They are washed down the drain. Unlike filters, the R.O. membrane is self-cleaning. As the source water flows through the module, it is divided into two streams. One stream is forced through the membrane by osmotic pressure created due to the water pressure on each side of the semi-permeable membrane. The second stream carries the rejected salts, dissolved pollutants and contaminates to the drain. Therefore, there is a minimum accumulation of debris on the pressure side of the membrane. Charcoal or carbon filters, on the other hand, become less efficient with each glass of water drawn through them. Carbon filters only remove a limited number of contaminants, some odors, and some tastes.