A model for the settling velocity of flocs; application to an aquaculture recirculation tank

Abstract

A general model for fl ocs settling velocity is still an open fi eld 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 fi sh excreta are a main concern in those tanks design. Excess concentrations of sediments can cause fi sh death or additional costs of energy for aeration. This research is aimed to understand the settling behavior of fl ocs 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 fl uid velocities, solid settling velocities, flocs shape and size. Different fl ow rates and solid concentrations were used to develop varied confi gurations in the system; models for floc settling velocity based on fractal theory were calibrated. Cohesive sediments from fi sh food were observed in long-term experiments at constant fl uid shear rate in the recirculation tank. A group of 50 images were obtained for every 5 min. Image analysis provided us with fl oc settling velocity data and fl oc size. Using fl oc settling velocity data, fl oc density was obtained for different diameters at equilibrium conditions, after 1 h or larger experiments. Statistical analysis of fl oc 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 fl oc settling velocity, which shows a reduction for larger fl ocs 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.