Sedimentation Process in Water Treatment

Objectives of Sedimentation

• Sedimentation is a key process in water treatment aimed at removing suspended solids, reducing chemical demand, and improving the performance of subsequent processes.

Theory of Sedimentation

• Discrete Settling: Occurs when particles settle independently without interacting with each other.
  • Settling velocity is influenced by particle size, density, fluid viscosity and density.
  • Governed by Stokes’ Law, which emphasizes the relationship between settling velocity and particle characteristics.
• Hindered Settling: Occurs when particles interact with each other, leading to a reduced settling velocity.
  • Influenced by concentration of solids in suspension, floc size and density.
• Settling of Flocculant Suspension: Encompasses the settling of larger flocs.
  • Settling velocity is faster than individual particles.
  • Influenced by floc structure and strength.

Ideal Settling Basin

• Design Characteristics:
  • Uniform flow distribution to ensure even settling.
  • Sufficient retention time for complete settling.
  • Absence of re-suspension of settled solids.

Efficiency of Settling Basin

• Efficiency is measured by the removal of suspended solids.
• Expressed as the percentage reduction in solids concentration between the inlet and outlet.

Design, Construction, and Operation of Sedimentation Tanks

• Design Considerations:
  • Surface area for settling.
  • Tank depth, typically 3-5 meters.
  • Retention time, typically 1-3 hours.
  • Flow rate to minimize turbulence.
• Construction:
  • Concrete or steel are commonly used for tank construction.
  • Rectangular or circular shape, depending on site requirements.
• Operation:
  • Inlets and outlets should be designed for even inflow and smooth outflow to prevent short-circuiting.

Sludge Removal and Disposal

• Sludge, the settled solids, is periodically removed for tank efficiency.
• Sludge Removal Methods:
  • Mechanical scrapers or pumps.
  • Gravity drainage.
• Sludge Disposal Options:
  • Landfill.
  • Dewatering for agricultural use.
  • Incineration, depending on regulations.

Tube and Plate Settlers

• Tube Settlers: Compact, inclined tubes that increase settling area and reduce tank size.
  • Enhance settling efficiency.
• Plate Settlers: Inclined plates that increase the effective settling area.
  • Improve solids removal in a smaller space.
  • Suitable for retrofitting existing tanks.

Problems on Design of Sedimentation Tanks

• Calculating Surface Overflow Rate (SOR):
  • SOR = (Flow Rate) / (Surface Area)
• Sizing the Tank:
  • Dimensions are determined based on required volume and surface area for efficient settling.
• Determining Sludge Volume:
  • Sludge production is estimated using empirical equations based on raw water quality and treatment processes.

Conclusion

• Sedimentation plays a vital role in water treatment, removing suspended solids and enhancing downstream process efficiency.
• Understanding the theory of sedimentation, design principles, and operating practices is crucial for optimizing sedimentation tank performance.
• Implementing effective sludge management and advanced technologies like tube and plate settlers further optimize treatment outcomes.

This quiz covers the fundamentals of sedimentation in water treatment, focusing on the objectives, theory, and basin design characteristics. It includes discrete and hindered settling concepts, as well as the behavior of flocculant suspensions. Test your understanding of how sedimentation improves water quality and treatment efficiency.