Introduction to Desalination
is the natural continuous process, which is essential
for the water recycle. Precipitation, such as rain or
snowfall on ground, which finally flows either in to
the sea or goes back to the atmosphere through evaporation
or percolates in the sub-soil is a process of natural
desalination. Living beings uses this water directly
from rain or from river, lakes or springs. During the
travel of the surface water towards sea it dissolves
minerals and other materials and becomes salty. Once
it arrives to the oceans natural evaporation removes
part of the water in to the atmosphere as cloud while
remaining water available in the ocean becomes very
salty. The evaporated water from the ocean is given
back to the earth in the form of rain or snow, which
again travels back to ocean and the cycle continues.
Availability of fresh water has been the main centre
of growth of civilisation. However, there are lots of
inequality existing on earth, which needs to be artificially
corrected through incorporation of technologies such
as thermal or membrane desalination. With the growth
of world population the need of fresh water has also
increased substantially which has resulted in growth
of desalination installation as well. Logically the
desalination activities are concentrated on those parts
of the earth where availability of water is scares.
This is precisely the reason why more than 80% of desalination
plants are located in the water scares Middle East region.
Unequal water distribution also exists within our country
and fresh water desalination technology is getting concentrated
more on water scares areas such as Gujarat, Tamil Nadu
and Rajasthan. Besides producing desalted water for
human consumption and Industrial requirement these technologies
are also found to be advantageous in the recovery of
water from waste streams. There is no reliable statistics
available on number of plants, their capacities, technologies
adopted and status on these plants in India. However,
rough indications are that there are more than 1000
membrane based desalination plants of various capacities
ranging from 20 m3/day to 10,000 m3/day. There are few
thermal based desalination plants also.
In order to compute a reliable data base the reader
is requested to provide as much information as he /
she can provide on the desalination plants, their installations
date, capacity, name of the supplier, present status
etc. This will help us in producing a reliable document,
which can be used by planners as well as users.
Commercially Available Desalting Processes
desalting device essentially separates saline water
into two streams: one with a low concentration of dissolved
salts (the fresh water stream) and the other containing
the remaining dissolved salts (the concentrate or brine
stream). The device requires energy to operate and can
use a number of different technologies for the separation.
The various desalting processes are listed below :
Multi Stage Flash Distillation (MSF)
Multiple Effect Distillation (MED)
Vapour Compression Distillation (VC)
Reverse Osmosis (RO)
60 percent of the world's desalted water is produced
with heat to distil fresh water from sea water. The
distillation process mimics the natural water cycle
in that saline water is heated, producing water vapour
that is in turn condensed to form fresh water. In the
laboratory or industrial plant, water is heated to the
boiling point to produce the maximum amount of water
For this to be done economically in a desalination plant,
the boiling point is controlled by adjusting the atmospheric
pressure of the water being boiled. (The temperature
required to boil water decreases as one moves from sea
level to a higher elevation because of the reduced atmospheric
pressure on the water. Thus, water can be boiled on
top of Mt. McKinley in Alaska [elevation 6200 meters]
at a temperature about 16°C less than boiling it
at sea level). The reduction of the boiling point is
important in the desalination process for two major
reasons: multiple boiling and scale control. To boil
water needs two import conditions: the proper temperature
relative to its ambient pressure and enough energy for
vaporisation. When water is heated to its boiling point
and then the heat is turned off, the water will continue
to boil only for a short time because the water needs
additional energy (the heat of vaporisation) to permit
boiling. Once the water stops boiling, boiling can be
renewed by either adding more heat or by reducing the
ambient pressure above the water. If the ambient pressure
is reduced, then the water would then be at a temperature
above its boiling point (because of the reduced pressure)
and will boil with the extra heat from the higher temperature
to supply the heat of vaporisation needed. As the heat
of vaporisation is supplied, the temperature of the
water will fall to the new boiling point.
To significantly reduce the amount of energy needed
for vaporisation, the distillation desalting process
usually uses multiple boiling in successive vessels,
each operating at a lower temperature and pressure.
This process of reducing the ambient pressure to promote
boiling can continue downward and, if carried to the
extreme with the pressure reduced enough, the point
at which water would be boiling and freezing at the
same time would be reached.
Aside from multiple boiling, the other important factor
is scale control. Although most substances dissolve
more readily in warmer water, some dissolve more readily
in cooler water. Unfortunately, some of these substances
like carbonates and sulfates are found in sea water.
One of the most important is gypsum (CaSC4), which begins
to leave solution when water approaches about 95°C.
This material forms a hard scale that coats any tubes
or containers present. Scale creates thermal and mechanical
problems and, once formed, is difficult to remove. One
way to avoid the formation of this scale is to keep
the temperature below boiling point of the water.
These two concepts have made various forms of distillation
successful in locations around the world. The process
which accounts for the most desalting capacity is multi-stage
flash distillation, commonly referred to as the MSF
Multi Stage Flash Distillation
the MSF process, sea water is heated in a vessel called
the brine heater. This is generally done by condensing
steam on a bank of tubes that passes through the vessel
which in turn heats the sea water. This heated sea water
then flows into another vessel, called a stage, where
the ambient pressure is such that the water will immediately
boil. The sudden introduction of the heated water into
the chamber causes it to boil rapidly, almost exploding
or flashing into steam. Generally, only a small percentage
of this water is converted to steam (water vapour),
depending on the pressure maintained in this stage since
boiling will continue only until the water cools (furnishing
the heat of vaporisation) to the boiling point.
The concept of distilling water with a vessel operating
at a reduced pressure is not new and has been used for
well over century. In the 1950s, a unit that used a
series of stages set at increasingly lower atmospheric
pressures was developed. In this unit, the feed water
could pass from one stage to another and be boiled repeatedly
without adding more heat Typically, an MSF plant can
contain from 4 to about 40 stages.
The steam generated by flashing is converted to fresh
water by being condensed on tubes of heat exchangers
that run through each stage. The tubes are cooled by
the incoming feed water going to the brine heater. This,
in turn, warms up the feed water so that the amount
of thermal energy needed in the brine heater to raise
the temperature of the sea water is reduced.
flash plants have been built commercially since the
1950s. They are generally built in units of about 4,000
to 30,000 cum/d (1 to 8 mgd). The MSF plants usually
operate at the top feed temperatures (after the brine
heater) of 90 -120°C. One of the factors that effects
the thermal efficiency of the plant is the difference
in temperature from the brine heater to the condenser
on the cold end of the plant. Operating a plant at the
higher temperature limits of 120°C tends to increase
the efficiency, but it also increases the potential
for detrimental scale formation and accelerated corrosion
of metal surfaces.
Multiple Effect Distillation
multi effect distillation (MED) process has been used
for industrial distillation for a long time. One popular
use for this process is the evaporation of juice from
sugar cane in the production of sugar or the production
of salt with the evaporative process. Some of the early
water distillation plants used the MED process, but
this process was displaced by the MSF units because
of cost factors and their apparent higher efficiency.
However, in 1980s, interest in the MED process has renewed,
and a number of new designs have been built. Most of
these new MED units have been built around the concept
of operating on lower temperatures.
MED, like the MSF process, takes place in a series of
vessels (effects) and uses the principal of reducing
the ambient pressure in the various effects. This permits
the sea water feed to undergo multiple boiling without
supplying additional heat after the first effect. In
an MED plant, the sea water enters the first effect
and is raised to the boiling point after being pre-heated
in tubes. The sea water is either sprayed or otherwise
distributed onto the surface of evaporator tubes in
a thin film to promote rapid boiling and evaporation.
The tubes are heated by steam from a boiler, or other
source, which is condensed on the opposite side of the
tubes. The condensate from the boiler steam is recycled
to the boiler for reuse.
Only a portion of the sea water applied to the tubes
in the first effect is evaporated. The remaining feed
water is fed to the second effect, where it is again
applied to a tube bundle. These tubes are in turn being
heated by the vapours created in the first effect. This
vapour is condensed to fresh water product, while giving
up heat to evaporate a portion of the remaining sea
water feed in the next effect. This continues for several
effects, with 8 or 16 effects being found in a typical
MED plants are. typically built in units of 2,000 to
10,000 cum/d (0.5 to 2.5 mgd). Some of the more recent
plants have been built to operate with a top temperature
(in the first effect) of about 70°C, which reduces
the potential for scaling of sea water within the plant
but in turn increases the need for additional heat transfer
area in the form of tubes. Most of the more recent applications
for the MED plants have been in some of the Carribbean
areas. Although the number of MED plants is still relatively
small compared to MSF plants, their numbers have been
Vapour Compression Distillation
vapour compression (VC) distillation process is generally
used for small and medium scale sea water desalting
units. The heat for evaporating the water comes from
the compression of vapour rather than the direct exchange
of heat from steam produced in a boiler.
The plants which use this process are generally designed
to take advantage of the principle of reducing the boiling
point temperature by reducing the pressure. Two primary
methods are used to condense vapour so as to produce
enough heat to evaporate incoming sea water: a mechanical
compressor or a steam jet. The mechanical compressor
is usually electrically driven, allowing the sole use
of electrical power to produce water by distillation.
With the steam jet-type of VC unit, also called a thermocompressor,
a vemturi orifice at the steam jet creates and extracts
water vapour from the main vessel, creating a lower
ambient pressure in the main vessel. The extracted water
vapour is compressed by the steam jet. This mixture
is condensed on the tube walls to provide the thermal
energy (heat of condensation) to evaporate the sea water
being applied on the other side of the tube walls in
VC Units are usually built in the 20 to 2,000 cum/d
(0.005 to 0.5 mgd) range. They are often used for resorts,
industries and drilling sites where fresh water is not
In nature, membranes play an important role in the separation
of salts. This includes both the processes of dialysis
and Osmosis that occur in the body. Membranes are used
in two commercially important desalting processes: electrodialysis
and RO. Each process uses the ability of membranes to
differentiate and selectively separate salts and water.
However, membranes are used differently in each of these
Electrodialysis was commercially introduced in the early
1960s, about 10 years before RO. The development of
electrodialysis provided a cost-effective way to desalt
brackish water and spurred considerable interest in
Electrodialysis depends on the following general principles:
salts dissolved in water are ionic, being positively
(cationic) or negatively (anionic) charged.
ions are attracted to electrodes with an opposite
can be constructed to permit selective passage of
either anions or cations.
dissolved ionic constituents in a saline solution such
as sodium (+), chloride (-), calcium (+ +), and carbonate
(- -) are dispersed in water, effectively neutralising
their individual charges. When electrodes connected
to an outside source of direct current like a battery
are placed in a container of saline water, electrical
current is carried through the solution, with the ions
tending to migrate to the electrode with the opposite
For these phenomena to desalinate water, membranes that
will allow either cations or anions (but not both) to
pass are placed between a pair of electrodes. These
membranes are arranged alternatively with an anion-selective
membrane followed by a cation-selective membrane. A
spacer sheet that permits water to flow along the face
of the membrane is placed between each pair of membranes.
The basic electrodialysis consists of several hundred
cell pairs bound together with electrodes on the outside
and is referred to as a membrane stack. Feed water passes
simultaneously in parallel paths through all of the
cells to provide a continuous flow of desalted product
water and brine to emerge from the stack. Depending
on the design of the system, chemicals may be added
to the streams in the stack to reduce the potential
for scaling. An electrodialysis unit is made up of the
following basic components :
power supply (rectifier)
Electrodialysis Reversal Process (EDR)
In the early 1970s, an American company commercially
introduced the EDR process for electrodialysis. An EDR
unit operates on the same general principle as a standard
electrodialysis plant except that both the product and
the brine channels are identical in construction. At
intervals of several times an hour, the polarity of
the electrodes is reversed, and the flows are simultaneously
switched so that the brine channel becomes the product
water channel, and the product water channel becomes
the brine channel.
The result is that the ions are attracted in the opposite
direction across the membrane stack. Immediately following
the reversal of polarity and flow, enough of the product
water is dumped until the stack and lines are flushed
out, and the desired water quality is restored. This
flush takes about 1 or 2 minutes, and then the unit
can resume producing water. The reversal process is
useful in breaking up and flushing out scales, slimes
and other deposits in the cells before they can build
up and create a problem. Flushing allows the unit to
operate with fewer pretreatment chemicals and minimises
Electrodialysis has the following
characteristics that lend it to various applications
for high recovery (more product and less brine)
usage that is proportional to the salts removed.
to treat water with a higher level of suspended solids
of effect by non-ionic substances such as silica.
chemical usage for pretreatment.
In comparison to distillation and electrodialysis, RO
is relatively new, with successful commercialisation
occurring in the early 1970s.
RO is a membrane separation process in which the water
from a pressurised saline solution is separated from
the solutes (the dissolved material) by flowing through
a membrane. No heating or phase change is necessary
for this separation. The major energy required for desalting
is for pressurising the feed water.
In practice, the saline feed water is pumped into a
closed vessel where it is pressurised against the membrane.
As a portion of the water passes through the membrane,
the remaining feed water increases in salt content.
At the same time, a portion of this feed water is discharged
without passing through the membrane.
Without this controlled discharge, the pressurised feed
water would continue to increase in salt concentration,
creating such problems as precipitation of supersaturated
salts and increased osmotic pressure across the membranes.
The amount of the feed water discharged to waste in
this brine stream varies from 20 to 70 percent of the
feed flow, depending on the salt content of the feed
An RO system is made up of the
following basic components :
membranes are made in a variety of configurations. Commercially
successful are spiral wound, hollow fibre, plate-and-frame
and Tubular. These configurations are used to desalt
both brackish and sea water, although the construction
of the membrane and pressure vessel will vary depending
on the manufacturer and expected salt content of the
Two developments have helped to reduce the operating
costs of RO Plants during the past decade: the development
of membranes that can operate efficiently with lower
pressures and the use of energy recovery devices. The
low-pressure membranes are being widely used to desalt
brackish water. The energy recovery devices are connected
to the concentrate stream as it leaves the pressure
vessel. The water in the concentrate stream loses only
about 1 to 4 bar (15 to 60 psi) relative to the applied
pressure from the high-pressure pump. These energy recovery
devices are mechanical and generally consists of turbines
of pumps of some type that can convert a pressure drop
to rotating energy.
The common element in all of these desalination processes
is the production of a concentrate stream (also called
a brine, reject or waste stream). This stream contains
the salts removed from the saline feed to produce the
fresh water product as well as some of the chemicals
that may have been added during the process. This stream
varies in volume depending on the process, but will
almost always be a significant quantity of water.
The disposal of this wastewater in an environmentally
appropriate manner is an important part of the feasibility
and operation of a desalting facility. If the desalting
plant is located near (he sea, the potential for a problem
will be considerably less.
The potential for a more significant problem comes when
a desalting facility is constructed inland, away from
a natural salt water body. Care must then be taken so
as not to pollute any existing ground or surface water
with the salts contained in the concentrate stream.
Disposal may involve dilution, injection of the concentrate
into a saline aquifer, evaporation, or transport by
pipeline to a suitable disposal point. All of these
methods could add to the cost of the process.
The means of properly disposing of the concentrate flow
should be one of the items investigated first in any
study of the feasibility of a desalination facility.
The cost of disposal could be significant and could
adversely affect the economics of desalination.
facilities exist in about 120 countries around the world.
The capital and operating cost for desalination have
tended to decrease over the years. Even though energy
prices have increased the desalting cost have been decreasing.
The cost of obtaining and treating water from conventional
sources has tended to increase because of the increased
levels of treatment being required in various countries
to meet more stringent water quality standards. This
rise in cost for conventionally treated water also is
the result of an increased demand for water, leading
to the need to develop more expensive conventional supplies
since the readily obtainable water sources have already
Many factors enter into the capital and operating costs
for desalination: capacity and type of plants, plant
location, feed water quality, labour cost, energy cost,
financing cost, ease of concentrate disposal, level
of instrumentation / automation and plant reliability.
However, as a guideline the reader can take the production
cost of a brackish water desalination plant to be Rs.
10 to 15 per m3 The production cost for a sea water
desalination plant varies between Rs. 40 to 50 per m3
Whereas the production cost of desalted water from effluent
varies from Rs. 15 to 50 per m3 depending upon the TDS
load in the effluent stream.
Desalination Technology has been extensively developed
over the past 40 years to the point where it is reliably
used to produce fresh water from saline sources. This
has effectively made the use of saline waters for water
resource development possible. The costs for desalination
can be significant because of its intensive use of energy.
However, in many arid areas of the world, the cost to
desalinate saline water is less than other alternatives
that may exist or be considered for the future. Desalinated
water is used as a main source of municipal supply in
many areas of the Caribbean, North Africa and the Middle
East. The use of desalination technologies, especially
for softening mildly brackish water, is rapidly increasing
in various parts of the world including India.
There is no "best" method of desalination.
Generally, distillation and RO are used for sea water
desalting, while RO and electrodialysis are used to
desalt brackish water. However, the selection of a process
should be dependent on a careful study of site conditions
and the application at hand. Local circumstances may
play a significant role in determining the most appropriate
process for an area. Combination of conventional effluent
treatment and RO (brackish or sea water) has been accepted
to be a reasonable technology for advanced effluent
treatment. Thermal Processes are also getting hooked
to these to achieve zero discharge.
The "best" desalination system should be more
than economically reasonable in the study stage. It
should work when it is installed and continue to work
and deliver suitable amounts of fresh water at the expected
quantity, quality, and cost for the life of a project.