Arrangement for separating suspension, in particular for treatment of wastewater. A 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 fluidization is maintained by the rising stream of liquid while 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. Thickened separated suspension in the form of flocks from a sludge blanket is withdrawn from a zone of the fluidized layer the velocity of upward flow in the fluidized layer decreasing in the upward direction.