Gravity thickeners and clarifiers

Gravity thickeners and clarifiers represent a class of solid-walled separator, where gravitational forces are used to raise the concentration of a suspension 

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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). 

The available gravity sedimentation equipment can be divided into batch operated settling tanks and continuously operated thickeners or claiiiiers. The batch settling tanks are still used where relatively small quantities of liquids are to be treated such as, for example, in spent lubrication oil reclamation and cleaning. The bulk of the processing by gravity sedimentation is, however, in continuously operated thickeners and clarifiers. 

Qo + Qr= Qi is the influent to the thickener (or secondary clarifier) gj is the recirculation flow and Q is the inflow to the overall treatment plant. A, is then compared with the clarifier area A the larger of the two is the one chosen for the design. For gravity thickeners. A, is automatically used, without comparing it to A because A is not actually calculated in designs of thickeners. 

Gravity sedimentation tests were conducted by a modified AEEP method (8) in which a 400-ml sample of effluent was examined in a 1-/ graduated cylinder giving a fluid depth of 140 mm. The height of the interface between the thickened sludge and clarified supernatant is plotted versus time. Vacuum filtration tests were conducted by a modified AEEP method ]9) in which a 0.05 ft circular Lucite leaf covered with an Eimco No. NY-415 monofilament filter cloth was used in a 1-/ beaker containing 417 ml of effluent sample. 

If the primary purpose is to produce the solids in a highly concentrated slurry then the process is called thickening and the equipment is known as the gravity thickener. The feed to a thickener is usually more concentrated than to a clarifier, the primary purpose of which is to clarify the feed. A correctly designed and operated thickener, however, can accomplish both clarification and thickening in one stage. 

Probably more relevant to the chemical industry is the scale-up of thickeners. Thickeners are basically gravity settling tanks that, apart from producing a clear overflow, are designed to have a thick underflow with as Htfle water content as possible. The feed into a thickener is generally more concentrated than the feed into a clarifier, and quite often exhibits zone-settling behavior because of the appHcation of flocculants. 

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