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Water Resource Augmentation Through Desalination
Shashikant S Raut, Associate Vice President, Project Sales, Ion Exchange (India) Ltd Fresh water is a scarce commodity; this fact is A logical solution for long term management, desalination solves the critical issue of growing fresh water demand, as the continuing challenge to manage the water resources is affected by industrialisation, agriculture expansion, urbanisation and population explosion.

The global climate change has a significant impact on resources everywhere and consequently there is a challenge to manage the water resources, as this will also impact fresh water resource availability. The fresh water sources are ground water, acquifers, rivers and lakes.

However there is a shift due to climate pattern change combined with population growth pressure and limited availability of new and inexpensive fresh water supply. The demand for water has increased in many fields due to various factors like - agriculture expansion, rapid industrialisation, population growth, urbanisation and economic development.

Also, there is scarcity of water due to unequal distribution of rain water, occasional draughts, exploitation of ground water, deterioration of water quality due to discharge of domestic and industrial effluent without adequate treatment Urbanisation, population and industries are putting great stress on aquatic environment.

The growing fresh water demand being a critical issue, industry and water bodies are shifting to water resource such as desalination process to supplement the effect in work supply.

World Ocean contains 97.2% water resource
It is drought-proof and limitless; therefore, sea water desalination is logical solution for long term management for growing fresh water demand. Desalination is a process that removes dissolved minerals including salts from sea water for industry and domestic use. The two types of major desalination techniques that are used are - thermal technology and membrane technology.
• Thermal technology
• Membrane technology

Desalination Technologies: Capacity Distribution
Thermal technology In thermal technology, there is heating of saline water and collection of the condensed vapour to produce pure water. Following are the different kinds of thermal technologies

Multi-stage flash distillation: This process of desalination was invented in 1950 and the first plant was set up in 1957 at Kuwait. In multi-stage flash distillation, distillation takes place through several (multistage) chambers. Feed water is first heated under high pressure and is led into first flash chamber where pressure is released causing the water to boil rapidly in sudden evaporation or flashing. This flashing continues at each stage because the pressure at each stage is lower than the previous stage. The vapour generated by flashing is converted into fresh water by condensed on heat exchangers.
• Multi-effect distillation (MED): The first MED plant was built in 1930 in Saudi Arabia. MED occurs in a series of vessels (effects) and uses the principles of evapouration and condensation at reduced ambient pressure water boils at lower temperature as pressure decreases, so the water vapour of the first level or effect serves as the healthy medium for the second and so on. The performance ratio increases with the number of vessels or effect i.e. they are directly proportionate.

Vapour compression distillation (VCD): This process has low energy requirement and it uses the heat pump principle of continuously recycling the latest heat exchanged in the evaporation condensation process. VCD is similar to MED. Vapour produced by evaporation of brine is not condensed in a separate condenser. Instead a compressor returns it to the steam side of evaporator in which it is originated where it condenses on the heat transfer surface giving up latent heat to evaporate an additional portion of brine.

Membrane technology (Reverse Osmosis)
RO uses a semi-permeable membrane with pumping at high pressure as a driving force to separate saline feed water into two streams - high quality product and high concentrate reject. The system consists of four major components – pretreatment, pressurisation, membrane separation and post treatment stabilisation.
The challenge for RO technology is to get the production cost of water comparable to alternate thermal desalination process. The plant cost depends on – intake type, feed water quality, plant size, process type and design, finance costs, O&M requirement and environment discharge.

Advances in sea water Reverse Osmosis
Technological advancement & decrease in water production cost over the decade has increased the supply of fresh water supply.

Sea water RO process is dominating the market with improvement on energy consumption and efficient membrane.

Membrane: With high surface area & tighter membrane the productivity of membrane has improved considerably. Although eight inch membrane elements are standard and widely used in large scale, the large diameter element of sixteen inches are commercialised reducing hardware cost and footprint of the plant.
Energy recovery system: RO process is energy intensive. The reuse of energy applied for reverse osmosis process is effectively utilised for reduction in energy cost of the plant. Pelton wheel turbines, pressure exchangers, turbochargers can reduce the energy requirement by 10 to 40 per cent. The future improvement of SWRO process is forecasted to advance the technology with development of membrane of higher salt rejection and reduce transmembrane pressure. The extension of membrane life beyond 10 years, advanced energy recovery development of a new generation high efficiency pump, alternative of stainless steel component and low cost brine treatment will be major factors for reduction in capital and operating cost.

Electro dialysis (EDI)
Electro dialysis is an electrochemical process whereby electrically charged particles, ions, are transported from a raw solution (retentate, dilute) into a more concentrated solution (permeate, concentrate) through ionselective membranes by applying an electric field.

Electro dialysis reversal (EDR)
Electro dialysis Reversal (EDR) is similar to ED but the polarity of the electrodes is regularly reversed, thereby freeing accumulated ions on the membrane surface. This process minimises the effect of inorganic scaling and fouling by converting product streams into waste streams. This process requires additional plumbing and electrical controls, but increases membrane life. EDR does not require added chemicals, and eases cleaning as well.

Modern Desalination Techniques There are several promising new technologies emerging in the desalination of seawater. These include

• Forward Osmosis,
• Carbon Nanotubes Membranes,
• Biomimetics

Forward Osmosis
Forward osmosis, like reverse osmosis, uses a semi-permeable polymer membrane to separate the dissolved solids from the liquid being treated. However, unlike reverse osmosis, forward osmosis does not need expensive energy consuming pumps. Forward osmosis uses a draw solution; a concentrated solution having a high osmotic pressure such as an ammonia carbon dioxide solution. This solution is fed from an adjacent tank into the osmosis cell, which, where due to its high osmosis pressure, draws the seawater through the membrane, with the membrane preventing the passage of solids.

The draw solution is then pumped into another tank where it is heated to convert it back to ammonia and carbon dioxide, before being recirculated to the osmosis cell. Meanwhile the residual fresh water left in this tank is pumped away for post processing and storage for future use.

Carbon nanotubes membranes
Desalination of seawater using carbon nanotubes technology has been tested and still under research and development by Lawrence Livermore National Laboratory. They have shown the feasibility of desalinating seawater using their nanotubes, with significant savings in energy and costs. Carbon Nanotubes are made from rolled sheets of carbon atoms.

Their micro size means that their smooth inside diameter can accept only seven water molecules side by side, being 50,000 times thinner than a human hair. Although this process is a new concept, it could replace conventional membranes in the desalination of seawater.

Biomimetics and Desalination
Biomimetics is the name given for mimicking nature’s processing techniques; the transfer of ideas from biology to technology. The majority of living cells contain minute water tubes that conduct water but exclude dissolved solids such as salt.

In water desalination technique, an example is to be seen in tropical coastal mangrove swamps. The reeds (species of Rhizophoraceae) grow in salty water, sucking up water through minute capillaries in their root system in a process known as transpiration. The membranes in the pipes allow water through, excluding the passage of the salt, a process that made biomimetics suitable for further research. Although this process is a new concept, it could replace conventional membranes in the desalination of sea water.

Desalination process is a source and logical solution for long term management with challenge to improve membrane process, concentrate management, recycle and alternative technologies to offer a low production cost for domestic, agriculture and industry use.