Influência da porosidade e massa específica na velocidade de sedimentação de agregados fractais de água com turbidez moderada

Abstract

This study aims to analyze the influence of porosity and specific mass on settling velocities of fractal aggregates formed with the application of two chemical coagulants: aluminum sulfate (SA) and aluminum polychloride (PAC). For each coagulant, the flocs were formed under the following speed gradients: 20, 30, 40, 50 and 60 s-1. The raw water, characterized by moderate turbidity of 50 ± 5 NTU, was prepared with deionized water and kaolin. The acquisition of aggregate data was made using the non-intrusive image monitoring technique during sedimentation. An algorithm was developed in order to identify and select the flocs of interest, since the manual procedure required a longer time than the computational analysis. Results showed that the code correctly assimilated more than 90% of the aggregates. Under the conditions tested, the flocs produced by coagulation with aluminum polychloride showed lower mean values of settling velocities for all gradients of the study, in comparison with the aggregates coagulated by the action of aluminum sulfate. Under Gf = 30 s-1, the mean sedimentation speeds were 9.2 mm/s for SA, while the value of 6.4 mm/s for PAC was calculated. At Gf = 60 s-1, the average value calculated for PAC represented 61% of the average speeds calculated for SA. In addition, it was observed that fractal dimensions and high specific masses (close to the maximum values determined in the samples) positively influenced the sedimentability of the flocs. When the fractals were compared with the impermeable Stokes spheres of the same specific mass, higher velocities were obtained for all coagulants and flocculation velocity gradients. The consideration of the porosity of the flocs in this work showed that the settling velocities of fractals were up to 3.82 times greater than those determined by Stokes' law.