Settling and Flotation

Settling and Flotation
CE 547 Settling and Flotation Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Settling Thursday, November 15, 2018Thursday, November 15, 2018
Al-Malack

What are the differences between Settling and Sedimentation?
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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

In settling basins, there are four functioning zones:
Inlet zone Settling zone Sludge zone Outlet zone Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Flow-through Velocity and Overflow Rate
vh = horizontal velocity of water (flow-through velocity) v0, vp = downward velocity Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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) = hours t0 (flocculent) = 4 – 6 hours Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

t0 can also be calculated using
V = tank volume As = tank surface area Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

For circular tanks Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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) Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Type 1: Discrete Settling
In dilute suspension, particles act independently (discrete) FG = body force FD = drag force FB = buoyant force Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

p, w = mass density of particle and water Vp = volume of particle
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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Substitute in equation (1) (FG – FB – FD = ma)
d = particle diameter, Ap = (d2)/4 (spherical) CD varies with flow regimes Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Intermediate values indicate transition flow
For laminar flow (CD = 24/Re) For non-spherical particles  = volume shape factor Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Example 1 Thursday, November 15, 2018Thursday, November 15, 2018
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Determine the settling velocity of a spherical particle having a diameter of 0.6 mm and specific gravity of Assume type 1 settling and water temperature of 22 C. Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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Determine the settling velocity of a worn sand particle having a measured sieve diameter of 0.60 mm and specific gravity of Assume a settling of type 1 and water temperature is 22 C. Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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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) Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

x can be plotted against vp (Fig 5.8c)
Study Example 5.5 (page 257) Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

the fractional removal
the removal efficiency is calculated using the same method as in discrete settling Study Example (5.8) Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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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. Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Secondary Clarification and Thickening
In secondary clarifiers: clarification thickening Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Solid Flux = Solids Concentration  Settling Velocity
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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Vc = velocity at the section of the settling zone
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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

min Vu can be obtained from a graph of [Xc] vs. Vu
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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Q0 + QR = influent to the tank QR = recirculation flow
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 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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Dissolved Air Flotation (DAF)
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f = fraction of saturation (0.5 – 0.8)
The dissolved air concentration of the wastewater in the air saturation tank (Caw,t) is: f = fraction of saturation (0.5 – 0.8) Casw,t,sp = saturation concentration of the dissolved air in wastewater in saturation tank at standard pressure (Ps) corresponding to the temperature of the wastewater P = pressure in tank Cas,t,sp = saturation concentration of dissolved air in tap water at standard pressure corresponding to the temperature equal to the temperature of the wastewater  = ratio of the dissolved air saturation value in wastewater to that in tap water or distilled water Cos,t,sp = saturation concentration of dissolved oxygen Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

The amount of air introduced into the flotation tank (Ai) is:
R = recirculation ratio Qi = influent flow Since Then Ps = 760 mm Hg = 1 atm pressure = 101,330 N/m2 As pressurized flow from air saturation tank is released into flotation unit, the pressure reduces to atmospheric, Pa. Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

f = 1 in the above equation Then air utilized for flotation is:
If, Cos,s,sp = saturation concentration of dissolved oxygen at standard pressure and prevailing ambient temperature of the flotation tank. After pressure is release, the remaining air (A0) in the recycled portion of the flow is: f = 1 in the above equation Then air utilized for flotation is: Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Since solids in influent = Qi[Xi] Then, air to solid ratio (A/S) is:
if there is no recycle, then Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

Laboratory Determination of Design Parameters
To design flotation unit, the following parameters are needed: overflow area pressure in the air saturation tank recirculation ratio 1. Overflow velocity, which is used to determine overflow area, is determined from lab setup (Fig 5.15) 2. (A/S) can also be determined from lab setup by measuring the clarity of water (from bottom of flotation cylinder) at different (A/S) values 3. In lab setup, no recirculation is used 4. Pressure in the air saturation tank and recirculation ratio can be designed based on the desirable (A/S) ratio. Study Examples 5.13 and 5.14 Thursday, November 15, 2018Thursday, November 15, 2018 Al-Malack

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Settling and Flotation

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