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AQUA FRESH Offers Reverse Osmosis Technology for Water purification

  • Since the early days of civilization mankind have adopted simple forms of membranes.
  • In early agriculture communities, household sieves were invented and developed to separate fin grain ground from coarse grain particles and shells.
  • Similarly, cheesecloth was made from cotton fibers and used to manufacture cheese.
  • Both forms of separation are based on differences in particle size.
  • However, developments in membrane technology have focused on adoption of other separation mechanisms,  such differences in solution and diffusion rates of various species across the membrane material.

 Elements of Membrane Separation

  • —A number of membrane-based desalination processes are used on industrial scale. The membrane-based processes include
    • —  reverse osmosis,
    • —  nanofiltration,
    • —  ultrafiltration, and
    • —   microfiltration.
  • Differences among membranes
    • —   Microfiltration operates on a particle size range of 0.10 µm to 0.15 µm.
    • —  Ultrafiltration operates on a particle size range of 0.15 µm to 5xl0-2 µm
    • —  Nanofiltration operates on a particle size range of 5xl0-2 µm to 5×10-3 µm
    • —  Reverse osmosis operates on a particle size range of 5xl0-3 µm to 10-4 µm.

Comparison between RO and other filtration processes

—  There is an inherent difference in the separation mechanism in all filtration processes and the reverse osmosis process.

  • In filtration, separation is made by a sieving mechanism, where the membrane passes smaller particles and retains larger ones.
  • In osmosis or reverse osmosis processes the membrane permeates only the solvent and retains the solute.
  • the microfiltration, ultrafiltration, and nanofiltration processes are used to separate the suspended material.
  • On the other hand, the reverse osmosis process is used to separate dissolved solids.

Reverse Osmosis

—Application of an external pressure to the salt solution side equal to the osmotic pressure will also cause equilibrium.

  •  Additional pressure will raise the chemical potential of the water in the salt solution and cause a solvent flow to the pure water side, because it now has a lower chemical potential.

Performance Parameters

—  The RO process is defined in terms of a number of variables:

  1. Osmotic and operating pressure
  2. Salt rejection
  3. Permeate recovery
  • —Membrane manufacturing companies define system specifications in terms of the feed quality, which includes salinity and temperature.

Osmotic and Operating Pressure

The magnitude of the pressure required to completely impede (inhibit) the flow of solvent is defined as the “osmotic pressure”

  • —  The osmotic pressure, π , of a solution can be determined experimentally by measuring the concentration of dissolved salts in the solution.
  • Operating pressure
  • —  Operating pressure is adjusted to overcome the adverse effects of the following:
    •    Osmotic pressure
    •    Friction losses
    •    Membrane resistance
  • —If the operating pressure is set equal to the sum of the above resistances the net permeate flow rate across the membrane would be minimal or equal to zero; therefore, the operating pressure is set at higher value in order to maintain economical permeate flow rate.

Features of the RO Membranes

  • The membranes are formed of thin film of polymeric material several thousand Angstroms thick cast on polymeric porous material.
  • Commercial membranes have high water permeability and a high degree of semi-permeability.
  • The membrane must be stable over a wide range of pH and temperature, and have good mechanical integrity.
  • The life of commercial membranes varies between 3-5 years. This depends on the feed water quality, pretreatment conditions, and stability of operation.