Ultra-broadband coherent open-path spectroscopy for multi-gas monitoring in wastewater treatment

Wastewater treatment plants significantly contribute to greenhouse gas emissions, including nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄). Current methods to measure these emissions typically target specific molecular compounds, providing limited scope and potentially incomplete emissions profiles. Here, we show an innovative ultra-broadband coherent open-path spectroscopy (COPS) system capable of simultaneously monitoring multiple greenhouse gases. This novel approach combines Fourier transform spectroscopy with a coherent, ultra-broadband mid-infrared light source spanning 2-11.5 μm at approximately 3 W power. Positioned above an aeration tank, the COPS system selectively detected absorption signatures for CH₄, CO₂, N₂O, ammonia (NH₃), carbon monoxide (CO), and water vapor (H₂O), enabling real-time, path-integrated concentration measurements with a temporal resolution of 40 s. Elevated concentrations of CH₄ and CO₂ were clearly identified within emission plumes traversing the beam path above the aeration tank. Additionally, CH₄ emission patterns closely tracked variations in ammonium loading from incoming wastewater, whereas CO₂ emissions correlated strongly with oxygen concentrations introduced during aeration. Measurements of N₂O, NH₃, and CO were stable and aligned closely with traditional point-based measurements from commercial gas analyzers. Our findings demonstrate that COPS offers a robust, comprehensive solution for the simultaneous real-time monitoring of multiple gases in complex and heterogeneous emission environments. This capability significantly enhances atmospheric and industrial emission assessments, potentially transforming the approach to emissions quantification and environmental management.