a paper fiber treatment system utilizes a screw revolving in a perforated cylinder as a tailings screen in place of conventional vibratory or other screens to separate undefibered fibrous materials or other undesirable solids from the free fibers . the tailings screen may be used , for example , in an otherwise conventional fiber treatment system to screen paper stock , downstream from a pulper for pulping waste paper or refuse containing a high percentage of paper fibers , or downstream of a digester for separating knots or other undefibered wood particles from the cooked fibers ejected from the digester .