Size reduction usually is an essential step in mechanical processing of mixed wastes. It reduces bulky items to particles and brings uniformity which is required in mechanical sorting systems. Therefore, source-separated materials, which generally have a relatively small maximum particle size, narrow particle size distribution, or both, do not need size reduction before other sorting systems.
Coarse shredding, usually to a maximum particle size of about 10cm, is typical of many mixed waste processing facilities, and secondary or even tertiary shredding are introduced whenever particle size is smaller than 10cm. Other circumstances that may need a small particle size are recovery and processing of some materials such as ferrous metals and glass.