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# المحاضرة 4-8

الكلية كلية الهندسة     القسم  الهندسة البيئية     المرحلة 4
أستاذ المادة محمد عبد مسلم عبد الله الطفيلي       04/07/2018 08:52:53

To perform sedimentation tests, a flocculating suspension similar in character to the one to be used in the settling tank is placed in the column in such a way as to ensure a uniform distribution of particles of all sizes from top to bottom. Samples are withdrawn from all the ports at various time intervals and the amount of suspended solids contained in the samples is determined. The percentage removal of suspended solids by sedimentation is then calculated. For example, if a suspension containing 500 mg/L of suspended solids was introduced into the column and the sample showed a concentration of 100 mg/L, then the percentage removed would be 80%. These percentages are plotted on a depth versus time graph. Points of equal percentage removals are joined and a plot as shown in Figure below is obtained. Overall removals of suspended solids at a certain detention time (t2) are given by:

where
Rt2 is the percentage removal at time t2,
R1, R2, . . . , R5 are percentage removals,
h5 = the total depth of water in column,
?h1, ?h2, ?h3, ?h4 are depth increments to successive percent removal curves at time t2.

Since the data obtained from the laboratory analysis represent ideal settling conditions. criteria for prototype design must account for the effects of turbulence, short circuiting, and inlet and outlet losses. The net effect of these factors is a decrease in the overflow rate and an increase in the detention time over that derived from the laboratory analysis. As a general rule, the overflow rate will be decreased by a factor of 1.25 to 1.75, and the detention period increased by a factor of 1.50 to 2.00.
The overflow rate SOR is the effective depth divided by the time required for a given percent to settle this distance. All particles having a settling velocity equal to or greater than SOR, will be 100 percent removed. Particles with a lesser settling velocity v will be removed in the proportion v/SOR.

EXAMPLE:
Using the settling curves of Figure (1), determine the overall removal of solids in a sedimentation basin (type 2 flocculant settling) with a depth equal to the test cylinder and at a detention time of 20 min. The total depth is 2.5 m.

SOLUTION:
From Figure (1), 40 percent of the particles will have a settling velocity of 0.1 m/min (2.5 m/25 min).
At t = 20 min, the volume of the test cylinder within ?h6 has 40% removal.
Determine percent removal of each volume of the tank. In the volume of the tank corresponding to ?h5 between 50 and 40 percent removal will occur. Similarly, in the tank volume corresponding to ?h4 between 60 and 50 percent will be removed. In like fashion, this is applied to other tank volumes.
Since 1/h = 1/2.5 = 0.4
?h1 = 0.23 m, ?h2 = 0.14m, ?h3 = 0.20 m, ?h4 = 0.32 m, ?h5 = 0.50 m
The overall removal at time 20 min Rt20 is:

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