Avaliação da morfologia e da distribuição do tamanho de partículas para pontos distintos do diagrama de coagulação/floculação

Abstract

Coagulation and flocculation are essential steps in the water treatment for public supply for the impurities removal. The knowledge of flocculent particle size distribution (PSD) and morphological characteristics is of great value to the understanding of the mechanisms that involve the flocs aggregation and subsequent removal. In this context, the present study has an objective the size distribution evolution and flocs structure monitoring formed in different chemical configurations (several pairs of pH of coagulation and coagulant dosage) during flocculation process in water with apparent high color (100 ± 5 uH), using aluminum sulphate as coagulant, from the laboratory simulation in Jar Test equipment submitted to eight shear rates (20, 30, 40, 50, 60, 80, 100 and 120 s-1). Particle measurement was performed by digital image analysis technique obtained by non-intrusive and non-destructive method, consisting of light plane generated by a laser and high-resolution camera, enabling the direct assessment of growth and rupture flocs dynamics. The results of particles distribution by size range indicated that the chemical coagulation condition interferes in the PSD evolution, stating that the larger aggregates formation occurs faster in regions acknowledged by the charges neutralization mechanism. The gradual shear rate increase promoted the flocs breakage, carrying in the more concentrated distribution in the size ranges smaller, which was practically unchanged for flocculation gradients higher than 60 s-1. The fractal dimension (Df) varied between 1.49 and 1.95. The speed gradient increase in flocculation has tended to reduce the Df value, promoting less compact and more branched flocs. It was observed a Df gradual increase in the initial flocculation minutes for lower aluminum dosages settings and more acidic pH, suggesting that particles morphological structure is directly linked to the coagulation mechanism.