Vehicle Routing for a Complex Waste Collection Problem

We consider a complex waste collection problem, where the residents of a certain region dispose of recyclable waste, which is collected using a fixed heterogeneous fleet of vehicles with different volume and weight capacities, fixed costs, unit distance running costs and hourly driver wage rates. Each tour starts and ends at one of several depots, not necessarily the same, and is a sequence of collections followed by disposals at the available recycling plants, with a mandatory disposal before the end of the tour. There are time windows and a maximum tour duration, which is interrupted by a break after a certain interval of continuous work. Moreover, due to the specificities of different collection regions, there are occasional site dependencies. The problem is modeled as a mixed binary linear program and the formulation is enhanced with several valid inequalities and elimination rules. To solve realistic instances, we develop a local search heuristic, which currently embeds much of the functionality of the mathematical model. The heuristic performs well, as indicated by an optimality gap of 2% compared to the exact solution on small instances. Future work will see improving the model formulation to solve larger instances to optimality and expanding the heuristic to include all of the features of the model.