To Measure or Not to Measure, That is the Question! Optical Remote Sensing of Greenhouse Gas Emissions from Wastewater Treatment

Municipal wastewater treatment plants (WWTPs) generate and emit greenhouse gases (GHG) throughout the plant via a number of activities and operations such as pumping, aeration, mixing, and sludge dewatering and digestion. As part of the collective efforts to achieve a 2050 'net-zero' emission goal, public utilities such as New York City, City of San Francisco (CA), City of Edmonton (Alberta), City of Calgary (Alberta) and Salt Lake City (UT) have started including quantitative sustainability indices as important factors to be evaluated when major capital improvements and O&M modifications are being planned for their water infrastructures. GHG emissions is one important sustainability index. In Canada, GHG emissions from major WWTPs are required to be reported to Environment and Climate Change Canada. A typical WWTP produces three major types of GHG: carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), both directly and indirectly. Direct GHG emissions occur at the site due to fuel consumption and the physical, chemical, and biological treatment processes. Those emissions are fugitive, heterogenous, and time-varying. Currently, three approaches are available for estimating GHG emissions from WWTPs. The first approach uses 'emission factors', such as the IPCC database, to estimate the rate at which a pollutant is released into the atmosphere (or captured) as a result of specific process activities. The second approach involves 'engineering principles and judgement' to estimate emissions based on engineering principles and judgment, using knowledge of the chemical and physical processes involved. Neither the first nor the second approach involves direct measurements. Rather, empirical coefficients are assumed which, often times, are not representative of the actual emissions at specific treatment facilities. The third approach may utilize emission monitoring systems such as the flux chamber method or the tracer method, which unfortunately are all based on point sampling, unable to capture the spatial and temporal heterogeneity of the GHG emissions at WWTPs. Due to the lack of monitoring method, so far, water utilities still rely on using the first two approaches for estimating their GHG emissions. Therefore, a robust field measurement approach is needed that is capable of capturing the dynamic, fugitive emissions at WWTPs. To achieve this, we have developed a hybrid method integrating the long range open-path Optical Remote Sensing (ORS) technology and inverse Dispersion Modeling (iDM) to continuously quantify GHG emissions from the wastewater treatment facilities on a real-time basis. The ORS instrument used in the field campaign consisted of an open-path tunable diode laser spectroscopy (OP-TDL) for CH4, a CO2 sensor, a portable meteorological station and 3-D sonic anemometer to record temperature, air pressure, relative humidity, and wind vector, and an open-path fourier transform infrared (OP-FTIR) for N2O. Both the OP-TDL and OP-FTIR scanned along multipaths to capture the 'big picture' of GHG plumes for dispersion model input for emission estimation. Backward Lagrange Stochastic model (bLS) was used to estimate the methane emission rate in real time. The system was first applied in the Bonnybrook WWTP, with an average daily flow of 324 million litre per day, located in Calgary Canada. Field measurements were conducted in autumn 2021, and spring of 2022. Signals were continuously captured and inverted to calculate the path-integrated concentrations of GHGs, with which the iDM model calculates the emission rates. The measured emission rate for methane ranged from 64-97 kg/hr, with the corresponding emission factor of 0.04-0.06 kgCH4/kgBOD5. For carbon dioxide, the measured emission was averaged at 0.29 ton/yr. Seasonal variations have been noticed. The team is currently working on calibrating a BioWin process model for simulating/predicting GHG emissions using the field measured emission data. Additional field measurements are also planned at a WWTP located in southeast US. This presentation will describe the newly developed ORS-iDS methodology and present the detailed results of continuous measurement from the field campaigns conducted during different seasons. The site-specific emission factors (EFs) developed based on the measurements have been compared with the literature and the IPCC values, which will also be presented to show the inaccuracy of using empirical, non-site specific emission factors. The diurnal and daily variation of emissions will also be characterized and presented.