A method of separating suspension, in particular for treatment of waste water, wherein the flocculating suspension is separated from the liquid by filtration in a fluidized layer in a sludge blanket wherein the flocks are created from the separated suspension and the fluidization is maintained by the rising stream of liquid, while the liquid with suspension enters the fluidized layer from the bottom and the liquid freed from suspension is discharged above the surface of the sludge blanket represented by the interface between the fluidized layer and the liquid without suspension. The thickened separated suspension in form of flocks from a sludge blanket is withdrawn from the zone of the fluidized layer, the velocity of upward flow in the fluidized layer essentially decreasing in the upward direction. The apparatus for performing this method contains an upward widening separator (1) provided by inlet (5, 38, 59) of treated suspension in its bottom part, and by means for withdrawing the liquid without suspension at its top. A separator (1) the inner space of which contains a separation space is provided with at least one withdrawal spot of the thickened suspension from separation space that is arranged above the inlet (5, 38, 59) into separator (1), predominantly at its outer wall (2) or outer walls (33, 34, 50, 51) and under the surface of the sludge blanket.