A method of treating an animal skin substrate comprising: agitating the animal skin substrate with a solid particulate material and a treatment liquor; separating the animal skin substrate from effluent comprising the solid particulate material, the treatment liquor and solid waste fragments derived from said substrate; transferring the effluent to a first separator, wherein the first separator is configured to allow the treatment liquor and at least a portion of the solid waste fragments to pass through the first separator but to prevent the solid particulate material from passing through the first separator, and providing a first filtrate stream comprising treatment liquor and solid waste fragments and a first residue comprising solid particulate material and residual solid waste fragments; treating said first residue to remove said residual solid waste fragments to provide cleaned solid particulate material, wherein said treating of said first residue comprises retaining said solid particulate material on a surface of a treatment separator during said treating, wherein the treatment separator is configured to allow liquid and residual solid waste fragments to pass through but to prevent the solid particulate material from passing through; and (i) directing a cleaning formulation onto said solid particulate material to provide a second filtrate stream comprising said cleaning formulation and said residual solid waste fragments and a second residue comprising cleaned solid particulate material; and/or (ii) agitating the treatment separator. agitating the animal skin substrate with a solid particulate material and a treatment liquor; separating the animal skin substrate from effluent comprising the solid particulate material, the treatment liquor and solid waste fragments derived from said substrate; transferring the effluent to a first separator, wherein the first separator is configured to allow the treatment liquor and at least a portion of the solid waste fragments to pass through the first separator but to prevent the solid particulate material from passing through the first separator, and providing a first filtrate stream comprising treatment liquor and solid waste fragments and a first residue comprising solid particulate material and residual solid waste fragments; treating said first residue to remove said residual solid waste fragments to provide cleaned solid particulate material, wherein said treating of said first residue comprises retaining said solid particulate material on a surface of a treatment separator during said treating, wherein the treatment separator is configured to allow liquid and residual solid waste fragments to pass through but to prevent the solid particulate material from passing through; and (i) directing a cleaning formulation onto said solid particulate material to provide a second filtrate stream comprising said cleaning formulation and said residual solid waste fragments and a second residue comprising cleaned solid particulate material; and/or (ii) agitating the treatment separator. (i) directing a cleaning formulation onto said solid particulate material to provide a second filtrate stream comprising said cleaning formulation and said residual solid waste fragments and a second residue comprising cleaned solid particulate material; and/or (ii) agitating the treatment separator.