(241a) A Multi-Objective Optimization Model to Plan City-Scale Water Systems with Economic and Environmental Objectives: A Case Study in Santiago, Chile

Climate Change has become an important issue in several regions worldwide since human behavior is conditioned by the effects of Global Warming on Earth. A direct effect is water scarcity, considered a global risk by the World Economic Forum, affecting two thirds of the world's population [1]. To address this issue, long term planification of large scale systems has become a key matter. Therefore, water management and planning under uncertainty has been a priority area of research aiming at improving large scale regional water systems. In this context, optimization techniques can be a valuable tool in the water resources management in order to redesign regional water systems, and this effort constitutes the main focus of this project.

In this work, we develop a novel superstructure to model the water system of a city. The problem is formulated as a Multi-Objective Mixed-integer Nonlinear Programming to determine the optimal configuration of a regional water system. The model is formulated to decide (i) the installation of new treatment plants, (ii) the actualization of the existing ones, and (iii) the connections within the new integration network. These changes in the water network allow to recycle and reuse water. The objective functions to minimize are: (i) Global Warming Potential, (ii) water usage from the source, and (iii) total cost of the water system. These functions integrate the economic and environmental impacts simultaneously in the model, in order to redesign the existing water system. New plants can achieve four water qualities: (i) freshwater; (ii) drinking water; (iii) irrigation water; and (iv) discharge water. The main novelties of this work are the large scale orientation of the formulation and the integration of economic and environmental objectives in the planning of a city-scale water system.

A case study is proposed in order to create a solution linked to reality, without losing adaptability to other contexts worldwide: the model is formulated inspired in Santiago, capital of Chile, with a population of six million people [2]. Santiago is the political, economic and institutional center of Chile and has been signed as a hydric-stress zone [3]. The participants of the network are classified into consumers (urban, industrial, and agricultural); sources (superficial and groundwater); distribution and collection nodes; treatment plants; and final disposal sinks.

First, a simplified problem is formulated and solved using Goal Programming strategy. The result shows a modification of the present network: the new system has no discharge to rivers. Furthermore, drinking and irrigation water demand can be supplied through new recycling plants with a direct connection. The solution does not recycle water to the source (freshwater quality). The proposed network is composed by large and small scale plants. The connection of these treatments with consumption and distribution/collection nodes reduces the transport cost.

Finally, the formulation of this problem is oriented to address large scale water systems with special focus on real regional water networks. The model is proposed to provide orientation to policy makers so as to plan future changes in city-scale water networks. The next stage in this project is to include uncertainty in water availability, demand, and other effects of Climate Change on the network.

References

[1] ALBRITTON-JONSSON, F., BREWER, J., FROMER, N., TRENTMANN, F. Introduction: Scarcity in the Modern World: History, Politics, Society and Sustainability, 1800–2075. 2019.

[2] NATIONAL STATISTICS INSTITUTE. CHILEAN GOVERNMENT. Primera entrega de resultados definitivos CENSO 2017: cantidad de personas por sexo y edad. 2018.

[3] VALDÉS-PINEDA, R., PIZARRO, R., GARCÍA-CHEVESICH, P., VALDÉS, J.B., OLIVARES, C., VERA, M., BALOCCHI, F., PÉREZ, F., VALLEJOS, C., FUENTES,R., ABARZA, A., HELWIG, B. Water governance in Chile: Availavility, management and climate change. Journal of Hydrology, 519, Part C: 2538-2567, Nov. 2014.