The membrane consists of several thin layers or sheets of film that are bonded together and rolled in a spiral configuration around a plastic tub (This is also known as a thin film composite or TFC membrane). The material of the membrane is semipermeble, it allows water molecules pass through while acting as a barrier to dissolved solids (i.e: mineral chemical contaminants). When the feed water stream passes across the surface of the membrane, the molecules penetrate the membrane surface, working their way around the sprial and collecting in the center tube. The remaining contaminants are concentrated and washed from the surace of the membrane down the drain.