Plastics from the municipal waste must be recycled on a large scale. This contribution describes the initial work to determine the surface energy of plastic waste. Results of such investigations were correlated with both sedimentation and flotation data. The penetration rate of a liquid into the horizontal powder layer was measured; basing on the rate of liquid penetration, the effective radius of the porous bed was calculated. For modification of the surface of plastics, tannic acid, sodium lignin sulfonate and microbial broth from Pseudomonas aeruginosa growth were used. In order to determine the effect of these reagents on the surface energy of plastic powders, the penetration and flotation experiments were carried out. For polystyrene and polyethylene the estimated surface energy was 28.6 [mJ/m2] and 24.5 [mJ/m2], respectively. These results were correlated with the sediment height extreme of plastic powders in solvent pairs. The adsorption of tannic acid, sodium lignin sulfonate and biosurfactants from microbial filtrate caused an increase of the surface energy of plastic particles. For instance, the free energy of polyethylene particles was 34.9 [mJ/m2] after the sodium lignin sulfonate treatment, and 35.57 [mJ/m2] for the tannic acid adsorption. The natural flotation of plastic particles in the presence of modifying reagents was shown to be dependent on the concentration of these reagents.