Abstract: An installation for the treatment of metals pickling solutions is disclosed. The invention provides for solutions to be treated by electrodialysis in a tank which is divided up to form sets of three compartments. Anode and cathode compartments in the form of removable cases are arranged alternately with spaces therebetween defining central compartments. In use, according to a preferred method of the invention, anolyte flows through the anode compartments while the picklng solution flows through the cathode and central compartments. Flow through the compartments may be either in series or in parallel.