Nonintrusive investigation of large Al-kaolin fractal aggregates with slow settling velocities

Rodrigo Moruzzi, Luiza C. Campos, Soroosh Sharifi, Pedro Grava da Silva, John Gregory

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Although a combination of aggregate characteristics dictate particle settling, it is commonly assumed that large particles have higher terminal velocities. This simplifying assumption often leads to overprediction of large aggregate settling velocities which in turn negatively impacts on estimates of sedimentation clarification efficiency. Despite its importance, little attention has been given to large aggregates with slow-settling velocities. This paper addresses this gap by investigating slow-settling velocities of large, heterodisperse and multi-shape Al-kaolin aggregates using non-intrusive methods. A particle image velocimetry technique (PIV) was applied to track aggregate velocity and a non-intrusive image technique was used to determine aggregate characteristics, including size (df), three-dimensional fractal dimension (Df), density (ρf), aggregate velocity (Vexp) and Reynolds number (Re). Results showed no strict dependence of settling velocity on large aggregate size, shape and density, as Al-kaolin aggregates with the same size exhibited different settling velocities. A comparison of the results with the well-known Stokes’ law for velocity modified by a shape factor showed that the settling velocities measured here can vary from 2 to 14 fold lower than the predicted values for perfect sphere-shape aggregates with the same density and size. Furthermore, results have also shown large Al-kaolin aggregate's drag coefficient (Cd) to be around 56/Re, for average fractal aggregate sphericity of around 0.58.
Original languageEnglish
Article number116287
Number of pages9
JournalWater Research
Early online date9 Aug 2020
Publication statusPublished - 15 Oct 2020


  • Flocculation
  • sedimentation
  • Aggregate density
  • Settling velocity
  • Fractal dimension
  • Sedimentation

ASJC Scopus subject areas

  • Water Science and Technology
  • Ecological Modelling
  • Pollution
  • Waste Management and Disposal


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