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FIGURE 5.11 Batch analysis of zone settling.

settling occurs. In this zone, the solids are further thickened by the compression, compaction, and consolidation processes.

Consider that initially at time to, a suspension of initial concentration [co] is poured into the graduated cylinder. Some time later at tb the four zones are formed. B retains the initial concentration and settles at a rate characteristic of this concentration. Since the concentration is constant at [co] throughout this zone, B settles at a constant velocity, thus the name uniform settling (velocity) zone. This velocity can be determined by following the interface A-B with time.

D is formed by particles piling on top of each other producing the largest concentration of the zones. There will be a gradation of concentration from D to B. Thus, zone C will have this gradation forming a concentration gradient in the zone. This is where the thickening process first occurs. The gradient will be more or less constant. As the particles pile up on top of another, zone D lengthens. To more or less maintain the concentration gradient, the concentration immediately below interface B-C and the concentration immediately above interface C-D must remain constant for a constant length of zone C. Because D is lengthening and to maintain the length of C and thus the gradient, the interface B-C must move up at the same speed as the lengthening of D. This means that zone B is eroded both at its top and its bottom. Hence, eventually, this zone must disappear. This happens at time t3.

After time t3, zone C starts to diminish and totally disappear at time t5 after which time pure compression commences. Pure compression continues until t7 where the slope of the curve now exhibits the tendency to become horizontal. After a very long time, the slope will, indeed, be horizontal. The time midway between t3 and t5,

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