Purpose: Life cycle assessment (LCA) and data envelopment analysis (DEA) have been combined in numerous occasions in order to identify the environmental efficiency of multiple units. In many cases, important differences in environmental performance have been identified among the units, referred to as decision-making units (DMUs). However, most studies have been limited to 1 year of assessment, preventing the study from identifying if efficiency values were linked to a specific trend of each DMU through time or to random annual changes. Therefore, the current study delves into the temporal variations in efficiency using DEA. Materials and methods: A DEA window analysis is applied to a set of wastewater treatment plants for a 4-year interval between 2009 and 2012 with the aim of understanding the annual behaviour of WWTPs in terms of environmental sustainability. A set of 47 WWTPs located in different regions of Spain were analysed using a window of 4 years of operation, to account not only for the differences in eco-efficiency between plants but also for changes due to the numerous temporal factors that may affect individual plants. Results: Results extracted from the assessment suggested that for the vast majority of the facilities, the efficiency standards tended to remain constant through time. Statistical tests (i.e. ANOVA and Friedman’s test) confirmed there were no significant differences between years in the different group sizes. This finding, confirmed that the use of the slacks-based measure of efficiency (SBM) model for one single year of operation is a good proxy for the evaluation of the environmental efficiency of these systems. In addition, the scale factor was confirmed as a significant driving force regarding environmental efficiency and significant differences in efficiency values were identified between large and medium WWTPS, on the one hand, and smaller plants, on the other. Conclusions: Significant differences were detected among plants with different legislation thresholds for their effluent withdrawal. WWTP discharging to nonsensitive water bodies appeared to be able to repeatedly attain efficiency values near the benchmark, whereas facilities with stricter thresholds (i.e. sensitive water bodies) struggled to achieve those values, especially in the case of the smaller plants.