there the free end of the hollow shaft carrying the filter plates 2 and which lies within the reactor is led into a gear 30 whose driven shaft 31 reaches up to the disk stacks 24 25 and drives these since the disk stack 24 25 are driven at a rotational speed which is significantly smaller in comparison to the filter plate stack the gear 30 is designed as a stepping down gear with this arrangement the cover 20 on the right side remains free of drive means for the biofilm reactor if a separate ventilation is necessary for the biofilm reactor then the cover 20 as is represented by way of fig1 to 15 may be provided on its inner side with an ejector pump 32 whose suction port runs out centrally within the reactor and which sucks air through a ventilation opening which is secured by a return valve the air being entrained by the delivered fluid flow in the manner of an ejector and flowing into the reactor in the region of the outer periphery the construction of this ejector pump 32 is visible in detail in fig1 to 15 the pump impeller 33 immerses into the suction opening 34 on the pump housing side the pump worm which connects radially to the pump impeller 33 is constructed in an ejector like manner and draws air in the ejector region 35 which is carried along in a finely distributed manner with the fluid and then exits close to the outer periphery within the reactor in the region of the opening 36 the pump impeller is driven by a drive motor 28 arranged outside of the cover 20 as may be seen in fig1 a fluid sensor.