in a method and an arrangement for treating a light fraction that is produced during the treatment of plastic - rich waste that is low in metal , at least the following steps are carried out consecutively : the light fraction is stressed by percussion and / or bashing , the light fraction is classified into at least two light fraction classes , at least one light fraction class is separated into at least one light material fraction and a heavy material fraction , at least one light material fraction is cleaned . the cleaning of the light material fraction , obtained after the separation , provides a very clean initial substance to be obtained , resulting in clearly improved material recycling and energy recovery .