WATER TREATMENT TECHNOLOGY (TAS 3010) LECTURE NOTES 9c - Sedimentation

Izan Jaafar, Engineering Science, FST, UMTIzan Jaafar, Engineering Science, FST, UMT

© SHAHRUL ISMAIL, DESc.University College of Science and Technology Malaysia

CHAPTER 3:Environmental Microbiology

CHAPTER 9c : CHAPTER 9c : CHAPTER 9c : CHAPTER 9c :

TAS 3101 : WATER TREATMENT TECHNOLOGY

Water Water

Treatment Treatment

Process :Process :

SedimentationSedimentation

Water Water

Treatment Treatment

Process :Process :

SedimentationSedimentation


Menu

1) Introduction

2) Settling Types

3) Settling Velocity

4) Design Calculation

5) Analysis


Process of heavier solid particles in suspension, settle to the body

of the tank by gravity

Designed to remove settled water from the basin without carrying

away any of the floc particles.

Depends on :

i) Velocity of flow

ii) Size and shape of particles

iii) Viscosity of water

Sedimentation - Intro


Type of particles :

1) Discrete / individual particle

- Size, velocity constant during the settling

- Density 2,000 – 2,200 kg/m3

2) Flocculant particles

- Size, velocity fluctuates during the settling

- Particles flocculate and grow bigger in size

- Density 1,030 – 1,070 kg/m3

Sedimentation – Particle Type


In ideal reactors, average velocity of water equal to its flow rate divided by

the area through which it flows,

v = Q (Eq. 1)

Ac

Where,

V = water velocity (m/s)

Q = water flow (m3/s)

Ac = cross surface area (m2)

Sedimentation - Concepts


- Settling velocity (Vs) must be determined to assure ideal sedimentation tank.

- Overflow rate (Vo) must be set at some value LESS THAN or EQUAL to Vs

- Vo = Q = H

As t

Where

Q = flow rate

As = surface area

H = depth of water

t = detention time

Settling Velocity


- Settling velocity is different for different types of particles.

- If Vs > Vo : Particles completely settled

If Vs < Vo : Particles do not settle unless particles at h level when

entering the sedimentation tank, where ;

h = Vs t

- Effective sedimentation tank : Vo < Vs

- Achieved by increasing the area of the tank (Vs = Q/As)

Settling Velocity

W

pWS

dGsgV

18

)997.0( 2


Settling Velocity


Three classes of particles settling :

Type 1

i) Particles settle discretely at a constant velocity

ii) Settle as individual particles and do not flocculate.

iii) E.g. : Sand, grit material

iv) Occurs during :

i) Presedimentation for sand removal

ii) Grit chamber

iii) Settling of sand during rapid sand filter cleaning

v) Concentration : very low

Settling Types


Type 2

i) Flocculate during sedimentation

ii) Size constantly changing

iii) Settling velocity is changing

iv) Settling velocity increase with depth and extent of flocculation.

v) Occurs during :

i) Alum or iron coagulation

ii) Primary Sedimentation basins

iii) Settling tanks in trickling filtration

vi) Concentration : low

Settling Types


Type 3 or Zone

i) Settle as mass and form a layer – “blanket

ii) Concentration high (greater than 1000 mg/L)

iii) Distinct clear zone and sludge zone are present.

iv) Occurs during :

i) Lime-softening sedimentation

ii) Activated sludge sedimentation

iii) Sludge thickeners

Settling Types


- Reduce the turbulence caused by rushing water leaves the

tank.

- Series of troughs, called “weirs” are put in place – feed into

central channel or pipe for transport of the settled water.

- Purpose :

i) Provide large area for the water flow to flow through.

ii) Minimize the velocity in the sedimentation tank near

outlet zone.

Sedimentation - Weirs


Weir arrangement - Rectangular


Weir arrangement - Circular


Basic design calculation :

1) Tank Volume

2) Detention Time

3) Surface Overflow Rate

4) Mean Flow Velocity

5) Percent Settled Biosolids

6) Particle Settling

7) Overflow rate

Sedimentation - Calculation


Determine criteria of the particles and the effectiveness of the tank

Method :

i. Use settle column : Depth of water is as same as actual water depth

ii. In steady state

iii. Record :

a) Initial concentration of the suspended solid, express as Co

b) Withdrawn sample at every sample ports, at selected time

intervals, Ct

iv. Calculate :

Percent removal, % R for every sample point and sampling time

% R = Co – Ct

Co

v. Graf : Plot % R at depth versus time

Sedimentation - Analysis


THANKS FOR THANKS FOR

YOUR ATTENTION…..YOUR ATTENTION…..

THANKS FOR THANKS FOR

YOUR ATTENTION…..YOUR ATTENTION…..

WATER TREATMENT TECHNOLOGY (TAS 3010) LECTURE NOTES 9c - Sedimentation

Documents