Water-energy-carbon synergies and trade-offs: A daily nexus analysis for wastewater treatment plants

Given the increasing interest in achieving carbon neutrality, water-energy-carbon synergy and trade-off mechanisms are critical for the sustainability of wastewater treatment plants (WWTPs). Based on daily opera-tion data of a WWTP in southern China from 2007 to 2021, we propose a new approach to explore the water--energy-carbon (WEC) nexus of WWTPs by developing a Water-Energy-Carbon Coupling Index (WECCI) and a four-quadrant clustering (FQC) method. It is found that the WECCI increased by 10.12% after the first standard upgrade and decreased by 8.20% after the second standard upgrade, which indicates that the transformation of WWTP may lead to unexpected change in the overall performance. Based on the FQC method, 85.64% of the operation data are mainly concentrated in the WEC best synergy zone after the first standard upgrade, whereas 58.52% of the operation data are concentrated in the low carbon trade-off zone after the second standard up-grade. This study indicates that WECCI and FQC are necessary tools to quantitively assess the integrated per-formance of WWTPs for better understanding the complex WEC mechanisms of WWTPs, which facilitate the sustainable transition of WWTPs towards carbon neutrality.