CE 547 Settling and Flotation 332021 AlMalack Settling

  • Slides: 63
Download presentation
CE 547 Settling and Flotation 3/3/2021 Al-Malack

CE 547 Settling and Flotation 3/3/2021 Al-Malack

Settling 3/3/2021 Al-Malack

Settling 3/3/2021 Al-Malack

A unit operation in which solids are drawn towards a source of attraction. We

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: n Rectangular n Circular 3/3/2021 Al-Malack

Rectangular Tanks or Basins Their length vary from 2 to 4 times their width

Rectangular Tanks or Basins Their length vary from 2 to 4 times their width n Their length may vary from 10 to 20 times their depth n Their depth vary from 2 to 6 meters n Solids which settle are removed by a sludge scraper continuously n Effluent flows out of the basin through a suitably deigned effluent weir and launder n 3/3/2021 Al-Malack

Circular Tanks or Basins Are easily upset by wind cross currents because they are

Circular Tanks or Basins Are easily upset by wind cross currents because they are conductive to circular streamlining n For the above reason, circular basins are typically designed for diameters not to exceed 30 meters in diameter n Influent is introduced at the center f the tank n Water flows from the center to the rim of the clarifier n 3/3/2021 Al-Malack

In settling basins, there are four functioning zones: Inlet zone n Settling zone n

In settling basins, there are four functioning zones: Inlet zone n Settling zone n Sludge zone n Outlet zone n 3/3/2021 Al-Malack

Flow-through Velocity and Overflow Rate vh = horizontal velocity of water (flow-through velocity) v

Flow-through Velocity and Overflow Rate vh = horizontal velocity of water (flow-through velocity) v 0, vp = downward velocity 3/3/2021 Al-Malack

For light suspension (flocculent), Vh 9. 0 m/hr For heavier suspension (discrete-particle) Vh 36

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 Z 0 = depth W = width L = length t 0 = detention time or retention time t 0 (discrete-particle) = 1 - 4 hours t 0 (flocculent) = 4 – 6 hours 3/3/2021 Al-Malack

t 0 can also be calculated using V = tank volume As = tank

t 0 can also be calculated using V = tank volume As = tank surface area 3/3/2021 Al-Malack

For circular tanks 3/3/2021 Al-Malack

For circular tanks 3/3/2021 Al-Malack

n n For a particle, having a settling velocity v 0, to be removed,

n n For a particle, having a settling velocity v 0, to be removed, the overflow rate of the tank q 0 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 m 3/hr per meter length of the weir for flocculent suspension and to 14 m 3/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) 3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

Settling Types Type 1: Removal of discrete particles Type 2: Removal of flocculent particles

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 3/3/2021 Al-Malack

Type 1: Discrete Settling In dilute suspension, particles act independently (discrete) FG = body

Type 1: Discrete Settling In dilute suspension, particles act independently (discrete) FG = body force FD = drag force FB = buoyant force 3/3/2021 Al-Malack

FG – FB – FD = ma m = particle mass a = acceleration

FG – FB – FD = ma m = particle mass a = acceleration (1) 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 3/3/2021 Al-Malack

Substitute in equation (1) (FG – FB – FD = ma) d = particle

Substitute in equation (1) (FG – FB – FD = ma) d = particle diameter, Ap = ( d 2)/4 (spherical) CD varies with flow regimes 3/3/2021 Al-Malack

Intermediate values indicate transition flow For laminar flow (CD = 24/Re) For non-spherical particles

Intermediate values indicate transition flow For laminar flow (CD = 24/Re) For non-spherical particles = volume shape factor 3/3/2021 Al-Malack

Example 1 3/3/2021 Al-Malack

Example 1 3/3/2021 Al-Malack

Determine the settling velocity of a spherical particle having a diameter of 0. 6

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. 3/3/2021 Al-Malack

Solution g = 9. 81 m/s 2; w = 997 kg/m 2; p =

Solution g = 9. 81 m/s 2; w = 997 kg/m 2; p = 2. 65(1000) = 2650 kg/m 2; dp = 0. 6 10 -3 m; = 9. 2 10 -4 Ns/m 2 3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

Example 2 3/3/2021 Al-Malack

Example 2 3/3/2021 Al-Malack

Determine the settling velocity of a worn sand particle having a measured sieve diameter

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. 3/3/2021 Al-Malack

Solution g = 9. 81 m/s 2; w = 997 kg/m 2; p =

Solution g = 9. 81 m/s 2; w = 997 kg/m 2; p = 2. 65(1000) = 2650 kg/m 2; dp = 0. 6 10 -3 m; = 9. 2 10 -4 N-s/m 2; d = 1. 24 0. 333 dp; (for worn sand) = 0. 86 d = 1. 24 0. 333 dp = 1. 24 (0. 86)0. 333 (0. 60 10 -3) = 0. 71 10 -3 m 3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

Settling Column Analysis n n n At time = zero, particle of d 0

Settling Column Analysis n n n At time = zero, particle of d 0 at water surface At time = t 0 (detention time), particle at sampling port and will be removed The settling velocity, (v 0 = Z 0 / t 0) Particles will be removed if their velocity v 0 x 0 fractions of all particles with velocity <v 0 (1 -x 0) fraction of particles with velocity v 0 (which will be certainly removed) 3/3/2021 Al-Malack

If R is the total removal n n 3/3/2021 If original concentration in the

If R is the total removal n n 3/3/2021 If original concentration in the column = C 0 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

x can be plotted against vp (Fig 5. 8 c) Study Example 5. 5

x can be plotted against vp (Fig 5. 8 c) Study Example 5. 5 (page 257) 3/3/2021 Al-Malack

Example 3 3/3/2021 Al-Malack

Example 3 3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

Type 2: Flocculent Settling particles have affinity towards each other and form flocs or

Type 2: Flocculent Settling particles have affinity towards each other and form flocs or aggregates n larger flocs settle faster than smaller ones n particles start small and become larger while settling n therefore, velocity of particle changes as the size changes n because velocity changes with depth, multiple sampling ports are provided n 3/3/2021 Al-Malack

the fractional removal the removal efficiency is calculated using the same method as in

the fractional removal the removal efficiency is calculated using the same method as in discrete settling Study Example (5. 8) 3/3/2021 Al-Malack

Example 4 3/3/2021 Al-Malack

Example 4 3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

Type 3: Zone Settling There are four zones: n n A: cleared of solids

Type 3: Zone Settling There are four zones: n n A: cleared of solids (clarification zone) B: uniform settling zone (solids concentration is constant, C 0) C: solids concentration increases (thickens) from B–C interface to CD 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 t 3 and t 5 (i. e. t 4) is called the critical time. 3/3/2021 Al-Malack

Secondary Clarification and Thickening In secondary clarifiers: clarification n thickening n 3/3/2021 Al-Malack

Secondary Clarification and Thickening In secondary clarifiers: clarification n thickening n 3/3/2021 Al-Malack

Solid Flux Method This is the method used in sizing the thickener area. The

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: n n 3/3/2021 gravitational settling conveyance effect of the withdrawal of sludge in the underflow of the tank Al-Malack

Therefore Gt = total flux Vc = velocity at the section of the settling

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 3/3/2021 Al-Malack

Generally n n n solids concentration is variable, so Gt is variable Gt used

Generally n n n 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 3/3/2021 Al-Malack

Then Q 0 + QR = influent to the tank QR = recirculation flow

Then Q 0 + QR = influent to the tank QR = recirculation flow Q 0 = 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 3/3/2021 Al-Malack

Example 5 3/3/2021 Al-Malack

Example 5 3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

Dissolved Air Flotation (DAF) 3/3/2021 Al-Malack

Dissolved Air Flotation (DAF) 3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

The dissolved air concentration of the wastewater in the air saturation tank (Caw, t)

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 3/3/2021 Al-Malack

The amount of air introduced into the flotation tank (Ai) is: R = recirculation

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/m 2 As pressurized flow from air saturation tank is released into flotation unit, the pressure reduces to atmospheric, Pa. 3/3/2021 Al-Malack

If, Cos, s, sp = saturation concentration of dissolved oxygen at standard pressure and

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 (A 0) in the recycled portion of the flow is: f = 1 in the above equation Then air utilized for flotation is: 3/3/2021 Al-Malack

Since solids in influent = Qi[Xi] Then, air to solid ratio (A/S) is: if

Since solids in influent = Qi[Xi] Then, air to solid ratio (A/S) is: if there is no recycle, then 3/3/2021 Al-Malack

Laboratory Determination of Design Parameters To design flotation unit, the following parameters are needed:

Laboratory Determination of Design Parameters To design flotation unit, the following parameters are needed: n n n 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 3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

Example 6 3/3/2021 Al-Malack

Example 6 3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

Example 7 3/3/2021 Al-Malack

Example 7 3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack

3/3/2021 Al-Malack