NOVELTY - A water separating method involves providing fat, oil and/or grease (FOG) containing liquid-type medium, cross-flow filtration cassette comprising an array of sheet members of rectangular and planar shape with main top and bottom surfaces, where the liquid-type medium is a viscous liquid, non- viscous liquid or an emulsion, the sheet members comprise a first retentate sheet, a first filter sheet, a permeate sheet, a second filter sheet and a second retentate sheet followed by retaining fatty acids and triglycerides in a retentate flow.    USE - Method for separating water from a fat, oil and/or grease-containing liquid-type medium for producing biodiesel (claimed).    ADVANTAGE - The method enables separating water with better safety in an effective manner.    DETAILED DESCRIPTION - A water separating method involves providing FOG containing liquid-type medium, cross-flow filtration cassette comprising an array of sheet members of rectangular and planar shape with main top and bottom surfaces, where the liquid-type medium is a viscous liquid, non- viscous liquid or an emulsion, the sheet members comprise a first retentate sheet, a first filter sheet, a permeate sheet, a second filter sheet, and a second retentate sheet, followed by retaining fatty acids and triglycerides in a retentate flow, having a diameter larger than pore size of the filter sheet, diffusing a portion of the liquid medium with permeate species through the filter sheets to enter the permeate sheet in a permeate flow. The FOG containing liquid-type medium is passed through the cross-flow cassette to separate water from the FOG. INDEPENDENT CLAIMS are also included for the following:    (1) a method for producing biodiesel fuel from trap grease    (2) a method for dewatering a liquid medium containing trap grease.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of components of a cross-flow filtration cassette.    Partition element (6)    Channel opening (8)    Inlet basin opening (10)    Outlet basin opening (12)    Permeate passage openings (13)