Greenhouse gas accounting methodologies for wastewater treatment plants: A review

Greenhouse gas (GHG) emissions from wastewater treatment plants (WWTPs) have become a major concern in the efforts to mitigate climate change and reduce overall emissions. Carbon emission accounting methods are direct strategies for obtaining carbon emission data from wastewater plants, but different accounting methods have certain limitations. In this study, the development history, the scope of application, the advantages and disadvantages of the emission factor, field monitoring, modeling methods, and machine learning methods were discussed. The reliability and accuracy of different accounting methods for direct and indirect emissions were compared and analyzed. In addition, the application progress of the accounting methods in developed and developing countries was discussed. This study shows that the specificity of emission factors directly affects the results obtained using the emission factor method. The field monitoring method is a relatively accurate approach, but it relies on new technology and a monitoring cycle of more than one year. The results obtained from long-term monitoring of the Anaerobic Oxic (AO) process are statistically greater (nitrous oxide (N2O) emissions averaging 2.25% of the nitrogen loading) than those obtained from short-term monitoring (N2O emissions averaging 0.032% of the nitrogen loading). The accuracy of the modeling approach relies on a good understanding of the pollutant fractions in the influent and the pollutant transformation mechanisms. In developed countries, compared to developing countries, most of the studies were conducted using on-site monitoring and modeling methods. In China, the emission factor method is used, which accounts for 44.4% of the total number of publications. Future research could refine the various GHG calculation methods and improve the accuracy of the calculations to meet the accounting needs of different wastewater treatment processes and different research objectives. This study provides insight into developing low carbon wastewater treatment processes and routes. It will also provide a reference for WWTPs to achieve carbon emission reduction.