D-D Replacement MembraneWhat is a membrane and how does it work? The membrane consists of several thin layers or sheets of film that are bonded together and rolled in a spiral configuration around a plastic tube, also known as a thin film composite or TFC membrane. The material of the membrane is semipermeable: 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 spiral and collecting in the center tube. The remaining contaminants are concentrated and washed from the surface of the membrane down the drain.How long will a membrane last? This is a somewhat difficult question to answer as it depends on how much the membrane is used and how well it has been looked after. Some areas in the country have much poorer tap water than others and in these areas the membrane will have to work harder to remove the impurities.In general a membrane will last about 2 years, especially if regularly flushed. The best way to test the membrane is using a TDS meter to determine if the quality of the water has deteriorated.