Next Generation Digestion for the Fugitive Methane Era

The Intergovernmental Panel on Climate Change (IPCC) found that anthropogenic methane emissions are responsible for 45 percent of current net warming (IPCC, 2023). The EPA estimates that centralized WWTPs in the United States are responsible for 14 percent of waste methane emissions. At WWTPs, methane emissions are most prevalent in the solids handling areas, as methane is produced as a byproduct from the microbial breakdown of organic waste during anaerobic treatment of wastewater and sludge. Reducing WWTP methane emissions plays a vital role in reducing our near-term greenhouse gas emissions, which is necessary to avoid climatic tipping points and reduce environmental impacts. This manuscript will summarize: 1.Why utilities should care about fugitive methane emissions at their WWTPs. 2.Approaches to identify and quantify sources of fugitive methane emissions at WWTPs. 3.Strategies for mitigating these fugitive methane emissions, and 4.How to fund these mitigation strategies. In addition to the summary, the manuscript will dive into the details on mitigation strategies, specifically: 1.Describing fugitive methane emission pathways around solids handling processes, primarily anaerobic digestion, biogas handling, and digestate processing and storage. 2.How to update our designs and O&M practices to mitigate these emissions. The City of Columbus's (Ohio) is at the forefront of identifying, quantifying, and mitigating fugitive methane emissions. This manuscript will use the work around their Southerly WWTP Bioenergy Project as a case study that other utilities with anaerobic digestion can learn from. The goal is for our industry to design better digestion systems to do our part in mitigating the worst impacts of climate change and provide the best value for our communities. City of Columbus's Southerly WWTP Bioenergy Project The City of Columbus (Ohio) has embarked on a Bioenergy program to transform the solids handling process at the Southerly WWTP (SWWTP). The project includes a 60,000 gallon per day organics receiving facility (ORF) to prepare waste organics for co-digestion, a 6 MG digestion process expansion to increase solids capacity and convert organics and sludge into biogas, and a 8MW cogeneration facility to make renewable electricity and recover heat from the biogas. Further, to expand on the City's cutting-edge work to identify and quantify fugitive methane emissions at the SWWTP, the project includes upgrades to the existing digestion system and redirecting foul air from the solids handling processes to the new waste gas burner facility. These improvements are projected to reduce identified fugitive methane by 91 percent and captures additional biogas for use within the cogeneration facility. Why Care About Fugitive Methane Emissions The capture and utilization of the fugitive methane identified at Columbus' SWWTP is equivalent to taking 18,000 cars off the road and saving the city $350,000 annually in combined heat and power savings that otherwise would have been lost to the atmosphere. Reducing methane emissions also: - Helps meet climate goals -As part of its Climate Action Plan, the City of Columbus (City) has set ambitious goals for the Columbus community to be carbon neutral by 2050, in alignment with the Paris Climate Agreement. An interim goal to reduce GHG emissions by 45 percent by 2030 was set to help meet the 2050 target. - Protects against Future Regulations-The U.S. Environmental Protection Agency (EPA) has moved to include GHGs, particularly methane, as regulated air pollutants under the Clean Air Act. This has led to the development of GHG reduction standards for the power, and oil and gas industries. For example, the Inflation Reduction Act includes provisions that indicate designated petroleum and natural gas facilities must calculate baseline methane emissions and be subject to a fee on excess methane emissions (Murphy, 2022). Similar regulations may eventually be adopted in the municipal wastewater sector. - Improves Public Perception - Methane emissions are gaining additional scrutiny from the public and climate activists. There is currently publicly available satellite imagery that can pinpoint large emitters of methane (including WWTPs) with many more satellites coming online in the next 12-24 months. Proactively identifying and mitigating fugitive methane transforms a negative publicity risk into a positive story of how the city is reducing emissions, improving health, and recovering resources. - Maximizes biogas beneficial use-To ensure the cogeneration facility is sized properly and biogas use, and related renewable energy production is maximized, it is critical to identify locations where fugitive methane can be captured. - Maximizes EPA GHG reduction grants-For the City to position for GHG reduction grants through the Inflation Reduction Act for future improvements, the City needs to justify and demonstrate the need to reduce GHG emissions at SWWTP. - Identify methane leaks posing safety risks-Leaking methane, along with associated constituents in biogas (i.e. hydrogen sulfide), pose a health and safety risk for plant staff. Identifying and mitigating these emissions will reduce the risks of potentially fatal accidents. How to Identify and Quantify Fugitive Methane Emissions Multiple measurement technologies were researched and deployed to identify and quantify fugitive methane emissions at the SWWTP; this included drone flux measurement (DFM), an optical gas imaging (OGI) camera, a gas analyzer, and a biogas capture box. A comparative analysis of the resulting data from these approaches was used to produce the emissions for the SWWTP. As expected, most of the methane emissions were associated with the solids handling parts of the facility, as outlined in Figure 1. How to Mitigate Fugitive Methane Emissions The field investigation helped to identify sources of fugitive methane in the solids handling and treatment processes that are associated with biological activity under anaerobic conditions, entrained biogas in the sludge, and dissolved methane in the sludge. Sources of fugitive methane identified at SWWTP are summarized in Figure 2, and include: - Annular space between floating covers and digester wall - Digester Pressure relief valves - Gaskets of digester cover hatches and manways - Flanges of digester gas piping - Leaks through concrete digester walls - Gas handling equipment - Exhaust stacks of odor control systems serving solids handling processes - Waste gas burners The manuscript will summarize evaluation of different mitigation strategies. The selected improvements share a common theme, which is to: 1.Maximize production and extraction of biogas in the anaerobic digesters, which reduces the potential for biogas to be emitted in downstream solids handling processes. 2.Improve biogas capture of the digestion and biogas systems. Some of the improvements include in the Bioenergy project are: - Improve digester feeding, mixing and withdrawal to maximize biogas production and extraction within the digesters. - Replace floating covers with fixed concrete covers rated for 60-inch water column gas pressure and embedded liner for corrosion protection and gas imperviousness. - Overhaul the biogas handling system to provide a larger delta between the operating pressure range and the safety pressure setpoints (waste gas burner backpressure control valves, pressure relief valves, emergency relief hatches, etc.). - Capturing the solids handling foul gas and combusting it in a vapor combustion unit (VCU), which is uniquely capable of combusting low percent methane gases. - Other miscellaneous advancements (liner penetration details, gasket specifications, instrumentation and control strategies, O&M practices). Most of these improvements have already been implemented and proven effective separately on previous digestion projects. Case studies of some of these examples will be highlighted to make the case that combining these improvements can provide an example of a new standard of care for designing and operating digestion facilities. How to Fund Fugitive Methane Mitigation Improvements A summary of how the Bioenergy project was structured to maximize the potential funding available from the Inflation Reduction Act (IRA); it is estimated that this project will receive over $50M from this program alone. Closing The City of Columbus's (Ohio) is at the forefront of identifying, quantifying, and mitigating fugitive methane emissions. This manuscript will use the work around their SWWTP Bioenergy Project as a case study that other utilities with anaerobic digestion can learn from. The goal is for our industry to design better digestion systems to do our part in mitigating the worst impacts of climate change and provide the best value for our communities.