Nanofiltration membrane rejects divalent ions in water such as the hardness components of water, calcium and magnesium ions, and, therefore, produces soft water. A nanofilter membrane allows most monovalent ions such as sodium and chloride to pass through, while it rejects divalent ions. monovalent ions create osmotic pressure and requires high pressure to pump water through a reverse osmosis membrane. Therefore, nanofilter membranes require much less pressure to pump water through the membrane because the hydraulic driving force does not have to overcome the effect of osmotic pressure from monovalent ions.