Considering Water Quality and Energy Efficiency to Define Optimum Operation of Integrated Systems of Water Supply and Value of Use of Water Quality

According to Gunkel et al. (2007), all rivers in the coastal region of Pernambuco´s state are influenced by sugarcane cultivation. Effects are often magnified as these same rivers supply cities as well and are generally heavily dammed for both urban water supply and electrical power generation. These reservoirs might also contribute to further water quality deterioration through eutrophication processes. Seeking to contribute with a decision support tool in the water supply area with this kind of problem, this study presents an optimization model that identifies the volume of water to be distributed from a set of integrated water supply systems monthly to each municipality in the Recife’s metropolitan region (Pernambuco´s state), in order to minimize expenses with electric power and water treatment. The optimization problem presented uses the linear programming techniques with constraints on the maximum supply capacity of each integrated water system, the installed pipeline network and the minimum volume of water to be distributed to each city. The model results showed that the optimization leads to a reduction of 4.82% of the costs, particularly from an increase of 20.73% in volume produced by Pirapama system, the newest and largest supply system of the state of Pernambuco, the standstill in some months of the smallest integrated systems in the region (Caixa D’Água and Marcos Freire), which also have the m³ of treated water more expensive in the region. At the same time requires a reduction of 15.38% in the activities of Tapacurá system, which suffers from eutrophication problems in the dam of the same name, its main source. By the reduced cost is possible to know how much the cost of the systems that do not operate at the optimal solution must be reduced so that they start to be considered as an option for use in catchment. Indeed, it may identify the opportunity cost of each of these systems, which is an important concept of economic theory. One of the scenarios studied considered the opportunity cost of Tapacurá system and the results showed a reduction in the value of the objective function of 6.98% (R$ 3,138,175.76) in relation to the optimal solution in scenario 1 (current conditions) and 11.47% (R$ 5,414,987.10) compared with that achieved by COMPESA in 2013. These results show the economy that the supply company would have for a year if invested in Tapacurá system to achieve lower energy and treatment costs, by installing more efficient pumps or policies to improve the water quality of the Tapacurá basin, for example. From the analysis of scenarios that consider different conditions of water quality of the main reservoirs of water catchment for integrated systems in the region, Pirapama and Tapacurá, the study estimates the direct use value of water quality for the company responsible for supplying water in the region, presenting a proposal for value the quality of water based on the avoided cost method. The direct use value of water quality from the Tapacurá system to the company responsible for the supply is R$ 3,874,662.18/year, which represents 8.21% of current expenses of the company on electricity and chemical products. The water quality of the dam Pirapama has a higher value, R$ 5,468,122.15/year, corresponding to 11.58% of the expenses of the company in 2013 with electricity and chemical products. The difference between these values results from the differentiated proportional share of them in the water supply of the metropolitan region of Recife.