Question of the Day: RO Salt Passage

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Question of the Day: RO Salt Passage
What properties determine how much of a "salt" or other compound will pass through a reverse osmosis membrane?

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5 Answers

  1. Size, charge, ionization, bonded pair, temperature.  Smaller molecules (sodium vs calcium) will be better rejected.  Divalent Ions are typically better rejected than similar sized monovalent ions. The ionization/dissocation of the molecule also has an important effect. Depending on the pH and temperature a large percentage of some molecules (ammonia, boron) will be in either a gas state or acid state) that is poorly rejected (gases are not rejected at all). Finally, bonded pair. Charge balance must be maintained across the membrane. A solution of Sodium Sulfate will be better removed than a solution of Calcium Chloride as the Sulfate is larger and better rejected and the sodium will be rejected to maintain the charge balance. The higher the temperature the more porus the membrane and the lower the feed viscosity both of which make it easier for salts to pass through the membrane.

  2. Most RO membranes cannot handle salt over 80,000 TDS.  Above 80,000 TDS, distillation is often used.

     

  3. If there are smaller molecules such as ammonia, the reject rate may be lower than materials such as salt.  We found that there was only a 60% reject rate as compared to 90% of calcium or chloride.

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  4. Generally, it firstly depends on the physical and chemical characteristics of the membrane itself. Second, it depends on network design, i.e. water recovery rate, concentration release rate, and the degree to which the surface of the membrane is "polarized" by a chemical potential gradient profile.  Surface concentration of the ions may be considerably higher than in the bulk of water passing along the length of the membrane.  Generally speaking, salt flux is proportional to the concentration of the salt at the membrane surface, so the less well transported the salt is from the surface, the higher polarization of the salt gradient, and the higher chemical potential it will exert at the membrane surface, resulting in higher transport across the membrane.  Salt flux will therefore take place whether or not there is net driving pressure for water to flow across the membrane.

  5. 1) in terms of the product or membrane the main will be the material but secondly the manufacturing process

    2) about the operating conditions is a combination of flow and pressure

    3) regarding to water parameters, the temp