• “If we could ever competitively, at a cheap rate, get fresh water from salt water, that it would be in the long-range interests of humanity which would really dwarf any other scientific accomplishments.” – John F. Kennedy in 1962
Introduction• Desalination, for the past four decades, has been hailed as the hope of mankind.• Certain group of people believe that desalination is the only answer for the increasing freshwater demand on a global scale.
Geography of Desalination• According to a recent report published by ‘Huntington Beach Seawater Desalination Facility’- there are more than 120 countries producing more than 3.5 billion gallons of potable water every day using desalination plants.• The total number of plants amounts to a staggering 21,000 in which 50% of them use sea water and the rest brackish water.• They are prevalent in countries situated in the Persian- Arabian Gulf, where there is acute scarcity of water.
Scope In India• India too uses the desalination technologies to meet the freshwater requirements of the people.• If we shift the focus to Tamil Nadu, it is estimated that the current water requirement is 1000 MLD (million liters per day). Due to urbanization the city the demand tends to rise.• Desalination plants in Minjur and Nemmeli, each provide 100 MLD whereas the plant at Pattipulam gives 200 MLD.• Experts predict that if there are no desalination plants there will be a shortage of 300 MLD in the next three years. Therefore, this technology is considered an absolute necessity.
Methods employed in DesalinationPlants• Sea water is converted into potable water using two methodologies – Phase Change and Non-Phase Change.• Here by phase change we mean the change in the state of a particular substance. For example, the phase change for water would be vapor when heated or ice when cooled.• Techniques like Electro-dialysis and Reverse Osmosis fall under non-phase change process. Low Temperature Thermal Desalination (LTTD) is a phase change process.
• The majority of desalination plants employ traditional technologies like Reverse Osmosis and Electro-dialysis. They produce around 300 MLD of water with a purity level of 400- 500 PPM (Parts Per Million).• The concept of Low Temperature Thermal Desalination has been envisaged in the 1960’s. Though the complete literature was known to the world, the implementation has been absent. The National Institute of Ocean Technology (NIOT) has developed and implemented a process using this theory.• Water derived from the LTTD method gives potable water with salinity levels of 20-200 PPM, which is a considerable achievement.
Working of Desalination Plant usingLTTD• It is a known fact that the surface temperature of sea water is around 28°C in the Indian conditions. The temperature of the water decreases as the depth of the sea increases.• LTTD uses this temperature difference (technically called ∆T) to establish an environment for desalination.
• The LTTD process contains a pressurized container along with a condenser. The boiling point of water is reduced from the original value (100° C) by applying a pressure of 1013 millibar. The hot surface water is passed into the pressurized container which turns the water into vapor.• It is then sent into a shell and tube condenser where cold water from the depths is pumped to convert the vapor back to water. This process is called flash evaporation.• The first of the LTTD desalination plants was set up in Kavaratti, Lakshadweep by the National Institute of Ocean Technology.
• In Reverse Osmosis the recovery rate is 45% which in turn has some environmental repercussions like release of hot water and brine discharge.• “In the LTTD process, at a low temperature difference of 6° C, the recovery rate is very low. Depending on the change in temperature we can increase or decrease the amount of water recovered from the sea. This percentage is usually in single digits. Though, the figure is small environment will be safe as the brine discharge is negligible. It is a renewable process with minimal stress on the environment,” G.Venkatesan, Scientist of NIOT who worked closely with the LTTD project.
• The post treatment process for the water obtained from the LTTD is inexpensive. It is passed through limestone, so that it attains the necessary nutrients and minerals. This stage changes and balances the chemical structure of water along with the pH level.• “A trade-off has to be done. LTTD consumes higher energy when compared to Reverse Osmosis and Electro- dialysis. The only achievement is the protection of environment. I think some new technologies will eventually come and reduce the consumption of LTTD,” adds Mr. Venkatesan.
The Disadvantages of Desalination Waste Disposal• The process of desalination requires pretreatment and cleaning chemicals, which are added to water before desalination to make the treatment more efficient and successful.• These chemicals include chlorine, hydrochloric acid and hydrogen peroxide, and they can be used for only a limited amount of time. Once theyve lost their ability to clean the water, these chemicals are dumped, which becomes a major environmental concern.• These chemicals often find their way back into the ocean, where they poison plant and animal life. Source: http://www.ehow.com/list_5961767_disadvantages-desalination.html
Brine• Brine is the side product of desalination. While the purified water goes on to be processed and put into human use, the water that is left over, which has a super saturation of salt, must be disposed of.• Most desalination plants pump this brine back into the ocean, which presents another environmental drawback.• Ocean species are not equipped to adjust to the immediate change in salinity caused by the release of brine into the area. The super-saturated salt water also decreases oxygen levels in the water, causing animals and plants to suffocate.
Health Concerns• Desalination is not a perfected technology, and desalinated water can be harmful to human health as well.• By-products of the chemicals used in desalination can get through into the "pure" water and endanger the people who drink it. Desalinated water can also be acidic to both pipes and digestive systems.
Energy Use• In an age where energy is becoming increasingly precious, desalination plants have the disadvantage of requiring large amounts of power. Other water treatment technologies are more energy efficient.
Vanishing Beaches of Tamil Nadu ----A report on RO Desalination Plant in Nemmeli district
Future Options of Desalination• Desalination is process primarily done in developed countries with enough money and resources. If technology continues to produce new methods and better solutions to the issues that exist today, there would be a whole new water resource for more and more countries that are facing drought, competition for water, and overpopulation.• Though there are concerns in the scientific world about replacing our current overuse of water with complete reliance on sea water, it would undoubtedly be at least an option for many people struggling to survive or maintain their standard of living.