The invention relates to a method for biologically treating waste water, using a set of micro-organisms having different metabolic spectra, in order to eliminate carbon and nitrogen, even phosphorus. Some of the micro-organisms are fixed to mobile solid carriers (2) and form a fluidised fixed biomass. Some of the micro-organisms are free to be used in an activated mud treatment. To this end, a first non-aerated treatment zone (21a) is followed by a second aerated treatment zone (21b); the treated effluent is subjected to a solid/liquid separation by flotation (6) at a speed higher than 10 m/H; and part of the mud recovered by flotation is recirculated (8) towards the activated mud treatment, said recirculation being controlled (9, 10, 11) so that the MES concentration of the effluent subjected to the liquid/solid separation remains compatible with the retained flotation.