IWA Pi Conference, May 15th 2017, ViennaAcademic research on benchmarking energy efficiency predominately focuses on two main streams of alternatives techniques: parametric and nonparametric. Since there is no agreement on the consistency of their estimates, the purpose of this paper is to investigate whether parametric ordinary least squares (OLS) and nonparametric data envelopment analysis (DEA) generate consistent wastewater treatment plant (WWTP) efficiency estimates. Our findings confirm that energy efficiency is function of several variables, including operational and exogenous factors. For it, a two-stage DEA process was followed in order to observe the overall effect produced by operation size and environmental conditions on efficiency index estimated by DEA. Based on the results of our analysis, the simply DEA in the variable return to scale assumption may not reflect the real efficiency of a WWTP. On the contrary a good consistency between OLS and 2-stage DEA was found. Conclusion of this study is that WWTP efficiency estimates may be sensitive to the method employed, and the use of multiple approaches for robustness checking is recommende
