A supercritical water oxidation reactor adapted to contain inside the reactor an aqueous fluid below and above its supercritical state, said fluid comprising organic and/or inorganic material, wherein the reactor comprising - a reactor body in the form of an elongate tubular element arranged, during use, with its longitudinal extension parallel to or substantially parallel to gravity, the reactor body being closed at its upper and lower ends thereby defining a cavity inside the reactor body; - a residue output connection having an inlet arranged at a first vertical height (h 1) inside the cavity and having an outlet arranged outside the reactor body, said residue output connection extends from its inlet downwardly towards the lower end of the reactor; - a distillate output connection having an inlet arranged at a second vertical height (h 2) inside cavity being higher than the first vertical height (h 1) and having an outlet arranged outside the reactor body, said distillate output connection extends from its inlet downwardly towards the lower end of the reactor; - an aqueous fluid inlet connection arranged at the lower end of the reactor body for inletting into the cavity the aqueous fluid to be brought into and above its supercritical state, - a plurality of individually controllable thermal elements arranged below the first vertical height (h 1) at, in the reactor body and/or inside the cavity and being adapted to provide an individually controllable heat flux into/out from the cavity so as to impose a vertical temperature profile on the fluid inside the cavity, - a plurality of temperature sensors arranged to measure a fluid temperature, or a temperature of the reactor body, at a plurality of different vertical positions, including a position above and a position below the first vertical height (h 1) and/or a salt concentration sensor arranged to measure the salt concentration in the fluid flowing through the residue output connection. - a reactor body in the form of an elongate tubular element arranged, during use, with its longitudinal extension parallel to or substantially parallel to gravity, the reactor body being closed at its upper and lower ends thereby defining a cavity inside the reactor body; - a residue output connection having an inlet arranged at a first vertical height (h 1) inside the cavity and having an outlet arranged outside the reactor body, said residue output connection extends from its inlet downwardly towards the lower end of the reactor; - a distillate output connection having an inlet arranged at a second vertical height (h 2) inside cavity being higher than the first vertical height (h 1) and having an outlet arranged outside the reactor body, said distillate output connection extends from its inlet downwardly towards the lower end of the reactor; - an aqueous fluid inlet connection arranged at the lower end of the reactor body for inletting into the cavity the aqueous fluid to be brought into and above its supercritical state, - a plurality of individually controllable thermal elements arranged below the first vertical height (h 1) at, in the reactor body and/or inside the cavity and being adapted to provide an individually controllable heat flux into/out from the cavity so as to impose a vertical temperature profile on the fluid inside the cavity, - a plurality of temperature sensors arranged to measure a fluid temperature, or a temperature of the reactor body, at a plurality of different vertical positions, including a position above and a position below the first vertical height (h 1) and/or a salt concentration sensor arranged to measure the salt concentration in the fluid flowing through the residue output connection.