Standard Size Reverse Osmosis Membrane Housing with built-in 1/4" quick-connect fittings. Standard Residential Membranes made By Filmtec, GE and many others, ranging from 18 to 100 Gallons Per Day, will fit in this housing. If you are replacing a membrane housing that has threaded fittings, you may need to replace your check valve. Many housings with threaded fittings have the check valve built-in to the threaded fitting in the center port on the end with two ports. A check valve is required for proper functioning of an under sink reverse osmosis system that uses a bladder tank.