Abstract: Apparatus is provided for dewatering an aqueous clay suspension by a process which includes preparing a partly dewatered filter cake and concurrently compressing the partly dewatered filter cake on a filtration drum while applying vacuum suction to it from inside the drum, and then removing the dewatered cake from the filter. The process also purifies the filter cake. An apparatus suitable for carrying out the process comprises a rotary vacuum filter, means for compressing the clay filter cake on the filter medium and means for applying vacuum filtration to the same portion of the cake being compressed. The compression means may comprise a rotatable roll mounted for rolling compressive contact with the filter cake. The rotatable roll may have a non-stick bearing surface to which solids of the filter cake are substantially non-adherent. The rotatable roll may be a pneumatic roll designed to conform to the filter drum and increase the size of the contact zone between the filter drum and the compression roll.