The treatment of wastewater (WW) and the resulting sludge in urban WW treatment plants (UWWTPs) is accompanied by a significant consumption of resources as well as the concomitant emission of pollutants into the environment. One of the objectives of the NEXT (next generation of wastewater treatment plants) project is to identify sufficiently long-term optimization opportunities for processes to be selected as best available techniques (BATs) within the meaning of the industrial emissions directive (IED). The methodological framework used in this article is based on multi-criteria statistical tools, processing data from industrial plants to classify the sites studied and identify the reference plants in order to propose BAT references and associated emission levels that are reachable. Two public databases available on the internet were used, and 1,010 plants served as samples. The application and results demonstrated the applicability of the methodology and validated the mathematical approach used, as the thresholds comply with current French and European regulations. In the framework of the analysis conducted, five techniques (membrane bioreactor, bacterial bed, sand filter, and high-load or medium-load activated sludge) were identified as BAT according to the range of regulatory classification requirements for WW treatment plants (WWTPs). In addition, an example of reference values that may constitute a possible basis for the regulatory thresholds has been proposed. In parallel, a few discussion points were identified, including the choice of the metric for the reference values, the lack of data for the sludge line limiting identification of the BATs to the water line alone, the approximate characterization in the databases of certain parameters, as well as the problem of technique coupling due to the identification of one main technique.