Emerging investigators series: revisiting greenhouse gas mitigation from conventional activated sludge and anaerobic-based wastewater treatment systems

Recent literature on carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions from wastewater treatment plants (WWTPs) has highlighted the poor consensus in total greenhouse gas (GHG) estimation (ranging from 0.243 to 2.4 kg CO2e per m(3)). In the present study, the major components of GHG emission variability in both conventional activated sludge (CAS) and mainstream anaerobic WWTPs are systematically investigated as a basis for delineating a roadmap to their future control and minimization. Through analysis of N2O generation pathways, it was determined that additional research via isotope labelling is necessary to elucidate distinct generation mechanisms in CAS WWTPs (e.g., nitrifier denitrification and hydroxylamine denitrification) and better predict N2O contributions to total GHGs. Conversely, mainstream anaerobic processes, although a potentially more sustainable alternative to conventional aerobic treatment, introduce effluent dissolved CH4 as a potentially significant GHG contributor. Sweep gas and vacuum driven membrane contactors are promising dissolved methane management strategies. However, further optimization of gas-to-liquid ratios and transmembrane pressures, respectively, are vital to balancing treatment efficiency with energy neutral/positive operation. Overall, a thorough elucidation of N2O generation pathways in CAS WWTPs and the development of effective dissolved CH4 management strategies for mainstream anaerobic processes will define their respective future roles in reducing wastewater-associated GHG emissions.