Some embodiments of the present invention provide a modular fluid treatment assembly and method in which modules of the system each have a head that can be connected to one or more heads (of one or more other modules) in different configurations. In some embodiments, the relationship between a cartridge of the module and its corresponding head prevents fluid from entering between the cartridge and an external shell of the module. Also, some embodiments provide a module having a head with substantially concentric inlet and outlet ports in fluid communication with a cartridge coupled to the head.