A process and apparatus serves to remove solid matter from an aqueous material suspension produced from printed used paper. As many interfering material particles are collected by floatation as possible in a floatation foam and diverted as a reject. A perforated separating element is located above the fibrous material suspension. Between the separating element and the floatation foam, an intermediate layer is formed. In the intermediate layer the fibers, which have been entrained by gas bubbles are again rinsed and retrieved.