Pretreatment

Pretreatment Objectives

To maximize the efficiency and lifespan of a desalination plant using reverse

osmosis, pretreatment aims to prevent clogging of osmosis membranes by :

Pretreatment System Objectives

– Turbidity< 0.5 NTU

– SDI < 3

– No oxidant

– Low residual metal (Fe, Al)

– Low organic carbon content

– No algae cells

– Low particle counts

removing algae/suspended solids/colloids

obtaining a low clog power of water

Source : http://reverseosmosispk.blogspot.kr/2011_05_01_archive.html

Pretreatment of seawater for desalination plants, Richard Dixon, ITT

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Source: http://dx.doi.org/10.1016/j.desal.2013.04.021

Pretreatment Types for RO

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Coagulation vs. Flocculation

Difference between Coagulation and Flocculation

Coagulation indicates the process

through which colloidal particles

and very fine solid suspensions are

destabilized so that they can begin

to agglomerate if the conditions are

appropriate

Flocculation refers to the process

by which destabilized particles

actually conglomerate into larger

aggregates so that they can be

separated from the seawater

Source: J van Leeuwen, 2010

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Principle of coagulation

What is Coagulation?

Source: J van Leeuwen, 2010

Coagulation is the destabilization

of colloids by addition of

chemicals that neutralizes the

negative charges

The Chemicals are known as

coagulants, usually higher

valence cationic salts (Al3+,

Fe3+, etc.)

Coagulation is essentially a

chemical process

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Coagulation Process

Source: Rosa-lee Cooke, Introduction to water & wastewater technology

Coagulation processes

1. Coagulant added

2. Coagulant forms precipitate, trapping impurities

3. Precipitate and trapped impurities settle to bottom

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Coagulants reactions

Iron salts

𝐹𝑒2 𝑆𝑂4 3 + 3𝐶𝑎 𝐻𝐶𝑂3 2 ⇒ 2 𝐹𝑒 𝑂𝐻 3 ↓ +3𝐶𝑎𝑆𝑂4 + 6𝐶𝑂2

Ferric sulfate

2𝐹𝑒𝐶𝑙3 + 3𝐶𝑎 𝐻𝐶𝑂3 2 ⇒ 2 𝐹𝑒 𝑂𝐻 3 ↓ +3𝐶𝑎𝐶𝑙2 + 6𝐶𝑂2

Ferric chloride

𝐹𝑒𝑆𝑂4 + 𝐶𝑎 𝐻𝐶𝑂3 2 ⇒ 𝐹𝑒 𝑂𝐻 ↓ +𝐶𝑎𝑆𝑂4 + 2𝐶𝑂2

Ferrous sulfate

Source: Coagulation and Flocculation Process Fundamentals, Minnesota Rural Water Association

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Coagulants reactions

Aluminum salts

𝐴𝑙2 𝑆𝑂4 3 + 3𝐶𝑎 𝐻𝐶𝑂3 2 ⇒ 2 𝐴𝑙 𝑂𝐻 3 ↓ +3𝐶𝑎𝑆𝑂4 + 6𝐶𝑂2

Aluminum sulfate (Alum)

Sodium aluminate

𝑁𝑎2𝐴𝑙2𝑂4 + 𝐶𝑂2 ⇒ 2 𝐴𝑙 𝑂𝐻 3 ↓ +𝑁𝑎𝐶𝑂3

𝑁𝑎2𝐴𝑙2𝑂4 + 𝑀𝑔𝐶𝑂3 ⇒ 𝑀𝑔𝐴𝑙2𝑂4 ↓ +𝑁𝑎2𝐶𝑂3

Source: Coagulation and Flocculation Process Fundamentals, Minnesota Rural Water Association

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Coagulant Reactions

Strategies for Al and Fe coagulant dosing

Colloid concentration

Alkalinity level

Destabilization mechanism

Al and Fe Dosing strategy

High

High Charge

Neutralization High dose or remove

alkalinity and add low dose

Low Charge

Neutralization Easy coagulation

Low

High Sweeping floc High dose,

Easy coagulation

Low Ineffective Increase alkalinity or Use other coagulant

Source: Piero M. Armenante, New Jersey Institute of Technology, 2011

Dosage depend primarily on pH, alkalinity and concentration

of the colloids and more.

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What is Flocculation?

The agglomeration of destabilized particles into a large size particles

known as flocs which can be effectively removed by sedimentation or

floatation

Flocculation is essentially a physical process

Principle of flocculation

Source: Shoalhaven Water, 2006

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1. Compensation of the negative particle charge by adding positively

charged ions to the surrounding water (salt flocculation)

2. Particle attachment by adsorption onto organic matter fixing particles

3. Particle collision through differences in particle settling velocity that

cause particles to collide

4. Particle collision through the Brownian motion by which particles

are randomly mover

5. Particle collision through the turbulent motion of the water by which

particles are brought together

Flocculation Process

Source: Tripathy, Tridib, and B. Rajan De. "Flocculation: a new way to

treat the waste water." Journal of Physical Sciences 10 (2006)

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Flocculator (Slow & Gentle mixing)

Flocculator is designed mainly to provide enough interparticle contacts to

achieve particles agglomeration so that they can be effectively removed by

sedimentation of floatation.

Transport Mechanisms

• Brownian motion: for relatively small particles which follow random motion

and collides with other particles

• Differential settling: Particles with different settling velocity in the vertical

alignment collides when one overtakes the other

• Mechanical mixing: Using different types of mechanical mixers to promote

particles contacts and thus their agglomeration

Source: Tripathy, Tridib, and B. Rajan De. "Flocculation: a new way to

treat the waste water." Journal of Physical Sciences 10 (2006)

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Conventional Pretreatment

1. Sedimentation

• Sedimentation is a treatment process in which the velocity of the

water is lowered below the suspension velocity and the suspended

particles settle out of the water due to gravity.

• The purpose of sedimentation is to decrease the concentration of

suspended particles in the water, reducing the load on the filters

1) Rectangular design - The simplest design

- It is used for large scale water treatment plants

- Advantage : predictability, cost-effective, low maintenance

- Disadvantage : It is required the large amount land area

2) Double deck rectangular basin

- It is composed of two rectangular sedimentation basins

- Advantage : It conserves land area

- Disadvantage : Higher operation and maintenance cost

3) Square or circular sedimentation basin

- Horizontal flow

- Disadvantage : It has short circuiting problems

Fig. Types of basins

Source : http://water.me.vccs.edu/

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Conventional Pretreatment

2. Media Filtration A media filter is a type of filter that uses a bed of sand, crushed granite or other material to filter water for drinking, swimming pools, aquaculture, irrigation, storm water management and other applications.

Table 1. Media used in multilayer filtration

• Dual and Multi-Media Filter

The more smaller size

- Depending on pore size e.g) Anthracite : light Garnet : very dense sand Source : http://water.me.vccs.edu/courses

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Media Filter

What is Media Filter?

A media filter is a type of filter that uses a bed of sand, crushed granite or other

material to filter water for drinking, swimming pools, aquaculture, irrigation,

stormwater management and other applications.

Media Filters may be classified according to the types of media used

as follows:

Single–media filters : These have one type of media,

usually sand or crushed anthracite coal.

Dual–media filters : These have two types of media,

usually crushed anthracite coal and sand.

Multi–media filters : These have three types of media,

usually crushed anthracite coal, sand, and garnet.

Source : http://en.wikipedia.org/wiki/Media_filter

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Basic Principle of Media Filter

Larger particles may be removed by straining

Particles may also be removed by sedimentation

Others may be intercepted by and adhere to the surface of the medium due to inertia

Filtration efficiency is greatly increased by destabilization or coagulation of the particles prior

to filtration

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Dual Media Filter

Single Media Filter Dual Media Filter

Seawater Parameters Single Media Filter Dual Media Filter

Turbidity(NTU) 1~2 < 10

Suspended Solids(mg/L) < 3 < 15

SDI 10~15 30

Algae(u/L) - 2,000

Chlorophyll(㎍/L) - < 5

DOC(mg/L) < 1.0 < 2.0

Limit recommended of the feed water

Source : A large Review of the pretreatment, Kader Gaid, Technical department, veolia water

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Dual Media Filter

Source : A large Review of the pretreatment, Kader Gaid, Technical department, veolia water

Single Media Filter(Oman Sur desalination Plant)

Dual Media Filter(Australian Desalination Plant)

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Conventional Pretreatment

Membrane treatment

Conventional treatment

Modern and continuously improving

Technology Developed in the 1800’s

Extremely compact Footprint Large land requirement

Physical barrier Separation process Gravity driven with coarse filtration

Fully automated with minimal chemical use

Operation Labor and chemical intensive

Membranes Vs Conventional Technology

Membrane treatment & Conventional Treatment

Source : http://www.gewater.com/pdf

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