24 Hours Operations Research

Consider a fleet of homogeneous vehicles hosted at a single depot, where spare empty containers are located and are available in limited quantity for each type. A set of service requests, one for each full container located at a recycling and disposal station, must be fulfilled. Optimization of the collection and disposal of recyclable waste is contributed by Bruglieri M., Malucelli F. , POLITECNICO DI MILANO ; Aringhieri R. ,Università di Milano; Nonato M. ,Università di Ferrara.

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The increasing of environmental concerns has affected several habits in the daily life of common people in Italy, including the attitude toward recycling, making people more receptive towards cooperative behaviours. Since many years, several municipal administrations provide regular services for the collection of restricted categories of recyclable household waste, such as small objects made of paper, plastic, metal, and glass. The service is implemented either according to a door-to-door collection pattern or by way of dedicated bins, usually located beside traditional garbage skips, which get periodically emptied on site by dedicated vehicles. While this kind of service is well established, bulky items are not contemplated, and their collection used to be operated on demand, subject to items restrictions. Only very recently in Italy, bulky waste recycling started to be extensively promoted on a large scale by local authorities, although the service is deployed according to a different logistic: items have to be conveyed by the owners to dedicated conveyance centres, rather than being periodically collected on site by company vehicles. Therefore, the success of this service heavily relies upon people’s individual behaviours.

Several conveyance centres have been established, so called Recycling and Disposal Stations (RDSs), usually located in the proximity of industrial settings, in the outskirts of large urban areas or incorporated within existing solid waste transfer stations. Residents bring their bulky waste material to the closest RDS and dispose it into dedicated containers. Several types of materials can be disposed at a RDS. The list includes batteries, garden trimyard, cardboard, furniture, electric appliances, metals, wood, rubble and many other materials except for industrial and construction debris. Each RDS hosts several containers, each one devoted to a single type of waste material.

Containers are classified depending on access side (left, right or rear access skips) and compacting equipment; however, the container type dedicated to a given material may vary from RDS to RDS, as it may depend on the site’s layout.

A container, once full, is first loaded as a single load on a vehicle, then carried to a Material Reclaimer Facility (MRF) for that kind of material and emptied there. MRFs are not necessarily located close to each other or to a RDS. There is a choice in the selection of the MRF when multiple MRFs for the same material are present in the same region. Once emptied, the container is repositioned by the same vehicle either to the original RDS or to any other RDS where an empty container of the same kind is required. In case that additional spare containers are available, stocked at some repository, empty containers coming from a MRF may be also delivered there; in the same way, empty containers can be picked up from the repository and brought wherever required.

The containers disposal service for an entire region is usually handled by the local municipal agency for waste collection and management. It is scheduled on demand on a daily base, and the vehicles routes are assigned to the agency drivers shifts. Since the quantity of collected materials constantly increases (almost doubling every two years in Italy), and more and more RDSs are being set up, emptying requests tend to become more frequent and to involve additional locations. Such trend calls for efficient computerized solution methods based on optimization algorithms, able to exploit all the advantages given by the integrated management of all the RDSs located in the same region.

A higher degree of freedom in the solution comes from splitting each request in two independent actions:

• bring the full container from its RDS to a MRF and empty it;
• deliver an empty container of the same type to the RDS.



The problem addressed here originates from an Italian waste management company (GESENU) and can be formalized as follows. Consider a fleet of homogeneous vehicles hosted at a single depot, where spare empty containers are located and are available in limited quantity for each type. MRFs for each material are given, and travel times are known along the road network connecting RDSs, MRFs and the depot. A set of service requests, one for each full container located at a RDS, must be fulfilled. Various requests can be present at the same RDS at the same time. Vehicles carry each container as a single load, one container at a time, either empty or full. The driver shift can last at most 360 minutes typically, starting at 8:00 a.m. We want to route the vehicles subject to a maximum route duration constraint (the driver shift), in order to serve all requests with the objective of minimizing the number of vehicles employed and, secondarily, the total travel duration. We call this problem the Ecological 1-Load Container Routing Problem (E1CRP).