The Industrial Emissions Directive 2010/75/EU aims at achieving a high level of protection of the environment as a whole. To succeed, reference documents, which consider techniques frequently used in the whole Europe and their environmental performance level, have to be elaborated. These techniques and their performances constitute the concept of Best Available Techniques (BAT), which is inevitable for industrial sectors concerned to consider. The treatment of wastewater and resulting sludge in Wastewater Treatment Plants (WWTP) is accompanied by significant consumption of resources (chemical products, energy, etc.) as well as the concomitant emission of pollutants into the environment. The NEXT project (ANR-14-CE04-10) is aimed at understanding the working of WWTPs by focusing on the sludge line. One of the objectives is to identify sufficiently long-term optimization opportunities for processes to be selected as Best Available Techniques (BAT) within the meaning of the Industrial Emissions Directive (IED). The methodological framework structured into five steps is essentially based on multi-criteria statistical tools processing data from industrial plants, in particular the consumption and emission values. The objective is to classify the sites studied and identify the reference plants. Two public databases available on the internet were used and 1010 French plants served as samples. The application and results demonstrated the applicability of the methodology used. Five techniques (membrane bioreactor, bacterial bed, sand filter and high-load or medium-load activated sludge) were identified as best available techniques according to the range of regulatory classification requirements for WWTPs. In addition, the reference values that may constitute the basis of the regulatory thresholds have been proposed and validated by scientific, legislative and expert judgments literature.