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Settling and Flotation

Learn about differences between settling and sedimentation in water treatment processes, types of settling tanks, design considerations, settling velocity calculations, and particle removal principles.

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Settling and Flotation

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  1. Settling and Flotation CE 547 Al-Malack

  2. Settling Al-Malack

  3. A unit operation in which solids are drawn towards a source of attraction. We are concerned with gravitational settling. What are the differences between Settling and Sedimentation? Sedimentation Is a condition whereby the solids are already at the bottom Settling Particles are falling down the water column in response to gravity Anyway, the two terms are used interchangeably Settling or sedimentation tanks are used to carry out settling of solids. There are two types of sedimentation tanks that are used in water and wastewater treatment plants: • Rectangular • Circular Al-Malack

  4. Rectangular Tanks or Basins • Their length vary from 2 to 4 times their width • Their length may vary from 10 to 20 times their depth • Their depth vary from 2 to 6 meters • Solids which settle are removed by a sludge scraper continuously • Effluent flows out of the basin through a suitably deigned effluent weir and launder Al-Malack

  5. Circular Tanks or Basins • Are easily upset by wind cross currents because they are conductive to circular streamlining • For the above reason, circular basins are typically designed for diameters not to exceed 30 meters in diameter • Influent is introduced at the center f the tank • Water flows from the center to the rim of the clarifier Al-Malack

  6. In settling basins, there are four functioning zones: • Inlet zone • Settling zone • Sludge zone • Outlet zone Al-Malack

  7. Flow-through Velocity and Overflow Rate vh = horizontal velocity of water (flow-through velocity) v0, vp = downward velocity Al-Malack

  8. For light suspension (flocculent), Vh 9.0 m/hr For heavier suspension (discrete-particle) Vh 36 m/hr A = vertical cross-sectional area Q = flow rate Z0 = depth W = width L = length t0 = detention time or retention time t0 (discrete-particle) = 1 - 4 hours t0 (flocculent) = 4 – 6 hours Al-Malack

  9. t0 can also be calculated using V = tank volume As = tank surface area Al-Malack

  10. For circular tanks Al-Malack

  11. For a particle, having a settling velocity v0, to be removed, the overflow rate of the tank q0 must be set equal to this velocity. • In the outlet zone, weirs are provided for the effluent to take off, therefore, weirs should be loaded with the proper amount of overflow (weir rate) • Weir overflow rates = 6 – 8 m3/hr per meter length of the weir for flocculent suspension and to 14 m3/hr.m for discrete particles. To calculate weir length, use: • Weirs are constructed along the periphery of the tank. If the periphery of the tank is not sufficient to meet the requirement, the inboard weirs may be used (Fig 5.7 b) Al-Malack

  12. Al-Malack

  13. Settling Types Type 1: Removal of discrete particles Type 2: Removal of flocculent particles Type 3: Removal of particles that settle in contiguous zone Type 4: Type 3 where compression or compaction of particles occur Al-Malack

  14. Type 1: Discrete Settling In dilute suspension, particles act independently (discrete) FG = body force FD = drag force FB = buoyant force Al-Malack

  15. FG – FB – FD = ma (1) m = particle mass a = acceleration p, w = mass density of particle and water Vp = volume of particle g = acceleration due to gravity v = settling velocity CD = drag coefficient Ap = projected area of the particle normal to direction of motion Since particle settles at its settling velocity, its acceleration = zero Al-Malack

  16. Substitute in equation (1) (FG – FB – FD = ma) d = particle diameter, Ap = (d2)/4 (spherical) CD varies with flow regimes Al-Malack

  17. Intermediate values indicate transition flow For laminar flow (CD = 24/Re) For non-spherical particles  = volume shape factor Al-Malack

  18. Example 1 Al-Malack

  19. Determine the settling velocity of a spherical particle having a diameter of 0.6 mm and specific gravity of 2.65. Assume type 1 settling and water temperature of 22 C. Al-Malack

  20. Solution g = 9.81 m/s2; w = 997 kg/m2; p = 2.65(1000) = 2650 kg/m2; dp = 0.6  10-3 m;  = 9.2  10-4 N-s/m2 Al-Malack

  21. Al-Malack

  22. Example 2 Al-Malack

  23. Determine the settling velocity of a worn sand particle having a measured sieve diameter of 0.60 mm and specific gravity of 2.65. Assume a settling of type 1 and water temperature is 22 C. Al-Malack

  24. Solution g = 9.81 m/s2; w = 997 kg/m2; p = 2.65(1000) = 2650 kg/m2; dp = 0.6  10-3 m;  = 9.2  10-4 N-s/m2; d = 1.240.333dp;  (for worn sand) = 0.86 d = 1.240.333dp = 1.24 (0.86)0.333 (0.60  10-3) = 0.71  10-3 m Al-Malack

  25. Al-Malack

  26. Settling Column Analysis • At time = zero, particle of d0 at water surface • At time = t0 (detention time), particle at sampling port and will be removed • The settling velocity, (v0 = Z0 / t0) • Particles will be removed if their velocity  v0 • x0 fractions of all particles with velocity <v0 • (1-x0) fraction of particles with velocity  v0 (which will be certainly removed) Al-Malack

  27. If R is the total removal • If original concentration in the column = C0 and after time of settling (t), the remaining concentration = C • The fraction of particles remaining in the water column close to the port Al-Malack

  28. x can be plotted against vp (Fig 5.8c) Study Example 5.5 (page 257) Al-Malack

  29. Example 3 Al-Malack

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  33. Type 2: Flocculent Settling • particles have affinity towards each other and form flocs or aggregates • larger flocs settle faster than smaller ones • particles start small and become larger while settling • therefore, velocity of particle changes as the size changes • because velocity changes with depth, multiple sampling ports are provided Al-Malack

  34. the fractional removal the removal efficiency is calculated using the same method as in discrete settling Study Example (5.8) Al-Malack

  35. Example 4 Al-Malack

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  39. Type 3: Zone Settling There are four zones: • A: cleared of solids (clarification zone) • B: uniform settling zone (solids concentration is constant, C0) • C: solids concentration increases (thickens) from B–C interface to C-D interface (thickening zone) • D: solids are compressed (compression zone, Type 4 settling). In this zone, solids are thickened by compression, compaction and consolidation processes. It has the highest solid concentration The time midway between t3 and t5 (i.e. t4) is called the critical time. Al-Malack

  40. Secondary Clarification and Thickening In secondary clarifiers: • clarification • thickening Al-Malack

  41. Solid Flux Method This is the method used in sizing the thickener area. The design of the thickener area considers zone C in Figure 5.11. Solid Flux = Solids Concentration  Settling Velocity In thickeners, solid flux is due to: • gravitational settling • conveyance effect of the withdrawal of sludge in the underflow of the tank Al-Malack

  42. Therefore Gt = total flux Vc = velocity at the section of the settling zone Xc = solid concentration at the section Vu = underflow velocity = Qu / At Qu = underflow rate of flow At = thickener area Al-Malack

  43. Generally • solids concentration is variable, so Gt is variable • Gt used in the design is called the limiting flux Gtl • Gt is equal to the rate of withdrawal of sludge in the underflow Therefore min Vu can be obtained from a graph of [Xc] vs. Vu Al-Malack

  44. Then Q0 + QR = influent to the tank QR = recirculation flow Q0 = inflow to the treatment plant After getting At, compare it with the clarification area, Ac, and the larger is chosen for design. If a thickener to be designed, At is taken for design. Study Example 5.12 Al-Malack

  45. Example 5 Al-Malack

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  50. Dissolved Air Flotation (DAF) Al-Malack

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