A multiple-effect evaporator is an apparatus for efficiently using heat from steam to evaporate water. In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last. Because the boiling temperature of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next, and only the first vessel (at the highest pressure) requires an external source of heat and thus saves energy and overall operational cost.
MEE is widely used for product concentration and has now been used for recovery of water from wastewater which can be reused for industrial operations thus reducing overall fresh water demand of the industry leading zero liquid discharge and sustainable development.