Screening is employed to separate materials of different sizes into two or more size distributions in waste sorting system. Screens will function to separate oversized and undersized materials as a preprocessing technique for other unit operations within the facility. The types of screens used in the MRF will be disc screens and trommels.
Trommels are rotating cylindrical screens that are inclined at a downward angle with the horizontal. Material is fed into the trommel at the elevated end and the waste separation occurs while the material moves down the drum. The tumbling action of the trommel effectively separates materials that may be attached to each other. Length, angle and diameter of the drum, depth of the material and the speed of rotation are important specifications in configuring the trommel to accomplish the desired goals. If necessary, the trommel can have flights that function to carry the materials to a higher location within the drum at lower rotational velocity.