Two Birds, One Test: Off-gas Testing for Assessing Scope 1 and Scope 2 Emissions

Background
The wastewater sector is rapidly adopting intensification technologies, driven by the need for improved process efficiency, reduced space requirements, and increased nutrient recovery. While intensification offers smaller footprint and potential operational benefits, their environmental trade-offs have not been thoroughly vetted. Technologies are likely to perform differently in terms of energy used and greenhouse gas (GHG) emissions. Bioreactor aeration is energy-intensive, contributing to most Scope 2 (indirect) emissions at water resource recovery facilities (WRRFs). Additionally, bioreactors can release nitrous oxide (N2O), a potent GHG with a global warming potential 270 times greater than carbon dioxide (CO<SUB>2</SUB>). While much focus has been placed on reducing Scope 2 emissions, direct Scope 1 emissions like N2O are also a significant contributor to WRRFs' overall emissions. N2O represent a significant portion of WRRFs' carbon footprint in conventional biological activated sludge processes. However, research and monitoring of N2O emissions from emerging intensification technologies is still in its infancy.

Off-Gas Testing as a Dual-Purpose Tool
Off-gas testing has long been employed to evaluate aeration efficiency and estimate the energy consumption of specific secondary treatment aeration zones. By measuring the air flux and making targeted assumptions, off-gas testing can estimate blower power demand through the adiabatic compression formula. When paired with data of the carbon intensity of the energy grid, Scope 2 emissions of the secondary treatment process can be also assessed. The combined ability to directly estimate oxygen transfer efficiency (OTE) and energy usage provides critical insights into the potential energy and GHG savings, which can be achieved by improving OTE or reducing air demand. Additionally, coupling this approach with GHG monitoring, such as N2O or methane release enables a comprehensive view of both Scope 1 and Scope 2 emissions. This approach offers insights into the GHG contributions from energy use versus direct emissions, proving valuable for assessing both existing conventional and new intensification technologies.

Tracking both Scope 1 and 2 emissions through off-gas testing allows utilities to observe the dynamic carbon footprint of their operations. This is important, as both direct and indirect emissions often show diurnal and seasonal variability. Understanding these fluctuations opens opportunities to develop control strategies that mitigate one or the other based on their respective impacts.

Methodology
An off-gas testing campaign including a hood and a portable analyzer was conducted at a WRRF to monitor N2O emissions and track Scope 2 emissions. The facility comprises four tanks operating in parallel, with one tank out of service for maintenance during the testing period. The reactors are configured in the Westbank configuration, with an average flow of 21.5 mgd. The hood used in this testing campaign (Figure 1), is a Carollo-developed, inflatable, self-ballasting hood designed for portability, ease of installation, and maximized sample representativeness. The hood measurements, combined with the portable analyzer measurements, enables high-frequency monitoring of airflow and gas species, allowing utilities to quickly, accurately, and cost-effectively estimate real-time GHG emissions.

Results and Discussion
The two-day off-gas testing, combined continuous measurements with sweep testing, revealed that N2O concentrations were proportional to CO<SUB>2</SUB> levels and inversely proportional to O2 concentrations, reflecting the effects of gas absorption and stripping in the aerated zones (Figure 2). This highlights the interplay between diurnal OTE variation and how it correlates to N2O stripping. Continuous N2O and airflow monitoring enabled the estimation of Scope 1 and 2 emissions. Figure 3 compares blower-related electricity use emissions and N2O emissions as CO<SUB>2</SUB> equivalents, showing higher N2O rates in the initial aerobic zones with higher airflow. Correlations between TKN and ammonia from daily samples yielded an N<SUB>2</SUB>O emission factor of 0.64% N2O-N/TKN-NPE -lower than the IPCC estimate of 1.6% but within the 0.03% to 1.29% range for secondary treatment bioreactors (Ahn et al., 2010; Kampschreur et al., 2009).

Extrapolated N2O emission represented 78% of the secondary treatment emissions due to the process' low specific energy use (530 kWh/MG). Figure 4 compares direct emissions of N2O to emissions from blower power use for a range of electricity emission factors representing various US states. It shows that the N2O emissions are larger, regardless of the carbon intensity of the electric grid, for this process configuration.

Conclusion
Intensification technologies hold promise for improving performance and capacity of WRRFs, but their N2O emissions are not yet fully understood. Direct, high-frequency monitoring of these emissions is essential for gaining an understanding of the carbon footprint associated with these technologies.
Real-time data on N2O emissions can support the development of new mitigation strategies based on assessment of Scope 1 and Scope 2 emissions. Process engineers and plant operators can use this data to fine-tune treatment processes, adjust operational parameters, and reduce energy use and GHG emissions.