A modular fluid processing system with at least one module comprising a frame and at least two housings, each housing having an interior configured to contain at least one fluid separation element; and at least one plumbing assembly with connection ports configured to connect in fluid communication with the interiors of the at least two housings in a first orientation and a second orientation. The at least one plumbing assembly is reversible in that the second orientation is substantially a 180.degree. rotation of the first orientation. An embodiment of the fluid processing system can have a first module including a frame and at least two housings with interiors configured to contain at least one fluid separation element; at least another module including a frame connectable to the frame of the first module, at least one housing, the at least one housing having an interior configured to contain at least one fluid separation element; and at least one plumbing assembly configured to connect in fluid communication with the interiors of the at least two housings of the first module and the at least one housing of the second module.