Development and validation of an off-gas tool for oxygen transfer efficiency and N₂O emission monitoring in wastewater treatment

Aeration flow operational costs are the most important economic factor in the operation of a wastewater treatment plant and nitrous oxide emissions have been reported as an important source of greenhouse gas emissions in wastewater treatment plant operations. The development of an off-gas analysis system capable of coupling the techniques for nitrous oxide measurements (N₂O) and oxygen transfer efficiency (OTE) evaluation was investigated. The developed tool was successfully applied in field measurements in the oxidation ditch wastewater treatment plant of Charneca de Óbidos (Portugal). The obtained results for OTE using the off-gas method (15.04 % ± 1.78) were validated by comparison with the OTE obtained using the in-situ oxygen uptake rate method (14.27 % ± 5.09). Several locations within the oxidation ditch were tested to study the variability of measurements along the reactor regarding OTE and N₂O, revealing a low level of variability between these locations. Thereafter, a central location with good mixing was studied for 30 days. On this characterization campaign, the results obtained for OTE ranged between a minimum value of 3.2 % and a maximum of 34.1 %, with an average value of 17.97 ± %5.38. Analysis of N₂O emissions averaged 0.012 ± 0.003 mgN-N2O/m3 in this plant, and the emission factor was estimated at 3.2 % ± 0.5. To the best of the authors' knowledge, this study represents the first report of simultaneous collection of oxygen transfer efficiency and N₂O emissions under real-world conditions. This approach enhances understanding of wastewater treatment plant performance, facilitates operational improvements, and contributes to the body of knowledge in environmental engineering and sustainability.