Storage tank assemblies and methods for water on water reverse osmosis systems

A method of fabricating a bladder type storage tank comprising:

• forming first and second injection molded plastic shells, which when joined together at a center region of each shell, form a tank enclosure, the first shell having a first opening therein away from the center region of the respective shell;
• forming a bladder;
• fabricating a plastic member and fastening the plastic member in the first opening in the first shell, the plastic member defining second and third openings in the first shell, and wherein;
• coupling the bladder to the second opening in the plastic member so that the bladder will be within the tank enclosure when the first and second shells are joined together at a center region of each shell, and so that the second opening is in fluid communication with an interior of the bladder;
• temporarily holding the bladder in a collapsed state away from the center region of the first shell with one or more strings and passing the strings through the third opening;
• welding the first and second shells together at the center region of each shell to form the tank enclosure;
• and
• withdrawing the strings to release the bladder.