Sedimentation: Tangent and Concentration

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Sedimentation is the process of separating a liquid mixture of suspended particles into clear supernatant liquid and denser slurry having a higher concentration of solids. This is usually accomplished by allowing the particles to settle through the force of gravity, mechanically using centrifugal force, or electrostatically using an electric current. Continuous sedimentation tanks are usually used in wastewater treatment facilities to separate suspended particles from wastewater.

This experiment aims to determine the effect of initial concentration and initial height of the slurry on its settling characteristics. Using a set of data obtained from the experiment, a continuous thickener or clarifier must then be
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Otherwise, a milky interface is formed. B is a suspended mixture of solids and liquid and has uniform concentration. Between B and C is a distinct transition zone. This zone is due to the rising liquid as the highly concentrated sludge D compacts. Zone c, unlike B, is a region of variable concentration. Note that regions A and D grow larger at the expense of B and C until such a point where maximum compaction of D is obtained. This point is called the critical settling point. At his point, only a single, distinct interface is formed between the concentrated sludge and the clear liquid.

To determine the effect of concentration or height on the settling characteristics of a suspension, batch sedimentation experiments are conducted. The data from these experiments can be used to design a continuous thickener.

Continuous thickeners consist of zones similar to a batch sedimentation process except that these zones are of constant height when steady state is achieved. The design of these thickeners s based on the identification of the concentration of the rate limiting layer.

First, batch settling data is obtained by plotting the height of the interface (between the clear liquid and slurry), z versus time. Please refer to the figure below. This uses the Talmadge and Fitch method.

A tangent line is drawn at the beginning (line B) and at the end (line D). The portions from which these tangent lines are drawn

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