Thickeners and clarifiers

Thickening and clarification are sedimentation processes, and the equipment used for the two techniques are similar. The primary purpose of thickening is to increase the concentration of a relatively large quantity of suspended solids whereas that of clarifying, 

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

Flocculating agents are often added to promote the separating performance of thickeners. 

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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... 

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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