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المحاضرة 4-10

الكلية كلية الهندسة     القسم  الهندسة البيئية     المرحلة 4
أستاذ المادة محمد عبد مسلم عبد الله الطفيلي       04/07/2018 08:55:11




PROBLEM:
Laboratory data were obtained on the settling of a textile mill waste as in Table (1). Design a settling tank to produce a maximum effluent suspended solids of 100 mg/l and with flow rate 5000 m3/d.
Table (1)
Initial solids, 500 mg/l
Time (min) Depth (cm)
50 100 150 200
Removal %
5 6 3 2 5
10 14 11 17 10
20 22 27 23 25
40 35 32 39 32
60 47 48 43 44
80 59 58 51 55
100 66 63 69 66
120 78 81 75 85


Hindered ( Zone ) Sedimentation (Type 3 settling)

In systems that contain a high concentration of suspended solids, both hindered or zone settling (type 3) and compression settling (type 4), in addition, discrete (free) settling (type 1) and flocculent settling (type 2) occur. Hindered (zone) settling occurs in sludge thickeners and at the bottom of a secondary clarifier in biological treatment processes.
The settling phenomenon that occurs when a concentrated suspension (such as activated sludge), initially of uniform concentration throughout, is placed in a graduated cylinder, is illustrated on Fig. S-3.




























Time
Figure S-3

Definition sketch of hindered (zone) settling with corresponding interface settling curve

Because of the high concentration of particles, the liquid tends to move up through the interstices of the contacting particles. As a result, the contacting particles tend to settle as a zone or blanket, maintaining the same relative position with respect to each other. The phenomenon is known as hindered settling. As the particle settle, a relatively clear layer of water is produced above the particles in the settling region. The scattered, relatively light particles remaining usually settle as discrete or flocculent particles. In most cases, an identifiable interface develops between the upper region and the hindered settling region on Fig. S-3. The rate of settling in the hindered settling region is a function of the concentration of solids and their characteristic .

Area Requirement Based on Single-Batch Test Results
A typical curve of interface height versus time for activated sludge is shown in Fig. S-4. From A to B, there is a hindered settling of the particles and this is called liquid interface. From B to C there is a deceleration marking the transition from hindered settling into the compression zone. From C to D there is a compression zone where settling depends on compression of the sludge blanket.
For purposes of design, the final overflow rate selected should be based on a consideration of the following factors: ( 1 ) the area needed for clarification (discrete settling of particles) (2) the area needed for thickening (settling of the interface between the discrete and hindered settling zones) and (3) the rate of sludge withdrawal.
The settling rate of the interface is usually the controlling factor.
Column settling tests, as previously described, can be used to determine the area needed for the free settling region directly. A column of height Ho is filled with a suspension of solids of uniform concentration Co. The position of the interface as time elapses and the suspension settles is given on Fig. S-4. The rate at which the interface subsides is then equal to the slope of the curve at that point in time. According to the procedure, the area required for thickening is given by :

A=(Q t_u)/H_o
Where:
A = Area required for sludge thickening, m2
Q = Flow rate into settling tank, m3/s
tu = time to reach a desired underflow (solids) concentration, s
Ho = depth of the settling column (initial interface height), m

From Fig. S-4, the critical concentration (C2) is determined by extending the tangent from the hindered and compression settling lines to their point of intersection and bisecting the angle formed. The bisector intersects the subsidence curve at C2 which is the critical concentration. The critical concentration controls the sludge-handling capacity of the tank at a height of H2.
A tangent is drawn to the subsidence curve at C2 and the intersection of this tangent with depth Hu, required for the desired underflow (or solids concentration Cu), will yield the required retention time tu. Since the total weight of solids in the system must remain constant, i.e. CoHoA = CuHuA, the height Hu of the particle liquid interface at the underflow desired concentration Cu is:

H_u=(C_o H_o)/C_u
The time tu can be determined as:
draw a horizontal line through Hu and draw a tangent to the subsidence settling curve at C2.
Draw a vertical line from the point of intersection of the two lines drawn above to the time axis to find the value of tu.
With this value of tu, the area needed for thickening can be calculated using equation:


المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .