Reverse osmosis (RO) (or ‘hyperfiltration’) and ultrafiltration (UF) are both unit operations in which water and some solutes in a solution are selectively removed through a semi-permeable membrane. They are similar in that the driving force for transport across the membrane is the pressure applied to the feed liquid. However, reverse osmosis is used to separate water from low-molecular-weight solutes (for example salts, mono-saccharides and aroma compounds), which have a high osmotic pressure. A high pressure, five to ten times that used in UF (4000–8000X 103 Pa), is therefore necessary to overcome this (hence the term reverse osmosis).