A mixing vessel includes a flexible container, elements for mixing its contents, aeration elements, a drain/lower bearing port having a rigid flange provided with a drain passage, attached in a tight manner to the lower portion of the container wall, supporting a lower bearing that is adjacent to the drain passage. The vessel includes an extended element for dispensing aeration gas and is spaced essentially radially from the drain/bearing port; at least one tubular element for dispensing aeration gas extends from the extended dispensing element, in the inside space, along the inside surface of the lower portion and the side portion of the wall of the container and extends on the outside of the container from the upper portion; at least one mixing element is spaced essentially from the lower portion of the container wall, from the lower bearing, and from at least one extended element for dispensing aeration gas.