A medium that allows water to pass through, but rejects dissolved solids, so that it can be used to separate solids from water. Certain molecules are capable of passing semipermeable membranes. The purpose of semipermeable membranes is for filtration, thereby aiding the process of advanced purification systems such as reverse osmosis, which specifically allows only water to pass. Semipermeable membranes are effective in not allowing solutes from going through it along with only permitting one solvent to pass through.