Modelando as eficiências de remoção de fósforo e clorofila-a em águas de reservatórios do semiárido com a utilização de materiais da técnica Floc and Sink

Abstract

Various techniques have been developed with the aim of mitigating eutrophication by controlling nutrients. The Floc and Sink technique is used to remove phosphorus in the water column through flocculation and sedimentation using coagulants and phosphorus adsorbing compounds. Characteristics of eutrophied reservoir water, such as pH and humic substances, can negatively affect the performance of the materials used in this technique. Models to evaluate the causes and effects of eutrophication have also been developed, becoming a tool in eutrophication management. Based on this, this study aimed to evaluate the interference of physical, chemical and biological factors in the waters of eutrophied reservoirs in the semi-arid region on the efficiency of the Floc and Sink technique. The hypothesis was that characteristics such as pH, alkalinity and the morphology of the dominant cyanobacterium interfere with the action of the coagulant PAC and BML clay, and that it is possible to model these relationships. To test this hypothesis, samples were taken at 5 points in four eutrophied reservoirs in the semi-arid region to characterize the physical, chemical and biological conditions (temperature, electrical conductivity, total dissolved solids, dissolved oxygen, turbidity, true and apparent color, total phosphorus (TP), soluble reactive phosphorus (SRP), pH, ORP, alkalinity, chlorophyll-a and cyanobacteria density). Flocculation-sedimentation tests were carried out to analyze the removal efficiency of phosphorus and chlorophyll-a from the water column. The treatments applied were: PAC (4, 8 and 16 mg Al L-1); BML (25, 50 and 100 mg L-1); and PAC + BML. The explanatory variables used to build the models were the physical, chemical and biological conditions, and the response variables were the phosphorus and chlorophyll-a removal efficiencies. As a result, the models built had a minimum R² of 0.39, p < 0.001 and a 95% confidence interval, defining pH, ORP, turbidity and DO as the variables that significantly affected the performance of the treatments. pH values above 7.3 can negatively influence the action of the materials, as well as ORP above 270 mV, but turbidity in the range between 5 and 18 NTU favored the application of the combined materials. There were also similarities between the phosphorus and chlorophyll-a removal efficiencies of the PAC and PAC+BML treatments, indicating that the application of PAC is more economically viable. The results of this study provide technical and scientific support for water resource management policies.