These are commonly called an RO storage tank. It is a bladder tank. Hydroneumatic, bladder, captive air, reverse osmosis storage & pressure tank are all common names for these tanks. All of these tanks fill a food safe bladder with water and that bladder is surrounded by a coushion of air .The air bladder located on the inside of this tank allows the tank to dispense water. Not all bladder tanks are created equal. There is a difference in tanks for regular water storage and tanks designed for pure water storage. Tanks rated for reverse osmosis water storage have stainless steel connection points vs. common steel which will corrode when exposed to pure water. These tanks can be used horizontally or vertically. The position makes no difference in it's ability to do its job.
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