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How Reverse Osmosis Purification Works

SEMIPERMEABLE MEMBRANES: THE CORE OF RO SYSTEMS

      The process of reverse osmosis (RO) represents the finest level of liquid filtration available today. While ordinary liquid filters use screens of various sizes to separate particles from the water streams, a reverse osmosis system employs a semipermeable membrane that separates an extremely high percentage of all unwanted material at the molecular level.

      For example, a semipermeable membrane may be permeable to water molecules of a dissolved salt solution, but not the molecules of the dissolved salt. If this membrane is placed between two compartments in a container, as shown in the first figure, then a salt solution is placed in one half of the container and pure water in the other, the water molecules will pass through the membrane while the salt molecules will not.

      The natural osmotic pressure across the membrane tends, over time, to bring the solutions on both sides of the membrane to the same concentration. That would move the pure water molecules across the membrane to make the salt solution more dilute.

      This the fundamental scientific principle that comes into play. That is to say, dissimilar liquid systems will try to reach the same concentration of materials on both sides of the membrane. Since the membrane is not permeable to the salt molecules, the only way for this to happen in our example is for pure water to pass through the membrane to the salt water side in an attempt to dilute the salt solution. This attempt to reach equilibrium is called osmosis.


APPLIED PRESSURE REVERSES NATURAL OSMOTIC FLOW

      However, the goal in our water purification system is to seperate the dissolved salt from the pure water. So it is necessary to reverse the natural osmotic flow by forcing the water from the salt solution through the membrane in the reverse direction. This can be accomplished by applying sufficient pressure to the salt water as it is fed into the system. This pressure creates the condition known as "reverse osmosis." Osmosis Principle


CROSS-FLOW FILTRATION PERMITS LONG-TERM OPERATION

      While the basic principles of reverse osmosis are simple, in practical terms the reverse osmosis process cannot go on indefinitely unless steps are taken to ensure that the membrane does not become clogged by the precipitated salts and other impurities forced against it by the pressurized stream of feed water.
      How these various factors are handled is one of the areas in which an excellant watermaking system differs from one that is "just OK." To significantly reduce the rate of membrane fouling, good reverse osmosis systems employ cross-flow filtration, shown in the second figure, which allows pure water to pass thourgh the membrane while the separated flow of concentrate sweeps rejected salts away from the membrane surface. Other system features to lengthen the membrane life include various provisions for input water pre-filtration.
Watermaker Operation


AquaMarine, Inc's Reverse Osmosis watermaker systems are designed for optimal system performance, minimal maintenance and long service life, resulting in the overall most economical installation.

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