The invention concerns a method for biological treatment of an effluent in order to purify it comprising treating said effluent in a first step which consists in an anaerobic biological treatment (13), with sulphate-reducing biomass fixed on a mobile support (13), producing a first effluent, then treating the first effluent in a second which consists in an anoxic biological treatment (14), with stationary sulphur-oxidizing biomass (14), producing a second effluent, and finally treating the second effluent in a third step which consists in an aerobic biological treatment (15), with stationary nitrifying biomass, producing a third purified effluent. Said method further comprises recycling part of the effluent present in the third step to the second step. The biomass present in the first step comprises sulphate-reducing bacteria, the biomass present in the second step comprises sulphur-oxidizing bacteria and the biomass present in the third step comprises nitrifying bacteria. The invention also concerns a device for implementing said method.