Transport of skips between recycling centers and treatment facilities

We study two versions of a skip pickup and delivery problem. In the first version, skips are picked up from recycling centers and, depending on their content, they are transported to specified treatment facilities where they are emptied before being returned to their origin. In the second version, the point of return of empty skips is flexible and determined by the skip type. We provide mathematical models for both versions of the problem and present a variable neighborhood search-inspired heuristic for solving them to near-optimality. Computational results are provided for real-life instances from four areas in Denmark.