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
 Wheaton, F.W., Aquaculture Engineering, Wiley–Interscience: New York, 708 pp., 1977.
 Droppo, I.G., Rethinking what constitutes suspended sediment. Hydrological Processes, 14, 653–667, 2001.
 Droppo, I.G. & Ongley, E.D., Flocculation of suspended sediment in rivers of south-eastern Canada. Water Research, 28, 1799–1809, 1994. doi: http://dx.doi.org/10.1016/0043-1354(94)90253-4
 Nicholas, A.P. & Walling, D.E., The signifi cance of particle aggregation in the overbank deposition of suspended sediment on river fl oodplains. Journal of Hydrology, 186, 275–293, 1996. doi: http://dx.doi.org/10.1016/S0022-1694(96)03023-5
 Droppo, I.G., Suspended sediment transport – fl occulation and particle characteristics. Encyclopaedia of Hydrological Sciences, ed. M.G. Anderson, John Wiley & Sons, Ltd: New York, 2005.
 Garcia-Aragón, J., Droppo I., Krishnappan B., Trapp, B. & Jaskot, C., Erosional characteristics and fl oc strength of Athabasca river cohesive sediments. Journal of Soil and Sediments, 11(2), 2011, 679–689. doi: http://dx.doi.org/10.1007/s11368-011-0345-4
 Watten, B.J. & Beck, L.T., Comparative hydraulics of rectangular cross fl ow rearing unit. Aquacultural Engineering, 6, 127–140, 1987. doi: http://dx.doi.org/10.1016/0144-8609(87)90010-0
 Summerfelt, S.G., Wilton, G., Roberts, D., Rimmer, T. & Fonkalrsrud, K., Developments in recirculating systems for Artic char culture in North America. Aquacultural Engineering, 30, 31–71, 2004. doi: http://dx.doi.org/10.1016/j.aquaeng.2003.09.001
 Timmons, M.B., Summerfeld, S.T. & Vinci, B.J., Review of circular tank technology and management. Aquacultural Engineering, 18, 51–69, 1998. doi: http://dx.doi.org/10.1016/S0144-8609(98)00023-5
 Hawley, N., Settling velocity distribution of natural aggregates. Journal of Geophysical Research, 87(C12), 9489–0498, 1982. doi: http://dx.doi.org/10.1029/JC087iC12p09489
 Sundaresan, S., Eaton, J., Koch, D. & Ottino, J., Appendix 2: report of study group on disperse fl ow. International Journal of Multiphase Flow, 29, 1069–1087, 2003. doi: http://dx.doi.org/10.1016/S0301-9322(03)00080-6
 Garcia-Aragon, J.A., Droppo, I., Krishnappan, B., Trapp, B. & Jaskot, C., Experimental assessment of Athabasca river cohesive sediment deposition dynamics. Water Quality Research Journal of Canada, 46(1), 87–96, 2011. doi: http://dx.doi.org/10.2166/wqrjc.2011.030
 Dyer, K.R., Cornelisse, J., Dearnaley, M.P., Fernessy, M.J., Jones, S.E., Kappenberg, J., McCave, I.N., Pejrub, M., Puls, W., Van Leussen, W. & Wolfstein, K.A., Comparison of in situ techniques for Estuarine fl oc settling velocity measurements. Journal of Sea Research, 36, 15–29, 1996.
 Sternberg, R.W., Berhane, I. & Ogston, A.S., Measurement of size and settling velocity of suspended aggregates on the Northern California Continental Shell. Marine Geology, 154, 43–53, 1999. doi: http://dx.doi.org/10.1016/S0025-3227(98)00102-9
 Li, D.H. & Ganczarczyk, J.J., Stroboscopic determination of settling velocity, size and porosity of activated sludge fl ocs. Water Research, 21(3), 157–262, 1987. doi: http://dx.doi.org/10.1016/0043-1354(87)90203-X
 Khelifa, A. & Hill, P.S., Models for effective density and settling velocity of fl ocs. Journal of Hydraulic Research, 44(3), 390–401, 2006. doi: http://dx.doi.org/10.1080/00221686.2006.9521690
 Kranenburg, C., The fractal structure of cohesive sediment aggregates. Estuarine Coastal Shelf Science, 39, 1665–1680, 1994. doi: http://dx.doi.org/10.1016/S0272-7714(06)80002-8
 Lau, Y.L. & Krishnappan, B.G., Measurement of size distribution of settling fl ocs. NWRI Publication No. 97-223. National Water Research Institute, Environment Canada, Burlington, Ontario, Canada, 1997.
 Rojak, S.N. & Flagan, R.C., Stokes drag on self-similar clusters of spheres. Journal of Colloid and Interface Science, 134, 206–318, 1990. doi: http://dx.doi.org/10.1016/0021-9797(90)90268-S
 Hribersek, M., Zajdela, B., Hribernik, A. & Zadravec, M., Experimental and numerical investigations of sedimentation of porous wastewater sludge flocs. Water Research, 45(4), 1729, 2011. doi: http://dx.doi.org/10.1016/j.watres.2010.11.019
 Neale, G., Epstein, N. & Nader, W., Creeping fl ow relative to permeable spheres. Chemical Engineering Science, 28, 1865–1874, 1973. doi: http://dx.doi.org/10.1016/0009-2509(73)85070-5
 Abade, G C., Cichocki, B., Ekiel-Je ewska, M.L., Nägele, G. & Wajnryb, E., Shorttime dynamics of permeable particles in concentrated suspensions. Journal of Chemical Physics, 132(1), 014503, 2010. doi: http://dx.doi.org/10.1063/1.3274663
 Garcia-Aragon, J.A., Salinas-Tapia, H., Guevara, J. & Diaz-Palomarez, V., Experimental observations of cohesive sediment dynamics in aquaculture recirculation tanks. WIT Transactions in Engineering Sciences, 79, 475–483, 2013.
 Johnson, C.P., Xiaoyan, L.I. & Logan, B.E., Settling velocities of fractal aggregates. Environmental Science and Technology, 30, 1911–1918, 1996. doi: http://dx.doi.org/10.1021/es950604g