O aprimoramento de sistemas de tratamento de água e de esgoto tem sido limitado pelo pouco conhecimento dos fenômenos ocorridos no interior das unidades e pela utilização de "parâmetros" não específicos (DBO, DQO etc.) no monitoramento e controle de processos e operações. Por outro lado, fenômenos como transferência de massa, adsorção, difusão e reações bioquímicas são todos influenciados pela distribuição de tamanho de partículas, de modo que o emprego da análise de tamanho de partículas pode auxiliar na escolha e no aprimoramento de tecnologias de tratamento e na adoção de procedimentos operacionais apropriados. Neste artigo, são apresentadas as principais técnicas de determinação de tamanho de partículas e os potenciais benefícios de sua utilização em pesquisas e no aperfeiçoamento de projetos e rotinas de operação de sistemas de tratamento de água e esgoto sanitário.

22The two-dimensional fractal dimension (Df) of large aggregates of kaolin (> 540 μm) 23 during the shear flocculation process for kaolin solution was investigated using non-intrusive 24 in situ image-based acquisition system. Separate experiments were also carried out for three

The accurate measurement of shallow flow velocities is crucial to understand and model the dynamics of sediment and pollutant transport by overland flow. In this study, a novel triple-tracer approach was used to re-evaluate and compare the traditional and well established dye and salt tracer techniques with the more recent thermal tracer technique in estimating shallow flow velocities. For this purpose a triple tracer (i.e. dyed-salted-heated water) was used. Optical and infrared video cameras and an electrical conductivity sensor were used to detect the tracers in the flow. Leading edge and centroid velocities of the tracers were measured and the correction factors used to determine the actual mean flow velocities from tracer measured velocities were compared and investigated. Experiments were carried out for different flow discharges (32-1813 ml s À1) on smooth acrylic, sand, stones and synthetic grass bed surfaces with 0.8, 4.4 and 13.2% slopes. The results showed that thermal tracers can be used to estimate shallow flow velocities, since the three techniques yielded very similar results without significant differences between them. The main advantages of the thermal tracer were that the movement of the tracer along the measuring section was more easily visible than it was in the real image videos and that it was possible to measure space-averaged flow velocities instead of only one velocity value, with the salt tracer. The correction factors used to determine the actual mean velocity of overland flow varied directly with Reynolds and Froude numbers, flow velocity and slope and inversely with flow depth and bed roughness. In shallow flows, velocity estimation using tracers entails considerable uncertainty and caution must be taken with these measurements, especially in field studies where these variables vary appreciably in space and time.

This study sought to evaluate the effect of form and particle size distribution of formed flocs during flocculation on the removal of particulate matter (PM) by settling. The understanding of the intrinsic processes of water treatment is fundamental for the improvement of these processes, and this understanding is often limited by the use of indirect measurements. For this reason, this study evaluated the relationships among the morphology, particle size distribution (PSD), and particle volume distribution of flocs formed with mean flocculation velocity gradients (G f) from 20 to 60 s −1 and settling velocities from 0.5 to 5.0 cm s −1 at flocculation times (T f) from 10 to 60 min. A correlation analysis of all of the parameters was performed to identify the most significant direct measurement for characterizing the removal of PM. Strong correlations among parameters were observed when considering the variation with G f for a given T f , and moderate correlations were observed when considering the variation with T f for a given G f. The strong correlations that resulted from the alteration of G f were a consequence of the significant changes in the morphological characteristics, PSD, and PVD of the flocs. The lower strengths of the correlations with the variation in T f can be explained by the rapid attainment of the equilibrium steady-state between aggregation and disaggregation forces.