Modelo de otimização quali-quantitativo para sistemas de recursos hídricos: aplicação no Sistema Engenheiro Ávidos e São Gonçalo - PB.

Abstract

Water resources are essential for life, which makes it necessary to make efforts to ensure the efficient use of water, especially in systems that have reduced water supply, multiple demands and effluents released into water bodies. The present study proposes to develop a qualitativequantitative optimization model, at monthly level, for application in water resources systems, allowing the modeling of nitrogen, phosphorus, dissolved oxygen and biochemical oxygen demand in water bodies. With the final product, it was possible to carry out the multi-objective, qualitative-quantitative optimization, based on linear programming, for several Scenarios, favoring decision making. Modeling, for rivers and reservoirs, of organic and inorganic forms of the nitrogen and phosphorus cycle, from domestic and agricultural effluents, the latter quantified from soil mass balances, was taken into account. The model was structured in a Matlab environment, with objective functions and restrictions of a quantitative and qualitative nature, with non-linearities being overcome by means of linearization artifices. As a case study, a system located in Alto Piranhas - PB, which comprises two reservoirs connected in series and the river stretches between them, was evaluated. Four Scenarios were analyzed, the first considering only the quantitative aspects of water resources and the others with the influence of domestic and agricultural effluents. After the optimization procedures, it was possible to meet urban demands throughout the period of operation, in the four Scenarios. For irrigated agriculture, mainly the Irrigated Perimeter of São Gonçalo, the allocated area was lower than the available area, due to the high demands associated with this activity. When the quality parameters were not prioritized in the optimization, the concentrations were high in several months, with the total phosphorus in the São Gonçalo reservoir, for example, presenting values up to 12.30 times higher than the maximum limits for this water body. Based on this objective, even when raw sewage was discharged, the levels in accordance with Conama Resolution nº 357/2005 were maintained. As the agricultural loads were inserted, there were months in which the concentrations of nutrients and dissolved oxygen, for São Gonçalo and the node between the reservoirs, were in disagreement with the current legislation, for the latter, for example, levels were observed of ammonia nitrogen about 1.75 times higher than the limit for the adopted Class. Thus, there were reductions in reliability and resilience, with greater vulnerability corresponding to the concentrations of total phosphorus in the node (38.40%), justifying results found in the literature about the impact of fertilizers on water bodies. The reuse of domestic effluents can be an alternative for the study area, favoring the water and nutritional supply of crops. The efficiency indices of the reservoirs indicated great relevance of Engenheiro Ávidos for the qualitative-quantitative supply of water bodies downstream, with 70.47% of its inflow being able to be used, mainly, for the assistance downstream. Finally, the optimization model proved to be satisfactory, enabling the system to be operated efficiently, so that the qualiquantitative aspects of water resources could be integrated, which makes the representation of the system more reliable.