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An Introduction to Desalination

Christos Charisiadis 2014

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Contents

1. Water Scarcity/ Water Salinity/ Brine/ Desalination Basics 3

1.1 Water Scarcity 3

1.2 Water Salinity 4

1.3 What is Salt (Brine)? 6

1. 4 Desalination Basics 7

2. Membrane technologies 8

2.1 Reverse Osmosis 8

2.2 Nanoflitration 12

2.3 Membrane Distillation 16

3. Thermal technologies 21

3.1 Multi-Stage Flash 22

3.2 The Multi-Effect distillation 24

3.3 Vapour Compression 25

3.4 Forward Osmosis 26

4. Ion-Exchange technologies 29

5. Negative viewpoints that surround the use of desalination 31

6. Future Options of Desalination 32

6.1 Ion concentration polarization process 32

6.2 Nanotechnology/Reverse osmosis 34

6.3 Graphene sheets 35

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DESALINATION

1. Water Scarcity/ Water Salinity/ Brine/ Desalination Basics

1.1 Water Scarcity

Water covers 70% of our planet, and it is easy to think that it will always be plentiful.

However, freshwater—the stuff we drink, bathe in, irrigate our farm fields with—is

incredibly rare. Only 3% of the world’s water is fresh water, and two-thirds of that is tucked

away in frozen glaciers or otherwise unavailable for our use.

As a result, some 1.1 billion people worldwide lack access to water, and a total of 2.7 billion

find water scarce for at least one month of the year. Inadequate sanitation is also a problem

for 2.4 billion people—they are exposed to diseases, such as cholera and typhoid fever, and

other water-borne illnesses. Two million people, mostly children, die each year from

diarrheal diseases alone.

Water scarcity involves water stress, water shortage or deficits, and water crisis. While the

concept of water stress is relatively new, it is the difficulty of obtaining sources of fresh

water for use during a period of time and may result in further depletion and deterioration

of available water resources. Water shortages may be caused by climate change, such as

altered weather patterns including droughts or floods, increased pollution, and increased

human demand and overuse of water. A water crisis is a situation where the available

potable, unpolluted water within a region is less than that region's demand. Water scarcity is

being driven by two converging phenomena: growing freshwater use and depletion of

usable freshwater resources.

Water scarcity can be a result of two mechanisms: physical (absolute) water scarcity and

economic water scarcity, where physical water scarcity is a result of inadequate natural

water resources to supply a region's demand, and economic water scarcity is a result of poor

management of the sufficient available water resources. According to the United Nations

Development Program, the latter is found more often to be the cause of countries or regions

experiencing water scarcity, as most countries or regions have enough water to meet

household, industrial, agricultural, and environmental needs, but lack the means to provide

it in an accessible manner.

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With human population expected to balloon another 50% by 2050, resource managers are

increasingly looking to alternative scenarios for quenching the world's growing thirst.

Desalination, the process of turning seawater into drinking water, is used by many countries

as a way of creating a more reliable water supply that doesn’t depend on rain.

1.2 Water Salinity

You may know that the oceans cover about 70% of the Earth's surface, and that about 97 %

of all water on and in the Earth is saline. The salt concentration is usually expressed in parts

per thousand (per mille, ‰) or parts per million (ppm). The United States Geological Survey

classifies saline water in three salinity categories. Salt concentration in slightly saline water is

around 1,000 to 3,000 ppm (0.1-0.3%), in moderately saline water 3,000 to 10,000 ppm (0.3-

1%) and in highly saline water 10,000 to 35,000 ppm (1-3.5%). Seawater has a salinity of

roughly 35,000 ppm, equivalent to 35 grams of salt per one liter (or kilogram) of water.

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Satellite view of La Plata River

,Uruguay discharge to the Atlantic

Ocean. One way minerals and salts

are deposited into the oceans is

from outflow from rivers, which

drain the landscape, thus causing

the oceans to be salty.

Some oceanographers dedicate their life to measuring very small changes in salinity, since

this can affect large-scale ocean circulation patterns and can also give valuable information

about changes in rainfall and storm patterns. In fact, small changes in salinity are what first

alerted scientists that global warming has already caused large-scale changes in the Pacific

and Atlantic Oceans.

By some estimates, if the salt in the ocean could be removed and spread evenly over the

Earth's land surface it would form a layer more than 166 meter thick, about the height of a

40-story office building. But, where did all this salt come from?

Why Seawater is saline?

First, “fresh” water is not entirely free of dissolved salt. Even rainwater has traces of

substances dissolved in it that were picked up during passage through the atmosphere.

Much of this material that “washes out” of the atmosphere today is pollution, but there are

also natural substances present.

1. From precipitation to the land to the rivers to the sea

The rain that falls on the land contains some dissolved carbon dioxide from the surrounding

air. This causes the rainwater to be slightly acidic due to carbonic acid. As rainwater passes

through soil and percolates through rocks, it dissolves some of the minerals, a process called

weathering. This is the water we drink, and of course, we cannot taste the salt because its

concentration is too low. Eventually, this water with its small load of dissolved minerals or

salts reaches a stream and flows into lakes and the ocean. The annual addition of dissolved

salts by rivers is only a tiny fraction of the total salt in the ocean. The dissolved salts carried

by all the world’s rivers would equal the salt in the ocean in about 200 to 300 million years.

Water salinity based on dissolved salts

Fresh water Brackish water Saline water Brine

< 0.05% 0.05–3% 3–5% > 5%

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The Mariana Arc, Mariana islands is

part of the "Ring of Fire" in the

western Pacific Ocean where

tectonic plates are moving

relatively quickly. Hydrothermal

vents, such as these, are present,

and they release large amounts of

carbon dioxide and minerals.

2. Salt comes up from below, too

Rivers and surface runoff are not the only source of dissolved salts. Hydrothermal vents are

recently-discovered features on the crest of oceanic ridges that contribute dissolved

minerals to the oceans. These vents are the exit point on the ocean floor from which sea

water that has seeped into the rocks of the oceanic crust has become hotter, has dissolved

some of the minerals from the crust, and then flows back into the ocean. With the hot water

comes large amounts of dissolved minerals. Estimates of the amount of hydrothermal fluids

now flowing from these vents indicate that the entire volume of the oceans could seep

through the oceanic crust in about 10 million years. Thus, this process has a very important

effect on salinity. The reactions between seawater and oceanic basalt, the rock of ocean

crust, are not one-way, however; some of the dissolved salts react with the rock and are

removed from seawater.

A final process that provides salts to the oceans is submarine volcanism, the eruption of

volcanoes under water. This is similar to the previous process in that seawater is reacting

with hot rock and dissolving some of the mineral constituents.

Will the oceans continue to become saltier? Not likely. In fact the sea has had about the

same salt content for many hundreds of millions if not billions of years. The salt content has

reached a steady state. Dissolved salts are being removed from seawater to form new

minerals at the bottom of the ocean as fast as rivers and hydrothermal processes are

providing new salts.

1.3 What is Salt (Brine)?

Brine is a solution of salt (usually sodium chloride) in water. Sodium chloride, also known as

salt, common salt, table salt or halite, is an ionic compound with the chemical formula NaCl,

representing equal proportions of sodium and chlorine.

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The attraction between the Na+ and Cl− ions in the solid is so strong that only highly polar

solvents like water dissolve NaCl well. When dissolved in water, the sodium chloride

framework disintegrates as the Na+ and Cl− ions become surrounded by the polar water

molecules.

2 NaCl(aq) + 2 H2O(l) → 2 NaOH(aq) + H2(g) + Cl2(g)

1. 4 Desalination Basics

Reducing salt water to its basic elements - salt and water - is so simple that it's become a

science lesson for first-graders. In fact, a "solar still" can turn salt water into fresh water in

just a few days. Simply fill a large bowl with salt water and set an empty glass at the center.

Then cover the bowl -- empty glass and all -- with plastic wrap that has a small hole poked in

the middle. Place the contraption in direct sunlight, and watch the water cycle at work: The

salt water evaporates, leaves salt crystals behind, and creates condensation that rises,

gathers on the plastic membrane and drips into the empty glass. The resulting fresh water is

good enough to drink.

But why remove salt in the first place? Turns out, drinking salt water can kill you. Ingesting

salt signals your cells to flush water molecules to dilute the mineral. Too much salt, and this

process can cause a really bad chain reaction: Your cells will be depleted of moisture, your

kidneys will shut down and your brain will become damaged. The only way to offset this

internal chaos is to urinate with greater frequency to expel all that salt, a remedy that could

work only if you have access to lots of fresh drinking water

Desalinization is the process in which salt is removed from water to make it consumable.

Desalinization still proves to be a costly method of acquiring freshwater. When it comes to

desalinating salt water, two of the main options are thermal distillation and reverse osmosis.

Thermal distillation involves boiling the water and collecting the resulting freshwater

condensation, while reverse osmosis involves pressurizing the salt water and forcing it

through a semi-permeable membrane, which will allow water molecules to pass through,

but not salt. And then there's membrane distillation which is a cross between thermal

distillation and reverse osmosis. It involves heating the salt water then passing it through a

tube made from a semi-permeable membrane, which allows water vapor to pass through

while not admitting salt molecules. All of these methods, however, require a considerable

amount of energy – not as environmentally sound as they could be, nor entirely practical for

use in developing nations, where electricity isn't readily available.

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Another problem with these methods of acquiring freshwater is not only their price, but the

fact that desalinization produces brine, which needs to be diverted back to the oceans or

seas, and creates an instability that was non-existent before. This by-product of desalinated

water contains high concentrations of salt and, when released back into a natural body of

water, can cause damage to marine life. That's because brine, which is usually denser than

the water into which it's released, settles atop low-lying sediment where it depletes

surrounding waters of oxygen. Also, the plants that carry out desalinization often emit some

type of pollutant, which can be a uneasy trade-off in many cases.

Desalination plants can have an indirect impact on the environment because many plants

receive energy from the local grid instead of producing their own. The burning of fossil fuels

and increased energy consumption allows more air pollution and gas emissions to occur.

Gaseous emissions from desalination stacks include carbon monoxide (CO), nitric oxide (NO),

nitrogen dioxide (NO2 ), and sulfur dioxide (SO2 ). These air pollutants can have a harmful

impact on public health.

Desalination techniques fall under 3 categories:

1. Membrane technologies

2. Thermal technologies

3. Ion-Exchange technologies

Reverse Osmosis is described as using the pressure difference in saltwater and pure water to

remove salts from water after the saltwater passes through membrane pores. Thermal

technologies use evaporation and distillation processes to separate the salt from saltwater.

According to WateReuse Research Foundation, 34% of the technology used by desalination

plants worldwide is thermal and 61% is Reverse Osmosis, which is contained under the

membrane technologies.

A simulation portrays how brine

plumes would disperse when

discharged from a proposed

desalination plant in California.

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1. Membrane technologies

1.1 Reverse Osmosis

Remember why it's so bad to drink salt water? When your cells pass water through the outer

membrane to keep you from dehydrating, osmosis is occurring. By moving the water

through the membrane, the cell is attempting to equalize its high internal salt concentration

with a low external salt concentration. That's called osmosis. Reverse osmosis occurs when,

for example, you put salt water on one side of a semi-permeable membrane and pressure

moves the water molecules through the filtering membrane as the larger molecules --

including salt molecules -- stay trapped behind. For salty sea or ocean water, a considerable

amount of pressure is required to move the water through a filter, where each pore is just a

fraction of the size of a human hair. This means a series of pumps are usually in play, all

exerting pressure on the.

RO tends to be a costly process due to the amount of energy needed to operate the pumps

to raise the pressure applied to feedwater. Extensive treatment must also be applied to the

feedwater, before being pumped, to protect the RO membranes. Chlorine cannot be used

due to how sensitive the membrane is to this chemical. Post-treatment of the membrane is

also required in some circumstances, depending on the use of the product water. The

amount of freshwater produced from the amount of feedwater, or the recovery rate, varies

from 30-80%. It could be argued that in some cases, people are not getting the “bang for

their buck” based on these percentages.

Here's a look at the step-by-step process of reverse osmosis desalination:

1. To set up a reverse osmosis desalinator, you first need an intake pump at the source of

the seawater.

2. Next, you need to create flow through the membrane. This will cause water to pass

through the salted side of the membrane to the unsalted side. Pressure comes from a water

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column on the salted side of the membrane. This will both remove the natural osmotic

pressure and create additional pressure on the water column, which will push the water

through the membrane. Generally, to desalinate saltwater, you need to get the pressure up

to about 50 to 60 bars [source: Lentech].

3. Feed water is then pumped into a closed container. As the water passes through the

membrane, the remaining feed water and salt solution become more concentrated. To

reduce the concentration of the remaining dissolved salts, some of the feed water and salt

solution is taken out of the container because the dissolved salts in the feed water would

continue to increase and thus require more energy to overwhelm the natural osmotic

pressure.

4. Once water is flowing through the membrane, and the pressure is equal on both sides, the

desalination process begins. After reverse osmosis has occurred, the water level will be

higher on the side where salt was added. The difference in water level is caused by the

addition of the salt and is called osmotic pressure; generally, the osmotic pressure of

seawater is 26 bars. The quality of water is determined by the pressure, the concentration of

salts in the feed water, and the salt permeation constant of the semi-permeable membrane.

To improve the quality of the water, you can do a second pass of membrane.

Once the freshwater and saltwater are separated, the freshwater should be stabilized; that

is, the pH should be tested to make sure it's fit for consumption.

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1.2 Nanoflitration

Nanofiltration (NF) membranes have applications in several areas. One of the main

applications has been in brackish and sea water treatment for drinking water production as

well as for wastewater treatment. NF can either be used to treat all kinds of water including

ground, surface, and wastewater or used as a pre treatment for desalination. The

introduction of NF as a pre treatment is considered a breakthrough for the desalination

process. NF membranes have the ability to remove turbidity, hardness, fluoride and nitrate

as well as a significant fraction of dissolved salts. Desalination can be performed with a

significantly lower operating pressure and becomes a much more energy‐efficient process.

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There are primarily four types of membranes, each representing a smaller pore size and

increased ability to capture contaminants. Moving from largest to smallest (in terms of pore-

size): microfiltration (0.1 microns ) and ultrafiltration (0.01 microns) are good, but

nanofiltration offers even smaller pores (0.001 microns) to filter out pollutants and bacteria,

and reverse osmosis (at 0.0001 microns) provides the smallest pores.

Nano-particles jointly form a large surface area or net to catch pollutants in water. One type

of nano membrane can be explained by a further analogy. Imagine a volleyball net stretched

across the pipe where the water enters. With a nano membrane that builds in ion-repelling

materials, the net of nano particles catches the contaminants (the volleyballs) by electrifying

the ions in the contaminants, such as toxic metals. This produces a net of electricity that

basically organizes the ions into channels based on their negative charge. This allows the

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neutral water to pass through the channel with the negative toxic ions lined up at the end of

the channel. So, in terms of the volleyball comparison, the volleyball toxins line up along the

surface of the net. Each hole in the volleyball net represents a channel where the water

molecules (the golf balls) line up and pass through.

While the technology for such reverse osmosis membranes have become conventional in

desalinization, nanofiltration is less common. The big difference–and the reason for the

interest in nano membranes–is that nanofiltration is much less expensive–about half the

cost of reverse osmosis, because nano membranes do not require the high pressure that

reverse osmosis membranes do.

When coupled with nano-filtration or NF membranes, reverse osmosis membranes are

called dense membranes. These two membranes can remove a whole range of “dissolved

species such as ions” in water. In other words, while combined RO and NF membranes are

more effective at removing contaminants (than traditional desalinization processes) when

they have smaller pores, they require more energy to push the water through the

membranes. So, the energy costs of electricity presumably go up once membranes are

combined. In some cases, the addition of membranes to the system may serve as a

pretreatment to prevent membrane fouling.

Nanofiltration; divalent cations and anions are preferentially rejected over the monovalent

cations and anions. Some organics with MW > 100 -500 are removed There is an osmotic

pressure developed but it is less than that of the R.O. process

Microfiltration and Ultrafiltration are essentially membrane processes that rely on pure

straining through porosity in the membranes. Pressure required is lower than R.O. and due

entirely to frictional headloss

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Requiring less time than conventional water treatment, nanofiltration typically occurs in a

single treatment and produces cleaner water. Nanofiltration seems particularly well suited

to treating ground water because of its ability to soften water and remove pesticides. Those

developing nations where salt removal is not the top priority and where farmers use nitrates

seem especially good candidates for low pressure nanofiltration systems. As water from

wells and underground sources is less available, rivers and lakes become a source of water.

Agricultural run-off from food crops and livestock make surface water more difficult to

purify because of such occurrences as bacteria from livestock. Nanofiltration utilizes several

polymer layers or sheets of filter materials. Most often, these sheets are rolled vertically to

fit into a cylinder, as seen in the following diagram.

There are environmental and health concerns about nanotechnology as well. These include

contamination resulting from the technology itself, the difficulty of containing the nano-

particles, and the absence of studies about the effects on humans. Within the water cleaning

membranes are nano-filters made of polymers that hold the nano-particles inside. This

cleans the water with the nano-particles going into the main stream of fresh water. In 2008,

there was a concern that the nano-particles are staying in the water, and scientists have not

yet studied the effects on humans. Some membranes use nano-particles such as metal,

dendrimers, and clays to clean the water. These are usually not considered toxic materials;

however, the concern is that their accumulation in the body could be toxic.

There are several health concerns about cleaning water with nanotechnology. One of these

concerns is its size. Since nanotechnology is so small, it can be hard to contain the nano-

particles. Researchers would need to make sure nano-particles do not contaminate the

environment and harm aquatic life. Because of its size, moreover, scientists do not know

what nano-particles can do to a human body once they are consumed in water or

concentrated in the body of fish. By comparison, pregnant women are warned not to eat

clams, for instance, because of the accumulated mercury contained in them. This means that

there may be traces of nano-particles in the water left behind after it is cleaned. People are

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concerned that nano-particles may be toxic because there has not been a study showing

that they are not.

1.3 Membrane Distillation

Membrane Distillation (MD) is a thermally-driven separation process in which only vapor

molecules are able to pass through a hydrophobic microporous membrane. The hydrophobic

nature of the membrane prevents the penetration of aqueous solution into the pores, thus

creating a vapor-liquid interface at each pore entrance. The driving force in MD is the vapor

pressure difference between the two sides of the membrane. The interest of using MD

process for desalination is increasing worldwide especially when using low grade heat

source.

By passing hot water through one side of the membrane, only the water vapour generated

from the seawater passes through. When the water reach the other side of the membrane

where cold seawater is flowing, it condenses into water droplets, producing fresh water.

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The advantages of MD compared to other process for desalination are:

1. Operating at low temperature below the boiling point, vapour space than MSF and

MED and can use alternative energy source such as solar energy, geothermal energy

and low cost

2. Operating at low pressure (atmospheric or vacuum pressure) and performance was

not limited by high osmotic pressure, while RO (50-80 bar for seawater) .

3. Reduced chemical interaction between membrane and process solutions.

4. Capable of treating highly concentrated solution more than seawater.

5. Higher salt rejection 99.99-100% while RO 95-98%

6. Less sensitive to fouling due to large pores 0.1- 1 μm and not limited by

concentration polarization

MD Configurations

A variety of methods may be employed to impose the vapour difference, which differ based

on the nature of the cold side processing on the permeate in general there are four kinds of

MD system configuration,

Direct contact membrane distillation (DCMD)

Air gap membrane distillation (AGMD)

Sweep gap membrane distillation (SGMD)

Vacuum membrane distillation (VMD)

Direct contact membrane distillation (DCMD)

In this configuration a hot feed solution is in direct contact with one side of the membrane

and colder water is in direct contact with the opposite side of the membrane. The vapor is

moved by the pressure difference across the membrane to the permeate side and condense

inside the membrane module. This is the simplest system in design because condensation is

carried out inside the membrane module. Advantage of this configuration high permeate

flux. The main drawback of this configuration is the heat loss by conduction.

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Air gap membrane distillation (AGMD)

The feed solution is in direct contact with the hot side of the membrane surface only.

Stagnant air is introduced between the membrane and the condensation surface. The

evaporated volatile molecules cross the membrane pore and the air gap to condense over

the cold surface inside the membrane cell. The advantage of this configuration is low heat

lost by conduction Due to the presence of air in the permeate side of the membrane.

Disadvantage of this type low permeate flux due to additional resistance to mass transfer

due to the presence of air gap between cold side of the assembly and permeate side of the

membrane.

Sweep gas membrane distillation (SGMD)

Inert gas is used to sweep the vapor at the permeate membrane side to condense outside

the membrane module. There is a gas barrier reduce heat loss by conduction but this is not

stationary which enhances the mass transfer coefficient and lead to high permeate flux. The

flux in SGMD is independent on the temperature of the sweep gas. The pressure drop of the

sweep gas increase as the velocity increase and the resistance in the boundary layer increase

The main disadvantage of this configuration is that a small volume of permeate diffuses in a

large sweep gas volume, requiring a large condenser.

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Vacuum membrane distillation (VMD)

In this configuration feed solution is brought in to contact with one side of hydrophobic

micro porous membrane and vacuum pulled on the opposite side to create a driving force

for mass transfer by a pump. Condensations take place outside the membrane module. VMD

can be characterized by the following steps:

1. Vaporization of the more volatile compounds at the liquid vapor interface

2. Diffusion of the vapor through the membrane pores

3. Withdrawal of water vapor from the membrane unit under vacuum

VMD has a number of advantage over conventional MD configuration, Production of pure

distilled water at lower operating temperature, resulting lower in cost and lower energy

requirements are need to achieve similar flux compared to other distillation and

desalination process. VMD is a promising technology that has the potential to become as

important as the conventional distillation and pressure driven membrane technology for

water desalination.

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2. Thermal technologies

Thermal-based technologies are often used for both energy production and water

desalination. Multi-stage flash (MSF) and vapor compression (VC) are the two most

common thermal technologies used, the former being used mainly in the Middle East. The

MSF process is a complex process in which saltwater travels through tubes and is preheated

by water vapor. The saltwater then empties into a brine pool, called a stage, where it

evaporates. The vapor from evaporation is used to heat the incoming saltwater. The vapor

then condenses to form potable water.

Vapor compression uses compressed vapor to heat saline or seawater in a distillation

process.

In a single-effect distiller, a heating element heats the water until it boils and eventually

becomes steam. The steam is then drawn away from the boiling chamber, where it cools,

condensing into highly treated distilled water. The contaminants in the original water are

left behind in the boiling chamber.

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2.1 Multi-Stage Flash

Unlike reverse osmosis, which relies on a membrane to filter out salt molecules, the

multistage flash method uses heat to convert salt water into fresh water. Why such an

unusual name? "Flash" refers to rapidly bringing the water to a boil, and this happens

multiple times, or in stages. As the salt water enters each stage of the conversion unit, it is

subjected to externally supplied steam heat and pressure. During each stage, water vapor

forms and is collected. This water vapor is fresh water and the left-behind salty concentrate

is known as brine. In multistage flash distillation -- as with reverse osmosis -- chemicals or

water softening agents are not usually added MSF can handle large quantities of water, but

requires the greatest amount of energy of all technologies.

Seawater enters the Brine Heater in a bank of tubes, where steam is condensing on the

outside. The heated seawater flows to the first stage, where it flashes upon entry. During

flashing, some of the water vapour (steam) is removed from seawater. The flashed vapour is

then condensed on the outside of the tubes carrying seawater feed to the brine heater.

The condensed steam is withdrawn as fresh water. The unflashed portion of seawater now

contains more salts, and is send to the second stage for further flashing. The second stage is

operated at a pressure lower than the first stage in order to lower the boiling point of

seawater.

At the second stage, more water vapour (steam) is flashed off, and is again recovered as

fresh water by condensing on the tubes carry seawater feed to the first stage. The remaining

seawater is then send to the third stage, at a lower pressure than the second stage, for more

separation.

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Multi-stage flash distillation plants built commercially has capacities ranging from 4,000 to

30,000 m3/day, and usually operates at the top feed temperature (after the brine heater) of

90 - 120 oC. A typical MSF plant can contain from 4 to about 40 stages, with each successive

stage operating at a lower pressure and temperature than the previous one. This allows the

reduction of boiling point of the seawater as it gets more concentrated in going down the

stages. Multiple boiling is thus possible without the supply additional heat after the brine

heater.

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2.2 The Multi-Effect distillation (MED)

In this process, sea water is warmed at 70-80°C. Evaporation takes place on an exchange

surface next to a boiler. The vapor produced in the first pool is condensed and goes out by a

pipe. The rest of the water goes in the second pool by another pipe. Water is warmed again,

vapor goes out in a pipe and the water in the third pool. Water is brought continually in the

pools and the bittern is evacuated . This technique is not very expensive because heat is only

necessary to warm the water of the first pool. For the others, the warm of the vapor in used.

The plant can be seen as a sequence of closed spaces separated by tube walls, with a heat

source in one end and a heat sink in the other end. Each space consists of two

communicating subspaces, the exterior of the tubes of stage n and the interior of the tubes

in stage n+1. Each space has a lower temperature and pressure than the previous space, and

the tube walls have intermediate temperatures between the temperatures of the fluids on

each side. The pressure in a space cannot be in equilibrium with the temperatures of the

walls of both subspaces. It has an intermediate pressure. Then the pressure is too low or the

temperature too high in the first subspace, and the water evaporates. In the second

subspace, the pressure is too high or the temperature too low, and the vapor condenses.

This carries evaporation energy from the warmer first subspace to the colder second

subspace. At the second subspace the energy flows by conduction through the tube walls to

the colder next space.

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2.3 Vapour Compression

Vapor compression uses compressed vapor to heat saline or seawater in a distillation

process.The effect of compressing water vapor can be done by two methods.

The first method utilizes an ejector system motivated by steam at manometric pressure

from an external source in order to recycle vapor from the desalination process. The form is

designated Ejecto or Thermo Compression.

Using the second method, water vapor is compressed by means of a mechanical device,

electrically driven in most cases. This form is designated mechanical vapor compression

(MVC). The MVC process comprises two different versions: Vapor Compression (VC) and

Vacuum Vapor Compression (VVC). VC designates those systems in which the evaporation

effect takes place at manometric pressure, and VVC the systems in which evaporation takes

place at sub-atmospheric pressures (under vacuum).

The compression is mechanically powered by something such as a compression turbine. As

vapor is generated, it is passed over to a heat exchanging condenser which returns the vapor

to water. The resulting fresh water is moved to storage while the heat removed during

condensation is transmitted to the remaining feedstock.

The VVC process is the more efficient distillation process available in the market today in

terms of energy consumption and water recovery ratio[citation needed]. As the system is

electrically driven, it is considered a "clean" process, it is highly reliable and simple to

operate and maintain.

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Some new technologies are either expected to be in use within the next 5 years, or

prototypes are already being set up.

2.4 Forward Osmosis

FO is a process that is expected to be marketed between 2010 and 2013. Like reverse

osmosis, the most state-of-the-art desalination method, forward osmosis uses a permeable

membrane to separate solutes from water. Unlike reverse osmosis (RO), forward osmosis

(FO) uses no energy to migrate water molecules from salt water into an even more

concentrated, but specialized, salt solution, which can then be evaporated at a much lower

heat. Carbon Nanotubes, expected to be implemented between 2013-2015, are added to

permeable membranes. The nano tubes are electrically charged, causing them to repel the

positively charged salt, but allowing water to pass with relatively little resistance.

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Osmotic pressure is the minimum pressure which needs to be applied to a solution to

prevent the inward flow of water across a semipermeable membrane.

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3. Ion-Exchange technologies

Electrodialysis; Electrodialysis reversal utilizes a membrane, like that in reverse osmosis,

but sends an electric charge through the solution to draw metal ions to the positive plate on

one side, and other ions (like salt) to the negative plate on the other. The charges are

periodically reversed to prevent the membrane from becoming too contaminated, as

typically found in regular electrodialysis. The ions located on both plates can be removed,

leaving pure water behind. Recently developed membranes reportedly have been chlorine

resistant, and generally remove more harmful ions (not just salt) than reverse osmosis. The

primary setback to electrodialysis reversal is the upfront cost to create the facility, as well as

the energy costs.

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Other new

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technologies are membrane distillation (MD), freeze separation (FS), and rapid spray

evaporation (RSE). MD uses differences of vapor pressure on two sides of a membrane to

allow for vapor to pass through the membrane. Only water vapor passes through the

membrane, but this method requires a a very large energy input. FS is a simple technique

that freezes saltwater, and creates a product of pure ice crystals. The ice crystals are then

separate from the brine, and melted. This method requires less energy that evaporation

techniques. Finally, RSE is an application that sprays water through nozzles at high

velocities. The water vaporizes as it exits, but the salt does not. RSE can use waste energy,

but is not viable for large applications, yet.

4. Negative viewpoints that surround the use of desalination

During the past five decades, public and private investment in developing desalination

technology has reached more than a billion dollars worldwide. And even with the progress

that's been made, the idea that desalination would do away with water scarcity is far from

reality. And that's because it's still really, really expensive to plan, build and manage

desalination. In fact, the average cost to turn one acre-foot -- about 325,000 gallons -- of salt

water into fresh water ranges from $800 to $1,400 and requires a significant amount of

energy.Producing fresh water using reverse osmosis costs about one-third less than

multistage flash, largely because of the costs of the thermal energy used by the latter

method in the boiling process.

The energy required to start up and power desalination plants is a huge expense and

because most current power sources are derived from burning fossil fuels, it is generally

looked upon as just a matter of choosing one environmental crisis over another. According

to Deutsche Welle, some of the most commonly used techniques for desalination, including

reverse osmosis (RO), use enough energy to release three times the amount of CO2

emissions that freshwater treatment plants release.

Brine, the salt-concentrated byproduct of water desalination, is receiving tremendous

attention from researchers around the world, but as of now, the effects of brine on marine

environments is vastly unknown. The disposal of brine by burying in 1 km underground was

discussed, but discarded due to the harmful effects the brine may have on groundwater for

future generations.

According to William Phillip from the University of Notre Dame, “membrane technology is

easily damaged by junk in the water.” Chemicals need to treat the water before membranes

are able to filter it, which potentially means another additive that may prove detrimental to

the environment at a later stage.

If regions situated away from the coast or in a high altitude try to use desalinated water, it is

an even more expensive process. Higher altitudes and far distances require great resources

to transport the water from the ocean or body of salt water.

Desalination is merely a route that should be considered to solve water scarcity issues, it is

in no way the only possibility.

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Why not Recycle our Water?

Wastewater becomes potable 'Safe Water'

Same Reverse Osmosis Technology

Costs less

o Wastewater needs to pass only once through RO filters (Seawater needs 2)

Less Brine produced

5. Future Options of Desalination

As the number of desalination plants worldwide continue to grow, so do concerns about

developing new technology to power the plants. Currently, large-scale desalination efforts

require a lot of energy to operate and often are high-maintenance affairs, thanks to lots of

working parts like membranes that tend to foul frequently.

5.1 Ion concentration polarization process

These cost and environmental concerns are all part-and-parcel of the next round of

improvements in desalination technology and processes. Light-weight portable desalination

devices are being developed by researchers in Korea and at the Massachusetts Institute of

Technology. The units produce enough fresh water to support several people. The process

uses gravity -- simply pour saltwater into the top of the device -- to remove salt and other

sediments using 1,600 filters.

(http://www.sciencedaily.com/releases/2010/03/100323161505.htm)

The system works at a microscopic scale, using fabrication methods developed for

microfluidics devices -- similar to the manufacture of microchips, but using materials such as

silicone (synthetic rubber). Each individual device would only process minute amounts of

water, but a large number of them -- the researchers envision an array with 1,600 units

fabricated on an 8-inch-diameter wafer -- could produce about 15 liters of water per hour,

enough to provide drinking water for several people. The whole unit could be self-contained

and driven by gravity -- salt water would be poured in at the top, and fresh water and

concentrated brine collected from two outlets at the bottom.

A single unit of the new desalination

device, fabricated on a layer of silicone.

In the Y-shaped channel (in red),

seawater enters from the right, and

fresh water leaves through the lower

channel at left, while concentrated brine

leaves through the upper channel.

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The basic principle that makes the system possible, called ion concentration polarization, is a

ubiquitous phenomenon that occurs near ion-selective materials (such as Nafion, often used

in fuel cells) or electrodes, and this team and other researchers have been applying the

phenomenon for other applications such as biomolecule preconcentration. This application

to water purification has not been attempted before, however.

ICP is an electrochemical

phenomenon that separates ions in

solution when a current is passed

through an ion selective membrane.

While this desalination method uses

ion-selective membranes, it is

considered a membraneless

filtration method because it uses

repulsive forces to direct ions away

from the membrane, only allowing

deionized water through. This

allows for the filtration system to

avoid challenges such as membrane

fouling and salt build-up that plague

other desalination techniques. This

method also uses less pressure and

less electricity than reverse osmosis

and electrodialysis, reducing the

resource and infrastructure costs of

installing a system based on this

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technology. Finally, since this technology does not rely upon filtration and it separates out

things like blood, viruses, bacteria and salt (all ionized species), the feed water needs little

treatment other than filtering out sand, large organic particles and seaweed.

Although the early results of this technology are promising, much more research and

engineering is necessary to create systems that could potentially be scalable. Engineers will

need to refine manufacturing techniques to ensure the chips can be made cost effectively.

Engineers must also work to create components that can be aggregated into stationary and

portable ICP systems for commercial desalination.

5.2 Nanotechnology/Reverse osmosis

RO systems require pumps to maintain sufficient pressure to force the water through the

membrane as well as cleaning procedures to clean bacteria that grows on the saltwater side

of the membrane, referred to as fouling.

Researchers are investigating the use of nanomaterials to reduce the pressure needed to

force water through the membrane and to reduce the capability of bacteria to grow on the

membrane.

Make water flow more easily through the membrane

We may be able to benefit from carbon nanotubes in the membranes that are used in

reverse osmosis to help with the process of desalination. For example, a company called

NanOasis is working on membranes that contain a very dense polymer film with carbon

nanotube pores.

Because the inside of carbon nanotubes is very smooth, water is transported through them

more easily. And, while the nano pores allow water to flow through, they stop salt ions,

making this method perfect for desalination. This method could reduce the energy required

for desalination by 30 to 50 percent.

Reduce bacteria in reverse osmosis

A company called NanoH2O adds nanoparticles to their membrane to optimize properties

such as surface roughness and charge. The company has been able to reduce the chance of

bacteria adhering to the membrane. Because bacteria on the membrane can reduce the

amount of water passing through, reducing the bacteria on the membrane means that you

don’t have to shut down the system for cleaning as often.

Explore capacitive deionization

A desalinization method called capacitive deionization has the potential to become more

cost effective than reverse osmosis. A capacitive deionization cell contains two electrodes,

one positively charged and one negatively charged.

The electrodes are charged because salt consists of negative and positive ions. Because

opposite charges attract, negatively charged ions are attracted to the positively charged

electrode and positively charged ions are attracted to the negatively charged electrode. For

example, if you dissolve regular table salt (sodium chloride) in water, the sodium and

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chloride separate to form

positively charged sodium ions

and negatively charged

chloride ions.

As seawater runs through the

cell, the salt ions attach to the electrodes and deionized water leaves the other end of the

cell. This technique doesn’t require high pressure to push the water through the membrane,

as in reverse osmosis, so it would be less expensive and use less power.

Researchers are developing electrodes made with nanomaterials to increase the electrode

surface area, which should increase the speed at which a cell can remove salt ions from

seawater. One interesting technique is the use of electrodes constructed from graphene

flakes, which researchers at the University of South Australia have demonstrated.

Researchers from around the world are also attempting to develop low-cost capacitive

deionization systems using nanostructured electrodes.

5.3 Graphene sheets

A new approach to water

desalination is that Graphene

sheets with precisely

controlled pores have

potential to purify water more

efficiently than existing

methods.

MIT researchers have come up

with a new approach using a

different kind of filtration

material: sheets of graphene,

a one-atom-thick form of the

element carbon, which they

say can be far more efficient

and possibly less expensive

than existing desalination

systems.

Working with graphene in

reality is more challenging

than filtering pixilated salt

from digital water molecules

on a computer. For starters,

although chemical etching and

ion beams can be used to

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create holes in graphene, it is difficult to produce holes of a specific size in an even

configuration. Nor does graphene eliminate the quandary of how much leftover brine can be

safely returned to the ocean without hurting underwater habitats. Toxicity could also be an

important issue, he says, "although there are no real answers right now in terms of

[graphene's] potential impact on [the safety of] drinking water."