Method for dewatering a porous wet web

A method for the removal of liquid from a porous web containing said liquid comprising the steps of

• providing a filtration medium in position to receive thereon a porous web containing a liquid disposed within said pores of said web, said filtration medium including a first layer made up of a thin porous membrane having pores extending through the thickness thereof and a porous substrate layer supporting said membrane, said substrate including a plurality of reservoirs defined therein and which are in fluid communication with said pores in said membrane, said reservoirs having an average volume which is several times greater than the average volume of said pores through said membrane,
• disposing said web on that surface of said filtration medium defined by said membrane in flat supported relationship thereon, to thereby establish fluid communication between said liquid in said web and the pores of said membrane,
• selecting the size, number and/or distribution of said pores over the area of said membrane to be sufficient to effect capillary transfer of liquid from said web into said pores of said membrane but which will preclude the unaided reverse flow of liquid from said pores of said membrane to said web, irrespective of the period of time over which said liquid is present in said pores of said membrane and/or within said reservoirs defined in said substrate,
• providing a reduced pressure on that surface of said filtration medium opposite the surface thereof on which said web is supported to produce a pressure differential across the thickness of said web and said filtration medium,
• adjusting the value of said pressure differential across said web and filtration medium to a value which results in the removal of liquid from said reservoirs of said substrate as said liquid is transferred to said reservoirs from said pores of said membrane,
• collecting or disposing of said liquid which is withdrawn from said reservoirs.