Sap may be concentrated via reverse osmosis. Only reverse osmosis and nano-filtration (ultra-osmosis) membranes are allowed. In the off-season, osmosis membranes shall be stored, in filtrate, in a hermetically sealed container and kept in a frost-free location. Sodium metabisulfite (SMBS) may be added to the filtrate to prevent mould growth. If SMBS is used, the membrane shall be rinsed before next use with a volume of water equal to the hourly capacity of the membrane [for example, 2728 L (600 gal. ) of water for a 2728 L/h (600 gal. /h) membrane]. Off-site storage of the membrane (for example, by the membrane supplier) shall be documented.