disclosed herein is a method and system for sorting recycled solid waste materials , such as plastics , by a liquid - fluidized bed classifier technique . the application of a lfbc to the separation of plastics , and in particular plastic particles , is shown to be advantageous for a number of reasons . firstly , the primary fluidization medium , i . e ., water , is present in large quantities during conventional chopping , washing , and flotation operations that are performed during plastics recycling operations . secondly , the natural density distribution of the major types of recycled plastics favors segregation by density in water , i . e ., the &# 34 ; lighter - than - water &# 34 ; polyolefins , polypropylene and polyethylene from the &# 34 ; heavier - than - water &# 34 ; polystyrene pvc and pet plastics . the latter types of plastics have been found to separate spatially in an upflow lfbc , while the &# 34 ; lighter - than - water &# 34 ; plastics can be collected at the top of the column , thereby combining flotation and classification in a single step . further separation of the polyolefins can be accomplished in a second lfbc operating in a downflow mode . thirdly , any overlap of density distributions between types of plastics is overcome via selective modification of plastic particle size / density . a fourth advantage of the lfbc spatial separation is that the separated plastic particles can be collected by hydraulic flow from the appropriate section of the column and supplied to simple gravity separators for particle recovery , and also for disengaging the fluidizing water for recycling , thereby enabling the continuous classification of plastic particles with both continuous feed and product removal .