Installation for treating a fluid comprising, in a section of a horizontal shell ring (1) a first particulate material filling a volume (V2), a deflector comprising 2 volumes V3 delimited by the internal shell of the shell ring and on the other hand by lower surfaces S3a and S3b making with a horizontal plane an angle (α) greater than the angle of repose of the first particulate material, a volume (V4) formed of a membrane backing up against the two volumes (V3) of the deflector over the entire length of the section of the shell ring, and of a second particulate material with which the volume (V2) is filled; the second particulate material being contained in the space formed by the membrane and the internal shell of the shell ring, the volumes V3 and V4 being situated in the upper half of the cross section of the horizontal shell ring.