Settling and Floatation – Part 2

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					Settling and Floatation – Part 2
• Particles settling in a water column may
  have affinity toward each other and
  coalesce to form flocs or aggregates.
• These larger flocs will now have more
  weight and settle faster overtaking the
  smaller ones, thereby, coalescing and
  growing still further into much larger
• The small particle that starts at the
  surface will end up as a large particle
  when it hits the bottom.
• The velocity of the growing flocc will
  therefore not be terminal (constant or
  one, but changes as the size changes.
• Because the particles form into flocs, this
  type of settling is called flocculent
  settling or type 2 settling.
Flocculation = Particle Growth with
• Because the velocity is terminal in
  the case of type I settling, only one
  sampling port was provided in
  performing the settling test.

• In an attempt to capture the
  changing velocity in type 2 settling,
  oftentimes multiple sampling ports
  are provided.

• The ports closer to the top of the
  column will capture the slowly
  moving particles, especially at the
  end of the settling test.
       Total removal efficiency
Total removal efficiency is determined using graphical methods as

Graphical method (1):

         n Dhn        Rn + Rn+1
    R=Σ      ------- -----------------
         1      H           2
Where H is the total depth of the settling column.

Or by graphical method (2)

                  ha                       hb
     R = Rc + -------------- (Rd - Rc) + -------------- (Re - Rd) + +
                 t2 Vpc                    t2 Vpc
Example ( )

 For the flocculation test results drawn in
    the Figure bellow, estimate the total
    removal efficiency at 30, 40 and 50
    min? Compare the results?
               ZONE SETTLING
• In systems that contain high concentrations of suspended
  solids, hindered (compression) settling occur in addition to
  discrete and flocculent settling.
• Because of high concentration of particles, the liquid tend to
  move up through the spaces between particles (interstices). As
  a result, particles tend to settle as a zone maintaining the
  relative position with respect to each other (see Figure 15). As
  settling continues a compressed layer of particles begin to
  form on the bottom of the tank (or cylinder) in what so called
  the compression settling zone.
• In the case of highly concentrated suspensions settling tests
  are required to determine the settling characteristics of the
• A column test, similar to that of flocculent settling test, is used
  to determine the size and removal efficiency of the
  sedimentation tank.
Type III Settling – Zone Settling

Type IV Settling – Compression
test and estimation procedure
1- A column of height ho is filled with the
  highly concentrated suspension with initial
  solids concentration of Co.

2- The position of the interface is monitored
  with time (hi, ti, ci).

3- A curve of hi versus ti is plotted (see Figure
  14). The slope of the curve, hi/ti,
represent the settling rate.
4- Select a design overflowrate, Qovr, then the area of the sedimentation
   tank, A, can be calculated;
     SETTLING RATE = Qovr =
Where also know that the settling rate equal settling velocity,
            Q * tn
            Co - Cn
     R = ------------
The height needed for settling, to reach the design
  underflow concentration of Cn, is Hn and can be
  estimated using the mass balance relationships as

     Ho * Co = Hn * Cn

           Ho * Co
     Hn = ------------
• 5 - Using Figure hi vs ti determine the point where
  there is a shift from hindered to compression
  settling by plotting the tangents and the bisecting
  angle. From this point we can determine the critical
  height, Hn, and the critical settling time, tn.

• 6 - Construct a tangent at the critical point. The
  intersection point of a horizontal line at height of Hn
  with this tangent will indicate the time tn. Once the
  time needed to reach the design underflow
  concentration tn is known, the area of the
  sedimentation tank can be estimated using the
•               Q * tn
•       A = --------------
•                 Ho
Solid Flux Concept For Hindered Settling
    ‫مبدء تدفق المواد الجامدة الكثيفة أو عالية التركيز‬

• The solids flux is the rate of solids
  thickening per unit area in plan view-in other
  words, the lb/hour-ft2 (Q * C)/A.
• As the solids settle in clarifiers and
  thickeners, they must be thickened from the
  initial concentration, Co, to the underflow
  concentration Cu, at the bottom of the tank
  (see Figure (.
• At any level in the settling tank, the movement of solids by
  settling is concentration times velocity:

Gs = Ct * vt
   = (Mass /Volume( * )Volume/Area-Time)
   = (Mass/Time-Area)

         Gs = solids flux by gravity;
         Ct = solids concentration;
         vt = hindered settling velocity.
First Step, hi versus ti
Vi = hi/ti
Second Step, vi = hi/ti Draw vs ci
          Gi = ci * vi Draw vs ci

      Gi = Vi * Ci
                  Bulk Flux
The movement of the solids due to bulk flow is
  given by

Gb = Cb * Vb

Gb = bulk flux;           Cs,Vs

Vb = bulk velocity.
                          Cb, Vb
Cb= bulk solids concentration
                              Qu, Cu
                           Total Flux
The total solids flux for gravity settling and bulk movement is

  Gt = Gs + Gb = Ct * Vt = Cs * Vs + Cb * Vb
        Gt = total flux.

The bulk velocity is given by
      Vb = Qu /A

        Qu = flow rate of the underflow;
        A = plan area of the tank.
The mass rate of solids settling-that is, the weight of the solids settling per unit

Mt = Qo Co = Qu Cu.

Mt = rate of solids settling;
Qo = influent flow rate to the tank;
Co = influent solids concentration.

The limiting cross-sectional area, A, required is given by
A = (Mt =Q C) / )GL= (Q C)/A)
      Qo Co
GL = limiting max flux = Gt.
Rearranging gives

Qu = Mt / Cu.

and combining this with Vb = Qu /A and

A = Mt / GL
  = Qo Co / GL

Vb = Qu /A = Mt / (Cu * A) = GL /Cu.
                                Step (3)

Step (1)



                           Step (4)


           Step (2)
Repeat 1-4 steps for various Cu
And see what GLand Gs and Gb distribution
you get and decide on the best option
     Example ( )
The following results were obtained from a hindered-zone
    settling test in basin with an area of 17500 ft2 and with
    average feed concentration of 3000 mg/l:

Settling Velocity, fps   6     5     4      3      2      1      0.75

Concentration, mg/l      550   950   1450   1850   2500   3500   5550

Draw the curve for the total solids flux knowing that the
    concentration of suspended solids in the underflow was
    (a) 11000 mg/l, (b) 14500 mg/l, and (c) 19000 mg/l.
Find from the graph the max allowable concentration and
    estimate the gravitational and the underflow solids flux at
    that point? If you need any additional information, state
    your assumptions.
• Flotation may be used in lieu of the normal
  clarification by solids-downward-flow
  sedimentation basins as well as thickening the
  sludge in lieu of the normal sludge gravity
  thickening. The mathematical treatments for
  both flotation clarification and flotation thickening
  are the same. As mentioned in the beginning of
  this chapter, water containing solids is clarified
  and sludge are thickened because of the solids
  adhering to the rising bubbles of air. The
  breaking of the bubbles as they emerge at the
  surface leaves the sludge in a thickened
            FLOTATION (2)

• Next Figure shows the flow sheet of a flotation
  plant. The recycled effluent is pressurized with
  air inside the air saturation tank. The
  pressurized effluent is then released into the
  flotation tank- where minute bubbles are formed.
  The solids in the sludge feed then stick to the
  rising bubbles, thereby concentrating the sludge
  upon the bubbles reaching the surface and
  breaking. The concentrated sludge is then
  skimmed off as a thickened sludge. The effluent
  from the flotation plant are normally recycled
Dissolved Air Floatation
Dissolved Air Floatation (1)
Dissolved Air Floatation (2)
Dissolved Air Floatation (3)
Dispersed Air Floatation will be Mostly
 Covered Under Mixing and Aeration
Hydraulics of Sedimentation Tanks
•   Pipes carrying water in and out
•   Channels (inlet and outlet zone)
•   Weirs
•   Valves ++

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