Energy efficiency evaluation of wastewater treatment plants: A methodological proposal for its benchmarking

To evaluate the energy performance of wastewater treatment plants (WWTPs), reliable, robust and holistic methods are needed. The data envelopment analysis (DEA) method, which allocates a flexible set of weights to input and output variables, has previously been used to benchmark the energy efficiency (EE) of WWTPs. However, this methodological approach suffers from discriminatory power, which makes it difficult to rank WWTPs and compare their performances because the EE scores are estimated under nonhomogeneous conditions. To overcome these limitations and to better understand the water-energy nexus, in this study, the EE of a sample of WWTPs was evaluated by allocating common weights to variables for all WWTPs in a DEA model (DEA-CSW). Evaluated WWTPs were shown to have a poor energetic performance, with an average EE score of 0.372. This means that WWTPs could save 62.8 % of their current energy use. Potential energy savings were estimated to be 118,206,789 kWh/year, which is equivalent to 29,552 tons of CO_2eq/year. Based on a DEA-CSW approach, only one WWTP was identified as energy efficient; therefore, it is the best performer among the assessed WWTPs. Significant differences in the weights allocated to energy and pollutants removed from wastewater were reported by the DEA-CSW and DEA allocating flexible weights. Hence, under the latter methodological approach, some relevant variables, from the functionality perspective of WWTPs, were ignored in the EE assessment. This study demonstrates the relevance of using suitable methods to benchmark the energy performance of WWTPs to avoid misleading conclusions therefore, avoiding misguided regulatory decisions.