The separation of different kind of plastic particles is required in the process of waste recycling. For the separation, drum processes with liquid can be used. The separation is based on the principle that particles either sink or float in a liquid depending on their densities. In this study, this process is numerically analysed for the separation of polyethylene terephthalate (PET) from polypropylene (PP) particles. The discrete element method coupled with the smoothed particle hydrodynamics method (DEM-SPH) is used for modelling purposes. The employment of the SPH for the modelling of the liquid exploits the strong side of this meshless method, namely, the relative ease in modelling large movements of the fluid with free surfaces and moving boundaries. This theoretical model is presented, and verification tests are performed, where a dam-break problem is considered as an example. Simulations of the plastic particle separation in the rotating drum are performed thereafter. The influences of the different operational and design parameters, such as the rotational velocity, feed rate, and number of lifters on the resultant purity of the plastic are estimated. It is expected that, in the future, the performed analysis will allow the numerical optimisation of drum separation processes. (C) 2017 Elsevier B.V. All rights reserved.