Even where fine screens are installed in the headworks of wastewater treatment plants, some debris will always end up in the sludge. Plastic parts and fibrous materials can severely disturb the operation of thickeners, digesters and dryers. Such solids impair the operation of pumps, heat exchangers, mixers, dewatering systems, pelletizers, etc. Hair and fibers often lead to the formation of tresses and grease balls. Where biosolids are land applied, plastic parts should be removed.