A membrane stack may be used, for example, in an electrodialysis or other electrically driven membrane separation device. The stack has a plurality of modules, each containing a number of membranes and spacers bundled together. A module can be removed from the stack, for example for diagnosis or repair, preferably by sliding the module out of the stack in a direction parallel to the plane of a membrane or spacer in the stack. A banding mechanism is described for compressing a stack but can be released to allow the stack to be dis-assembled. Preferably, the banding mechanism is also capable of lifting at least an upper end plate or electrode from the stack. Ports communicate with parts of a stack and may be used to perform diagnostic tests. Preferably there stack has at least two modules as described above and each of the two modules has at least one port.