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

Abstract

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 CO_2e per m³). 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.