Continuous Monitoring of Fugitive Methane in Wastewater Treatment Plants Using Ground Sensors

Introduction Greenhouse gas (GHG) emissions, primarily comprising carbon dioxide (CO2), methane (CH), and nitrous oxide (N2O), are a major contributor to climate change, driving global warming and environmental instability (Gautam et al., 2021). These gases originate from various human activities, including energy production, transportation, agriculture, oil and gas industry, wastewater treatment, and waste management (Varon et al., 2021). Among these sources, methane is particularly potent, having a global warming potential significantly higher than CO2 over a 20-year period. This makes methane emissions a critical target for mitigation efforts, as reducing them can have an immediate impact on curbing climate change and its negative influence on the environment (Liu et al., 2019). Wastewater treatment plants (WWTPs) are considered an active source of GHG emissions, especially methane, which is generated during the biological processes that involve breaking down the organic matter during the anaerobic digestion processes (Khalil et al., 2024). There are multiple key emission hotspots within WWTPs including the sludge treatment areas, anaerobic digesters, and equalizer tanks, where anaerobic conditions can lead to significant methane release. In addition, there might be additional unexpected sources of GHG emissions that may contribute to the overall emissions from these WWTPs such as aeration tanks. As these emissions can be episodic and location-specific, continuous monitoring is essential for capturing real-time data on emission patterns and variabilities. Despite its significance, a major limitation in the wastewater sector is developing an affordable and continuous methane monitoring systems. Therefore, the use of advanced sensing technologies, such as ground and drone-mounted sensors, has been gaining greater attention during recent years. These technologies enable more precise and continuous monitoring, allowing for effective detection of fugitive methane emissions and identification of the hotspots that might otherwise go unnoticed. Continuous monitoring not only provides critical insights into emission dynamics but also supports timely interventions, helping to reduce the carbon footprint of WWTPs and contribute to broader climate goals. Objectives The main objectives of the current study are to: (1) install a group of ground sensors to isolate the methane emissions from different wastewater treatment processes; (2) identify the hotspots of methane emissions within the wastewater treatment facility based on continuous timeseries of fugitive methane emissions from different sources; and (3) compare the methane emissions from the ground sensors with other sensing technologies such as drone-based techniques. Methods and Materials In the current study, 16 ground sensors were installed in a local wastewater treatment facility (WWTF) in Ontario, Canada. The WWTF comprises the typical treatment processes including the primary and secondary treatment, anaerobic digesters, and solids management facilities. In addition, this WWTF receives high organic loading rates which can contribute to the fugitive methane emissions. The ground sensors were distributed around the treatment processes and building to continuously monitor the fugitive methane from these processes. For example, four ground sensors were installed around the aeration tanks to continuously monitor the methane concentrations and emissions from the basins. It should be noted that the observation period started in June 2024 where the ground sensors observed the methane concentrations on a 15-minute interval. Besides the ground sensors, a drone and Optical Gas Imaging (OGI) sensing campaigns were carried out in the WWTF in Summer 2024 to compare the methane concentrations and emissions from different monitoring levels including the ground and aerial sensing techniques. Results According to the preliminary insights from the time-series of methane concentrations using the ground sensors, it was observed that there are multiple hotspots for methane emissions within the WWTF. The average methane concentration at each ground sensor varied according to the location of sensor, the isolated process, and wind properties that play a crucial role in conveying the methane plume. For example, the average methane concentration over four months (i.e., from beginning of June until the end of September) ranged between 7.2 to 11.4 ppm for the sensors surrounding the aeration tanks (Fig. 1), indicating that these basins are a potential source of methane emissions. In addition, the average methane concentration was approximately 10.3 ppm near the primary clarifiers (Fig. 2a). Moreover, the range of the average methane concentration of the sensors installed around the sludge treatment facilities was 5.7 -- 11.2 ppm. Based on these observations, it can be emphasized that most of the treatment processes are considered hotspots for fugitive methane emissions with the WWTF. These findings were validated using the drone and OGI sensing approaches where the drone flights and OGI camera confirmed the locations of the hotspots for emitting methane within the treatment processes (Fig. 2). Conclusions and summary The deployment of ground-based sensors for the quantification and detection of fugitive methane emissions at the WWTF provided valuable insights into the spatial and temporal distribution of emissions. The sensors, strategically placed near the expected and unexpected emitters such as sludge handling units, anaerobic digesters, and aeration tanks, captured real-time data on methane concentrations, enabling the identification of persistent emission sources. Over the monitoring period, fluctuations in emission levels were observed that were linked to the operational activities and environmental conditions. This data underscored the variability of methane emissions and emphasized the importance of continuous monitoring to capture short-term spikes and long-term trends. Ultimately, the ground sensor campaign highlighted the potential for such technology to complement aerial sensing, providing granular, location-specific data critical for effective emission mitigation strategies in wastewater treatment operations.