A method and device used for treating effluents by the biological aerobic activated sludge process with hydrodynamic separation, sludge collection, internal sludge recirculation and atmospheric air or oxygen dissolution functions implemented by a device that accumulates the functions of solids retention and gas dissolution in the biological reactor, increasing the hydraulic load capacity of the secondary settler, as well as the load absorption capacity of the biological reactor, thus almost doubling the treatment capacity in relation to a conventional activated sludge process.