Membrane washing units are designed to wash the membranes at specified intervals, against any kind of membrane blockage. The membrane surface is covered with a plate formed by the settling of sulphate, carbonate, silica and similar chemical salts by crystallisation. These salts reduce the efficiency of the membrane by blocking its pores and channels. Blockage can be eliminated by the washing process. During washing TO units have to be turned off. The washing process may be applied to all the membranes as well as to covered single membranes depending on the amount of pollutants. Washing procedures specified according to the type of pollution are applied. The most important issue regarding the washing process is depicting the type of blockage on the membranes and using the chemical solution that will solve the blockage.