A membrane is a physical barrier between two fluids (feed side and product side) that selectively allows certain components of the feed fluid to pass. The fluid that passes through the membrane is called permeate and the fluid retained on the feed side is called retentate. The equipment needed for the separation is deceptively simple. It consists of the membrane plus the container to hold the two fluids. The simplest arrangement is to use a stirred tank that is separated into two volumes via a membrane. Stirred-tank systems are used in laboratories but not commonly in large-scale separations.