Storage tanks are used to contain high purity water or act as capacity buffers, eliminating the need for oversized reverse osmosis systems that feed directly into processes. Storage tanks must preserve the integrity of the water. Materials of construction include stainless steel, fiberglass reinforced plastic (FRP) or premium grade medium density plastics. The tank manufacturing process must adhere to quality standards for use with high purity water. Stainless steel tanks are available as atmospheric or pressure rated vessels with varied interior and exterior finishes. Stainless steel is also the material of choice for hot water applications such as water for injection (WFI) production in pharmaceutical processes. Tanks constructed of other materials are atmospheric and usually vented with sub-micron filters. Storage tanks are sized according to the demand of the water system or application.