A Model for the Settling Velocity of Flocs; Application to an Aquaculture Recirculation Tank

A Model for the Settling Velocity of Flocs; Application to an Aquaculture Recirculation Tank

J. A. Garcia-Aragon H. Salinas-Tapia J. Moreno-Guevara V. Diaz-Palomarez S. Tejeda-Vega

Centro Interamericano de Recursos del Agua, Faculty of Engineering, Univ. Aut. Estado de México, Toluca, México

Instituto Nacional de Investigaciones Nucleares, Estado de México, México

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A general model for flocs settling velocity is still an open field of research in the scientific literature. In this work, a reduced model of an aquaculture recirculation tank was used to validate a model for floc settling velocity. Cohesive sediments from non-used food and fish excreta are a main concern in those tanks design. Excess concentrations of sediments can cause fish death or additional costs of energy for aeration. This research is aimed to understand the settling behavior of flocs when subjected to a liquid shear rate. A reduced scale model of an aquaculture recirculation tank was build in Plexiglas in order to use particle image velocimetry and particle tracking velocimetry techniques to measure fluid velocities, solid settling velocities, flocs shape and size. Different flow rates and solid concentrations were used to develop varied configurations in the sys- tem; models for floc settling velocity based on fractal theory were calibrated. Cohesive sediments from fish food were observed in long-term experiments at constant fluid shear rate in the recirculation tank. A group of 50 images were obtained for every 5 min. Image analysis provided us with floc settling velocity data and floc size. Using floc settling velocity data, floc density was obtained for different diameters at equilibrium conditions, after 1 h or larger experiments. Statistical analysis of floc velocities for different floc sizes allowed us to obtain an expression for the drag coefficient as a function of floc particle Reynolds number (Rep). The results were compared with floc settling velocity results from different researchers. The model is able to define the general behavior of floc settling velocity, which shows a reduction for larger flocs that is not taken into account in classical models. Only two parameters of the drag coefficient model for a permeable spherical particle are needed to be calibrated, for different types of sediments, in order to have more general applicability.


aquaculture, drag coefficient, flocs, flocs density, fractal dimension, permeable particle, PIV, PTV, recirculation tank, settling velocity


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