Abstract: The present invention relates to a high-precision, high-efficiency recirculation system for preserving multiple-density biomass and attached biofilm in wastewater treatment bioreactors, which is characterized in that it consists of a bioreactor tank in which a central recirculation duct is centrally and vertically disposed, said duct comprising a diffuser cone at the bottom thereof, close to the bottom of a baffle of the bioreactor, and an upper guide cone at the upper end thereof, wherein the central recirculation duct is designed for an inductor/nozzle to be inserted there into up to the limit of the upper wall of the bioreactor baffle, said parts forming together a central assembly containing, in vertical attitude, a drive shaft which comprises, at the upper end thereof, above the inductor/nozzle, sealing means for hermetic sealing at the upper wall of the bioreactor baffle, wherein the upper end of the drive shaft is coupled to a low-revolution motor reducer for rotating the drive shaft, which comprises o