Thickening and Clarifying

TABLE 11.6. Porosities and Permeabilities of Some filter Media 

Equation (11.24) cannot be entirely valid because it predicts zero resistivity at zero pressure, whereas cakes do have structures and significant resistivities even at minimal operating pressures. Modified Eq. (11.12) is extrapolatable, and is rewritten here as 

Some data fitted to these equations by Tiller et al. (1979) are in Table 11.8 here the constant k is the same for both a and e, although this is not necessarily generally the case. Unfortunately, these data show that the parameters are not independent of the pressure range. Apparently the correlation problem has not been solved. Perhaps it can be concluded that insofar as the existing filtration theory is applicable to real filtering behavior, the approximation of Almy and Lewis may be adequate over the moderate ranges or pressures that are used commonly, somewhere between 0.5 and 5 atm. 

The expecting filtering area is expected to be substantial, measured in tens of m2. 

TABLE 11.8. Parameters of Equations for Resistivity a and Porosity e of Some Filter Cakes 

Fi re 114. Data of compressibilities and porosities of filter cakes, (a) Parameters of the correlation a = an(AP) for resistivitv of CaSiO, 

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The following types of devices are commonly applied to measure the various operational parameters of thickeners and clarifiers. They have been used in conjunction with automatic valves and variable-speed pumps to achieve automatic operation as well as to simply provide local or remote indications. 

TABLE 18-7 Typical Thickener and Clarifier Design Criteria and Operating Conditions... 

The design and construction of thickeners and clarifiers is described by Dahlstrom and Cornell (1971). 

Figure 10.11. Types of thickener and clarifier (a) Bridge supported (up to <40 m dia.) (b) Centre column supported (<30 m dia.) (c) Traction driven (<60 m dia.)...

The process of thickening involves the concentration of a slurry, suspension, or sludge, usually by gravity settling. Because concentrated suspensions and/ or fine particle dispersions are often involved, the result is usually not a complete separation of the solids from the liquid but is instead a separation into a more concentrated (underflow) stream and a diluted (overflow) stream. Thickeners and clarifiers are essentially identical. The only difference is that the clarifier is designed to produce a clean liquid overflow with a specified purity, whereas the thickener is designed to produce a concentrated underflow product with a specified concentration (Christian, 1994 Tiller and Tarng, 1995 McCabe et al., 1993). 

Coagulant and/or Flocculant Selection Coagulants and flocculants are widely used to enhance the settling rate which reduces thickener and clarifier size and improves overflow clarity and/or underflow sluriy density. The terms coagulation and flocculation are sometimes used interchangeably however, each term describes separate functions in the particle agglomeration process. 

Tiller, F.M., and D. Tarng. 1995. Try deep thickeners and clarifiers Chemical Engineering Progress March 75-80. 

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