To help minimize this boundary layer buildup, membrane designers have used a method known as tangential-flow or cross-flow filtration that relies on high-velocity fluid flow pumped across the membrane’s surface as a means of reducing the boundary layer effect.
In this method, membrane elements are placed in a plate-and-frame, tubular, or spiral-wound cartridge assembly, through which the substance to be filtered (the feed stream), is pumped rapid-ly. In cross-flow designs, it is not economic to create shear forces measuring more than 10-15 thousand inverse seconds, thus limiting the use of cross-flow to low-viscosity (watery) fluids. In addition, increased cross-flow velocities result in a significant pressure drop from the inlet (high pressure) to the outlet (lower pressure) end of the device, which leads to premature fouling of the membrane that creeps up the device until permeate rates drop to unacceptably low levels. These typical Industry systems use carbon media filters that are frequently disposed of in landfills creating more solid waste.