TY - JOUR
T1 - New hydraulic insights into rapid sand filter bed backwashing using the Carman–Kozeny model
AU - Kramer, Onno J.I.
AU - de Moel, Peter J.
AU - Padding, Johan T.
AU - Baars, Eric T.
AU - Rutten, Sam B.
AU - Elarbab, Awad H.E.
AU - Hooft, Jos F.M.
AU - Boek, Edo S.
AU - van der Hoek, Jan Peter
N1 - Funding Information:
This research is part of the project “Hydraulic modelling of liquid-solid fluidisation in drinking water treatment processes” carried out by Waternet (the water utility of Amsterdam and surroundings), Delft University of Technology and HU University of Applied Sciences Utrecht. Financial support came from Waternet's Drinking Water Production Department. We acknowledge and thank our students from Delft University of Technology, HU University of Applied Sciences Utrecht and Queen Mary University of London and in particular Victor Shao and Cas van Schaik for the precise execution of many laboratory and pilot plant experiments. This research project did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.
Publisher Copyright:
© 2021 The Author(s)
PY - 2021/6/1
Y1 - 2021/6/1
N2 - Fluid flow through a bed of solid particles is an important process that occurs in full-scale water treatment operations. The Carman–Kozeny model remains highly popular for estimating the resistance across the bed. It is common practice to use particle shape factors in fixed bed state to match the predicted drag coefficient with experimentally obtained drag coefficients. In fluidised state, however, where the same particles are considered, this particle shape factor is usually simply omitted from the model without providing appropriate reasoning. In this research, it is shown that a shape factor is not a constant particle property but is dependent on the fluid properties as well. This dynamic shape factor for irregularly shaped grains increases from approximately 0.6 to 1.0 in fluidised state. We found that unstable packed beds in moderate up-flow conditions are pseudo-fixed and in a setting state. This results in a decreasing bed voidage and simultaneously in a decreasing drag coefficient, which seems quite contradictory. This can be explained by the collapse of local channels in the bed, leading to a more uniform flow distribution through the bed and improving the available surface for flow-through. Our experimental measurements show that the drag coefficient decreases considerably in the laminar and transition regions. This is most likely caused by particle orientation, realignment and rearrangement in particles’ packing position. A thorough hydraulic analysis shows that up-flow filtration in rapid sand filters under backwash conditions causes the particle bed to collapse almost imperceptibly. In addition, an improved expression of the drag coefficient demonstrated that the Carman–Kozeny model constant, however often assumed to be constant, is in fact not constant for increasing flow rates. Furthermore, we propose a new pseudo-3D image analysis for particles with an irregular shape. In this way, we can explain the successful method using optimisation of the extended terminal sub-fluidisation wash (ETSW) filter backwashing procedure, in which turbidity and peaks in the number of particles are reduced with a positive effect on water quality.
AB - Fluid flow through a bed of solid particles is an important process that occurs in full-scale water treatment operations. The Carman–Kozeny model remains highly popular for estimating the resistance across the bed. It is common practice to use particle shape factors in fixed bed state to match the predicted drag coefficient with experimentally obtained drag coefficients. In fluidised state, however, where the same particles are considered, this particle shape factor is usually simply omitted from the model without providing appropriate reasoning. In this research, it is shown that a shape factor is not a constant particle property but is dependent on the fluid properties as well. This dynamic shape factor for irregularly shaped grains increases from approximately 0.6 to 1.0 in fluidised state. We found that unstable packed beds in moderate up-flow conditions are pseudo-fixed and in a setting state. This results in a decreasing bed voidage and simultaneously in a decreasing drag coefficient, which seems quite contradictory. This can be explained by the collapse of local channels in the bed, leading to a more uniform flow distribution through the bed and improving the available surface for flow-through. Our experimental measurements show that the drag coefficient decreases considerably in the laminar and transition regions. This is most likely caused by particle orientation, realignment and rearrangement in particles’ packing position. A thorough hydraulic analysis shows that up-flow filtration in rapid sand filters under backwash conditions causes the particle bed to collapse almost imperceptibly. In addition, an improved expression of the drag coefficient demonstrated that the Carman–Kozeny model constant, however often assumed to be constant, is in fact not constant for increasing flow rates. Furthermore, we propose a new pseudo-3D image analysis for particles with an irregular shape. In this way, we can explain the successful method using optimisation of the extended terminal sub-fluidisation wash (ETSW) filter backwashing procedure, in which turbidity and peaks in the number of particles are reduced with a positive effect on water quality.
KW - Drinking water treatment
KW - Dynamic particle shape factors
KW - Filter-backwash
KW - Hydraulics drag relations
KW - Multiphase flows
KW - Particle orientation
UR - https://www.scopus.com/pages/publications/85104089307
U2 - 10.1016/j.watres.2021.117085
DO - 10.1016/j.watres.2021.117085
M3 - Article
C2 - 33862394
AN - SCOPUS:85104089307
SN - 0043-1354
VL - 197
JO - Water research
JF - Water research
M1 - 117085
ER -
SDG 6 Clean Water and Sanitation