Coupling Coordination Analysis of Water, Energy, and Carbon Footprints for Wastewater Treatment Plants

It is urgent for the wastewater treatment sector to respond to global climate change. Although studies related to the water–energy–carbon (WEC) nexus have been widely conducted, the application of the coupling coordination indicator is still limited in the wastewater treatment sector. This study fills such a research gap by linking water footprint (WF), energy footprint (EF), and carbon footprint (CF) together and testing these indicators in 140 wastewater treatment plants (WWTPs) in Shandong province, China. Both the EF and CF of these WWTPs were calculated by conducting hybrid life cycle assessments, while WF was calculated by using a WF method. The results show that gray WF generated from 1 m3 of wastewater ranged from 9.58 to 12.90 m3, while EF generated from 1 m3 of wastewater ranged from 9.42 × 10−2 to 0.22 kg oil eq and CF generated from 1 m3 of wastewater ranged from 0.58 to 1.27 kg CO2 eq. Also, the total WF, EF, and CF of these WWTPs in Shandong were 4.26 × 1010 m3, 5.32 × 108 kg oil, and 3.35 × 109 CO2 eq in 2021, respectively. Key factors contributing to the overall greenhouse gas (GHG) emissions were the on-site GHG emissions and off-site electricity-based GHG emissions. Meanwhile, total nitrogen was the dominant contributor to the gray WF. In addition, the coupling coordination indicators of WF, EF, and CF ranged from 0.7571 to 0.9293. Finally, this study proposed several policy recommendations to improve the overall sustainability of this wastewater treatment sector by considering local realities, including adopting multi-dimensional indicators, decarbonizing current electricity grids, promoting the utilization of renewable energy, and initiating various capacity building efforts.