a device for biological digestion of sewage utilizes rotationally symmetric rotatable growth surfaces , preferably plastic immersion percolating filters , which filters are immersed in the sewage as they rotate . the growth surfaces are mounted upon an immersion cylinder that is rotatably driven and mounted within a gas - tight container according to a preferred embodiment . the drive for the immersion cylinder includes a drive pinion having teeth which engage stud bolts connecting rings forming part of the immersion cylinder .