Drone-Based Imaging and Sensing: Quantification of Fugitive Methane Emissions from Full-Scale Wastewater Treatment Facility

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.