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Osmosis and Reverse osmosis

Osmosis-

 During natural osmosis, water flows from a less concentrated solution through semi permeable membrane to a more concentrated saline solution until concentrations on both sides of the membrane are equal. 

Reverse Osmosis:

It requires external pressure to reverse natural osmotic flow. As pressure is applied to the saline solution, water flows from a more concentrated saline solution through the semi permeable membrane.
• Gear pump or Reciprocating pump
• 60 bar pressure
• Chemical dosing- Sodium Hexametaphosphate sheets to assist washing the cartridge material
• Types of membrane- Cellulose acetate, aromatic polyamide membrane & polysulphonate
• Polyamide or polysulphonate (membrane not permeable to salt)- spirally wound- for Sea water
• Asymmetric membrane- Cellulose acetate membrane- hollow fine fiber- for brackish water
• Supported by Spiral wound cartridge 
• Central Porous tube core for pure water.
• Many envelopes 2 type of membranes
• Envelopes separated by coarse gauge sheets
• Cartridge when open- appearance of opened book
• Everything is then packed in stainless steel cartridge
• Relief arrangement required for the system
• Chlorine in pure water is added for chlorination
• Sterilization of pure water
• De-chlorination- pure water passing through compressed carbon filter
• Output of pure water – increases- putting cartridge tube in parallel
• Quality of pure water- increases- putting cartridge tube in series

Pretreatment 

Dosage- 5mg/ltr- inhibits precipitation of calcium carbonate. 10 micron cartridge filter used to reduce suspended solids in the membrane. Feed often heated to 400C to improves output but most systems are designed for 250C.
Post treatment
Sterilization by chlorination & to overcome flat taste soda ash can be used. 

Safety device in RO system

• Sea water H.P relief valve
• Permate relief valve
• Conductivity meter
• D.P alarm
• L.P cutout 

Advantages of RO system

• Small in size and light in weight
• Quiet operation
• Operation not effected by rough sea conditions
• Simple and fast maintenance
• No jacket water temperature is required
• Produce instantly

Disadvantage of RO system

• Careful pretreatment needed for long membrane life
• Membrane are expensive
• Output dependent on membrane condition
• For very high quality 2-pass system required


Reverse Osmosis Membrane: 

  • A reverse osmosis membrane has a thin microporous surface that rejects impurities, but allows water to pass through.
  • The membrane rejects bacteria, pyrogens, and 85%-95% of inorganic solids. Polyvalent ions are rejected easier than monovalent ions. 
  • Organic solids with molecular weight greater than 300 are rejected by the membrane, but dissolved gases pass through. 
  • Before purification TDS (total dissolved solid) allowed for this system is 45,000ppm and after purification it will be 300-1000 ppm. 
  • Material for reverse osmosis membrane is poly-sulphonate & polyamide.

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