Abstract: A multistage membrane distillation device includes a plurality of membrane distillation cells each having at least one membrane. Each membrane defines a feed space at one surface thereof and a vapor space at an opposite surface thereof, and is configured to allow a part of a feed flowing in the feed space to evaporate and pass through the membrane as a vapor phase into the vapor space where the vapor phase is condensed to a distillate including a volatile and condensable substance, and the non-evaporated feed to exit the feed space as a concentrated fluid. The device further includes a fluid connection for allowing the distillate from an ith cell to flow as a feed into the feed space of an (i+1)th cell to produce a further distillate with a higher concentration of the volatile and condensable substance. The concentrated fluid from each cell is prevented from entering the feed space of other cells.