An electrodialysis unit comprising at least two electrodialysis stacks (ED1 and ED2) connected in series, wherein: (a) stacks ED1 and ED2 comprise anion exchange membranes and cation exchange membranes; and (b) the anion exchange membranes in stack ED1 have a lower electrical resistance than the anion exchange membranes in stack ED2 and the cation exchange membranes in stack ED1 have a lower electrical resistance than the cation exchange membranes in stack ED2. Also claimed is a process for purifying liquids, e.g. for desalinating sea water or brackish water.