settling and floatation - part 2

7/27/2019 Settling and Floatation - Part 2

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Settling and Floatation Part 2


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FLOCCULENT SETTLING (1)

Particles settling in a water column mayhave affinity toward each other andcoalesce to form flocs or aggregates.

These larger flocs will now have moreweight and settle faster overtaking the

smaller ones, thereby, coalescing andgrowing still further into much largeraggregates.

The small particle that starts at thesurface will end up as a large particle

when it hits the bottom. The velocity of the growing flocc will

therefore not be terminal (constant orone, but changes as the size changes.

Because the particles form into flocs, this

type of settling is calledflocculentsettlingor type 2 settling.


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Flocculation = Particle Growth with

Time


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FLOCCULENT SETTLING (2)

Because the velocity is terminal inthe case of type I settling, only onesampling port was provided inperforming the settling test.

In an attempt to capture thechanging velocity in type 2 settling,oftentimes multiple sampling portsare provided.

The ports closer to the top of thecolumn will capture the slowlymoving particles, especially at theend of the settling test.


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Total removal efficiency

Total removal efficiency is determined using graphical methods asfollows:

Graphical method (1):

n Dhn Rn + Rn+1R = ------- -----------------

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


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For the flocculation test results drawn inthe Figure bellow, estimate the total

removal efficiency at 30, 40 and 50

min? Compare the results?

Example ( )


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ZONE SETTLING

In systems that contain high concentrations of suspendedsolids, hindered (compression) settling occur in addition todiscrete and flocculent settling.

Because of high concentration of particles, the liquid tend tomove up through the spaces between particles (interstices). Asa result, particles tend to settle as a zone maintaining the

relative position with respect to each other (see Figure 15). Assettling continues a compressed layer of particles begin toform on the bottom of the tank (or cylinder) in what so calledthe compression settling zone.

In the case of highly concentrated suspensions settling testsare required to determine the settling characteristics of the

suspension. A column test, similar to that of flocculent settling test, is used

to determine the size and removal efficiency of thesedimentation tank.


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Type III Settling Zone Settling

Type IV Settling CompressionSettling


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test and estimation procedure

1- A column of height ho is filled with thehighly concentrated suspension with initialsolids concentration of Co.

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

3- A curve of hi versus ti is plotted (see Figure14). The slope of the curve, hi/ti,

represent the settling rate.


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4- Select a design overflowrate, Qovr, then the area of the sedimentationtank, A, can be calculated;

Q

SETTLING RATE = Qovr =

A

Where also know that the settling rate equal settling velocity,

Ho

SETTLING RATE =

tn

or

Q * tn

A =

Ho

Co - Cn

R = ------------

Co


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The height needed for settling, to reach the design

underflow concentration of Cn, is Hn and can beestimated using the mass balance relationships asfollows:

Ho * Co = Hn * Cn

or

Ho * Co

Hn = ------------Cn


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5 - Using Figure hi vs ti determine the point wherethere is a shift from hindered to compression

settling by plotting the tangents and the bisectingangle. From this point we can determine the criticalheight, 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 Hnwith this tangent will indicate the time tn. Once thetime needed to reach the design underflowconcentration tn is known, the area of thesedimentation tank can be estimated using the

equation: Q * tn

A = --------------

Ho


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Solid Flux Concept For Hindered Settling

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 andthickeners, they must be thickened from the

initial concentration, Co, to the underflow

concentration Cu, at the bottom of the tank

(see Figure (.


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Waste


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At any level in the settling tank the movement of solids by


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At any level in the settling tank, the movement of solids bysettling is concentration times velocity:

Gs = Ct * vt

= (Mass /Volume( * )Volume/Area-Time)

= (Mass/Time-Area)

whereGs = solids flux by gravity;

Ct = solids concentration;

vt = hindered settling velocity.


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First Step, hi versus ti


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Vi = hi/ti


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Second Step, vi = hi/ti Draw vs ci

Gi = ci * vi Draw vs ci

Gi = Vi * Ci

Gt


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Bulk Flux

The movement of the solids due to bulk flow isgiven by

Gb = Cb * Vb

where

Gb = bulk flux;

Vb = bulk velocity.Cb= bulk solids concentration

Qu, Cu

Cs,Vs

Cb, Vb

T t l Fl


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Total FluxThe total solids flux for gravity settling and bulk movement is

therefore

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

whereGt = total flux.

The bulk velocity is given by

Vb =Qu /A

Also

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


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g , g g p

time-is

Mt=Qo Co = Qu Cu.

where

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

where,

GL = limiting max flux = Gt.


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Rearranging gives

Qu = Mt / Cu.

and combining this with Vb =Qu /A and

A = Mt / GL

= Qo Co / GL

gives

Vb=

Qu/

A = Mt / (Cu * A) = GL /Cu.


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GL

Gs

Gb

Step (1)

Step (2)

Step (3)

Step (4)


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Repeat 1-4 steps for various Cu

And see what GLand Gs and Gb distribution

you get and decide on the best option


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Example ( )The following results were obtained from a hindered-zone

settling test in basin with an area of 17500 ft2 and withaverage 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 theconcentration 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 andestimate the gravitational and the underflow solids flux atthat point? If you need any additional information, stateyour assumptions.


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CL


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FLOTATION

Flotation may be used in lieu of the normalclarification by solids-downward-flowsedimentation basins as well as thickening thesludge in lieu of the normal sludge gravity

thickening. The mathematical treatments forboth flotation clarification and flotation thickeningare the same. As mentioned in the beginning ofthis chapter, water containing solids is clarifiedand sludge are thickened because of the solids

adhering to the rising bubbles of air. Thebreaking of the bubbles as they emerge at thesurface leaves the sludge in a thickenedcondition.


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FLOTATION (2)

Next Figure shows the flow sheet of a flotationplant. The recycled effluent is pressurized withair inside the air saturation tank. The

pressurized effluent is then released into theflotation tank- where minute bubbles are formed.The solids in the sludge feed then stick to therising bubbles, thereby concentrating the sludgeupon the bubbles reaching the surface and

breaking. The concentrated sludge is thenskimmed off as a thickened sludge. The effluentfrom the flotation plant are normally recycled


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Dissolved Air Floatation

Dissolved Air Floatation (1)


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Dispersed Air Floatation will be Mostly

Covered Under Mixing and Aeration


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Hydraulics of Sedimentation Tanks

Pipes carrying water in and out

Channels (inlet and outlet zone)

Weirs

Valves ++

settling and floatation - part 2

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