Water quality modeling: a Brazilian experience in water resource management for decision making in wastewater treatment plants

In order to reduce water conflicts and to attend to multiple water uses, this study evaluated water quality in the Ribeirão das Botas Stream basin in Brazil's Central-West region, in regards to the evolution of legal and institutional aspects at the federal and state scope, mainly with the inclusion of the legal instrument to authorize water use rights. Through a self-purification study with effluent discharge simulated from a wastewater treatment plant, four points along the river were evaluated during the dry season, using the QUAL-UFMG model, for the parameters DO (dissolved oxygen), BOD (biochemical oxygen demand) and total coliforms. Three effluent discharge simulations were carried out. The first scenario encompassed a single criterion: maximum BOD concentration discharged according to legislation (120 mg.L-1), where it resulted in DO below 7 mg.L-1 and BOD outside of the classification class 2, exceeding the value of 5 mgBOD.L-1. The second scenario used the limit of the diluted concentrated value in the receiving water for the BOD (<5 mg.L-1) and DO (> 5 mg.L-1) parameters. The third scenario related the quantitative aspect linked to the authorization instrument to accommodate the uses of the river basin. The physical-chemical and biological analyzes with the presence of E. coli and total phosphorus parameters was in disagreement with the current standards, which shows that there is a negative impact due to anthropic activities in the region caused by inadequate urban effluent discharge and agricultural activities. It was observed that in scenarios 2 and 3, the evaluated parameters respected the river’s support capacity, which presented DO above 7 mg.L-1 and BOD below 5 mg.L-1. However, for the stream’s support capacity to be met without changing its classification, the wastewater treatment plant that is to be implemented should have a maximum flow of 0.43 m3.s-1 and a minimum efficiency of 96%, not 65% or 88% according to scenarios 1 and 2.This study permits assistance to the water resource management council for decision making in the face of multiple use conflicts with the use of mathematical modeling for self-purification studies according to the evolution in the legislation, taking into account not only qualitative aspects, but also quantitative and diverse water uses.