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Fresh Water Generation by Reverse Osmosis
Reverse Osmosis process has been explained in the earlier article. It requires the Raw Water to be pumped across the Semi-permeable Membrane at a pressure in excess of its Osmotic Pressure which depends on the extent of dissolved impurities in the Raw Water. Raw Water can be Seawater, Brackish Water or Effluent Water. Depending on the quality of Raw Water, a pretreatment is required to be done on the Raw Water before starting the actual Reverse Osmosis process. We shall consider here Seawater as Raw Water.
Fresh Water Generation Process
The Seawater may contain suspended impurities liable to damage the delicate Semi-permeable Membranes made of polymer materials. The suspended impurities from Seawater are removed using micro filters. Any colloidal type suspended impurities may require ultra filtration as pre-treatment using Semi-permeable Membranes of that type.
Some polymer materials for membranes like Polyamide are adversely affected by any Chlorine in Raw water. Therefore, if there is any Chlorine in the Seawater, it must be eliminated by pre-treatment. The condition of Raw Water prior to processing by Reverse Osmosis should be crystal clear free of suspended impurities and Chlorine.
Reverse Osmosis Process
The pre-treated Seawater is then pumped into a number of pressure vessels, depending on the capacity of the plant, containing Semi-permeable Membrane in form of cylindrical roll cartridges. Each of the pressure vessels has one inlet and two outlets. Seawater enters from inlet and divides into two streams for two outlets namely clean fresh water also termed Permeate and the concentrated Seawater (Brine) which is discarded to sea.
Reverse Osmosis Process
The Seawater enters the vessels at high pressure (usually from 50 to 70 bar, depending on the concentration of Seawater.
Permeate quality (i.e. salt content in the product water) depends on the characteristics of the Semi-Permeable Membranes. The membranes are available with salt rejection rates ranging up to 99.9% in which single stage process removes the complete salt content. Alternately, the process can be carried out in two stages if membranes with high salt rejection rate are not available.
Reverse Osmosis Membrane Cartridge
The Permeate (product water) from the above process may contain small quantities of dissolved salts and is suitable as drinking water if the salt content is below 250 ppm, well within the WHO limit. However, if the water is to be used for boiler feed, it has to be completely free from salts and the product water may require further treatment to get rid of small quantities of salts by Ion Exchange Treatment.