Assessing eco-efficiency of drinking water treatment plants: A synthetic index approach within water-energy-carbon nexus

Drinking water treatment plants (DWTPs) are essential facilities significantly contributing to greenhouse gas (GHG) emissions and operational costs within the provision of drinking water services. Unlike previous studies that evaluated the performance of DWTPs based on a single criterion, this study introduces a synthetic index to assess the eco-efficiency of a sample of 36 DWTPs from a holistic approach. This index integrates three key variables such as the volume of water produced and its quality, operational costs, and GHG emissions. The Efficiency Analysis Trees (EAT) method has been applied which, unlike other traditional multi-criteria approaches, also allows estimating optimal operating costs and GHG emissions based on varying produced water volumes. Results reveal substantial variations in optimal operating costs and GHG emissions, ranging from $0.023 to $0.519 per cubic meter and 0.050 kgCO 2 equivalent to 0.584 kgCO 2 equivalent per cubic meter, respectively. These divergences were also evident in the potential savings in operational costs, which ranged from $0.013 to $0.044 per cubic meter, and in the reductions in greenhouse gas (GHG) emissions, which varied between 0.005 gCO 2 equivalent and 67.899 gCO 2 equivalent. The average eco-efficiency score among the DWTPs was 0.595, ranging from 0.022 to 1.000. The number of eco-efficient DWTPs was 7 representing 19.4% of the sample. Variability in the performance of DWTPs underscores the limitations of uniform regulatory targets, advocating for customized targets that consider individual DWTP capacities.