1b of the circular sieve 1 both from the outside and from the inside as best seen by referring to fig3 the spray jets of the spray nozzles 11 39 are directed essentially perpendicular to the surface of the circular sieve or screen 1 and they are dispositioned at a location of the circumference of the circular sieve 1 which viewed in the direction of rotation of such circular sieve is disposed at an angle u03b1 fig3 of about 60 u00b0 to 80 u00b0 with respect to the topmost location or crown t of the circular sieve 1 as also will be recognized by reverting to fig1 to 3 the circular sieve or screen 1 is provided with impervious regions 13 which in each instance form troughs or vats 14 owing to the domed or curved configuration of such regions 13 the troughs or vats 14 are suitable for the formation of a liquid sump or pool s the related base or floor of the troughs or vats 14 are formed in each instance by the impervious regions 13 of the circular sieve or screen 1 viewed in the axial direction of the circular sieve 1 the troughs or vats 14 are.