Trommels are widely used in recycling operations for many purposes basically to separate materials by their size.. 

First is to get rid of small pieces out of PET and HDPE bottles after the bale breaker; in fact small pieces are lose caps, small stones, broken glass and, many times, small pieces of PVC.

This because PVC is more brittle than PET, specially if the bale of bottles is aged and it has been stored outside for a while.

Moreover it does a beautiful job dosing the amount of bottles going to the following sorting conveyor belt making life way easier to both manual or automatic sorting.

Some manufacturers use it also with the scope of pre-washing to remove paper labels and outside surface dirt by means of water spray or, sometimes, with hot water.

Going with water means , as a consequence, to ground bottles with a wet granulator and a dryer to separate dirty water from flakes before going to washing.

So, if it sounds grate from one point of view, it is a little less from both investment, capacity and operating costs.

Trommels are also used in MRF operations combined with manual sorting to separate different materials by size