A reverse osmosis membrane is a very tightly wrapped microfiber that has a .001 micron rating. Water is forced through the membrane by water pressure. Only the purest water molecule can penetrate the membrane. There are two kinds of water coming out of the membranes: the pure and the waste water. The pure water is also called the permeate, product or dilute solution. The waste water is also know as the brine or concentrate. The pure water has low TDS. The waste water, however, is normally higher in TDS than your tap water. This is because of the pre-treatment of water before the RO membrane.