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Drinking Water Engineering and Science An interactive open-access journal
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Preprints
https://doi.org/10.5194/dwes-2020-30
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/dwes-2020-30
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  15 Oct 2020

15 Oct 2020

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This preprint is currently under review for the journal DWES.

Can terminal settling velocity and drag of natural particles in water ever be predicted accurately?

Onno J. I. Kramer1,2,3,4, Peter J. de Moel5, Shravan K. R. Raaghav2, Eric T. Baars3, Wim H. van Vugt4, Wim-Paul Breugem2, Johan T. Padding2, and Jan Peter van der Hoek1,3 Onno J. I. Kramer et al.
  • 1Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Water Management, P.O. Box 5048, 2600 GA, Delft, the Netherlands
  • 2Delft University of Technology, Faculty of Mechanical, Maritime and Materials Engineering, Department of Process and Energy, Leeghwaterstraat 39, 2628 CB, Delft, the Netherlands
  • 3Waternet, P.O. Box 94370, 1090 GJ, Amsterdam, the Netherlands
  • 4HU University of Applied Sciences Utrecht, Institute for Life Science and Chemistry, P.O. Box 12011, 3501 AA, Utrecht, the Netherlands
  • 5Omnisys, Eiberlaan 23, 3871 TG, Hoevelaken, the Netherlands

Abstract. Natural particles are frequently applied in drinking water treatment processes in fixed bed reactors, in fluidised bed reactors, and in sedimentation processes to clarify water and to concentrate solids. When particles settle, it has been found that in terms of hydraulics, natural particles behave differently when compared to perfectly round spheres. To estimate the terminal settling velocity of single solid particles in a liquid system, a comprehensive collection of equations is available. For perfectly round spheres, the settling velocity can be calculated quite accurately. However, for naturally polydisperse non-spherical particles, experimentally measured settling velocities of individual particles show considerable spread from the calculated average values.

This work aimed to analyse and explain the different causes of this spread. To this end, terminal settling experiments were conducted in a quiescent fluid with particles varying in density, size and shape. For the settling experiments, opaque and transparent spherical polydisperse and monodisperse glass beads were selected. In this study, we also examined drinking water related particles, like calcite pellets and crushed calcite seeding material grains, both applied in drinking water softening. Polydisperse calcite pellets were sieved and separated to acquire more uniformly dispersed samples. In addition, a wide variety of grains with different densities, sizes and shapes were investigated for their terminal settling velocity and behaviour. The derived drag coefficient was compared with well-known models such as Brown–Lawler.

A sensitivity analysis showed that the spread is caused to a lesser extent by variations in fluid properties, measurement errors and wall effects. Natural variations in specific particle density, path trajectory instabilities and distinctive multi-particle settling behaviour caused a slightly larger degree of spread. In contrast, greater spread is caused by variations in particle size, shape and orientation.

Onno J. I. Kramer et al.

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Videos of terminal settling experiments in water: path instabilities O. J. I. Kramer, S. K. R. Raaghav, and W. P. Breugem https://doi.org/10.4121/uuid:3ffdfa51-38f0-4188-aec5-8cd8fc8f1941

Onno J. I. Kramer et al.

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Short summary
Our work investigates the settling behaviour of natural granules applied in drinking water treatment plants. We show that these natural granules have a tendency to show a considerably large deviation in terms of their settling velocity; this is contrary to what many velocity prediction models assume. In the current work we present and discuss the factors which contribute to the observed deviation in drag and settling velocity. It connects full-scale operations and research.
Our work investigates the settling behaviour of natural granules applied in drinking water...
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