The water shaft consists of rings (2) between which spacers are provided. The spacers can be shaped so as to fit over the rings, and can themselves be rings with radial grooves in their top and bottom surfaces, or alternatively of porous material. Water-distribution rings can also be provided, mounted on the outside of the shaft rings. One or more spacers can be combined with water-distribution equipment, grooves and/or bosses being provided on the side of the latter towards the shaft rings. The gap between the shaft rings can be between 2 mm and 30 mm wide.
