Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev

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Computer Science Engineering (CSE) : Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev

The document Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev is a part of the Computer Science Engineering (CSE) Course Environmental Engineering - Notes, Videos, MCQs & PPTs.
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TYPES OF GRAVITATIONAL SETTLING PHENOMENON 

(i) Discrete particle settling: Applicable for very low concentration solids 

  • Particles settle as individual entities  
  • No interaction between particles  

(ii) Flocculation settling: Applicable for dilute suspension of particles that coalesce or flocculate  

  • By flocculation, particle size increases and terminal velocity increases.  
  • Settling can be increased by addition of some ballasting agent such as polymers.  

(iii) Hindered settling 

  •  For suspension of intermediate settling. 
  • In this case, particles are such close together that the inter-particle force due to one hinders the settling of other particle. 
  • The particles remain in fixed position with respect to each other and particles settles as a whole

(iv) Compression settling

  • Case in which particles are in such high concentration that a whole structure is formed.  
  • Compression takes place due to weight of whole mass which continuously increases. 
  • A clear water is formed above compression zone  

 

CLASSIFICATION OF SEDIMENTATION TANKS 

  • Grit chamber: For removal of sand, grits, etc.  
  • Plain sedimentation tank: For removal of settleable solids. 
  • Chemical precipitation tank: for removal of very fine suspended particles by adding coagulants, etc 
  • Septic tanks: For doing sedimentation and sludge digestion together in households 
  • Secondary settling tanks: After activated sludge or trickling filter treatment systems.  


SCOUR VELOCITY  

Maximum horizontal velocity though the tank which does not allows resuspension (scouring) of settled particles. It is given as [1]:  

 Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev (3.8.1)  
 Where, f is the Darcy–Weisbach friction factor (unit-less) and its value varies in the range 0.02- 0.03; k is cohesion constant that depends upon the type of material being scoured (unit-less). Its value varies in the range of 0.04- 0.06. For sticky interlocking matter k=0.6 whereas for ungrounded sand k=0.4.  

Important point in design of sedimentation tank  Assume t is the detention time for which a suspension is detained in the settling tank having height H, length L and width W. Also assume, VH is the horizontal velocity and ut is the terminal settling velocity of the target particle. Now, Cross-sectional area of tank (AC)=H×W Surface area of tank (A)=L×W If Q is the flow rate of wastewater into the tank, 

Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev  (3.8.2)  
 Since the target particle should not re-suspend during its flow along the length of the tank, therefore, detention time 

Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev (3.8.3) 
 Also, the target particle should settle down before it reaches the outlet, therefore, 

Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev (3.8.4)  
 Combining,  

Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev (3.8.5) 

This expression gives following important points: 

  • The terminal velocity should be ≥ surface loading of the tank. 
  • Surface area is more important than the height of the settling tank. 
  • Higher the surface area, higher will be the removal efficiency and more will be the removal of finer particles. 
  • All particles having settling velocity ut ≥ vo will be completed removed.  
  • For particles having ut < vo, only ut/vo only fraction will be removed. 


Problem 3.8.1: A municipal wastewater plant is to be designed to treat maximum flow rate of 60000 m3/d. Target particle for settling has the following characteristics: DP=200×10-6 m, k=0.05, f=0.025, ρP=1.25×103 kg/m3. For a rectangular classifier having ratio of length to width>6, overflow rate is at-least four times the settling velocity and horizontal velocity at-most one-third of the scour velocity.  (a) Find the dimensions of the rectangular tank  (b) Determine detention time 

Solution:  

Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev

Actual horizontal velocity=VH/3=0.02951 m/s.

Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev
Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev

Overflow rate=3×u= 21 .7 6 x 10 -3 m / s

If W is the width, L is the length and H is the height of the rectangular settling basin, 

Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev

Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev

Also given: Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev

Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev

W=2.305 m,  
 L=6×2.305=13.83 m  
 H=23.54/2.305=10.21 m
 Volume of tank, V=LWH=325.47 m3

Detention time, Setting and Sedimentation (Part - 2) Computer Science Engineering (CSE) Notes | EduRev

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