When the thin-film membranes are manufactured they are dry. These dry membranes have an indefinite shelf life, when stored properly. Membranes become wet when they are flushed or tested with water. Once wet, the membranes can not be dried. The wet membranes must be preserved to prevent the growth of micro-organisms on them. This is done usually by using a 1-2% solution of sodium metabisulfite. For more details look for information on storing membranes.