Flocculation process solids concentration

The factors which may make CCD a preferred choice over other separation systems include the following rapidly settling solids, assisted by flocculation relatively high ratio of solids concentration between underflow and feed moderately high wash ratios allowable (2 to 4 times the volume of hquor in the thickened underflows) large quantity of sohds to be processed and the presence of fine-size sohds that are difficult to concentrate by other means. A technical feasibihty and economic study is desirable in order to make the optimum choice. 

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. 

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

The most common thickener is the circular basin type shown in Figure 7. After treatment with flocculant, the feed stream enters the central feed well which dissipates the stream s kinetic energy and disperses it gently into the thickener. The feed finds its height in the basin where its density matches the density of the inside suspension and spreads out at that level. Solids concentration increases downward in an operating thickener giving stability to the process. 

Different concentration limits of the filler arise from the CCA concept [22]. With increasing filler concentration first an aggregation limit O is reached. For >+, the distance of neighboring filler particles becomes sufficiently small for the onset of flocculation and clusters with solid fraction A are formed. Dependent on the concentration of filler particles, this flocculation process leads to spatially separated clusters or, for 0>0, a through going filler network that can be considered as a space-filling configuration of fractal CCA-clusters. The different cases for spherical filler particles are shown schematically in Fig. 1. 

Colloid behavior in natural soil-water systems is controlled by dispersion-flocculation processes, which are multifaceted phenomena. They include surface electrical potential (El-Swaify, 1976 Stumm and Morgan, 1981), solution composition (Quirk and Schofield, 1955 Arora and Coleman, 1979 Oster et al., 1980), shape of particles, initial particle concentration in suspension (Oster et al., 1980), and type and relative proportion of clay minerals (Arora and Coleman, 1979). When suspended in water, soil colloids are classified according to their settling characteristics into settleable and nonsettleable solids. 

Operation When operated correctly, thickeners require a minimum of attention and, if the feed characteristics do not change radically, can be expected to maintain design performance consistently. In this regard, it is usually desirable to monitor feed and underflow rates and solids concentrations, flocculant dosage rate, and pulp interface level, preferably with dependable instrumentation systems. Process variations are then easily handled by changing the principal operating controls—underflow rate and flocculant dose—to maintain stability. 

UV spectrum of urban effluent from a chemical treatment plant has thus a featureless shape, like the one of raw wastewater. The absorbance value globally decreases as the wavelength increases. However, the effect of the coagulation-flocculation process can be noted. Indeed, the absorbance value above 250 nm is very low, showing that the concentration of suspended solids and colloids the optical, effect of which being more sensible in this wavelength range (see Chapter 6), is also very low. 

Oscillatory structural forces appear in thin films of pure solvent between two smooth solid surfaces and in thin liquid films containing colloidal particles including macromolecules and surfactant micelles (Israelachvili 1992). In the first case, the oscillatory forces are called the solvation forces and they are important for the short-range interactions between solid particles and dispersions. In the second case, the structural forces affect the stability of foam and emulsion films as well as the flocculation processes in various colloids. At lower particle concentrations, the structural forces degenerate into the so-called depletion attraction, which is found to destabilize various dispersions. 

The natural process of bringing the particles (and also the polyelectrolytes) together by Brownian motion (called perikinetic flocculation ) is often assisted by orthokinetic flocculation which increases particle collisions through the motion of the fluid and velocity gradients in the flow. This is the idea behind the use of in-line mixers or paddle-type flocculators in front of some separation equipment such as gravity clarifiers. The rate of flocculation in clarifiers is also increased by recycling of the floes in order to increase the rate of particle-particle collisions through the increase in solids concentration. 

Sometimes the effect of the detention time is so strong that the overflow rate can be ignored and the scale-up is based on the so-called second order test procedure (as the flocculation process usually follows a second order reaction model ). The required detention time is determined by testing the residual solids concentrations in the supernatant under the conditions of mild shear. 

Various designs are used to achieve either a clarification of liquors, or a significant increase in solids concentration resulting in the production of a wet solid filter cake of minimal moisture content. This can be driven by either an applied pressure to the substrate over a porous filter medium (i.e. pressure filtration), or a vacuum applied behind the filter medium (vacuum filtration). The process can be on a batch basis, or via a continuous operation where, by a mechanical rotary means, new filtration areas are offered to the substrate. Chemical conditioning, including flocculation and coagulation, can be used to increase throughput, cake solids or fines capture. 

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