Flocculation processes

Although reduction or elimination of the repulsion barrier is a necessary prerequisite of successful flocculation, the actual flocculation in such a destabilized suspension is effected by particle—particle collisions. Depending on the mechanism that induces the collisions, the flocculation process may be either perikinetic or orthokinetic. 

When the overflow clarity is independent of overflow rate and depends only on detention time, as in the case for high soHds removal from a flocculating suspension, the required time is deterrnined by simple laboratory testing of residual soHd concentrations in the supernatant versus detention time under the conditions of mild shear. This deterrnination is sometimes called the second-order test procedure because the flocculation process foUows a second-order reaction rate. 

The stabilization of an emulsion iavolves slowiag the destabilization, primarily the flocculation process. This may be achieved ia two principal manners by reduciag the mobiHty of droplets through enhanced viscosity or by inserting an energy barrier between them (see also Dispersants Flocculating agents). 

In previous works [18-20,23,102] water-soluble polymers such as polyacrylamide (pAM), polysodium acrylate (pAA Na), poly(acrylamide-sodium acrylate) (pAM-AA Na), poly(acrylamide-diallyethylamine-hy-drochloride) (pAM-DAEA-HCl), and poly(acrylamide-sodium acrylate-diallyethylamine-hydrochloride) (pAM-AANa-DAEA-HCl) were used in the recovery of cations and some radioactive isotopes from aqueous solutions. It was found that the floe is formed between the added polymer and ions of the solution in the flocculation process with the formation of a crosslinked structure. The formed cross-linked structure is characterized by [103-105] ... 

Both preformed and in situ ferrite lowered plutonium concentrations in simulated process waste from 10-4 g/1 to 10-8 g/1 in one treatment step. Two or three flocculant precipitations, as currently used for waste processing, were required to achieve the same result. Ferrite waste treatment produced 4.1 g/1 solids, while production waste processing during the past year, using the flocculant process, produced 7.9 g/1 solids. 

Amuda, O.S. and Alade, A., Coagulation/flocculation process in the treatment of abattoir wastewater, Desalination, 196, 22-31, 2006. 

Amuda, O.S., Amoo, I.A., Ipinmoroti, K.O., and Ajayi, O.O., Coaguation/flocculation process in the removal of trace metals present in industrial wastewater, Journal of Applied Sciences and Environmental Management, 10 (3), 159-162, 2006. 

With regard to reversible flocculation kinetics, the problem is even more challenging- Detailed models for the deflocculation process as well as the flocculation process are required computer simulation is probably going to be the only way forward here ... 

The results of this study clearly show the complex dependence of the flocculation process on polymer dosage and charge density. It is seen that the form of dependence varies markedly among the responses monitored. In addition to the factors studied here, it can also be expected to depend upon several other physicochemical conditions of the system, including the type of mixing. The final state of flocculation achieved by a mineral/polymer system will depend upon many interactions in the system as determined by various chemical and hydrodynamic properties of the particles, polymer, dissolved organics and the fluids. 

The two major theories of flocculation, the bridging model (1) and the electrostatic patch model (2, 3 ), provide the conceptual framework for the understanding of polymer-aided flocculation, but they do not directly address the kinetics of the process. Smellie and La Mer (4) incorporated the bridging concept into a kinetic model of flocculation. They proposed that the collision efficiency in the flocculation process should be a function of the fractional surface coverage, 0. Using a modified Smoluchowski equation, they wrote for the initial flocculation rate... 

This approach is based on the assumption that polymer adsorption is fast ("instantaneous") compared with flocculation. In other words the surface coverage is taken to be constant during the flocculation process. Equation (1) states that the flocculation rate tends to zero when 0 tends to 0 or 1. The maximum rate occurs at 0 = 0.5, i.e., at 50% surface coverage. 

Flocculation rate limitation. The adsorption step was rate limiting for the overall flocculation process in this system. Polymer adsorption rate measurements for dispersed systems reported in the literature (2,26) do not lend themselves to direct comparisons with the present work due to lack of information on shear rates, flocculation rates, and particle and polymer sizes. Gregory (12) proposed that the adsorption and coagulation halftimes, tA and t, respectively, should be good indications of whether or not the adsorption step is expected to be rate limiting. The halftimes, tA and t, are defined as the times required to halve the initial concentrations of polymer and particles, respectively. Adsorption should not limit the flocculation rate if... 

Finally, as pointed out in the results of the present study, there is a possibility of determining a "pseudo" OFC in an orthokinetic experiment if the extent of flocculation is not taken to its equilibrium value. This may erroneously lead to the conclusion that the polymer adsorption step is not rate limiting for the overall flocculation process. 

The treatment technology was based on a selective precipitation/-flocculation process specifically developed by WISMUT in collaboration with leading German research institutions ... 

In reviews on the use of in situ sensors" or optical sensor systems" for bioprocesses, UV-vis does not play a major role. An example for the application of at-line UV-vis spectroscopy was presented by Noui et al. The selective flocculation processes of Saccharomyces cerevisiae homogenates were followed with a newly developed direct UV spectrophotometer. The results from a PLS regression model were in good agreement with those from off-line chemical assays. 

As mentioned previously in Section 7.2.4, the coagulation/flocculation process was found to be affected by the presence of surfactants in the raw water or wastewater. Such interference was observed for both alum and ferric sulfate coagulant, but the use of certain organic polymer... 

Soluble polymers are widely used to control the state of dispersion of fine-particle suspensions. Depending on the polymer, and how it is applied, they can serve to enhance stability (dispersants) or to promote aggregation of the particles (flocculants). The topics covered in this chapter are intended as an overview of the use of polymers for stability control in mineral-particle suspensions with particular emphasis on flocculation processes. A brief discussion of stabilisation by polymers is included for completeness. 

In general, flocculation processes involve two basic steps ... 

Continuous flocculation in stirred tanks shows similar trends with regard to agitation/mixing conditions as for batch systems. The overall performance of continuous flocculation processes tends to be inferior to that which can be obtained in batch systems. This is attributed to a closer approach to equilibrium in the continuous case. In-line flocculation, by injection of polymer into turbulent flow in a pipe, is widely practised in industry and offers certain advantages. Multiple addition at several points along the length of a pipe is preferred to single-point addition. 

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