PROSPECTS OF AFFORDABLE FREHWATER THROUGH SEA WATER REVERSE

OSMOSIS DESALINATION

By

Hadi Ali Madkhali

A THESIS

Submitted to

Michigan State University

in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

Mechanical Engineering

2012

ABSTRACT

PROSPECTS OF AFFORDABLE FREHWATER THROUGH SEA WATER REVERSE

OSMOSIS DESALINATION

By

Hadi Ali Madkhali

This work investigates the prospects of freshwater production through Seat Water

Reverse Osmosis (SWRO) desalination. The process of SWRO is highly dependent on

desalination Energy Consumption, Technology, and Cost. To establish a baseline for the work,

an extensive literature study and analysis was carried out on SWRO desalination processes and

technology. Different types of SWRO systems were studied by arrangement configuration,

energy consumption and energy recovery systems. Because energy consumption by the SWRO

system is the single important factor affecting the production of freshwater, energy consumption

and processes efficiency analyses were carried out. Furthermore, the work includes the

development of an analytical procedure for predicting the performance and cost of the SWRO

desalination processes and systems. Finally, the work concludes on future trends in SWRO

desalination processes and systems.

iii

ACKNOWLEDGEMENTS

I would like to express my appreciation to the many who have given of their time and

consideration in helping me carry out this research. First of them is Professor Abraham Engeda.

Thanks for staff of seawater desalination plant in Jeddah-Saudi Arabia, and thanks for Biological

Department at MSU. I also want to thank my mother, my father, and my wife who always

encourage me.

iv

TABLE OF CONTENTS

LIST OF TABLES……………………………………………………………………………..vi

LIST OF FIGURES…………………………………………………………………………….vii

CHAPTER 1

INTRODUCTOPN TO SEAWATER REVERSE OSMOSIS DESALINATION…………......1

History……………………………………………………………………………………….1

Definitions…………………………………………………………………………………...2

RO Process…………………………………………………………………………………..4

CHAPTER 2

LITERATURE ANALYSIS ON SEAWATER REVERSES OSMOSIS DESALINATION......7

CHAPTERS 3

THE DIFFERENT TYPES SEAWATER REVERSES OSMOSIS DESALINATION

PROCESSES BY CONFIGURATION AND ENERGY RECOVERY SYSTEM……………..11

Hydraulic to Mechanical-Assisted Pumping…………………………………………………12

Hydraulically driven pumping in series……………………………………………………....13

Hydraulically driven pumping in parallel…………………………………………………….14

CHAPTER 4

BASICTERMS AND EQUATIONS OF REVERSE OSMOSIS………………………………..17

CHAPTER 5

ENERGY ANALYSIS OF SEAWATER REVERSE OSMOSIS DESALINATION

PROCESSES AND SYSTEMS………………………………………………………………….21

Theoretical Energy of Separation……………………………………………………………..21

Actual Energy of Separation………………………………………………………………….22

CHAPTER 6

DEVELOPMENT OF AN ANALYTICAL PROCEDURE FOR PREDCTING THE

PERFORMANCE AND COST OF SEAWATER REVERSE OSMOSIS DESALINATION

PROCESSES AND SYSTEMS…………………………………………………………………24

Performance of Reverse Osmosis Units……………………………………………………...24

Effect of Temperature……………………………………………………………………..27

Effect of Pressure………………………………………………………………………….30

Effect of Salinity on RO Performance…………………………………………………….31

Effect of Recovery on RO Performance…………………………………………………..32

Cost of Seawater Reverse Osmosis…………………………………………………………..32

CHAPTER 7

FUTURE TREBDS IN SEAWATER REVERSE OSMOSIS DESALINATION PROCESSES

AND SYSTEMS………………………………………………………………………………...34

v

Membrane System……………………………………………………………………………34

Energy Recovery Devices…………………………………………………………………….35

Alternative Energy……………………………………………………………………………37

CHAPTER 8

CONCLUSION…………………………………………………………………………………..39

APPENDIX……………………………………………………………………………………....40

REFERENCES…………………………………………………………………………………..45

vi

LIST OF TABLES

Table 3.1: The Energy Consumption for Isobaric ERD……………………………………..16

Table 3.2: The Energy Consumption for a Turbine ERD……………………………………16

Table 5.1: The energy consumption of the seawater reverse osmosis desalination plant using

turbines and usage of pressure exchangers…………………………………………………..22

Table 5.2: Turbine operation for power saving……………………………………………...23

Table 5.3: Pressure exchangers for power saving (estimate)………………………………...23

Table 6.1: Impact of temperature on the osmotic pressure for TDS=35,000mg/l…………...25

Table 6.2: Expected Increase in Flux Due to Temperature Rise…………………………….29

Table 6.3: Values of Product Concentration and Feed Pressure at Various Temperatures…30

vii

LIST OF FIGURES

Figure 1.1: Osmosis and Reverse Osmosis Process…………………………………………...4

Figure 1.2: A schematic graph of the three important parts in the reverse osmosis technique.5

Figure 1.3: A module of a reverse osmosis membrane………………………………………..6

Figure 2.1: The decreasing of the power consumption of the reverse osmosis from 1970 to

2008……………………………………………………………………………………………8

Figure 3.1: The relation between the energy recovery potential and the TDS………………11

Figure 3.2: Hydraulic to Mechanical-Assisted Pumping…………………………………….13

Figure 3.3: Hydraulically Driven Pumping in Series………………………………………...14

Figure 3.4: Hydraulically Driven Pumping in Parallel………………………………………15

Figure 4.1: General Schematic of RO System……………………………………………….17

Figure 6.1: Effect of Feed Concentration on the Applied Pressure………………………….25

Figure 6.2: Impact of the applied feed pressure on the permeability membrane coefficient...26

Figure 6.3: The relation between the feed temperature and the product flow rate…………..29

Figure 6.4: Increasing the feed temperature leads to increase TDS and decreases Pf……….30

Figure 6.5: Impact of the applied pressure on the flux and the salt rejection………………..31

Figure 6.6: Typical cost for a RO Desalination Plant………………………………………..33

Figure 7.1: Evolution of permeability and salt passage of seawater composite polyamide

membranes…………………………………………………………………………………...35

Figure 7.2: Comparing between three ERDs………………………………………………...36

Figure 7.3: Fluid Switcher ERD……………………………………………………………..37

1

Chapter 1: Introduction to Seawater Reverse Osmosis Desalination

I. History:

If we go back to the fourth century, we will find Greek sailors were desalinating seawater by

evaporating it, so desalination technology is not a new invention, it is known as long time ago. In

the same century, Aristotle observed the principle of distillation; it is a desalination process using

distillation method. In 1869, desalination concept reported the first patent, and in 1944, United

Kingdom built the first desalination plant in the world. However, the first desalination plant

using reverse osmosis process has been operated in Jeddah-Saudi Arabia in 1978 [3]. Studies on

using reverse osmosis process for desalination of seawater have begun since 1748 by Jean

Antoine Nollet, and then many researchers who were interested in this field continue studying

the reverse osmosis technique. Reid is one of many scientists who studied reverse osmosis in the

late 1950s and discovered that cellulose acetate RO membranes were able to separate the salt

substances from the saline water and obtain fresh water; however, the amount of water flux was

very low. In 1960, Loeb and Sourirajan from the University of California- Los Angles developed

this type of membrane and improved it to produce high amounts of water flux and reject high

amounts of salt. Then, the reverse osmosis technique became valid to be applied in plants of

desalination of the sea water. In fact, not only development of membranes had helped in

progressing the seawater desalination process, but also they used for another reverse osmosis

applications; such as, wastewater treatment, water softening, and food processing. Williams has

mentioned in his paper to that quick progress in sales of the membranes. “an estimate indicated

that sales of RO membrane products had grown to $118 million yearly in 1990, with great

potential for continued growth” (Williams 1). As a good economic point that indicates to the fast

improvement of the seawater reverse osmosis desalination, the early reverse osmosis plants were

2

consuming around 20 KWh/m3, this power consumption dropped to be 3.5 Kwh/m

3 in 2000. The

production water using this amazing technique became very popular in many countries because

the remarkable advance of the reverse osmosis membrane, reduction cost of the water product

and the energy recovery devices. After this short story about the desalination, we should define

the desalination process and other basic characteristics that relate to it.

II. Definitions

Desalination as a simple definition is getting potable water by reducing dissolved salts and other

unhealthy substances from feed water sources. For example, sea water in normal circumstances

is not suitable for the domestic purpose because it has excessive salt, but by reducing the salts

and the other undesirable substances utilizing desalination process, it will be valid as drinking

water and for other daily purposes. Therefore, desalination of seawater is a perfect solution that

can help in providing fresh water resources. Perhaps, someone wants to ask if there is any

specific amount of the dissolved salt in the potable water, or how many of saline or mineral

substances are contained in water for making it as drinking water. TDS is a measurement or

parameter that used to measure quantity of the dissolved salt in the water; TDS is a shortcut of

Total Dissolved Solids and (mg/L) milligram per liter or (ppt) parts per thousand is unit of TDS.

Based on the World Health Organization (WHO) and the United States Environment Protection

Agency (EPA) and under the Safe Water Drinking Act, 500 mg/L is a maximum concentration of

the TDS as a standard for the potable water. Therefore, when the TDS concentration is higher

than 500 mg/L and lowers than 15,000 mg/L, it is classified as brackish water. In addition, if the

water’s TDS concentration is higher than 15,000 mg/L, such as ocean and bay waters, they are

classified as seawater that represents the percent of water in our plant (Voutchkov 2010), see

table A-3 in the appendix for TDS of different sources of seawater. There is another important

3

factor that has to be in the account in the desalination processes; it is the potential of hydrogen

(PH). PH is measured on a scale that runs between 0 and 14. Lower than seven means acid is

present, and above than seven means alkaline is present. According to the Environmental

Protection Agency, EPA, the PH of most drinking water is between 6.5 and 8.5. However,

adjustment the PH of the feed water can help in improvement the desalination plant; it considers

an important factor in performance of the desalination plant. As we noted, discovering

desalination of the sea water is not a new concept; however, it is the progress of using new and

professional technology that help in optimizing quality and quantity of the production, thus, the

improvement of energy consumption. Using desalination of the sea water became very important

in order to supply healthy water for humans, especially in the Middle East, since seawater is

considered the main source for drinking water. According to Global Water Intelligence and the

International Desalination Association, the most countries in the world that use desalination

technology are Saudi Arabia, UAE, USA, USA, Spain, Kuwait, Algeria, Chine, Qatar, Japan,

and Australia. As a comparing process between the United States of America and Saudi Arabia

in producing potable water per day, Saudi Arabia produces10, 759,693 m3/day but the USA

produces 8,133,415 m3/day even though the number of people who live in the United State of

America is around 313,029,090, but the population number in Saudi Arabia is around 26.1

million [2]. As a result, that statistic process indicates the importance of desalination of seawater

in Saudi Arabia. We know now what desalination is, so we should to see how this process can be

achieved. In order to obtain fresh water using the desalination process, there are several

techniques to attain this purpose. These techniques can be classified into three types based on

the process principle. First, there is a process based on the physical change in the state of water,

such as the freezing process or distillation. Second, there is a process using membranes, such as

4

OSMOSIS REVERSE OSMOSIS

Semipermeable Membrane

Πosm

.

reverse osmosis. Third, there is process acting on chemical bonds, for example, ion exchange.

Basically, this research will focus on the reverse osmosis process. The osmosis process, as a

simple definition, is a natural phenomenon in which two solvents of water are separating by a

semipermeable membrane, where one of them has a higher solute concentration than the other.

Spontaneously, the water that has low concentration will pass through the semipermeable

membrane to the region of the high solute concentration, see figure (1.1). Therefore, in order to

reduce the salts from the water that has the higher concentration, reverse osmosis has to exist.

III. RO Process

Reverse Osmosis (RO) is a treatment process for production of fresh, low salinity potable water

from saline water source (seawater or brackish water) via membrane separation by applying a

high pressure to the salty water which means consumes high energy for creating a high pressure

in order to force the water through a membrane. As a result, only the water molecules pass

through the membrane while the solid particles remain suspended and cannot pass; therefore,

producing freshwater will be attained, see figure (1.1).

Figure (1.1): Osmosis and Reverse Osmosis Process.

Applied pressure to overcome osmotic pressure

results in reverse osmosis.

High Concentrated

Solution

Low Concentrated

5

There are three important components in the RO technique, high pressure pump, energy recovery

devices, and RO membrane, see figure (1.2). As we know, achieving RO process needs a high

pressure, so the feed water is highly pressurized before entering the membrane unit. Temperature

and salinity of the water are the two main factors that affect value of the high pressure; typically

it is in range 55-85 bars. When the feed water pressurized into the membrane, it is divided into

potable water and brine that still has a high pressure. The pressure drop of the brine is about 1.5

to 2 bars. Therefore, it is a good idea to use recovery energy devices for this high pressure for

energy consumption. Energy recovery turbine and pressure exchanger are two good examples

for the energy recovery devices. The third basic factor in the RO is the membrane, see figure

(1.3). As a simple definition, the membrane is a flat surface with selective permeability. In order

to accomplish the goal of acquiring potable water, the membrane has to exist in the RO process.

It is used for removing salt and retains undesirable components in the water. The undesirable

components are materials that make the water unhealthy; for example, micro-algae, bacteria,

certain viruses, micro-organisms, and micro-pollutants are considered undesirable materials. The

most common RO membrane used in desalination are spiral wound, a thin-film composite. They

consist in a flat sheet sealed like an envelope and wound in a spiral (Voutchkov 2010).

Figure (1.2): A schematic graph of the three important parts in the reverse osmosis technique.

ERD

Permeate Flow, Pp

Raw Water, Po

High Pressure Pump

Brine, Pe Brine, Pc

Membran

e

Feed, Pf

6

Figure (1.3): A module of a reverse osmosis membrane [9]. “For Interpretation of the references

to color in this and all other figures, the reader is referred to the electronic version of this thesis.”

As previously mentioned, the membrane considers a basic element in reverse osmosis facilities.

It plays an important role in the cost of the treatment process; for instance, its replacement costs

5% of treatment costs. Even though there are many advantages of using the membrane in the RO

technique, there are some disadvantages. Membrane clogging is a popular problem in the reverse

osmosis. According to Michel Dutang, Director of Research, Development and Technology at

Veolia Environment, “The aim of the Membrane Center of Expertise (ARAMIS), created in

2004 at Anjou Recherche (Veolia Environnement’s water research center) is to identify the

matter responsible for clogging, and recommend efficient and durable treatment solutions

(pretreatment and appropriate cleaning cycles).” In addition, there is a thesis about the membrane

clogging at the University of Poitiers and the Banyuls Oceanographic Institute in France. This

thesis works to identify the compounds liable to clog reverse osmosis membranes.

Feed Water

Feed Water

Permeate

Concentrate

7

Chapter 2: Literature Analysis on Seawater Reverse Osmosis Desalination

Recently, construction of new seawater desalination plants is increasing in order to

augment water resources even though seawater desalination consumes more energy than

conventional technology for the treatment of fresh water. This chapter will focus on the possible

methods that can recover the energy consumption, and the potential role of advanced materials

and innovative technologies in improving performance.

Seawater considers a basic source for fresh water in many countries, and over one –third

of the world’s population live in these countries. According to Elimelech and Phillipt, “Presently,

over one-third of the world’s population lives in water-stressed countries and by 2025, this figure

is predicted to rise to nearly two-thirds.” Therefore, demand fresh water is in increasing;

however, the huge growth in population and industrials, climate change, and pollution of the

freshwater resources has an obvious effect on providing ample and safe water. Since importance

of the water in our daily life, there are many technological solutions for this problem.

Controlling in supplying water, such as water conservation, infrastructure repairing, and

distribution systems, is a solution that can help improve the use of existing water resources, but it

cannot help in increasing the fresh water resources. Desalination of seawater is an optimal ways

for providing fresh water resources. Seawater reverse osmosis is a wonderful solution for

providing healthy and ample fresh water; in addition, this technique considers the most energy

efficient technology for seawater desalination. In the past 40 years, the amount of power needed

to derive desalination in seawater reverse osmosis has decreased because of progressing

development in the reverse osmosis technique, such as installation energy recovery devices,

using more effective pumps, and including higher permeable membranes, see figure (2.1).

8

Figure (2.1): The decreasing of the power consumption of the reverse osmosis from 1970 to

2008.

Besides to the energy recovery devices that as we mentioned play clear role in advance recovery

of the energy for the RO system, also permeability membranes can help in recovery the energy

required for this process. According to American Association for the Advancement of Science,

“It is argued that increasing the membrane permeability will reduce the pressure needed to drive

permeation, thereby reducing the energy demand of reverse osmosis desalination.” The above

graph encourages and gives a good motivation for competition in order to develop and improve

the seawater reverse osmosis plants.

As we know that reverse osmosis process means separation between pure water and some

solute materials. Therefore, there is amount of energy has to apply for achieving segregation or

dismantle between the product water and the other materials. In fact, one of many ways that can

help declining the energy demand in the reverse osmosis process is by understanding the

minimum amount of energy that required for separating dissolved solids from pure water. As a

reversible thermodynamic process, this minimum amount of energy can be calculated by using

the following equation:

0

2.5

5

7.5

10

12.5

15

17.5

20

1970 1980 1990 2000 2004 2008

Pow

er C

onsu

mpti

on (

KW

h/m

3)

YEAR

9

( ) RT [lnawdnw + lnasdns] = PsVwdnw (2.1)

( ) = ΔH TΔS (2.2)

This equation shows that the free energy of mixing equals to the required energy for achieving of

separation process. They are equal in magnitude and opposite in their sings.

where is the free energy of mixing, R is the ideal gas constant, T is the absolute

temperature, aw is the activity of water, as is the activity of salt, nw is the number of moles of

water, ns is the number of moles of salts, Vw is the molar volume of water. Since we are

concerned with a process that removes pure water from seawater, dns = 0. Naturally this

minimum energy is less than the actual energy because desalination plants do not operate as a

reversible thermodynamic process. Also, the desalination plants are finite in size.

In the desalination plants that are operating by reverse osmosis process, the membrane

elements are installing in series. Therefore, the pressure applied to feed water at the first element

must be at least equal to the osmotic pressure of the concentrate water that is leaving the last

membrane. The system is described as a thermodynamic limit when the applied pressure equals

the osmotic pressure of the concentrate. “For a system at the thermodynamic limit, highly

permeable membranes may help reduce capital costs by reducing the membrane area needed, but

the will not reduce energy consumption.” (Elimelech, Menachem 715). Currently, SWRO plants

are operating at applied pressure greater than the concentration pressure by 10-20%, so this

means they are close to the thermodynamic limit.

Before entering feed water into the membrane system, it needs to pretreatment process.

Process of pretreatment is another consumption of energy. There is energy of more than 1

KWh/m3 consumes by the intake, pretreatment, and brine discharge in the desalination of

10

seawater using the reverse osmosis technique. Removal the toxic elements of the feed water in

order to achieve post treatment process for getting potable water consider another source of

energy consumption. For example, The World Health Organization recommends a boron

concentration in drinking water below 0.5 mg/L since this element is one of the toxic elements

that affect human health. Quantity of boron concentration in the seawater varies between 4 to 5.5

mg/L; it is proportional to the seawater salinity. Therefore, removal of boron and also chloride

element considers part of the energy consumption.

Furthermore, reverse osmosis membranes have clear effect on energy consumption.

Despite the excellent performance of thin film composite membranes that have begun on 1980s,

there are still some hindrances that lead to increase the energy consumption of the plant. For

instance, surface properties of membrane suffer of fouling problems. One of the recent

inventions of the membrane that can use for the seawater reverse osmosis desalination is

nanotube membranes that help slightly in save of energy. In addition, there are researches on

sulfonated block copolymers to fabricate chlorine resistant membranes in order to improve the

membrane system and then helps in energy consumption.

11

Chapter 3: The Different Types Seawater Reverse Osmosis Desalination processes by

Configuration and Energy Recovery System

In seawater RO desalination, a significantly large amount of energy is involved in pressurizing

the seawater for driving it through the RO membrane. This pressurization is achieved with the

help of a high-pressure pump, which is the most significant energy consumer in a SWRO plant.

Since the brine reject produced in this process has a high pressure, simply dumping it back into

the sea is a waste of energy. This pressure can be reused and thus, the energy could be recycled.

It is a good idea now to see who much energy the brine may be has. There is a parameter called

Energy Recovery Potential that can be defined as the ratio of the hydraulic energy in the reject

stream to the hydraulic energy in the feed stream.

( ) (3.1)

Where Pf is the feed pressure, is pressure loss in membrane array, Pex is brine exhaust

pressure (disposal pressure), and RR is the reject ratio. As shown in the following graph that

ERP increases the TDS increases.

Figure (3.1): The relation between the energy recovery potential and the TDS.

0

0.1

0.2

0.3

0.4

0.5

0.6

10000 15000 20000 25000 30000 35000 40000 45000 50000

ER

P

TDS,mg/l

12

Energy of the brine led to the innovation of energy recovery devices (ERDs) that prevent the

wastage of energy in the SWRO process. Therefore, there are three different types of seawater

reverse osmosis desalination process based on the energy recovery devices.

1. Hydraulic to mechanical-assisted pumping, such as turbine.

2. Hydraulically driven pumping in series, such as turbocharger.

3. Hydraulically driven pumping in parallel, such as isobaric ERD.

I. Hydraulic to Mechanical-Assisted Pumping, see figure (3.2):

As shown in the figure (3.2), the recovery device here is a turbine that receives the brine from the

membrane system in order to use the high energy that brine has and then help in recover of

energy. The concentrate brine is ejected at high velocity through one or more nozzles onto a

turbine wheel. The turbine is attached to a shaft that is connected to the high pressure pump and

a motor where this shaft operates on the main feed. The most common types of the high pressure

pumps that are connecting with a turbine using a shaft are a kinetic centrifugal type and a

positive displacement type. In fact, there is a main disadvantage of this type of energy recovery

devices. The hydraulic to mechanical-assisted pumping system has double energy conversion.

One of them occurs when hydraulic energy of the brine is converted to mechanical energy of a

rotating shaft. The other conversion occurs when the mechanical energy of the shaft is then

converted to the hydraulic energy of feed. Therefore, the energy is lost because the energy that is

transformed by the turbine and the impeller pump.

13

Figure (3.2): Hydraulic to Mechanical-Assisted Pumping [11].

This system of energy recovery devices is highly inefficient and does not significantly lower the

costs associated with the process. Therefore, the search was still on for a more efficient ERD. To

see how much energy the plant will consume if it installs a turbine as a recovery energy device,

the following equation can calculate the energy consumption.

( ) ( ) (3.2)

Where Qhp is the high-pressure pump flow rate, Php is the high-pressure pump differential

pressure, Qr is the turbine flow rate, Pr represents the turbine differential pressure, ηt is the

turbine efficiency, ηhp is the high-pressure pump efficiency, and ηhpm is the high-pressure pump

motor efficiency.

II. Hydraulically Driven Pumping in Series:

Membranes

Turbine Shaft

High Pressure Pump

Seawater Supply Pump

14

The hydraulically driven pumping in series has an impeller and a turbine, which are coupled to a

shaft within the same casing, see figure (3.3). Hydraulic turbocharger, Pelton-drive pump, and

hydraulic pressure booster are examples of this type of energy recovery devices. The main feed

pump and the impeller and runner are placed in series. This type of energy recovery devices are

used in small and midsized desalination plants. However, using it in larger plants, such as the

plants in the Middle East is limited because of its size limitations. Moreover, these systems failed

to address the problem of converting energy from hydraulic to mechanical and then back to

hydraulic, thereby hindering the efficiency of operation.

Figure (3.3): Hydraulically Driven Pumping in Series [11].

III. Hydraulically Driven Pumping in Parallel:

This type of energy recovery device has designed based on “theory of work exchange”. Theory

of work exchange means involved a direct transfer of hydraulic energy of brine to hydraulic

Membranes

High Pressure Pump

Turbocharger

5 PSI (0.345 bar)

45.5 (m3/h)

62 bar, 68.1 m3/h

0.35 bar, 68.1 m3/h

63.4

bar

113.6

m3/h

113.6

m3/h

2 b

ar

113.6 m3/h

41 bar

15

energy of feed. As shown in figure (3.4) that the main feed pump is placed in parallel to the

device and operates on a portion of the feed, which is equal to the amount of the permeate water.

The device operates on the other portion of the feed whose amount is equal to the spent brine.

This is based on the concept of "work exchange". In these ERDs, the hydraulic energy of brine is

directly converted to hydraulic energy of feed, leading to over 90% energy efficiency. Isobaric

energy recovery device is an example of this type of the hydraulically driven pumping in

parallel.

Figure (3.4): Hydraulically Driven Pumping in Parallel [11].

These devices achieve energy-transfer efficiencies of up to 98% (Sanz and Stover, 2007).

As explanation to Isobaric ERDs, this devices transfer pressure from the high-pressure brine

reject to a portion of feed water by putting them in direct contact in pressure-equilibrating or

isobaric chambers. Concentrate rejected by the membranes flows to the isobaric ERD(s), driven

Membranes

Circulation Pump

Hig

h P

ress

ure

Pum

p

PX-260 Device

16

by a circulation (booster) pump. The ERD replaces the concentrate with feed water. Pressurized

feed water merges with the discharge of the high-pressure pump to feed the membranes. Some

mixing occurs between the concentrate and feed water in the ERD resulting in a slight increase in

the membrane feed salinity and a corresponding increase in the membrane feed pressure. The

energy consumption in the reverse osmosis system with using Isobaric energy recovery device

can be calculated using the following equation:

( ) ( ) ( ) ( ) (3.3)

Where Qhp is the high-pressure pump flow rate, Php is the high-pressure pump differential

pressure, Qcp is the circulation pump flow rate, Pcp represents the circulation pump pressure, ηhp

is the high-pressure pump efficiency, ηcp is circulation pump efficiency, ηhpm is the high-

pressure pump motor efficiency, and ηcpm is the circulation pump motor efficiency.

Comparison between turbine ERD and isobaric ERD at constant permeate flow rate (Qp = 227

m3/hr.) and at permeate recovery rate (Pr = 40%):

Table (3.1): The Energy Consumption for Isobaric ERD:

Table (3.2): The Energy Consumption for a Turbine ERD:

Qhp,m3/s Ph,pa Qcp,m3/s Pcp,pa ηhp ηhpm ηcp ηcpm EC,W EC,kw

0.06417 6600000 0.0939 200000 0.9 0.9 0.9 0.9 547415.7 547.45

Qhp,m3/s Ph,pa QT,m3/s ηhp ηhpm ηT EC,W EC,kw

0.1578 6600000 0.0931 0.9 0.9 0.83 1268606 1268.61

17

Permeate

QP

Feed Water

QF

Concentrate

QC

Chapter 4: Basic Terms and Equations of Reverse Osmosis

There are basic terms and equations that describe concept of the reverse osmosis process, so it is

a good idea to understand them before going to other chapters. The following figure (4.1) in its

simple way can describe the essential connotation of the RO units and it considers a starting

point for understanding some basic parts in the reverse osmosis system; such as, QF that

represents the saline feed water flow, QP is the permeate flow, and QC is the concentrate flow.

Figure (4.1): General Schematic of RO System.

As show in the above figure that feed water flow rate is a summation of the permeate flow rate

and the brine flow rate.

Qf = Qc + Qp (4.1)

Qf TDSf = TDSp Qp + TDSc Qc (4.2)

Osmotic Pressure

πs = RT∑ (mi) (4.3)

πs is the osmotic pressure of the seawater ,R is the universal gas constant and it equals

0.082L.atm/mol oK, T is the water temperature in Kelvin, and ∑ (mi) is the sum of the molar

concentrations of all constituents in the saline water. An approximation for the osmotic pressure

can be given by:

Membrane

18

πs = ( ( )) ( ) (4.5)

Water and Salt Transport

The rate of water passage through the reverse osmosis membrane is given by the following

relation:

QP = Kw (ΔP – Δπs)

(4.6)

Where QP is the rate of water flow through the membrane, ΔP is the hydraulic pressure

differential across the membrane, Δπs is the osmotic pressure differential across the membrane,

Kw is the membrane permeability coefficient for water, A is the membrane area, and X is the

membrane thickness.

The rate of salt flow through the membrane is given by the following relation:

Qs = Ks ΔTDS

(4.7)

Where Qs is the flow rate of salt through the membrane, Ks is the membrane permeability

coefficient for salt. ΔTDS = (TDSf - TDSp ) is the salt concentration differential across the

membrane, where TDSf is the feed water concentration and TDSp is the product (permeate)

water concentration.

Permeate Recovery Rate

Pr = (Qp/Qf) Χ 100 (4.8)

Cr = (Qc/Qf) Χ 100 (4.9)

19

Pr represents permeate recovery rate, and Cr is concentrate recovery rate. See figure (4.1) for Qf,

Qc and Qp. As a typical seawater reverse osmosis, the permeate recovery rate is 40% to 65%.

Total Dissolved Solids

TDSc=(TDSf-Pr/100*TDSp)/(1-Pr/100) (4.10)

TDSc is the actual concentration of the brine, TDSf is feed water concentration, and TDSp is the

actual concentration of permeate. For example, if we assume the recovery rate is 50%, TDS of

the feed water is 35,000mg/L, and permeate salinity of 200 mg/L.

By using the equation (4.10) for getting TDSc=69,000mg/L

Usually if the TDSf =35ppt (parts per thousands), TDSc is 50 to 70ppt, and TDSp is 2 to 5ppt.

Net Driving Pressure

TMP = Fp - (Qpfc+Pp+0.5Pd) (4.11)

Where TMP is trans membrane pressure; in other words, it is the net driving pressure NDP. NDP

is the actual pressure that drives the transport of fresh water from the feed side to the fresh water

side of the membrane. Fp is the applied feed pressure of the saline water to the membrane, Qpfc is

the average osmotic pressure on the feed/concentrate side of the membrane, Pp is the permeate

pressure, typically is 1 to 2 bars. In addition, Pd is the pressure drop across the feed/concentrate

side of the RO membrane.

Salt Passage

SP = (TDSp/TDSf) 100% (4.12)

Sale Rejection

20

Sr = 100% - Sp (4.13)

Concentration Polarization

The Concentration Polarization Factor (CPF) can be defined as a ratio of salt concentration at the

membrane surface (Cs) to bulk concentration (Cb). To explain, there is a boundary layer is

formed at the membrane surface; this boundary layer comes from the water that flows through

the membrane and the salts that are rejected by the membrane. Increasing the concentration

polarization leads to greater osmotic pressure at the membrane than in the bulk feed solution,

reduce permeate flow rate, and then increases the salts rate.

CPF = Cs/Cb (4.14)

Specific Factors

TCF = exp(K*(1/(273+Tf) - 1/298)) (4.15)

Where TCF is temperature correction factor, K is a constant characteristic for a given membrane

material, and Tf is feed water temperature in degrees Celsius.

21

Chapter 5: Energy Analysis of Seawater Reverse Osmosis Desalination Processes and

Systems

In the seawater reverse osmosis desalination, there are two main energy needs. Energy for

pumping feed water from the sea, and then transfers it through pretreatment devices. Also,

discharge the concentrate to the sea. Second energy demand in the seawater reverse osmosis

desalination is by increasing the feed water pressure way above the osmotic pressure of the

concentrate flowing through the membrane passage.

I. Theoretical Energy of Separation

As we mentioned before that minimum energy of separation equals to the free energy of mixing

in magnitude and opposite in signs. The minimum isothermal reversible work of separation at a

temperature T, which is applicable to any desalination process regardless of the separation

mechanism, is given by:

(5.1)

W is the minimum isothermal reversible work of separation, represents the change in enthalpy

between the final and the initial stages, T is the absolute temperature of the solution, is the

changes in entropy, and is the change of the free energy. After substitute equation (1.5) to molar

concentration of the salt in water:

∫ ∫ ∫

dn (5.2)

Where aw is the water activity, it is the ratio of the water vapor pressure of the solution to that of

pure water at the same temperature.

(5.3)

After integrating the above equation (2.5), the final expression is given by:

∫

(5.4)

22

W is the theoretical minimum amount of energy of separation in KWh/m3.

II. Actual Energy of Separation

One of many important features of seawater reverse osmosis desalination is that the energy

consumption of RO processes is now close to the theoretical thermodynamic minimum energy

comparing with other desalination processes that require much higher specific energies.

Experimentally, 3.5 – 4.2 KWh/m3 of energy is consumed by

reverse osmosis seawater

desalination at permeate recovery of 50%. As it is mentioned at beginning of this chapter, the

energy consumption for the reverse osmosis process is distributed into energy for pumping the

feed water, and then transferring into filtration and pretreatment devices. In addition, since the

osmotic pressure of the concentrate flowing through the membrane passage increases, energy of

the feed pressure will increase. Moreover, membrane cleaning techniques effects the energy

consumption. If seawater RO plant operates at a lower pressure, this way will lead to reduce the

energy consumed by the high-pressure pump. However, more equipment will be required to

maintain the same recovery level, such as piping, membranes, and pressure vessels. The

following tables show the energy consumption in the seawater reverse osmosis desalination plant

using two different types of energy recovery devices, turbines and usage of pressure exchangers.

Table (5.1): The energy consumption of the seawater reverse osmosis desalination plant using

turbines and usage of pressure exchangers [8]

Pumps

No.

Flow,m3/h

Diff. Head,

bar

Energy,

KWh/pump

Energy,

KWh, total

Specific Energy,

KWh/m3

Intake 6

2,200 1.0 77 462 0.07

Raw Water

Supply

6 2,200 2.5 192 1,154 0.18

Feed

Booster

12

1,042 7.7 281 3,368 0.54

23

Table (5.2): Turbine operation for power saving [8]

Pumps

No.

Flow,m3/h

Diff.

Head,

bar

Energy,

KWh/pump

Energy,

KWh,

total

Specific Energy

KWh/m3

High-

Pressure

Aggregate:

Pumps

12

1,042

69.3

2,381

28,567

Turbine

12

521

73.0

-980

-11,763

Motors

12

1,444

17,323

2.77

Auxiliary

+ Lighting

400

400

0.06

Total 3.63

Table (5.3): Pressure exchangers for power saving (estimate) [8]

Pumps

No.

Flow,m3/h

Diff.

Head,

bar

Energy,

KWh/pump

Energy,

KWh,

total

Specific Energy

KWh/m3

High-

Pressure

Aggregate:

Pumps

6

1,042

69.3

2381

14284

Pressure

exchangers

Depend

on

size/n

6252/n

6252/n

Auxiliary

pumps

6

1042

3.3

132

792

Motors

12

1,444

15,076

2.41

Auxiliary

+ Lighting

400

400

0.06

Total 3.26

24

Chapter 6: Development of an Analytical Procedure for Predicting the Performance and

Cost of Seawater Reverse Osmosis Desalination Processes and Systems:

The basic goal for the reverse osmosis technique is to produce potable water with high quality

under paying attention in side of economic element. Therefore, it is important now to understand

performance and costs of seawater reverse osmosis desalination processes and systems.

A. Performance of Reverse Osmosis Units

In fact, product flow rate and salt rejection are the major performance parameters. Pressure,

temperature recovery and feed water salt concentration are the main variables that affect product

flow rate and salt rejection. If we go back to chapter four and solve the equations, we will see

that the applied feed pressure is proportional to feed salinity as shown on Figure 6.1. In other

words, as the feed salt concentration increases the required feed pressure increases too.

Obviously, the water flux will drop if there is increasing in the feed concentration with keeping

value of the feed pressure; feed pressure is constant. Another important point that is increasing in

salt concentration will raise the osmatic pressure and then offsets the feed driving pressure. For a

constant TDS of fluid and with changing in the fluid temperature, osmotic pressure will change

and then the applied pressure changes too, see table (6.1). Also from the basic RO Equations in

chapter four, it is obvious that the rate of water flow through a membrane is increasing when the

net driving pressure differential increases and it is decreasing when the net driving pressure

decreases. Net driving pressure is proportional to the applied feed pressure.

25

0

10

20

30

40

50

60

70

80

90

0 1000 2000 3000 4000 5000 6000 7000 8000

Figure 6.1 Effect of Feed Concentration on the Applied Pressure

T,C T,K Πs, bar

10 283 26.09051

12.5 285.5 26.32099

15 288 26.55147

17.5 290.5 26.78195

20 293 27.01243

22.5 295.5 27.24291

25 298 27.47339

27.5 300.5 27.70388

30 303 27.93436

32.5 305.5 28.16484

35 308 28.39532

37.5 310.5 28.6258

40 313 28.85628

Table (6.1) Impact of temperature on the osmotic pressure for TDS=35,000mg/l.

There is an important phenomenon in the reverse osmosis system that is known as membrane

compaction. To explain this phenomenon, naturally, quantity of the water flux will increase with

increasing in the applied pressure. Therefore, the membrane permeability coefficient for water,

Kw will not still constant if the pressure rose, as show in Figure 6.2

Compaction will lead to increase in the density of membrane material which will decrease the

rate of diffusion of water and dissolved constituents through the membrane.

Feed Concentration, ppm

Appli

ed P

ress

ure

, psi

26

Figure (6.2): Impact of the applied feed pressure on the permeability membrane coefficient.

As a result of compaction phenomenon, the density of membrane material will increase, so this

increasing in the density causes decreasing the rate of diffusion of water and dissolved

constituents through the membrane. In addition, In order to maintain the design permeate flow,

higher pressure has to be applied. Since total dissolved solids of seawater are higher than in

brackish water, the feed pressure of seawater reverse osmosis is much higher than in brackish

applications, and then the compaction process will be more significant in seawater RO.

The following equation that is from chapter four, equation (4.7):

Qs = Ks ΔTDS

This equation represents the rate of salt flow Qs where it is mainly proportional to the

concentration differential across the membrane. The concentration gradient across the membrane

acts as a driving force for the flow of salt through the membrane. As a result of increasing feed

concentration across the membrane, the water flux will decline. In other side, salt flux increases

0

0.0000002

0.0000004

0.0000006

0.0000008

0.000001

0.0000012

20 25 30 35 40 45 50 55

Applied Pressure, bar

Kw

,m3/m

2s

Kpa

27

when feed concentration rises. Based on the previous equations, water flux through the

membrane will be affected by increasing in operating pressure. Increasing of the feed pressure

helps in process of salt rejection, but there is an upper limit to the amount of salt that can be

excluded by increasing the feed pressure. There is a specific limit where some salt flow with

water flowing through the membrane. When the applied pressure decreases, salt passage

increases since reducing pressure means decreasing permeate flow rate that cause a dilution of

salt. Another important parameter that can play in performance of the reverse osmosis system is

the recovery rate. The recovery rate affects salt passage and product flow. Based on the equation

(4.2) in chapter four that the recovery rate when it increases, the salt concentration on the feed-

brine side of the membrane increases. Rising of the salt concentration on the feed-brine side of

the membrane will cause an increase in salt flow rate across the membrane. Also, a higher salt

concentration in the feed-brine solution leads to raise the osmotic pressure, and then reduces the

NDP. Therefore, the higher salt concentration will cause reducing the product water flow rate.

The salt concentration of the feed water considers an important factor that can determine the

maximum recovery possible. Therefore, in order to increase the recovery rate, treatment the feed

water to prevent precipitate of the salt is a good step for increasing the recovery.

I. Effect of Temperature:

The effect of temperature on membrane performance is the most important parameter. When

temperature of feed water is increased for constant product flow the required applied feed

pressure decreases and the product water salinity increases. Energy consumption is decreased as

the applied pressure decreases. If the permeate flow is let to increase as the temperature increase

fewer membrane elements will be required. This leads to a considerable saving in the water

28

production cost. As a rule of thumb membrane capacity increases about 3% per degree Celsius

increase in water temperature [8].

Saudi Arabia is the country that depends on the reverse osmosis technology to get potable water.

As a hot weather in this country in general, raw water from deep wells is pumped at a

temperature in the range of 50-60 Co. Therefore, the first step in treatment of this water is

cooling where they try to cool the row water till be in a range 30-35 Co to meet the RO

membrane specifications. Permeate and salt passage increase with increasing the feed water

temperature. There is about 3 % increase in water production rate for each degree rise in

temperature. However, the increase in feed water temperature accelerates the rate of membrane

degradation. High temperature also affects the membrane retention coefficient. Low membrane

retention is obtained at high temperature, so optimizing of the operation of reverse osmosis

system should be studied in order to maintain the desired product water quality at the optimum

operating variables. The rate of water permeation through the membrane increase as the feed

water temperature increases since the viscosity of the solution is reduced and higher diffusion

rate of water through the membrane is obtained. Increasing feed water temperature will yield

lower salt rejection or higher salt passage due to higher diffusion rate for salt through the

membrane. Using the following equation to see how temperature can affect the permeate flux if

we assume we have seawater at initial temperature 25Co,

Look at table (6.2).

TCF = (6.1)

Where TCF is temperature correction factor, T is feed water temperature in degrees Celsius. A

temperature of 25 Co is used as a reference point, with TCF = 1, and is constant between 1.024

and 1.03. The next results are based on the above equation. From the results, as temperature of

the feed water raises permeate flux increases. In other words, the rate of the permeate flux

29

changes around 3% for each degree. Another results in a graph (6.3) shows the relation between

the temperature of the fluid and the permeate flux at TDS equals 35,000 mg/l.

Temperature, Co

TCF

30 1.159

35 1.344

40 1.558

45 1.806

Table (6.2) Expected Increase in Flux Due to Temperature Rise.

Figure (6.3): The relation between the feed temperature and the product flow rate.

We find that as temperatures increase, the percentage of permeate flux rises where we see that

the rate of change in permeate flux is about 3 % per degree. Therefore, if the temperature

increases, the permeate flow rate will increase, and then fewer membrane elements will be

required. In addition, when temperature of the feed water goes up, this means the applied

pressure will go down, so energy consumption will decrease too. Water permeability of the

membrane is influenced by increasing the feed water temperature. Based on data from Water

0.0055

0.0056

0.0057

0.0058

0.0059

0.006

0.0061

0.0062

0.0063

25 28 31 34 37 40 43 46 49

Qp,m

3/s

ec.

T,Co

30

Treatment in Riyadh-Saudi Arabia, around 1.5% per degree is expected to increase in water

permeability of the reverse osmosis membrane. Table (6.3) shows another excellent data that has

achieved by Riyadh water treatment plants where they hold the permeation rate to be constant at

378 m3 /day and recover at 75%.

Temperature, Co TDS, mg/l Pressure, bar

5

10

15

20

25

30

35

40

45

50

12

13

14

16

17

19

21

23

26

28

27.3

23.1

19.7

17.0

14.8

14.8

11.9

10.8

9.8

9.0

Table (6.3) Values of Product Concentration and Feed Pressure at Various Temperatures

Figure (6.4): Increasing the feed temperature leads to increase TDS and decreases Pf.

II. Effect of Pressure

Look at the following graphs that are based on a simulation and optimization of full scale reverse

osmosis desalination plant by Sassi and Mujtaba- School of Engineering Design and Technology

0

5

10

15

20

25

30

5 10 15 20 25 30 35 40 45 50

TDS of Product,mg/l

Applied Feed Pressure,bar

T, Co F

eed P

ress

ure

(bar

), a

nd T

DS

(m

g/l

)

31

- University of Bradford- Bradfor. As we see from figure (6.5) Salt rejection increases linearly at

low to moderate pressure. At high pressure, salt rejection decreases because the increase in

osmotic pressure along the feed channel. Also as it is clear from the graph that permeate flux in

the lower pressure region increases linearly which illustrates a linear relationship between the

permeate flux and the driving pressure. However, in the higher pressure region water flux

increases slowly. This result may be due to the accumulation of the salt along the membrane

channel that exerts an increasing osmotic pressure [13].

Figure (6.5): Impact of the applied pressure on the flux and the salt rejection [7].

III. Effect of Salinity on RO Performance

At a constant feed pressure, and by assuming the recovery is constant too, the net driving

pressure decreases if the feed water salinity increases. As a result fresh water production will

decline. In case of salt transport, when the feed water salinity increases, the sale concentration

gradient increases. Therefore, the salt rejection will decrease.

The following chart shows operating factors influence the performance of RO membranes:

Pf increases the product quality increases

Pf decreases the product quality decreases

Tf increases the product quality increases

Per

mea

te F

lux

(m

/s)

10

-5

Pressure (bar)

Sal

t R

ejec

tion %

32

Tf decreases the product quality decreases

TDSf increases the product quality decreases

TDSf decreases the product quality increases

Pp increases the product quality decreases

Pp decreases the product quality increases

IV. Effect of Recovery on RO Performance

Effect of the recovery on the performance of the reverse osmosis system occurs clearly when

the concentration polarization reaches to a high level that can cause to lower the permeate

flux. Also, precipitation of soluble salts has a clear impact on the fresh water production and

salt rejection.

B. Cost of Seawater Reverse Osmosis

Generally, the total cost of reverse osmosis plant consists of two terms: capital cost and

operation/maintenance cost; look at figure (6.6). Capital cost includes implementation of

construction, engineering jobs, administrative, and financing activities. Operation and

maintenance costs consist of plant operation costs such as energy, chemicals, replacement of

consumables, and labor. Also, maintenance costs for plant equipment, buildings, and utilities.

Expenditures for the operating and maintenance costs are expressed per year. As producing the

fresh water, the operating cost is expressed per volume; it means dollar per m3.

As a good benchmark for the reverse osmosis technique, the capital and operating costs of

seawater desalination plants have decreased for some reasons. For example, as capital costs,

process design improvements, membrane performance development, manufacturing methods and

increased competition are helped in lower the capital costs of the plants. Development the

33

performance of the processes, such as raising the membrane life, reducing corrosion, and

improvement the energy efficiency have reduced the operating costs. This great progress in

lower the capital and the operating costs of the reverse osmosis plants due to the completion in

improvement manufacturing techniques. Using the high pressure will increase lowering the

energy costs.

Figure (6.6) Typical cost for a RO Desalination Plant

Typical Costs For a Reverse Osmosis Desalination Plant

Electrical Energy

Fixed Cost

Labor

Membrane Replacement

Maintenance and Parts

Consumable

34

Chapter 7: Future Trends in Seawater Reverse Osmosis Desalination Processes and

Systems

Reverse osmosis technology has proved its ability in desalination of seawater. It has achieved big

successes in producing high quality of potable water and its great consumption of energy

compared with other types of desalination processes. Also, in coming decades, population in

growth, industries in increasing, that means provision of fresh water becomes a most important

issue for attaining stable life. Therefore, there are huge competitions in improvement this

technique. Membrane system, energy recovery devices, and alternative energy are the most

import parts that help in develop and improve reverse osmosis process.

I. Membrane System

Since the membrane system is the key important part in the reverse osmosis process,

improvement of this part is in progressing, see figure (7.1). Development water permeability of

the membrane system in order to reduce energy consumption is possible. However, for seawater

desalination, any significant future reduction of energy requirement is limited by osmotic

pressure of the concentrate and apparent coupling of water and salt transport. For current

commercial RO membranes the increase of water permeability is associated with increase of salt

transport and increased permeate salinity.

The nominal salt rejection of commercial seawater membranes is about 99.85%. In order to

maintain the same permeate salinity at lower feed pressure, membranes with higher water

permeability have to maintain the same salt transport rate, which translates to a proportionally

reduced salt passage i.e. increased nominal salt rejection. Better understanding of the

mechanism of water transfer and salt rejection in RO membranes at the molecular level will lead

to a new era of membrane technologies. Increase membrane resistance to oxidizing agents and

35

chlorine, development of large-size membrane elements and membrane compaction techniques,

and research on the long-term behavior of membranes at elevated temperatures are interesting

and good topics for membrane’s future that can help in improvement RO desalination plant.

Figure (7.1): Evolution of permeability and salt passage of seawater composite polyamide

membranes [11].

II. Energy Recovery Devices

A high pressure pump provides the pressure required for RO treatment. Because of the relatively

high energy requirements, most SWRO systems are equipped with an energy recovery device

that recovers energy from the pressurized RO concentrate leaving the system. The energy

recovery system typically recaptures approximately 50% of the initial pumping energy. There are

a number of devices available commercially that are capable of reducing the unit power

consumption of reverse osmosis units. However, there is a criteria has to be considering for

achieving an excellent goal of the recovery devices. This criterion means paying attention to the

selection of the most suitable, efficient and cost effective device in Sea Water Reverse Osmosis

desalination plants. For example, the Pelton wheel has a high efficiency and low cost motors can

36

be used. This would not only reduce energy consumption but would also save costs of the

equipment and motors required. Therefore, Pelton wheels are extremely useful for reducing

energy consumption as well as costs incurred for operating the HP feed pump; however, this

device is good for small plants. The large plants that have capacity of 200,000m3/day, energy

recovery turbines are most appropriate, see figure (7.2) [11].

Figure (7.2): Comparing between three ERDs [11].

As new energy recovery devices, fluid switcher is a new invention device and it is under

research. There three components of this device, namely, the rotary fluid switcher, and check

valve nest along with two pressure cylinders, , see figure (7.3). Simply, the working principle of

the fluid switcher depends on two strokes. The first stroke occurs when the first cylinder as

shown in figure (7.3) receives high pressure brine, and at the same time the low feed pressure is

pressurized and then pumped out. The second stroke occurs when the high pressure

concentration in the second cylinder is depressurized and drained out by the incoming low

pressure feed. Once the Fluid Switcher energy recovery device completes both strokes, the

37

switcher rotates and the second working phase begins wherein a motor drives it at 7.5 rpm. Thus,

alternative stroke modes are achieved in the cylinders. In order to avoid the intermixing, the feed

and brine are isolated from each other using a piston. As a result, no intermixing will occur

between the feed and the concentration fluid [11].

Figure (7.3): Fluid Switcher ERD [9].

III. Alternative Energy

Reverse osmosis process has a great a chance to invest the renewable energy or what we can call

it hybrid system in order to improve level of this technology. Solar and nuclear energy are

wonderful sources that can support the reverse osmosis system in reduction the energy costs.

A. Solar Energy

Solar energy is a great alternative energy for the energy consumption in the reverse osmosis

plants. Saudi Arabia is one of the most important sources of the solar energy, so there is a great

opportunity for using this type of energy in many applications, such as seawater desalination

plants. Reverse osmosis technique can abandon using the conventional desalination that depends

on fossil fuels especially in Saudi Arabia because the huge amount of solar energy. Therefore,

living in health environment and reducing costs of the desalinations plants will be achieved.

High Pressure

Brine

Cylinder 1 Piston

Cylinder 2

Low Pressure

Brine

Rotary Fluid

Switcher

Motor

Low Pressure

Seawater

Pressurized

Seawater

38

“The experience with solar desalination is investigated based on the analysis of 79 experimental

and design systems worldwide. Our results show that photovoltaic-powered reverse osmosis is

technically mature and — at unit costs as low as 2–3 US$ m–3 — economically cost-competitive

with other water supply sources for small-scale systems in remote areas.” (Messalem, Rami, et

al. 285). This is a good indicator that proves importance of using the solar energy in the reverse

osmosis plants since the obvious consumption of energy that can be attained. In fact, the design

option that has been implemented most frequently in solar driven RO desalination systems is a

combination of RO membranes and arrays of photovoltaic (PV) modules.

B. Nuclear Energy

Nuclear energy could be an option for electricity supply; it can also be used as an energy source

for seawater desalination. RO membrane permeability is improved as feed water temperature

into the system is increased. This results in the possibility of “preheating” the feed water

temperature above ambient seawater temperature, thereby increasing the potential to reduce the

cost of water production.

39

Chapter 8: Conclusion

Comparing with the other different types of desalination processes, seawater reverse osmosis

desalination has proved its great ability in produces high quality of product and consumes less

energy than other desalination processes. Based on its history, in a short time the reverse osmosis

achieved successful progress; for instance, consumption dropping from 20 KWh/m3, to be 3.5

Kwh/m3 in 2000. Another important development of the reverse osmosis technology is becomes

able to remove up to 99.9of the dissolved solids. In fact, this technology is continues toward a

perfect future. For example, the reject or the brine stream that contains high energy has given a

great chance for the competitions to creating new devices that cover this energy and then

optimize the plant. Moreover, the great progress of this type of desalination has made its actual

energy of separation close to the theoretical separation energy. However, in order to attain

purpose of quality and quantity and improve performance of the plant, there are parameters have

to be in account in the reverse osmosis process, such as temperature, pressure, membrane

permeability coefficient, and effect of brine. Reverse osmosis technique has ability to contribute

with another source of energy such as solar and nuclear energy. If these sources of energy use

with the reverse osmosis application, huge energy will saved especially in the countries that are

located in the Middle East such as Saudi Arabia.

40

APPENDIX

41

Table A-1: The palatability of water according to its concentration of total dissolved solids, TDS

(WHO, 1984)

Palatability TDS, mg/l

Excellent Less than 300 mg/l

Good Between 300 and 600 mg/l

Fair Between 600 and 900 mg/l

Poor Between 900 and 1200 mg/l

Unacceptable Greater than 1200 mg/l

Table A-2: The Classification of water according to its concentration of dissolved solids

(National

Research Council, 2004)

Description TDS, mg/l

Potable Water < 1000

Mildly Brackish Water 1000 to 5000

Moderately Brackish Water 5000 to 15000

Heavily Brackish Water 15000 to 35000

Average Seawater 35000

Table A-3: Different salinities in seawaters

Seawater Source Typical TDS

Concentrate, mg/l

Temperature, Co

Pacific/Atlantic

Ocean

35,000 9 - 16 (Avg. 18)

Caribbean 36,000 16 - 35 (Avg. 26)

Mediterranean 38,000 16 – 35 (Avg. 26)

Gulf of Oman, Indian

Ocean

40,000 22 – 35 (Avg. 30)

Red Sea 41,000 24 – 32 (Avg. 28)

Arabic Gulf 45,000 16 - 35 (Avg. 26)

Table A-4: Range of concentrate to which different desalination processes can be applied

Process Concentration Range TDS, mg/l

Ion Exchange 10 – 800

Reverse Osmosis 50 – 50,000

Electro dialysis 200 – 10,000

Distillation Process 20,000 – 100,000

42

Table A-5: Calculation of molar concentration of Pacific Ocean water salts, TDS =35,000 mg/l

Seawater Constituents Concentration, mg/l Concentration, moles/l

Cations

Calcium 403 0.0101

Magnes. 1298 0.0534

Sodium 10693 0.4649

Potass. 387 0.0099

Boron 4.6 0.0004

Bromide 74 0.0009

Total 12859.6 0.5396

Anions

Bicrbonat 142 0.0023

Sulfate 2710 0.0392

Chloride 19284 0.5432

Fluride 1.3 0

Nitrate 78.1 0

Total 22215.4 0.5847

Total, Cations + Anions 35075 1.1243

43

Table A-6: Periodic Table of Chemical Elements

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

I II IIIb IVb Vb VIb VII

b VIIIb Ib IIb III IV V VI VII VIII

1H

1.00

79

2He

4.00

26

3Li

6.94

12

4Be

9.01

21

5B

10.8

11

6C

12.0

10

7N

14.0

06

8O

15.9

99

9F

18.9

98

10Ne

20.1

79

11Na

22.9

89

12M

g

24.3

05

13Al

26.9

81

14Si

28.0

85

15P

30.9

73

16S

32.0

65

17Cl

35.4

53

18Ar

39.9

48

19K

39.0

98

20Ca

40.0

78

21Sc

44.9

55

22Ti

47.8

67

23V

50.9

41

24Cr

51.9

96

25M

n

54.9

38

26Fe

55.8

45

27Co

58.9

33

28Ni

58.6

93

29Cu

63.5

46

30Zn

65.4

09

31Ga

69.7

23

32Ge

72.6

41

33As

74.9

21

34Se

78.9

63

35Br

79.9

04

36Kr

83.7

98

37Rb

85.4

67

38Sr

87.6

21

39Y

88.9

05

40Zr

91.2

24

41N

b

92.9

06

42M

o

95.9

42

43Tc

98.9

06

44Ru

101.

07

45Rh

102.

90

46Pd

106.

42

47A

g

107.

86

48Cd

112.

41

49In

114.

81

50Sn

118.

71

51Sb

121.

76

52Te

127.

60

53I

126.

90

54Xe

131.

29

55Cs

132.

90

56Ba

137.

32

57La

138.

90

72Hf

178.

49

73Ta

180.

94

74W

183.

84

75Re

186.

20

76Os

190.

23

77Ir

192.

21

78Pt

195.

08

79A

u

196.

96

80Hg

200.

59

81Tl

204.

38

82Pb

207.

21

83Bi

208.

98

84Po

208.

98

85At

209.

98

86Rn

222.

01

87Fr

223.

01

88Ra

226.

02

89Ac

227.

02

104R

f

261.

10

105D

b

262.

11

106S

g

266.

12

107B

h

264.

12

108H

s

109M

t

110D

s

111R

g

112U

ub

113U

ut

114U

uq

115U

up

116U

uh

117U

us

118U

uo

Lanthanoids 58Ce

140.

11

59Pr

140.

90

60N

d

144.

24

61P

m

146.

91

62S

m

150.

36

63Eu

151.

96

64G

d

157.

25

65Tb

158.

92

66D

y

162.

50

67Ho

164.

93

68Er

167.

25

69T

m

168.

93

70Y

b

173.

04

71Lu

174.

96

Actinoids 90Th

232.

03

91Pa

231.

03

92U

238.

02

93N

p

237.

04

94Pu

244.

06

95A

m

243.

06

96C

m

247.

07

97Bk

247.

07

98Cf

251.

07

99Es

252.

08

100F

m

257.

09

101M

d

258.

09

102N

o

259.

10

103L

r

260.

10

Alkali

metals

Alkali earth

metals

Transition

metals Other metals

Semi-

metals

Non-

metals Noble gases

44

Table A-7: Ten largest SWRO plants in the world (2004)

Country Location Capacity

(m3/h)

Year of

construction

Membrane

manufacturer

Module

United

Arab

Emirates

Fujairah 7,083 2004 Hydranautics/

Nitto

Spiral wound

Saudi

Arabia

Yanbu 5,333 1998 Toyobo Hollow fiber

Spain Carboner

as

5,000 2003 Hydranautics/

Nitto

Spiral wound

Trinidad

and

Tobago

Point

Lisas

4,542 2002 Hydranautics/

Nitto

Spiral wound

USA Tampa

Bay

3,917 2003 Hydranautics/

Nitto

Spiral wound

Saudi

Arabia

Al Jubail 3,750 2002 DuPont/

Toray

Hollow fiber/

spiral wound

Spain Cartagena 2,708 2002 Hydranautics/

Nitto

Wickel element

Saudi

Arabia

Jeddah I 2,367 1989 Toyobo Hollow fiber

Saudi

Arabia

Jeddah II 2,367 1994 Toyob Hollow fiber

Spain Marbella 2,350 1998 DuPont Hollow fiber

45

REFERENCES

46

REFERENCES

1. Amjad, Zahid. Reverse Osmosis: Membrane Technology, Water Chemistry, and

Industrial Applications. New York: Van Nostrand Reinhold, 1993. Print.

2. “Desalination Facts.” Water Webster. Web. 30 June, 2008. 10 Feb, 2012.

3. Dutang, Michel.“The magazine of scientific chronicles.”Seawater desalination. 1-3, web.

2005. 13 Feb, 2012.

4. Elimelech, Menachem et al. The Future of Seawater Desalination: Energy, Technology,

and the Environment. American Association for the Advancement of Science. December

1-7, 2011. 20 Jan, 2012.

5. Humphery, Jimmy L., and George E. Keller. Separation process technology. New York:

The McGraw-Hill, 1997. Print.

6. Messalem, Rami, et al. Desalination and Water Treatment: Solar-driven desalination

with reverse osmosis. 2009.

7. Mujtab, and Kamal Sassi. Simulation and Optimization of Full Scale Reverse Osmosis

desalination Plant. 2010. Web. 13 March 2012.

8. Raphael, Semiat et al. Energy Aspects in Osmotic Processes.2009. Web. 24 November

2011.

9. “Perth Seawater Desalination Plant, Australia.” Water Technology. Web. 03 May, 2012.

10. Sourirajan, S. Reverse Osmosis. New York: Academic Press. Inc., 1970. Print.

11. Stover, R.L. Energy Recovery Devices in Desalination Applications. Web. 11 January

2012.

47

12. Voutchkov, Nikolay et al. Introduction to Reverse Osmosis desalination. 2010. Web. 12

March 2012.

13. Williams, Michael. A Brief Review of Reverse Osmosis Membrane Technology. 2003.

Web. 09 23 December 2011.