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.