Settling classes for fine suspended particles (original) (raw)
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Characteristics of Resuspension, Settling and Diffusion of Particulate Matter in a Water Column
Environmental Fluid Mechanics, 2005
Experiments were conducted in order to characterize the distributions of concentrations of suspended particulate matter (SPM) in water columns of lakes and reservoirs. The experiments, in a reduced model of the water column, used a set of oscillating grids. Runs were done with particles denser than water as well as with light particles. The results were in good agreement with analytical solutions for steady-state, and non-steady-state conditions. An approximate analytical solution was derived and found to be in agreement with the full solution. The threshold for resuspension was measured, and characterized in terms of a modified Shields parameter, which is appropriate to a zero-shear environment. All experiments showed that the distribution of SPM exhibited a layer near the bottom that is thought to be analogous to the benthic nepheloid layer (BNL) observed in larger lakes. The thickness of the nepheloid layer increases with the turbulence intensity.
Chapter 15 Settling velocity of sediments at high concentrations
Proceedings in Marine Science, 2008
This paper describes two semi-empirical formulas for the hindered settling velocity at high concentration. The first formula is based on the Richardson and Zaki formula (1954) with the inclusion of the effect of the maximum volume concentration of matter cmax≈0.65. The second formula is based on the solid-fluid mixture theory. Both expressions produce the best results among the studied formulas in the case of non-cohesive particles. The formula based on the fluid-mixture theory tends, however, to be sensitive to the estimation of the viscosity of the mixture. In the case of cohesive particles, as larger uncertainties exist due to unknown parameters as the density and the size of the flocs, no conclusion could be found. The Winterwerp (1999) formula gave similar results to the two proposed formulas. The estimation of the gelling concentration is also an important challenge, and appeared to correspond to for a volume concentration of the flocs =max≈0.8.
Modelling the Physical Properties of Fine Suspended Sediments
Asian and Pacific Coasts 2009, 2009
This paper presents new formulations for estimating the effective floc density (Δρ f) and the relative viscosity (μ r) of mixture of fine suspended sediments. Fitting parameters of the new models were calibrated using data available in the literature. Good comparisons were obtained when the settling velocities estimated based on the proposed models were compared against the settling velocity estimated from existing models found in the literature. The average value of the primary particle size, D p for the data used in the analysis, inferred from the new model for Δρ f was found to vary from 0.05 μm to 100 μm with a mean value of 2.5 μm. The new model for μ r is applicable to mixtures of both non-cohesive and cohesive sediments.
Can terminal settling velocity and drag of natural particles in water ever be predicted accurately
2021
Natural particles are frequently applied in drinking water treatment processes in fixed bed reactors, fluidised bed reactors, and 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 aims 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, which are 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 the one of Brown and Lawler (2003). 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 the spread. In contrast, a greater spread is caused by variations in particle size, shape, and orientation. In terms of robust process designs and adequate process optimisation for fluidisation and sedimentation of natural granules, it is therefore crucial to take into consideration the influence of the natural variations in the settling velocity when using predictive models of round spheres. Published by Copernicus Publications on behalf of the Delft University of Technology. 54 O. J. I. Kramer et al.: Can velocity and drag of particles in water be predicted accurately?
Modelling the settling of suspended sediments for concentrations close to the gelling concentration
Continental Shelf Research, 2011
This paper deals with the sedimentation of highly concentrated sediment suspensions (cohesive as well as non-cohesive) and the beginning of the consolidation of cohesive sediments. Based on a comparison of existing empirical formulas and experimental data, the particle Reynolds number was shown to be of importance for the behaviour of particularly non-cohesive sediments. In addition it plays a role in determining whether one or two interfaces develop during the sedimentation phase. In the case of cohesive sediments, the estimation of the gelling concentration, although difficult, seems to be fundamental. Some suggestions on the estimation of the permeability coefficient and total settling function are then given in order to improve the modelling of the sedimentation and consolidation behaviour for concentrations close to the gelling concentration.
International Journal of Mineral Processing, 2003
This paper presents a unified theory of solid-liquid separation of flocculated suspensions including sedimentationthickening, centrifugation and filtration. After identifying the variables and equations for each of the operations, thickening, centrifugation and filtration, and establishing the compatibility between them, we show that these processes can be described by variants of one scalar hyperbolic-parabolic strongly degenerate partial differential equation with appropriate initial and boundary conditions. To complete the description, constitutive equations should be postulated for the solid-fluid interaction forces in the suspension and for the permeability and the compressibility of the porous medium, which is either a sediment or a filter cake. A particular unit operation can then be simulated by solving these equations numerically. The mathematical analysis of the resulting model confirms the well-posedness of the mathematical model and support the design of robust numerical simulation methods. These methods are employed to calculate a variety of examples from thickening, centrifugation and filtration, which illustrate the theory.
Settling characteristics of particles in a suspension of medium to high solids concentration
1992
During the thickening process of sludges with intermediate to high solid concentrations three settling regimes are typically encountered, namely, zone, transition, and compression regimes. Recent studies have indicated that the validity of Kynch's formulation, which is the most widely used for sizing settling basins, is limited to the zone settling regime. His formulation is based on the solids mass balance equation and does not consider the role of the rising sediment at the bottom of the settling basin. This limitation is ...
Significance of Aggregation of Fine Sediment Particles in Their Deposition
Estuarine, Coastal and Shelf Science, 2002
The significance of aggregation processes, by which the properties of suspended fine sediment particles or flocs change during transport, is examined for the simple case of deposition of estuarine sediments in a flume. A multi-class model for aggregation processes is combined with a one-dimensional, unsteady, multi-class sediment transport model to calculate the deposition rate for two flume experiments—one with no
International Journal of Mineral Processing, 2004
Based on a numerical method introduced by Bürger and Karlsen [J. Eng. Math. 41 (2001) 145], a software was developed for the simulation of batch and continuous thickening. The paper recalls the application of the phenomenological theory of sedimentation-consolidation processes to batch settling and continuous thickening of flocculated suspensions. The software presents two alternatives, one for each of these possibilities. For batch thickening, the initial and critical concentration and the height of the initial suspension must be entered together with the parameters of the flux density function and the effective solid stress. The output is a settling plot showing as many lines of constant concentration as requested and a plot of the concentration profile for selected times. For continuous thickening, only the steady state is simulated. The input is the solid feed flux and the required underflow concentration or volume underflow rate. If the thickener area is known, the capacity and the concentration profile in the equipment can be predicted. On the other hand, if the capacity is known, the required settling area and the resulting concentration profiles are predicted. Several examples show the application.