The present disclosure is directed at a modular apparatus for a saltwater desalinating system, and a method for using same. The apparatus includes multiple internal modules that are compressively coupled to each other. Each of the internal modules includes a pair of rigid end plates located at opposing ends of the internal module, and a stack of membrane bounded compartments that are layered from one of the end plates to the other. The modular apparatus can be used in a membrane based desalination system, which includes concentration difference energy systems, electrodialysis reversal systems, and membrane distillation systems. The modular apparatus helps to mitigate problems such as leakage and buckling in such systems, and can be used to increase membrane packing density and, accordingly, desalination efficiency.