Desalination is a technology that involves removing of dissolved salts from sea water. The uses of the process of desalination has been mentioned in works of Aristotle and Pliny. By making use of this process mariners keep themselves alive during long ocean trips. In US a nuclear reactor aircraft carrier has the capability to generate 1,50,000 l of water per day.
The advances in desalination process began around 1900’s and there was a major step in world war || as there was a need for sending water to military troops in remote, arid areas. At 1980, it had become commercially viable and at 1990 it had become common. There are around 16,000 desalination plant across the globe producing 20 billion gallons of water every day. With technological advances it may increase to 30 billion gallons every day. The world water consumption is 1200 gallons every day.
The earth has 300 trillion cubic miles of water , each mile occupying 1 trillion gallons of water. But the problem is 97% of this water is saline and cannot be used for common use as the average salt content is 35,00 parts per million. Quoting ‘How Desalination Works’ by Laurie Dove: “Ingesting salt signals your cells to flush water molecules to dilute the mineral. Too much salt, and this process can cause a really bad chain reaction: Your cells will be depleted of moisture, your kidneys will shut down and your brain will become damaged. The only way to offset this internal chaos is to urinate with greater frequency to expel all that salt, a remedy that could work only if you have access to lots of fresh drinking water.”
3% of 30,000 gallons of water is still a lot of water. But this water is stored as icecaps and glaciers. Some of this water is stored as water vapour and most of it is in ground. And ground water is easily not accessible due to its depth and location . Hence we are not even making 1% of productive use of our most precious resource.
Based on the strength of salt in water, water can be classified as:
Black water is water obtained by mixing sea water and fresh water usually present at sites where sea meets rivers.
There are many technologies in desalination. the older technology was heating salt water under sun and allowing water to evaporate and condense leaving behind salt. But now, technologies like Reverse Osmosis(R0), Multi Flash Distillation (MFD), Multi Effect Distillation (MSD) are used for distillation. RO is the most popular one as it doesn’t require thermal energy but mechanical energy is done to push salt water to another compartment through a membrane leaving the salts behind. In RO 1kwh of power is applied to 1 cubic meter of water.
Basic Desalination Process
Desalined Water is costlier than pure water obtained from rivers, lakes which involves series of treatment and conservation. However, if there is no water alternative available, desalined water costs around $1 per litre .
Desalination plants are present in around 160 countries and are under construction in other countries. And hence desalined water is the future source of water.
In response to increasing water insufficiency, over the last thirty years desalinisation has evolved into a viable alternative water system. It permits United States of America to tap non-traditional water resources with well potential to produce a sustainable, drought-proof water system.
Desalination provides solely around 1 % of the world’s drinkable, however this proportion is growing year-on-year. An unexpected US$10 billion investment in the next five years would add 5.7 million cubic meters per day of new production capacity. This capacity is expected to double by 2030.
Desalinated water is made by either using salt water (water with salt content of less than 10,000 mg/L), or water that salinity in an exceed Desalinated water is made by either using salt water (water with salt content of less than 10,000 mg/L), or water that salinity in an exceedingly vary of 30,000 to 44,000 mg/L. whereas desalinisation of salt water offers opportunities to provide lower price water, it’s unlikely to be a main supply of other facility within the future. the entire volume of salt water worldwide is restricted (less than I Chronicles of the world’s water) and, in most arid regions of the globe, it’s nearly totally utilized.
The world’s oceans contain over ninety seven. 2 % of the planet’s water resources. The high salinity of ocean water, and also the vital prices related to saltwater desalinisation, suggests that most of the world’s facility has historically come back from water sources: groundwater aquifers, rivers and lakes. However, ever-changing climate patterns, combined with population growth pressures and restricted availableness of recent and cheap water provides, are shifting the water industry’s attention – the globe is looking to the ocean for water.
The ocean has 2 distinctive features as a water supply – it’s drought-proof and is much limitless. Over 50 % of the world’s population lives in urban centres bordering the ocean. In several arid elements of the globe like the center East, Australia, Northern continent and Southern CA, the population concentration on the coast exceeds 75 %. water desalinisation provides a logical resolution for the property, long-run management of growing water demand.dingly vary of 30,000 to 44,000 mg/L. whereas desalinisation of salt water offers opportunities to provide lower price water, it’s unlikely to be a main supply of other facility within the future. the entire volume of salt water worldwide is restricted (less than I Chronicles of the world’s water) and, in most arid regions of the globe, it’s nearly totally utilized.
The high salinity of ocean water, and also the vital prices related to saltwater desalinisation, suggests that most of the world’s facility has historically come back from water sources: groundwater aquifers, rivers and lakes. However, ever-changing climate patterns, combined with population growth pressures and restricted availableness of recent and cheap water provides, are shifting the water industry’s attention – the globe is looking to the ocean for water.
The ocean has 2 distinctive features as a water supply – it’s drought-proof and is much limitless. Over 50 % of the world’s population lives in urban centres bordering the ocean. In several arid elements of the globe like the centre East, Australia, Northern continent and Southern CA, the population concentration on the coast exceeds 75 %. water desalinisation provides a logical resolution for the property, long-run management of growing water demand.
At the end of 2015, there have been around 18,000 desalinisation plants worldwide, with a complete installed production capability of 86.55 million m3/day or 22,870 million gallons per day (MGD). Around a quarter mile of this capability (37.32 million m3/day or nine,860 MGD) is found within the Middle East and North Africa. whereas desalinisation in this region is projected to grow endlessly at a rate of 7 to 9 % per annum, the “hot spots” for accelerated desalinisation development over consequent decade are expected to be Asia, the United States of America and Latin America.
Materials ; Methodologies
A Desalination process involves separation of salt water into two concentrations – solution with concentration less than the water feed and solution with salt concentration higher than water feed called as “brine concentrate”. All the technologies in desalination can be classified as Thermal process technology and Membrane technology based on their source of energy. In both technologies , energy is taken and pure water is produced.
Thermal processes are multi-step process in which desalination is done by distillation ranging from small scale to commercial production. This technology originated before 60years in Middle-East, where it is still the main technology to desalinate water in order to use it for drinking and irrigation.
Principal thermal processes include:
• Multi-flash distillation (MFD) – The heated water is boiled at very high pressure and is allowed to condense on the surface. This condensed water on the surface heats the feed water, thus utilising the heat of vaporization, and also preheating the feed water. The characteristics of this distillation are high volume, high corrosion and high chemicals required.
• Multi-effect distillation (MED) – It may have vertical or horizontal tubes through which steam is passed, The condensation of steam takes place on one side, and heat transfer to the feed water takes place on the other side. The pressure gradually decreases as temperature decreases due to transfer of heat to the liquid medium.
• Vapor compression distillation (VCD) — Here the vapour is compressed using heat transfer sheets where salt water is present on other side. This helps in preheating of salt water. Pressure applied on vapour side is high and on Salt water side is low to decrease it’s boiling point.
In thermal distillation, even chemical impurities are removed as there is only 1 plate used in each voltizer plate. High molecular weight impurities (eg. Algal toxins) cannot be removed by just steam distillation, hence they are removed as concentrates. Thermal processes are high energy incentive device, hence membrane processes are preferred.
Membrane process involves the transit of water via a semipermeable membrane under application of pressure. The pressurization revokes the spontaneous transport of water which occurs from the dilute side to the more concentrated side to regularize the free energy of the fluids. Various types of membranes are plied in wastewater treatment, e.g., membrane bioreactors, but desalination processes require reverse osmosis (RO) membranes; nanofiltration can have several applications.
The pore sizes of the membranes range from 0.1 to 1 micrometer (µm) for microfiltration, 0.001 to 0.1 µm for ultrafiltration, +/- 0.001 µm for nanofiltration, and 0.0001 to 0.001 µm for RO. Membranes are also applied for electrodialysis processes.
The membranes were initially cellulose acetate, but now-a-days they are usually layered polyamides and Poly sulfones with a spongy support material. Ceramic membranes are also becoming available that can provide greater roughness and oppose substances that can destruct polymers.
MF and UF membranes have various applications including pretreatment to reduce particulate loading and fouling on RO and NF membranes. NF membranes can diminish multivalent ions, but they are not efficient for monovalent sodium and chloride ions.
RO membranes are carried out at high pressure in the range of 265 to 1,000 psi to improve their efficiencies. RO membranes can elimiminate up to 99 percent of ions, and organic chemicals with molecular weights of nearly 200 daltons and larger. Solvent chemicals can be a obstacle because they may get dissolved in the polymeric membrane and may be transported to the processed water side. Water recoveries can reach upto about 85 percent, more or less depending upon the composition of the feedwater and design. Recoveries can be enhanced with subsequent brine retreatment. Periodic treatments are required to eliminate scaling.
Granular activated carbon is usually applied to dechlorinate the water just before it arrives the RO membrane because free chlorine can harm most membranes that can permit microbial regeneration in the GAC column. Carbon fines can also be released, which can add up to membrane fouling, so supplemental prefiltration may be necessary. Non-free chlorine