A containment vessel bladder for retaining material to be stored within a containment vessel. The bladder comprises a flexible enclosure and a drain facilitating element. The enclosure has a sealed interior, a floor portion connected to an enclosed top by one or more wall portions, and an opening for the delivery of material to the sealed interior. The drain facilitating element is positioned on the floor portion the enclosure and is expandable from a deflated position to an expanded position. When deflated the drain facilitating element is foldable with the enclosure to facilitate storage and transport of the containment vessel bladder. When expanded the drain facilitating element has one or more sloped surfaces aiding in the direction of material stored within the sealed interior toward a pre-determined location within the sealed interior to help facilitate the removal of material from the containment vessel bladder.