Settling and Floatation – Part 2
FLOCCULENT SETTLING (1)
• 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
FLOCCULENT SETTLING (2)
• 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
• 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)
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?
• 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
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,
SETTLING RATE =
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 = --------------
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)
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
The movement of the solids due to bulk flow is
Gb = Cb * Vb
Gb = bulk flux; Cs,Vs
Vb = bulk velocity.
Cb= bulk solids concentration
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)
GL = limiting max flux = Gt.
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.
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
• 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
• 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)
• Valves ++