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.

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.

iii

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
iv

Membrane System……………………………………………………………………………34
Energy Recovery Devices…………………………………………………………………….35
Alternative Energy……………………………………………………………………………37
CHAPTER 8
CONCLUSION…………………………………………………………………………………..39
APPENDIX……………………………………………………………………………………....40
REFERENCES…………………………………………………………………………………..45

v

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

vi

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

vii

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

1

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

2

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

3

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).
Applied pressure to overcome osmotic pressure
results in reverse osmosis.

Low Concentrated
Πosm.

High Concentrated
Solution

OSMOSIS

REVERSE OSMOSIS

Figure (1.1): Osmosis and Reverse Osmosis Process.
Semipermeable Membrane

4

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).
Raw Water, Po
Brine, Pe

Brine, Pc
ERD
Membran
e

High Pressure Pump

Feed, Pf

Permeate Flow, Pp

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

5

Feed Water

Permeate

Concentrate

Feed Water

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.

6

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).

7

Power Consumption (KWh/m3)

20
17.5
15
12.5
10
7.5
5
2.5
0
1970

1980

1990

2000

2004

2008

YEAR

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:
8

(
(

)

RT [lnawdnw + lnasdns] = PsVwdnw

) = ΔH

(2.1)

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
9

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.

10

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.
(

)

Where Pf is the feed pressure,

(3.1)
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.

0.6
0.5

ERP

0.4
0.3
0.2
0.1
0
10000 15000 20000 25000 30000 35000 40000 45000 50000
Figure (3.1): The relation between the energy recovery potential and the TDS.

11

TDS,mg/l

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.

12

Membranes

High Pressure Pump

Turbine

Shaft
Seawater Supply Pump

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:
13

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

113.6 m3/h
2 bar

hydraulic, thereby hindering the efficiency of operation.

Membranes

113.6 m3/h
41 bar

63.4 bar

113.6 m3/h

High Pressure Pump

62 bar, 68.1 m3/h
Turbocharger

5 PSI (0.345 bar)
45.5 (m3/h)

0.35 bar, 68.1 m3/h

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
14

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.

High Pressure Pump

Membranes

Circulation Pump

PX-260 Device

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
15

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 highpressure 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:
Qhp,m3/s Ph,pa
Qcp,m3/s Pcp,pa
ηhp
ηhpm ηcp

ηcpm

EC,W

EC,kw

0.06417

0.9

547415.7

547.45

6600000 0.0939

200000

0.9

0.9

0.9

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

16

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.

Feed Water

Permeate

QF

QP
Membrane
Concentrate
QC
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:

17

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)

18

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
19

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.

20

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,
changes in entropy, and

is the

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)
21

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]

Intake
Raw Water
Supply
Feed
Booster

Pumps
No.
6

Flow,m /h
2,200

Diff. Head,
bar
1.0

Energy,
KWh/pump
77

Energy,
KWh, total
462

Specific Energy,
KWh/m3
0.07

6

2,200

2.5

192

1,154

0.18

12

1,042

7.7

281

3,368

0.54

3

22

Table (5.2): Turbine operation for power saving [8]
Pumps
No.

Motors

Energy,
KWh/pump

Energy,
KWh,
total

12

1,042

69.3

2,381

28,567

12
12

521

73.0

-980

-11,763

1,444

HighPressure
Aggregate:
Pumps
Turbine

Flow,m /h

Diff.
Head,
bar

17,323

2.77

400

400

0.06
3.63

3

Auxiliary
+ Lighting
Total

Specific Energy
KWh/m3

Table (5.3): Pressure exchangers for power saving (estimate) [8]
Pumps
No.
HighPressure
Aggregate:
6
Pumps
Pressure Depend
exchangers
on
size/n
Auxiliary
pumps
6

Flow,m3/h

Diff.
Head,
bar

Energy,
KWh/pump

Energy,
KWh,
total

Specific Energy
KWh/m3

1,042

69.3

2381

14284

6252/n

6252/n

1042

3.3

132

792

1,444

15,076

2.41

400

400

0.06
3.26

Motors
12
Auxiliary
+ Lighting
Total

23

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.

24

90
80

Applied Pressure, psi

70
60
50
40
30
20
10
0
0

1000

2000

3000

4000

5000

6000

7000

8000

Feed Concentration, ppm
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.
25

0.0000012

Kw,m3/m2s Kpa

0.000001
0.0000008
0.0000006
0.0000004
0.0000002
0
20

25

30

35

40

45

50

55

Applied Pressure, bar
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

26

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 feedbrine 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

27

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

28

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.
0.0063
0.0062

Qp,m3/sec.

0.0061
0.006
0.0059
0.0058
0.0057
0.0056
0.0055
25

28

31

34

37

40

43

46

49

T,C
o

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
29

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
5
10
15
20
25
30
35
40
45
50

TDS, mg/l
12
13
14
16
17
19
21
23
26
28

Pressure, bar
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

Feed Pressure (bar), and TDS (mg/l)

30
25
20
TDS of Product,mg/l

15

Applied Feed Pressure,bar
10
5
0
5

10

15

20

25

30

35

40

45

50

T, Co

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

30

- 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

Salt Rejection %

Permeate Flux (m/s) 10-5

channel that exerts an increasing osmotic pressure [13].

Pressure (bar)
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
31

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
32

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.
Typical Costs For a Reverse Osmosis Desalination Plant

Electrical Energy
Fixed Cost
Labor
Membrane Replacement
Maintenance and Parts
Consumable

Figure (6.6) Typical cost for a RO Desalination Plant

33

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

34

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
35

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

36

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].
Low Pressure
Seawater

High Pressure
Brine
Piston

Cylinder 1

Low Pressure
Brine
Cylinder 2
Motor
Pressurized
Seawater Figure (7.3): Fluid Switcher ERD [9].

Rotary Fluid
Switcher

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.

37

“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.

38

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.

39

APPENDIX

40

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
Pacific/Atlantic
35,000
Ocean
Caribbean
36,000
Mediterranean
38,000
Gulf of Oman, Indian 40,000
Ocean
Red Sea
41,000
Arabic Gulf
45,000

Temperature, Co
9 - 16 (Avg. 18)
16 - 35 (Avg. 26)
16 – 35 (Avg. 26)
22 – 35 (Avg. 30)
24 – 32 (Avg. 28)
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

41

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

42

Table A-6: Periodic Table of Chemical Elements
1
2
3
4
5
6
7
8
9 10
VII
I
II IIIb IVb Vb VIb
VIIIb
b
1H
1.00
79
3Li 4Be
6.94 9.01
12 21
12M
11Na
g
22.9
24.3
89
05
25M
Fe Co 28Ni
19K 20Ca 21Sc 22Ti 23V 24Cr
n 26 27
39.0 40.0 44.9 47.8 50.9 51.9
55.8 58.9 58.6
54.9
98 78 55 67 41 96
45 33 93
38
41N 42M
Tc Ru Rh 46Pd
37Rb 38Sr 39Y 40Zr
b
o 43 44 45
85.4 87.6 88.9 91.2
98.9 101. 102. 106.
92.9 95.9
67 21 05 24
06 07 90 42
06 42

11

12

13

14

15

16

Ib

IIb

III

IV

V

VI VII VIII

104R 105D 106S 107B

f
b
g
h
223. 226. 227.
261. 262. 266. 264.
01 02 02
10 11 12 12
58Ce 59Pr

60N

18

2He

4.00
26
5B
6C
7N
8O
9F 10Ne
10.8 12.0 14.0 15.9 18.9 20.1
11 10 06 99 98 79
13Al 14Si 15P 16S 17Cl 18Ar
26.9 28.0 30.9 32.0 35.4 39.9
81 85 73 65 53 48

29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br 36Kr

63.5 65.4 69.7 72.6 74.9 78.9 79.9 83.7
46 09 23 41 21 63 04 98
47A

g
107.
86
79A
55Cs 56Ba 57La 72Hf 73Ta 74W 75Re 76Os 77Ir 78Pt
u
132. 137. 138. 178. 180. 183. 186. 190. 192. 195.
196.
90 32 90 49 94 84 20 23 21 08
96
87Fr 88Ra 89Ac

17

48Cd 49In 50Sn 51Sb 52Te

53I

54Xe

112. 114. 118. 121. 127. 126. 131.
41 81 71 76 60 90 29
80Hg 81Tl 82Pb 83Bi 84Po 85At 86Rn

200. 204. 207. 208. 208. 209. 222.
59 38 21 98 98 98 01

108H 109M 110D 111R 112U 113U 114U 115U 116U 117U 118U

s

t

s

61P

62S

63Eu

g
64G

ub

66D

uq

up

43

67Ho 68Er

uh

us

69T

70Y

uo

Lu
d m m
d
y
m
b 71
140. 140.
151.
158.
164. 167.
174.
144. 146. 150.
157.
162.
168. 173.
11 90
96
92
93 25
96
24 91 36
25
50
93 04
A C
F M N L
93N
Pu 95 96 97Bk 98Cf 99Es 100 101 102 103
90Th 91Pa 92U
p 94
m m
m d
o
r
Actinoids
232. 231. 238.
244.
247. 251. 252.
237.
243. 247.
257. 258. 259. 260.
03 03 02
06
07 07 08
04
06 07
09 09 10 10
Alkali
Alkali earth
Transition
SemiNonOther metals
Noble gases
metals
metals
metals
metals
metals
Lanthanoids

65Tb

ut

Table A-7: Ten largest SWRO plants in the world (2004)
Country Location Capacity
Year of
Membrane
(m3/h)
construction manufacturer
2004

Hydranautics/

Module

United
Fujairah
Arab
Emirates

7,083

Spiral wound

Saudi
Arabia

Yanbu

5,333

1998

Toyobo

Hollow fiber

Spain

Carboner
as

5,000

2003

Hydranautics/

Spiral wound

Trinidad
and
Tobago

Point
Lisas

4,542

USA

Tampa
Bay

3,917

Saudi
Arabia

Al Jubail

3,750

Spain

Cartagena 2,708

Nitto

Nitto
2002

Hydranautics/

Spiral wound

Nitto
2003

Hydranautics/

Spiral wound

Nitto
2002

DuPont/
Toray

spiral wound

Hydranautics/

2002

Hollow fiber/

Wickel element

Nitto
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

44

REFERENCES

45

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47