Abstract: The present invention relates to a filtering apparatus and the method of washing such an apparatus. The filtering apparatus with an upward flow of a fluid to be treated in an enclosure (7) closed at least at one end (15) and containing a fixed floating bed of filtering particles (16) of a density less than that of the fluid to be treated, is characterized in that the pipe (3) for discharging the filtered materials in suspension has the form of a lyre with vent (5) and dimensions allowing the column of fluid contained in the enclosure to be emptied at a speed higher than the minimum fluidization speed, the filtering particles having a characteristic mass per unit volume between 5 kg/m.sup.3 and 500 kg/m.sup.3 and a pipe (1) for intake of the fluid to be treated is disposed in a zone at the lower end of the enclosure situated below the lower level of the fixed floating bed and above the upper level (30) of the waste discharge pipe (3).