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Chapter 9. Sedimentation
2016 Physicochemical Water Treatment Engineering
1. Introduction
1) Definitions
Sedimentation, also known as settling, may be defined as the removal of solid particles from a suspension by settling under gravity.
Clarification is a similar term, which usually refers specifically to the function of a sedimentation tank in removing suspended matter from the water to give a clarified effluent. In a broader sense, clarification could include flotation and filtration.
Thickening in sedimentation tanks is the process whereby the settled impurities are concentrated and compacted on the floor of the tank and in the sludge-collecting hoppers.
Concentrated impurities withdrawn from the bottom of sedimentation tanks are called sludge, while material that floats to the top of the tank is called scum.
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2) Applications
- WTP
i. Plain sedimentation prior to slow sand filtration
ii. Settling prior to rapid sand filtration
iii. Settling in a lime-soda softening
iv. Settling in an iron or manganese removal plant
- WWTP
3) Sedimentation basins
- reinforced concrete
- circular (10.7 to 45.7 m in dia.), square (10.7 to 61.0 m),rectangular (1.52 to 30.5 m by up to 76.2 m)
2. Types of settling
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2. Types of settling - continued
i. Type I: Free settling (No interaction between particles)
- Dilute suspensions, discrete process, no aggregation
- Particles settle as separate units
- Example: plain sedimentation of surface waters, settling of sand in gritchambers
- vs = f(particles, fluid)
ii. Type II: Flocculent settling
- Dilute suspensions, aggregation (particles flocculating while settling)
particle size increases
settling velocity increases with depth
- Example: primary clarifier
2. Types of settling - continued
iii. Type III: Zone/hindered settling
- Moderate concentration of particles
- Neighboring effects
- Fixed relative position
> settling at constant velocity
- Example: secondary clarifier
iv. Type IV: Compression settling
- High particle concentration
- Particles are touching
- Compaction/consolidation
- Example: lower depths of activated sludge clarifier
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3. Type I settling: Free Settling
1) Stokes’ law
velocity terminalparticle
tcoefficien drag
gravity todueon accelerati
densitywater
density particle
area sectional cross particle
volumeparticle
t
D
w
p
p
p
V
C
g
ρ
ρ
A
2
2t
wPDd
VACF
pp gW
pwb gF
dbt FFW
dt
dVm Force balance (zero acceleration)
W
dFbF
1) Stokes’ law - continued
bd FWF
gV
AC wppt
wPD )(2
2
wPD
wppt
AC
gV
)(2 2
dAp
p
3
2 3
3
4rp
2rAp
Assume a sphere
w
wp
D
t
C
gdV
3
4 2
w
wp
D
tC
gdV
3
4
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ii. Transitional flow 1 < Re < 104
iii. Fully turbulent flow Re > 104
i. Laminar flow Re < 124
ReDC
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2dgV wpt
w
wpt
gdV
3.0 0.4DC
(Re)fCD For the spheres,wtdVRe
34.0Re
3
Re
24DC
“Stokes’ equation”
- Much of the settling in dilute suspensions in WTP and WWTP follows this eq.
Graphical determination of settling velocity of spherical particles
=> Fig. 9.5
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2) Ideal Basin Theory (Camp)
- Assumptions
i) discrete settling
ii) uniform distribution of particles throughout the depth of the entrance
zone
iii) even distribution of the flow entering/leaving the basins
iv) sludge zone
Vo
Vi
• vo = settling velocity of the smallest particle size that is 100% removed
• Settling distance for a particle with vi: d-e (=hi), which is equal to b-c (=hi)
• Assume even distribution of particles at the entrance zone
• Removal efficiency (R; fraction removed) for a particle with vi is h
hi
Q
Ah
Q
Vt time) retention(
• Q/A = surface loading (rate) or overflow rate; importance of A
Vo
Vi
ii tvh
removal 100% for velocity settling : 0 vA
Q
A
Qth
AQ
v
h
hR ii
/
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3) Settling column analysis
- Obtain velocity distribution and estimate total removal efficiency
4) Consideration on designing basin depth
- Depth of clarifier doesn’t enter into the calculation. Therefore, the most economical design would be one where depth is minimum. But practical limits on clarifier depth are:
1) Space needs for sludge accumulation and removal mechanisms.
2) Influence of wind, thermal instabilities and density currents.
3) Difficulties in attaining flow that is uniformly distributed at entrance and exit of sedimentation basin.
5) Design criteria for sedimentation tanks
- Minimal turbulence (inlet baffles)
- Uniform velocity
- No scour of settled particles
- Slow moving particle collection system
- Q/A must be small (to capture small particles)
Settling zone
Sludge zoneInle
t zo
ne Ou
tlet
zo
ne
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4. Type II settling – Flocculent settling
- Primary settling of wastewaters and the settling of chemically coagulatedwaters and wastewaters
- Plain sedimentation of domestic wastewaters commonly achieves removalof 50 to 60% of the suspended solids for nominal detention times (=BODof 30 to 35%)
- Difficult to model the process due to continuously changing size and shape
- The greater the tank depth, the greater is the opportunity forcontact among particles [thus analyze through the columnlength]
- No satisfactory formulation available for predicting flocculation effect onsedimentation
Settling column analysis
- Column
> height: can be of any diameter but should be equal in height to thedepth of the proposed tank (150 mm dia., 3 m height or at least depth ofthe proposed settling tank)
> dia.: at least 130 to 205 mm to minimize side-wall effects
- Procedure
> uniform distribution of the particles initially throughout the height of thecolumn
> ports at equal intervals (0.6m intervals)
> samples removed through the ports at periodic time intervals and ssconcentrations are determined ==> obtain % removal
xij = (1-Cij/Co)*100 : % removed at ith depth at the jth time interval
> plot % removal vs. settling time
+ interpolation made ==> isoremoval lines are drawn
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- Procedure - continued
> obtain removal efficiency for assumed detention times
=> % removal vs. detention time
>> approximation can be refined by adding more terms and decreasingthe interval between the isoremoval lines
> scaling factor to compensate for the side-wall effects : 1.25-1.75 fordetention time, 0.65-0.85 for
> overflow rates for the various settling times (where the R curvesintercept the x-axis)
for the curve Rc, vo = H/tc*proper conversions (settles H during time tc)
0v
Fig. 9. 11 & Fig. 9.14 in text
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5. Type III and IV settling
1) Zone or hindered settling
- Settling of an intermediate concentration of particles in which particlesare so close together that interparticle forces hinder the settling ofneighboring particles. The particles remain in a fixed positionrelative to each other, all settle at constant velocity. The mass ofparticles settle as a zone. Distinct solid-liquid interface
- settling that occurs in the intermediate depths in a final clarifier
2) Compression settling
- Particles touch each other; settling that occurs only by compressionof the compacting mass
- At lower depths of a final clarifier for the activated sludge
Fig. 9.20, 9.21
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- fig. 31
2) Circular Settling Tank
i) Center feed
- If D > 9.14m, inlet pipe runs beneath the tank
- If D < 9.14m, inlet pipe enters through the wall
- side water depth
- weir channel around the periphery
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ii) Peripheral feed
- flow moves around
the periphery in an
orifice
- periphery entry does
not give as uniform flow
as the previous tanks
3) Fluid flow, retention time
- dead spaces, eddy currents, wind currents, thermal currents
> most fluid elements passing through a time shorter than the theoretical
detention time
> dead spaces, eddy currents ==> reduces vol. for settling; cannot
utilize the whole detention time, V/Q
> wind & thermal currents => creates flow that passes directly from the
inlet to the outlet of the basin
- can be measured by tracer studies: a slug of tracer is added in the inlet
conc. at the outlet is measured
- if ideal basin theory is applied to slightly flocculent particles, it will be conservative.
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Tracer studies on circular and rectangular settling tanks
Fig. 37
Time (min) 0 58 77 91 114 154 250
Conc. remaining(mg/L) 650 560 415 325 215 130 52
Homework
1. 9.4 in text.
2. A settling column analysis assuming type I settling was carried out with a raw water and a column with a height of 2 m. Initial suspended solids concentration in the column was 650 mg/L, and solids concentrations over the sampling periods are tabulated below. When the raw water is treated with a surface loading of 2.4×10-2 m/min, calculate the removal efficiency in the sedimentation basin.
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