Sedimentation of flocculent suspensions: State of the art

Abstract

Three modes or types of sedimentation are recognized in flocculent suspensions: Clarification, in which floccules are separated and settle independently; zone settling, in which floccules are incorporated into some solids structure so that all are constrained to subside at more or less the same rate; and compression or compaction, in which the solids structure is strong enough to exhibit a compressive yield value. Current models for sedimentation in the three moses are reviewed, with particular emphasis on their use thickening presented in the literature derive from a basic partial differential equation for force balance. Models differ in which terms of the equation are disregarded. Several of the theories of compaction are essentially equivalent solutions for the same model, but with different sets of independent variables. and reliability (or lack of it) for sedimentation basin design. Most models ofSedimentation is here defined as the entire process by which particles settle out of fluid suspension. All steps of the process are profoundly affected by interfacial phenomena. Van der Waals or London forces, if not overpowered by electrostatic repulsions (zeta potentials) or fluid-dynamic shear, cause particles to cohere on contact. This gives rise to three different modes or regimes of sedimentation. Which one is followed depends upon the solids concentration, and on the relative tendency of the particles to cohere. The factors are important because they correspond to different sedimentation models or mechanisms.Types of sedimentation are represented paragenetically in Figure 1. The left side represents particles with little tendency to cohere; the right side, those for which interparticle cohesion is strong. The vertical axis represents particle concentration, with more concentrated suspensions at the top.At low concentrations, there occurs a regime called clarification. Particles are, on the average, far apart and free to settle independently, but collisions occur. If the particles then cohere, they grow into clumps, or floccules, whose settling rate increases as they grow. If, on the other hand, they do not cohere, each bumps downward at its characteristic rate. Thus there are two types of settling in clarification: flocculent, and particulate. There is no sharp boundary between the two; one grades gradually into the others. And, it is common practice to modify behavior in this regime by adding flocculants to aid clarification.Clarification behavior is easily recognized in a batch test. Slower settling particles string out behind faster settling ones. The upper layers gradually thin out, or clarify, Settled solids collect at the bottom in a layer whose upper boundary rises as solids settle into it.As particles become more concentrated, crowded closer together, they Enally reach a point where each is in contact with others. If they then have any tendency to cohere, they link into some sort of floc structure. Whether the structure formed is continuous (Fitch 1982(Fitch , 1972 consists of a bed of closely-spaced...

Predictive control of decantation in batch sedimentation process

“…Decantation of clarified liquid in batch sedimentation 1 is considered in this article. Little attention has been paid in the literature cited to the actual control strategy for the decanting process.…”

Numerical Model of Sedimentation/Thickening with Inertial Effects

“…A number of researchers (Michaels and Bolger 1962;Fitch 1966Fitch , 1975Fitch , 1979Fitch , 1983Shirato et al 1970;Tiller 1981) noted that Kynch's theory is not valid in the compression zone of flocculated suspensions, where the particle velocity is dependent on the solids stress gradient as well as the concentration (Fitch 1983). Studies by Been (1980), Tiller (1981), andFitch (1983) have generalized Kynch's theory to include the consolidation process.…”

“…The gravity thickening models utilize a force balance (or momentum) equation. The differences between the various models are the terms that are considered and the constitutive relationship assumptions utilized (Fitch 1979;Vaccari and Uchrin 1989).…”

Tracer study of batch sedimentation. [Resin beads - 45 and 62. mu. m in diameter]

“…be uniform to within 6%, and theoretical attenuation calculations verified that the tracer signal was linearly proportional to the particle concentration. 2. Detector alignment and variations in thickness of collimating slits were identified as significant sources of measurement error.…”

“…Still neglecting solids stresses, Dixon solved the force balance [Eq. (2)] numerically, both with and without local acceleration terms. When these terms were neglected, Dixon predicted a· graded concentration zone as did the Kynch theory; however, when local acceleration terms were included, no graded concent'ra.ti on zone was predicted, and thus, Dixon argued that these terms could not be neglected.…”