(204k) Optimal Planning for the Sustainable Management Municipal Solid Waste
The high production of municipal solid wastes currently represents a problem of major importance; this is because the existing landfills are not sufficient for the secure disposal of the wastes generated in most of the developing countries. As a result, this situation causes pollution to water, land and air. Although the significant development presented for the adequate treatment and management of municipal solid wastes, the major problem is that the garbage is mixed (i.e., different types of materials are mixed and they require to be separated for the adequate treatment), which increases the separation costs and make more difficult and costly the treatment process, and also this yields insanity conditions. Most of the actions that have been implemented to solve this problem are focused only on one type of waste separately, without taking into account the interaction between the garbage composition and distribution, and without considering the entire supply chain optimization as well as the economies of scale; this way, the distribution of wastes, products and the design and selection for the processing facilities are crucial to yield a proper solution for the entire problem.
An alternative to help solving the inadequate waste disposal problem integrating sustainability considerations, in this paper is proposed a multi-criteria optimization model for the adequate municipal solid waste management; the optimization model accounts for the optimal planning of the reuse of municipal solid wastes to maximize the economic benefits considering sustainability criteria. The optimization model simultaneously selects the processing technologies, markets, cities producing wastes, amount and type of wastes reused, amount and types of products yielded, and size and location of processing facilities.
Figure 1 shows a schematic representation of the addressed problem, which considers a set of cities , these cities have a specific production rate of wastes, the wastes can be plastics, metals, papers, waste of food, etc. The wastes from cities can be separated in different subtypes of waste, for example polyethylene and chloride of polyvinyl for plastics as well as green and amber for glasses. After separating the wastes, these can be transported to processing plants located in the cities ( ) to treat the wastes and produce several products. The produced product depends on the used technology and raw material. A set of technologies for each class of waste is considered. Finally the products are delivered to the products consumers, in this case the cities ( ). Also, each city has a specific demand of products and wastes production, the processing plants are located in different regions distributed geographically. Each processing plant can treat the wastes using several technologies. Each technology has a specific operational and capital cost depending on its upper and lower limits. There is also a transportation cost for raw materials and products. Then, the problem consist in determining the optimal supply chain network for the waste recycling, determining the wastes reused from each city, the location for the processing facilities, the type of technologies used in each place, the distribution of products accounting for economic and sustainability objectives.
The economic objective function is the maximization of the total net annual profit ( ), which takes into account the profit for the selling of products ( ) minus the total separation cost ( ), the total capital cost ( ), the total operational cost ( ), the transportation cost ( and ) the no separation cost ( ). This economic objective function is stated as follows:
The reused waste corresponds to an environmental and social objective function. This is because for a greater reuse of the produced garbage, the garbage deposited in the landfills decreases; and this way, the environmental problems associated to the land and water pollution decrease. In the same way, the reuse of garbage represents a positive impact to society, this is because the generated jobs required for the deferent stages involved in the reusing process as well as the life quality improvement. Therefore, the percentage of the consumed waste ( ) is considered in this work as a measure for the social impact, and this objective function is stated as follows:
The application of the proposed methodology has been illustrated through a case study of a distributed system for treating of municipal solid wastes in a central-west region of Mexico. Results show that it is possible to implement a distributed processing system to reuse municipal solid wastes while the net annual profit is maximized. In addition, the results can be used for governments to make decisions about the wastes management and define the amount of wastes that must be reused to obtain several products. It should be noted that the results include the supply chain design associated to the municipal solid wastes.
Figure 1. Superstructure for a supply chain of a waste refinery.