Increase of population, development of industry, environmental problems and continuous energy consumption in the world render renewable energy resources crucial. This paper focuses on multiple criteria decision making (MCDM) models for energy saving and using energy resources efficiently in order to maintain sustainable operations at wastewater treatment plants (WWTPs). An integrated fuzzy quality function deployment (FQFD) framework and technique for order preferences by similarity to an ideal solution (TOPSIS) selection algorithm is proposed. Fuzzy analytic hierarchy process (FAHP) is used to determine importance weights of attributes in MCDM model to avoid inconsistent results of crisp QFD analyses caused by the variability of human judgment. Fuzzy analytical network process (FANP) is used to consider both symmetrical and asymmetrical relationships between customer requirements (CRs) and engineering characteristics (ECs) for constituting internal and external correlation matrices in the house of quality (HOQ). The comprehensive weight vector of ECs is used as the weights of the selection criteria in the TOPSIS side of the integrated methodology. The proposed MCDM methodology is used for the goal of choosing the ideal gas fuel at WWTPs. Regarding to the results of application of the proposed MCDM model at a WWTP in Turkey, the most important EC regarding to the goal is hydrocarbons with the weight value of 0.32, while gas processing system with 0.21 is the second and impurities with 0.19 is the third EC. Biogas is chosen as the ideal gas fuel. In addition, natural gas is the second, liquefied natural gas is the third, compressed natural gas is the fourth and landfill gas is the fifth gas fuels in turn at the WWTP in Turkey. (C) 2017 Elsevier Ltd. All rights reserved.