Reverse Osmosis 

Reverse Osmosis is a process for removing dissolved mineral salts, submicron particulate matter, organic molecules and compounds and microorganisms from water by forcing water, under increased pressure, through a semi-permeable membrane.  This process is the “reverse” of the natural osmotic process in which fluids with a low concentration of dissolved solids pass through a membrane into an area of higher concentration.  With reverse osmosis, water is made to pass from a state of high concentration to a state of low concentration.  Since reverse osmosis does not occur naturally it must be created by applying pressure to the high solids water in order to force it through the membrane.  The membrane material must be strong and resistant enough to withstand the high pressures of RO operation.  Most membrane applications such as the processing of potable water utilize pressures that range between 200 and 400 psi.  Other applications such as seawater desalination may require pressures as high as 1000 to 1200 psi.  The pressure applied to the inlet/feed side of the reverse osmosis membrane must be significantly higher than the natural osmotic pressure of the water in order for the osmotic process to be reversed.  As a result high pressure pumps are generally used to create the pressure needed to produce product/permeate flow rates that are economically acceptable.

 

The product flow termed as “permeate” of a reverse osmosis unit is mainly a function of temperature and pressure.  Some membranes require a tempered water (77 ºF) in order to optimize their production while others require higher pressures.  The quality of a reverse osmosis product/permeate water is based on a percentage of the dissolved solids supplied to the membrane.  Typical “rejection” of dissolved solids will range between 96% - 99.5% depending upon the chosen membrane.   Since reverse osmosis membranes are “semi-permeable” they must have a reject/waste stream termed as “concentrate” to carry away the impurities that have been removed/rejected.  System “recovery” (product water divided by feed water) is limited by the characteristics of the feed water.  Single pass reverse osmosis units typically have a recovery of between 50 and 75%.  System recovery can be increased/controlled with a recycle stream. 

 

In order to maintain the effective and efficient operation of a reverse osmosis system an economic balance between product water quality and system recovery needs to be determined.  High recovery rates will appear to make a system more efficient and decrease waste/concentrate water however they will also increase the concentration of dissolved solids in the system which will hinder the quality of the product/permeate water.

 

Pretreatment of water prior to a reverse osmosis unit is almost always required.  The majority of reverse osmosis membranes in service today are of TFC (thin film composite) construction.  TFC membranes have high salt rejection rates, exhibit good performance over a wide range of pH and temperature conditions and are not degradable by microorganisms.  When using thin film composite (TFC) reverse osmosis membranes chlorine removal is a must.  The chlorine may be eliminated by using activated carbon, injection of sodium metabisulfite or ultraviolet light.  In addition, it is generally recommended that hardness minerals such as calcium and magnesium be removed from the feed water so as to prevent scale formation on the reverse osmosis membranes.  Turbidity, iron and other impurities must also be controlled for optimum performance of a reverse osmosis system.

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