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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Concentrated impurities withdrawn from the bottom of sedimentation tanks are called sludge, while material that floats to the top of the tank is called scum.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

18

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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Fig. 5-24 M&E

p. 373 M&E

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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. 9.20, 9.21

5. Actual Sedimentation Basins

1) Rectangular Settling Tank

- fig. 29, 30

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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.