**5. Conclusions**

The assessment of energy requirements for urban water supplies has emerged as a relevant topic. The few previous studies that have evaluated the energy efficiency of DWTPs have employed conventional DEA models, ignoring the deterministic nature of this method. To overcome this limitation, a double-bootstrap DEA model was used in this study to evaluate the energy efficiency of a sample of DWTPs.

Empirical application with a sample of Chilean DWTPs provided three primary conclusions. First, the energy efficiency of the sample of DWTPs evaluated was very low; less than 5% of facilities were energy efficient, and the DWTPs could reduce the energy consumed by >50% while maintaining the same level of pollutant removal. Many water companies have focused on the optimization of energy use in WWTPs in recent years; the findings of this study show that improving the energy efficiency of DWTPs is also challenging. Second, the integration of data variability in the energy efficiency assessment notably affected the results. The ranking of DWTPs based on original and bias-corrected energy efficiency scores differed significantly. Water regulators that use benchmarking to regulate water companies or to set water tariffs should integrate data variability in their performance assessments, as in this study, to avoid the generation of biased results and conclusions. Third, among the variables studied, the determinants of energy efficiency were the volume of raw water treated (i.e., facility capacity), DWTP age, and the main technology used to treat raw water (i.e., pressure filters or rapid gravity filters). Plant size and the technology used to treat raw water are structural variables that cannot be modified by DWTP managers; thus, the improvement of energy efficiency in existing facilities is difficult in the short term. However, in consideration of long-term energy efficiency, these features must be taken into account when planning the construction of new DWTPs. In addition, facility age positively influenced energy efficiency, revealing the important role of equipment maintenance and replacement in the energy efficiency of water treatment facilities.

The improvement of DWTP energy efficiency is essential to achieve global climate goals and provide affordable drinking water for all people. Several regulations, such as the Revised Energy Efficiency Directive (which defines the European Union energy efficiency targets), and international agreements are focused on the reduction of consumers´ energy requirements. The European Water Framework Directive, which establishes full cost recovery for water services, was implemented in 2000. It mandates that all costs of urban water services, including DWTPs, must be transferred to citizens via water tariffs. The improvement of DWTP energy efficiency contributes to the achievement of the UN's Sustainable Development Goal 6 and to the fulfillment of climate change agreements. In this context, to support decision-making, it is essential not only to assess the energy efficiency of DWTPs, but also to identify the determinants of energy efficiency using a reliable and robust methodological approach, as was done in this study.

**Supplementary Materials:** The Supplementary Materials are available online at http://www.mdpi.com/1996- 1073/12/4/765/s1.

**Author Contributions:** Conceptualization, M.M.-S. and R.S.-G.; methodology, R.S.-G.; software, R.S.-G.; validation, M.M.-S. and R.S.-G.; formal analysis, M.M.-S.; investigation, M.M.-S. and R.S.-G.; resources, M.M.-S.; data curation, R.S.-G.; writing—original draft preparation, M.M.-S.; writing—review and editing, M.M.-S.; visualization, R.S.-G.; supervision, M.M.-S. and R.S.-G.; project administration, M.M.-S.; funding acquisition, M.M.-S.

**Funding:** This research was funded by CONICYT, programme Fondecyt (11160031).

**Conflicts of Interest:** The authors declare no conflict of interest.
