A treatment column includes a treatment zone which is completely filled with a bed of granular treatment material. A normal treatment operation includes passing a fluid to be treated in a downward direction through the bed of treatment material, thereby treating the fluid. Solid particles suspended in the fluid are removed by the upper portion of the bed of treatment material and are retained therein. Regeneration of the bed is achieved in a countercurrent direction within the treatment column. Retained solid particles are removed from the upper portion of the bed by periodically removing the upper portion only of the bed from the treatment column and transferring such upper portion to a separate washing column whereat the portion of the bed is cleaned and washed. The thus cleaned and washed treatment material is then returned to the upper portion of the treatment zone within the treatment column to replenish the bed.