Introduction

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

A 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 :

Major Processes

• Thermal
  Multi Stage Flash Distillation (MSF)
  Multiple Effect Distillation (MED)
  Vapour Compression Distillation (VC)
• Membrane
  Reverse Osmosis (RO)
  Electrodialysis (ED)
  

Minor Processes

• Freezing
• Membrane Distillation
• Solar Humidification
  

Thermal Process

Over 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 vapour.

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 process.

Multi Stage Flash Distillation

In 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.

Multi-stage 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

The 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 large plant.

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 increasing.

Vapour Compression Distillation

The 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 the vessel.

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 readily available.

Membrane Processes

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 processes.

Electrodialysis

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 this area.

Electrodialysis depends on the following general principles:

• Most salts dissolved in water are ionic, being positively (cationic) or negatively (anionic) charged.
• These ions are attracted to electrodes with an opposite electric charge.
• Membranes can be constructed to permit selective passage of either anions or cations.
  

The 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 charge.

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 :

• Pre-treatment
• Membrane Stack
• Low-Pressure Circulation pump
• DC power supply (rectifier)
• Post-treatment

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 membrane fouling.

Electrodialysis has the following characteristics that lend it to various applications :

• Capability for high recovery (more product and less brine)
• Energy usage that is proportional to the salts removed.
• Ability to treat water with a higher level of suspended solids than RO.
• Lack of effect by non-ionic substances such as silica.
• Low chemical usage for pretreatment.

Reverse Osmosis

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 water.

An RO system is made up of the following basic components :

• Pretreatment
• High-Pressure Pump
• Membrane Assembly
• Post-treatment

RO 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 feed water.

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.

Concentrate Disposal

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.

Economics

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 been used.

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.

Summary

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.