according to the present invention there is provided a novel type of membrane unit for use in electrodialysis comprising a heat - sealable frame framing and forming an integral entity with an electrodialysis membrane . the frame may be a regular frame , or a two - frame structure framing a single membrane . the invention further relates to electrodialysis devices such as stacks , bags , cells , spiral units and the like based on such membrane units , which are hermetically bonded by heat - sealing to similar units or to polymeric or other members .