The present invention provides an electrochemical apparatus for processing a fluid comprising: an electrode; a solid electrolyte; and means for providing forced flow through the apparatus of a fluid to be processed, wherein the apparatus is arranged such that when the fluid to be processed is introduced into the apparatus there exists at least one three phase interface between the electrode, the solid electrolyte and the fluid to be processed and such that the fluid forming part of the three phase interface undergoes forced flow through the apparatus, and methods in which the electrochemical apparatus is used.