Test: Sedimentation Tanks - Civil Engineering (CE) MCQ

Continuous Flow Type Sedimentation Tank

• In this water is not allowed to rest.
• Flow always takes place but with a very small velocity. During this flow, suspended particles settle at the bottom of the tank.
• The flow may be either in the horizontal direction or vertical direction.

Detention time:
It is defined as the average theoretical time required for water to flow along the length of the sedimentation tank.
In continuous-flow sedimentation tanks, the actual detention period used in practice should be more than twice the theoretical detention period. 
It is given by the ratio of the volume of the tan to the rate of flow:
  [Valid for both rectangular and circular tanks]
It can also be calculated as:
 [Valid for rectangular tank only]
Here,
V - Volume of the tank (m³)
For a rectangular tank, 
V = L × B × H
For a circular tank, 

Where
L - Length of the tank (m), B - Width of the tank (m), H - Height of the tank (m)
d - Diameter of the circular tank (m), Q - Rate of flow (m³/sec) = A × v
A - Area of c\s (m²) = B × H, t_d - Detention time (sec)
v - Average velocity of the flow (m/sec)

Important Points
The following table shows the detention period for different types of sedimentation: 

Settling basins are ponds constructed for the purpose of removing entrained solids by sedimentation. Clarifiers are tanks built with mechanical means for continuous removal of solids being deposited by sedimentation.

Clarifiers are tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. Settling basins are ponds constructed for the purpose of removing entrained solids by sedimentation.

The efficiency of the Sedimentation Tank:

• The efficiency of the sedimentation tank indicates the overall percentage removal of suspended matter at a given overflow rate or surface loading rate.
• The efficiency of the sedimentation tank increases if the overflow rate reduces (more time available to particles for settle).

From the equation, it is clear that if the surface area (B × L) of the tank increases, the overflow rate reduces, and efficiency increases for a given discharge.
Efficiency depends on the following parameter during sedimentation:

• The velocity of flow: Efficiency increases if the velocity of flow reduces
• Viscosity: Efficiency increases if viscosity reduces (Viscosity changes but we can not do)
• Size of particle: Efficiency increases if the size of particle increases.

Sewage treatment plants consist of the following procedures:

• Screening
  • These are placed before grit chambers so as to trap and remove floating matter, such as pieces of cloth, paper, wood cork, hair, fiber, kitchen refuse, fecal solids, etc in the sewage.
  • Those floating materials if not removed will choke the pipes or adversely affect the working of the pumps.
• Grit removal basins
  • Grit chambers or detritus tanks are the sedimentation basins placed in front of the wastewater treatment plant to remove inorganic particles such as sand, gravel, grits, shells, bones, and other non-putrescible material.
  • They are usually placed after the fine screens to remove particles of size up to 0.2 mm or so.
• Primary sedimentation
  • The primary sedimentation tanks are thus designed to remove a part of the organic matter from the sewage effluent coming from the grit chambers.
  • These are designed for effecting the settlement of particles by reducing the settling velocity or by detaining the sewage in them.
  • Lower is the settling velocity, better is the efficiency of the tank, So settling velocity governs the design of the tank
• Aeration
  • From the primary sedimentation tank, the sewage flows to the aeration tank and is mixed with activated sludge.
  • These are rectangular tanks in which air is continuously introduced into these tanks.

A layer of accumulated solids called sludge forms at the bottom of the tank and is periodically removed. Prior to sedimentation, coagulants are added to the water in order to facilitate the settling process, which is followed by filtration and other treatment steps.

Based on the method of operation, there are two types of sedimentation tanks. The first one is filled and draw type tank and the second one is the continuous flow type tank.

Based on the shape of the sedimentation tank, there are two types of sedimentation tank. They are circular sedimentation tank and rectangular sedimentation tank.

In case of fill and draw type sedimentation tank, water from the inlet is stored for some time. The time may be 24 hours. The suspended particles settle down. After 24 hours, the water is discharged through the outlet.

• The efficiency of the sedimentation tank indicates the overall percentage removal of suspended matter at a given overflow rate or surface loading rate.
• The efficiency of the sedimentation tank increases if the overflow rate reduces (more time available to particles for settle).

• From the equation, it is clear that if the surface area (B × L) of the tank increases, the overflow rate reduces, and efficiency increases for a given discharge.

Important Points
Efficiency depends on the following parameter during sedimentation:

• The velocity of flow: Efficiency increases if the velocity of flow reduces
• Viscosity: Efficiency increases if viscosity reduces (Viscosity changes but we can not do)
• Size of particle: Efficiency increases if the size of particle increases.