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Category Archives: Seawater desalination

CAPZ (Clean water at affordable price with zero discharge) is a new desalination concept that separates seawater into fresh drinking water and industrial salt both simultaneously using only sun and wind power. Seawater is nothing but fresh water in abundance with valuable mineral salts dissolved in it. These minerals include Sodium, Potassium, Lithium and Magnesium and a host of other minerals in traces. It requires a holistic approach to separate them in their pure form along with fresh water for potable and industrial applications. CAPZ technology precisely attempts to do that so that the seawater intake can be put into huge economic usage while reducing or eliminating completely the discharge of highly saline effluent contaminated with chemicals back into the sea. With increasing demand for fresh water and depleting sources of fresh water due to natural and man-made causes , sea water has become an important source for fresh drinking water. Sea water is not only a source of fresh water but also a new source of clean Hydrogen an energy of the future. It requires only sun ,sea and wind to achieve this!

CAPZ application

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Seawater desalination is a technology that provides drinking water for millions of people around the world. With increasing industrialization and water usage and lack of recycling or reuse, the demand for fresh water is increasing at the fastest rate. Industries such as power plants use bulk of water for cooling purpose and chemical industries use water for their processing. Agriculture is also a major user of water and   countries like India exploit ground water for this purpose. To supplement fresh water, Governments and industries in many parts of the world are now turning to desalinated seawater as a potential source of fresh water. However, desalination of seawater to generate fresh water is an expensive option, due to its large energy usage. However, due to frequent failure of monsoon rains and uncertainties and changing weather pattern due to global warming, seawater desalination is becoming a potential source of fresh water, despite its cost and environmental issues.

Seawater desalination technology has not undergone any major changes during the past three decades. Reverse osmosis is currently the most sought after technology for desalination due to increasing efficiencies of the membranes and energy-saving devices. In spite of all these improvements the biggest problem with desalination technologies is still the rate of recovery of fresh water. The best recovery in SWRO plants is about 50% of the input water. Higher recoveries create other problems such as scaling, higher energy requirements and O&M issues and many suppliers would like to restrict the recoveries to 35%, especially when they have to guarantee the life of membranes and the plant.

Seawater is nothing but fresh water with large quantities of dissolved salts. The concentration of total dissolved salts in seawater is about 35,000mgs/lit. Chemical industries such as Caustic soda and Soda ash plants use salt as the basic raw material. Salt is the backbone of chemical industries and number of downstream chemicals are manufactured from salt. Seawater is the major source of salt and most of these chemical industries make their own salt using solar evaporation of seawater using traditional methods with salt pans. Large area of land is required for this purpose and solar evaporation is a slow process and it takes months together to convert seawater into salt. It is also labor intensive under harsh conditions.

The author of this article has developed an innovative technology to generate fresh water as well as salt brine suitable for Caustic soda and Soda ash production. By using this novel process, one is able to recover almost 70% fresh water against only 40% fresh water recovered using conventional SWRO process, and also recover about 7- 9% saturated brine simultaneously. Chemical industries currently producing salt using solar evaporation are unable to meet their demand or expand their production due to lack of salt. The price of salt is steadily increasing due to supply demand gap and also due to uncertainties in weather pattern due to global warming. This result in increased cost of production and many small and medium producers of these chemicals are unable to compete with large industries. Moreover, countries like Australia who have vast arid land can produce large quantities of salt   with mechanized process  competitively; Australia is currently exporting salt to countries like Japan, while countries like India and China are unable to compete in the international market with their age-old salt pans using  manual labor. In solar evaporation the water is simply evaporated.

Currently these chemical industries use the solar salt which has a number of impurities, and it requires an elaborate purification process. Moreover the salt can be used as a raw material only in the form of saturated brine without any impurities. Any impurity is detrimental to the Electrolytic process where the salt brine is converted into Caustic soda and Soda ash. Chemical industries use deionized water to dissolve solar salt to make saturated brine and then purify them using number of chemicals before it can be used as a raw material for the production of Caustic soda or Soda ash. The cost of such purified brine is many times costlier than the raw salt. This in turn increase the cost of chemicals produced.

In this new process, seawater is pumped into the system where it is separated into 70% fresh water meeting WHO specifications for drinking purpose, and 7-10% saturated pure brine suitable for production of caustic soda and Soda ash. These chemical industries also use large quantities of process water for various purposes and they can use the above 70% water in their process. Only 15-20% of unutilized seawater is discharged back into the sea in this process, compared to 65% toxic discharge from convention desalination plants. This new technology is efficient and environmentally friendly and generates value added brine as a by-product. It is a win situation for the industries and the environment. The technology has been recently patented and is available for licensing on a non-exclusive or exclusive basis. The advantage of this technology is any Caustic soda or Soda ash plant located near the seashore can produce their salt brine directly from seawater without stock piling solar salt for months together or transporting over a long distance or importing from overseas.

Government and industries can join together to set up such plants where Governments can buy water for distribution and industries can use salt brine as raw material for their chemical production. Setting up a desalination plants only for supplying drinking water to the public is not a smart way to cut the cost of drinking water. For example, the Victorian Government in Australia has set up a large desalination plant to supply drinking water. This plant was set up by a foreign company on BOOT (build, own and operate basis) and water is sold to the Government on ‘take or pay’ basis. Currently the water storage level at catchment area is nearly 80% of its capacity and the Government is unlikely to use desalinated water for some years to come. However, the Government is legally bound by a contract to buy water or pay the contracted value, even if Government does not need water. Such contracts can be avoided in the future by Governments by joining with industries who require salt brine 24×7  throughout the year, thus mitigating the risk involved by  expensive legal contracts.

 

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 has 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 multi flash distillation (MFD). Both these processes are energy intensive.RO process requires electrical energy and MFD requires thermal energy. Most of the countries in Pesian  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.

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 much. 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 Ocean 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 sign 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.

Wind is a potential source of renewable energy, especially for islands with an average wind velocity of 5mts/sec and above. Many islands in pacific ocean  have some common problems like sea erosion, shortage of power and drinking water. These small islands with little population are fully depending on diesel fuel. In fact their life depends on diesel fuel and any increase in price significantly affects their daily life. Their main source of income is only by fishing and they live day to today.

I had a personal experience of visiting a small island off Port Moresby in Papua New Guinea. They call it Dougo Island or ‘Fisherman’s island’ with population of less than 700 people. It is about 4.5km wide and 2km long. It is a coral atoll pushed out of the sea. One can take stroll on the beach and it is one of the most beautiful experiences one can have. It gives a feeling that you are far away from the rest of the world. There is a small abandoned World War II Airfield. The people in the island do not have any electricity or drinking water and most of them are fishing on small boats. Their boats are fuelled by diesel. They will go to nearby city of Port Moresby and sell their fish and with that money they will buy drinking water and diesel in cans and return to the island. This is their daily life.

Such an island is an ideal location to set up a wind turbine and a small sea water desalination plant, that can easily solve their problem of water and power. The trade wind from the Coral Sea in the island of Papua New Guinea blows almost 7-8 months in a year and their wind velocity averages 7 mts/sec. Two wind turbines of each 250 kW capacity and a small seawater desalination SWRO plant of capacity 15,000lts/day will be sufficient to solve their problems. The desalination plant will consume about 4.5Kwhrs/m3 of water generated. About 2000 kwhrs/day of power can be supplied to the village, each family consuming about 2.85 khrs/day for 6 hours/day and also for the desalination plant. The system will generate  a surplus power.

Renewable wind energy is the best option for such islands to generate on-site power and also to desalinate seawater for supply of drinking water. With increasing global warming and sea level rising, these small island face seawater intrusion and inundation. Many islands are slowly disappearing into the vast sea. Moreover, these islands are the most vulnerable to the fluctuating diesels prices and they are walking on a tight rope.Industrialised countries with an average power consumption of several kilowatt-hours per day are crying foul about rising energy cost while people in such small islands barely manage their food and shelter after paying for the diesel.

Recently the Government of Maldives conducted their cabinet ministers meeting under the sea, to showcase their plight due to sea level rise caused by global warming, to the rest of the world. Small islands can cry loud but their voice  is muffled by roaring sea, while rest of the world carries on their business as usual.

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