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Removing dissolved minerals from seawater, brackish groundwater, or treated wastewater —appears at first glance to be the ideal answer to freshwater shortages. What could be more attractive than harnessing the planet’s seemingly inexhaustible 1.34 quadrillion (that’s 15 zeros) megaliters of seawater?
Ocean Average Salinity (gms./lit)Atlantic 35.4Indian 34.8Pacific 34.5Brackish Waters 0.5 to 3
Water for agriculture accounts to over 90% of water use in India (Royal Academy of Engg 2012) By 2030, 2/3rds of the world population will be suffering from water shortages (HSBC Optimised Global Water Index, 2008)
Over15,000 desalination plants in operation worldwide, producing upto 55.6 cu.m. water a day (0.5% of global reqmt) 60% are located in the Middle East. Total world capacity is approaching 30 million m3/day of potable water The worlds largest plant in Saudi Arabia produces 128 Million Gallons a Day (MGD) of desalted water.
Intrinsically, the feed stream contains .. Dissolved solids Silt Algae Bacteria Various flora and fauna TEPs (Transparent Exopolymer Particles)**TEP : Conc. : 20-5000 particles/ ml Size: 0.002 to 0. 2 mm Exist as amorphous blobs, clouds and sheets Hence, Pretreatment Method is generally a customised form of Coagulation
How it works Separate saltwater and freshwater with a membrane that blocks salt ions, and the freshwater rushes to the salty side by the natural process of osmosis. Reverse osmosis (RO) uses hydraulic pressure to shove water molecules in the opposite direction, with the membrane holding back the salt. Upside Comparatively low energy cost. Downside Toxic brine; can’t completely filter potentially harmful substances like boron, arsenic, lithium, and some pharmaceutical compounds. When membranes become clogged, they must be scraped and bleached or they stop working; cleaning, however, reduces the expensive membranes’ lifetime. Pretreating the water to remove the gunk slows the rate of fouling but requires a lot of real estate. Best for Brackish groundwater, which contains on average only 3 to 5 grams of salt per liter. RO is also increasingly being used to desalinate seawater, however. Energy trade-off The pressure needed to push water through the membrane is proportional to the water’s salinity. Higher pressure means higher energy cost. On average, RO demands at least 3.5 MWh/ML produced from brackish water
How it works Capacitive deionization works without membranes. It filters impurities by streaming the water between two charged electrodes. The electrodes attract ions in the water, which stick to them, leaving freshwater. The attached ions eventually clog the electrodes, but cleaning is easy: Simply reverse the electrical polarity to flush the ions back out. Good candidates for electrodes are advanced materials such as carbon aerogel and mesoporous carbon. Upside Easy to clean; requires less power. The process could theoretically go on forever without changing electrodes. Downside Works only for brackish water; in practice, electrodes can foul. Does not mitigate toxic brine. Best for Brackish water. Energy trade-off Far less pressure means less energy. What’s next This year, a test reactor will be unveiled in New Mexico, part of an international project led by Campbell Applied Physics, in Rancho Murieta, Calif., and several U.S. national laboratories.
Adequate & Safe disposal of brine poses a significant environmental challenge Brine Salinity depends upon: Salinity of feedwater Desalination Method Recovery rate of plant Along with high salt levels, can contain Mg, Pb, I as well as chemicals introduced via urban & agril. runoff
Reduce volume of brine to be discharged and minimize the adverse chemicals found therein. Usage of better artificial filters or even natural filters, to reduce the amt. of chemicals during the pre treatment process
Coastal Methods: -Discharge to oceans -Confined Aquifers Inland Methods: -Deep well injection -Evaporation Ponds -Solar energy ponds -Shallow Aquifer storage for future use
There’s no definitive ans. to this Qs. Since the Cost comprises of a no. of factors: -Labor -Chemicals -Peripherals -Maintenance -Electricity -Capital/Amortisation
The plant has been constructed in the New Jubail II Industrial Zone in the Saudi Arabia, Kingdom’s Eastern Province. Provides 8,00,000 cu.m. of freshwater for cities in the Eastern Province, Generates 2,750MW of electricity. Freshwater produced by the $3.8 billion desalination plant will be transported
The Minjur desalination plant consists of 8,600 sea water RO membranes, 248 pressure vessels, 23 pressure exchangers, five high-pressure pumps, 16 pressure filter vessels, electrical, automation and control systems, and a 1,200m of HDPE pipeline of 1,600mm diameter. The CMWSSB has laid a 33km pipeline with a cost of INR930m ($20m) to carry the treated fresh water from Minjur to Red Hills. The project also includes infrastructure for the collection of seawater. A 110kV/22kV sub-station has been set up by the Tamil Nadu Electricity Board for uninterrupted power supply to the desalination plant. A thorough environmental impact study was conducted in the planning stage.
Chennai Water Desalination (CWDL) is executing the project for the CMWSSB on a design, build, own, operate and transfer (DBOOT) basis. In September 2005, the CMWSSB signed a bulk water purchase agreement (BWPA) with CWDL to purchase water from the Minjur desalination plant at a cost of INR48.66/m³ ($1.03/m³). It will be sold to industries at a rate of INR60/m³ ($1.27/m³). At the end of the 25-year agreement the contract shall be renewed.
As promising as the process sounds, there’s still a price to pay. As long as the cost of desalination continues to depend on the cost of energy, these technologies won’t help much of the energy-starved developing world that needs them the most. Also, there is the problem of the toxic sludge generated as a by product. Throwing the brine back into the ocean can kill fish and smaller denizens of the food chain. Several contenders might make history of these concerns. New methods in the pipeline reduce desalination’s energy demands in innovative and intriguing ways. Further off are technologies that could turn desalination’s Achilles’ heel into a source of strength: In the future, desalination might just be powered by the very waste it filters out.
http://www.fwr.org Desalination, with a grain of salt- A California perspective, Cooleym, Gleick, Woolf Fundamentals of Water Desalination, Dessouky and Elsevier, 2002 Desalination: A National Perspective, National Research Council Committee on Adv. Desalination Tech., USA 2008 http://www. Idadesal.org http://www.desline.com