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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]:

(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,

(3.8.2)

Since the target particle should not re-suspend during its flow along the length of the tank, therefore, detention time

(3.8.3)

Also, the target particle should settle down before it reaches the outlet, therefore,

(3.8.4)

Combining,

(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 u
_{t}≥ v_{o}will be completed removed. - For particles having u
_{t}< v_{o}, only u_{t}/v_{o}only fraction will be removed.

**Problem 3.8.1: A municipal wastewater plant is to be designed to treat maximum flow rate of 60000 m ^{3}/d. Target particle for settling has the following characteristics: D_{P}=200×10^{-6} m, k=0.05, f=0.025, ρ_{P}=1.25×10^{3} kg/m^{3}. 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: **

**Actual horizontal velocity=V _{H}/3=0.02951 m/s.**

Overflow rate=3×u_{t }= 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,

Also given:

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 m^{3}

Detention time,

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