Net-zero greenhouse gas emission from wastewater treatment: Mechanisms, opportunities and perspectives

Net-zero emission and self-sustaining carbon-neutral have been widely encouraged and intensively explored in wastewater treatment, as the wastewater industry accounts for approximately 1.6% of global greenhouse gas (GHG) emissions. However, how to achieve net-zero GHG emission in wastewater treatment from a biological standpoint is still lacking. In this paper, the status of GHG (i.e. nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2)) production and emission in wastewater treatment was comprehensively reviewed. Generally, N2O emission contributes the majority of the total GHG emissions from wastewater treatment, and the dissolved N2O and CH4 in the effluent should be paid more attention for the further GHG mitigation. Based on the newly identified microbial communities capable of nitrogen removal with less GHG emissions, several alternative integration systems for GHG mitigation were discussed. Coupling denitrifying anaerobic methane oxidation with anammox process shows high nitrogen removal performance and low GHG emissions, which is regarded as a more promising integration system for GHG mitigation. Moreover, the novel strategies to manage N2O and CH4 as energy sources were also demonstrated, aiming to offset certain carbon footprint in wastewater treatment and thus attain net-zero GHG emission. In addition, future challenges and perspectives of above integration systems and strategies were discussed to facilitate the development of environmentally sustainable wastewater treatment. Finally, a prospective hybrid system integrating emission reduction and energy recovery was proposed as an example for achieving net-zero GHG emission from wastewater treatment.