessons help ensure that the time for THP downtime, temporary operations and impacts to the rest of the solids handling processes are minimized. Most importantly, other WRFs will benefit from the experiences of the collaborative process conducted by HRSD to bring an idea from MOs to plant and HRSD leadership-- when everyone comes to the table with an open mind to reach a common goal. By empowering staff's creativity, WRFs can realize cost and labor saving innovations and at the same time demonstrate the value that everyone brings to the utility. This presentation will be presented by HRSD Maintenance Operators who were instrumental in planning and identifying innovative approaches for minimizing downtime and were key in executing this shortest Cambi shutdown on record. They will speak directly to their experiences, and how good ideas can be brought to supervisors for the good of the WRF. These speakers will be especially engaging to Operators and Maintenance staff. This presentation will allow time for and encourage questions and dialogue with the audience to learn not only about THP O&M details, how to collaborate as a team, bringing Operations and Maintenance together to execute complicated shutdowns and maintenance activities, but most importantly, how to foster a WRF culture that encourages and empowers staff at all levels to bring forth creative ideas for solving complex issues and how to support the execution of these ideas.