the screen belt means 21 as well as any cleaning liquid added by spray tubes 12 and 13 to a suitable collection area not shown in the embodiment of fig1 the direction of travel of the returning section of the lower screen belt means 21 is depicted by the arrow generally clockwise about roller means 30 26 33 33 31 and 32 and the direction of rotation of rotary brush means both 10 and 11 is also clockwise as viewed in fig1 referring to both fig1 and 2 a similar cleaning apparatus is arranged and situated generally as at the upper portion of the top or upper screen belt means 22 as possibly best seen in fig2 a first drum type or generally cylindrical rotary brush means 10 39 is situated above the returning section of the upper screen belt means 22 and positioned as to have the generally radiating bristles thereof applied as from above against the returning section of the upper screen belt means 22 further rotary brush means 10 39 is situated as to have its axis of rotation transverse to the direction of travel of the returning section of the upper screen belt means 22 a second drum.