NOVELTY - Forming alkyl ester involves: (A) mixing in vessel oil source containing triglycerides and/or free fatty acids with alcohol in presence of catalyst to form alkyl ester; and (B) separating alkyl ester by cross-flow filtration cassette comprising array of sheets including first retentate, first filter, permeate, second filter, and second retentate sheets, where at least alkyl esters of diameter larger than filter sheet pore size, are retained in retentate flow, and portion of medium with permeate diffuses through filter sheets and enters permeate sheet and permeate flow.    USE - For producing alkyl ester for use as fuel (claimed).    ADVANTAGE - The method for forming alkyl ester provides treatment of fats, oils and grease (FOG) such as trap grease, brown grease, and/or yellow grease, for separation of target products and efficient production of biodiesel while reducing disposal of solid or liquid waste matter into landfills or water treatment facilities, by using cross-flow filtration cassette. Thus overcomes major costs/drawbacks of current fatty acid methyl esters (FAME) production process; where current systems used for separation of desired products and contaminants in biodiesel production involved exorbitant volumes of water to wash soaps and contaminant out of produced FAME. Such shortcomings relating to use of excessive water also involve high energy costs to provide FAME product that meets ASTM specs. The system of cross-flow filtration cassette used in method has ability to enable waterless production of FAME by preferentially passing FAME and restraining soaps and particulates, thus avoids use of water altogether. Ester production occurs directly in fuel, eliminating isolation and purification prior to blending. The end by-products (glycerol, water and enzyme) may be separated from biofuel by using cross-flow filtration cassettes. The process includes separation of remaining alcohol, produced glycerol and water in single step using cross-flow filtration cassette, leaving behind purified biodiesel product. The enzyme and reaction products are retained in retentate stream and glycerol and water in permeate stream is removed thus maintain effective level of water and/or glycerol in reaction vessel. Alternatively, if production process does use water, water can be easily cleaned to remove contaminants, and as such, can be then recycled to be used again rather than discarded to drain system. The cross-flow filtration cassettes can be used in combination with conventional spiral technology or preferably perform separation and permeate clean up, thus rendering multiple reuses of cleaned permeate. The method thus involves efficient production and purification of biodiesel and other valuable components while separating important additional components and reducing disposal of solid or liquid waste matter into landfills or water treatment facilities, and produces minimal amount of negative effects on sewer systems, wastewater treatment facilities and/or landfill sites. By cross-flow filtration cassette method further provides advantage of deodorizing system to deal with obnoxious smell of using brown grease. The constituents that cause obnoxious smell, are passed through membrane into permeate, and can enable retained concentrated grease/fat/food particulate to be used as animal feed.    DETAILED DESCRIPTION - Method for producing an alkyl ester involves:    (A) mixing in a reaction vessel an oil source containing triglycerides and/or free fatty acids with an alcohol in the presence of a biocatalyst or chemical catalyst to produce an alkyl ester; and    (B) separating the alkyl ester from at least the alcohol and catalyst, by using a cross-flow filtration cassette, where the cross-flow filtration cassette comprises: an array of sheet members of rectangular and planar shape with main top and bottom surfaces, where the sheet members include in sequence in the array a first retentate sheet, a first filter sheet, a permeate sheet, a second filter sheet, and a second retentate sheet, in which at least the alkyl esters, having a diameter larger than the filter sheet's pore size, are retained in the retentate flow, and at least a portion of the liquid medium with any permeate species diffuses through the filter sheets and enters the permeate sheet and permeate flow.    DESCRIPTION OF DRAWING(S) - The figure shows system for isolating the desired biodiesel product and purification of reaction products.