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Monday, March 26, 2012
Seawater is the largest source of fresh water as well as the source of Hydrogen energy.However; seawater cannot be used directly for these applications and it requires further treatment. Seawater contains a number of dissolved salts and the TDS, total dissolved solids, of seawater is about 35,000ppm (parts per million).The commonly used industrial desalination process is by RO (reverse osmosis) as well as by multistage flash distillation (MFD). Both these processes are energy intensive.RO process requires electrical energy and MFD requires thermal energy. Most of the countries in Persian Gulf use desalination process to convert seawater into drinking water as well as industrial water. These oil rich countries depend on the desalinated seawater as their main source of drinking water supply. In the desalination process by RO, the TDS level of seawater is reduced from 35,000ppm to 500ppm, meeting the WHO (World Health Organization) specifications for drinking purpose. The advantage with reverse osmosis process is it can remove even the smallest bacteria and virus, during the desalination. The water can further be disinfected by the injection of Chlorine before distributing for drinking purpose. The majority of desalination plants use RO process because it is economical. There is a worldwide shortage for safe drinking water and more and more SWRO plants are coming up in various parts of the world. The technology of RO has advanced so much that the cost of desalinated seawater can compete with surface water in many parts of the world, especially in Gulf region where the energy cost is low. The rapid increase in population and industrial growth has created a greater demand for fresh water. In conventional SWRO process, only 35-40% of fresh water is recovered and the balance 60-65% is discharged back into the sea as a highly saline brine, with TDS levels exceeding 65,000pm, almost double the salinity of seawater. Similarly most of the power plants located on sea coasts are using seawater for cooling purpose. In once through cooling system, the seawater is circulated into the power plant to condense steam in turbines and returned back to the sea. The temperature and salinity of the returning water into the sea is always higher than the intake water. Some oceanographers feel that such slow increase in salinity of seawater affects the temperature of the sea and the climate. However, discharge of highly saline brine into the sea has become routine and EPA (Environmental and Pollution Authority) of various countries routinely approve such discharge, claiming it does not affect the marine life significantly. The environmental impact study conducted in one country is routinely followed by many countries and invariably conclude that such discharge has a very little or no impact to the environment. Human beings are concerned only with their environment and not with the marine environment where variety of marine species live. Our oceans have been heavily polluted from the time of industrial revolution by oil spills, toxic industrial effluent discharges, desalination and power plant discharges. The TDS levels of seawater in Gulf region has considerably increased in the past few decades. The TDS levels are about 50,000 ppm against conventional levels of 35,000PPM.The oceans are acidified by absorption of excess carbon dioxide from the atmosphere due to greenhouse gas emissions. The power required to desalinate seawater is directly proportional to the osmotic pressure of seawater. The osmotic pressure increase as the TDS level increases, which in turn increases the energy consumption by desalination plants. A recent report from US government says that fresh water will become a serious issue after a decade and even wars may be waged between countries for the sake of fresh water. The human activities not only cause global warming but also changing the chemistry of our oceans. Steadily dwindling fish population is a clear indication of changing chemistry and biology of our oceans. In the absence of a proven scientific evidence to show that human beings cause these changes in the ocean, we will carry on our business as usual until we reach a point of no return. If you add salt to the water, it will not boil at 100C at 1 atmospheric pressure but slightly at a higher temperature. It is high school physics. When the salinity of the ocean increases from 35,000ppm to 50,000ppm, does it not affect the evaporation of the sea, which condenses into a cloud and come back as a rain? Does it mean there will be less precipitation in the future? Even if the ocean is under constant circulation, the overall salinity level keeps increasing.