Contaminated material is unwanted content in the feedstock material. Using feedstock with contaminates means a lower quality finished product containing contaminated materials. This unwanted content may including rocks, dirt, tramp metal, and plastic. All of these materials can cause problems with the finished material, such as plastic in compost.
Waste wood such as railroad ties and pallets often have unwanted metal and nails.
Contaminates may also cause additional wear to grinder machine parts.