A vertically elongate and oriented container (1) of wire cloth material contains a vertically oriented nozzle assembly (2), which sprays a fiber suspension against the inner wall of the container. The nozzle assembly is revolved around the axis of the container and is displaced backwards and forwards vertically so that the suspension sprays, which in the direction radially outward widen against the shell wall of the container (1) afford a cleaning effect in respect of deposited fibers on the inner wall of the container (1) at the same time as the liquid of the suspension efficiently is separated through the container wall in order to then be collected in a housing (9) surrounding the container (1).