Reverse Osmosis:

Reverse Osmosi

Reverse osmosis (RO) has become a popular water treatment technology in nearly every industry requiring separation of a dissolved solute from its solvent, the solvent usually being water. The most common application of RO is the purification of water, involving simply the removal of undesirable contaminants. Industry makes heavy use of this application of RO for producing highly purified process water, and for treating industrial wastewater.
Dissolved solutes can be salts, or they can be organics, such as sugar or dissolved oils. The solutes are the species of lesser concentration held in solution by molecular attraction with the solvent, which is in greater concentration.
The basic process of reverse osmosis uses a pump and a semipermeable membrane. The pump provides the driving force. The semipermeable membrane passes water in preference to the solute that is dissolved in the water.

The process of reverse osmosis is not the same as filtration. Filtration is the removal of particulates by size exclusion. Particulate are removed by filtration because they are too large to fit through physical pores in the filter media, whereas water molecules can readily fit through pores.
The mechanism of reverse osmosis is different from filtration in that physical holes do not exist in a RO membrane. Such holes have never been found, even with microscopes of higher magnification. It is more likely that water and smaller-molecular weight organics are able to diffuse through the membrane polymer by bonding between the segments of the polymer’s chemical structure. Dissolved salts and larger molecular weight organics will not permeate the membrane easily, however, because of their size and charge characteristics. The RO membrane also is able to obtain nearly absolute removal of suspended solids that have means of traversing the membrane except via mechanical lesions.

Reverse osmosis is also a process of separation. The feed water stream is separated into a stream of purified water and a stream of concentrated solutes and particulates.

Sizes of Some Common Particles:

Substance Microns
Table salt 100
Human hair Human hair 50 – 80
Sand 50 and more
Smallest visible to naked human eye 30 – 50
Talcum powder 10
Cocoa 8 – 10
Iron rust 5
Carbon in oils 1 – 10
Clay 0.1 – 1.0
Pigments 0.2 – 0.4
Bacteria 2.0 – 0.4
Viruses 0.3 – 0.015
(Courtesy Osmonics Inc.)

Reverse Osmosis Theory:

Reverse osmosis systems separate dissolved solutes from water via a semipermeable membrane that passes water in preference to the solute. An RO membrane is very hydrophilic, implying that water is attracted to its chemical structure. Water can bond with the ends of the polymer segments making up the membrane. This gives water the ability to readily diffuse into and out of the polymer structure of the membrane.

Water (H2O) is a very polar molecule. There is a strong separation of charge that occurs across its molecule. When water is formed by the coming together of two hydrogen atoms with an oxygen atm, each of the two hydrogen atoms shares its one electron with the oxygen atom. This puts hydrogen atom in more stable state. The oxygen atom is also in a more stable energy state when it borrows the electrons from the hydrogen atoms.
The two hydrogen atoms will orient themselves on one side of the oxygen atom. Because it has given up its negatively charged electrons, the remaining positively charged proton of each hydrogen atom is able to exert its positive charge on its surroundings. The electrons that have migrated to the oxygen atom will tend to exert their negative charge on the far area of the oxygen atom. The end result is a separation of charge between the ends of the water molecule.
If this separation of charge is compounded by the large number of water molecules present in a typical solution, a substantial force is present that is capable of pulling apart the oppositely charged components of a salt. The oxygen end of water molecule will attract the positively charged cationic components of the salt. As the salt molecules separate into their components, they go into solution.
While in solution, the charged ions will remain surrounded by water molecules that are attracted to the charge of the ions. In essence, an ion group is created. The size of this ion group will depend on the size of the ions at its center and the extent of the charge characteristics of the ion. How the size of this ion group compares to the spacing between the polymer segments has a lot to do with how well the ion will diffuse through the membrane.
Although separated by water molecules, the dissolved cations will still maintain an attraction to the dissolved anions in the solution.A hydrated cation is going to resist diffusing through the membrane if a corresponding hydrated anion cannot also pass through. Otherwise, a charge imbalance would be created between the feed side and the permeate side of the RO membrane
The percent passage of any particular dissolved salt will depend upon ionic component with greater size and charge characteristics. For example, if the hydrated sulfate anion of a Sodium Sulfate solution cannot diffuse through the area between polymer segments, the sodium cation will not permeate through either. If calcium chloride is added to that solution, the hydrated sodium ion will then be able to diffuse more readily through the membrane because the hydrated chloride anion is better able to permeate the membrane.
The ability of an RO membrane to diffuse certain salts while rejecting others is not absolute. For any particular membrane, the percent passage of smaller ions or lesser charged ions, will be relatively greater than that of larger ions, or ions with greater charge characteristics. Generally cations anions with greater valence numbers will be better rejected than ions with lower valence.
How well an organic molecule permeates an RO membrane will depend on its physical size and shape and on its chemical characteristics. Generally the closer an organic molecule is structurally to that of the membrane polymer, the more readily it will diffuse through the polymer. Smaller organic molecules with polar characteristics will tend to diffuse better than the larger neutral ones. A very general rule of the thumb is that organic molecules with molecular weight less than 200 are more likely to permeate the membrane that ones with molecular weight greater than 200.

RO Pretreatment:

The purpose of RO pretreatment is to optimize the performance and life of the RO membrane elements. Pretreatment equipment and chemical feeding is engineered to accomplish this task by meeting the following three objectives:-

  • Insure the compatibility of the feed stream with the RO membrane.
  • Reduce or eliminate the potential for scale formation on the RO membrane or in the element flow channels.
  • Reduce the RO chemical cleaning frequency by reducing the fouling potential of the feed stream.

    The main components of the RO block are:-

  • Pressure vessel or Membrane element housing
  • RO structural frame
  • Membranes
  • Piping material
  • High pressure pump
  • Electrical motors
  • Instrumentation
  • RO Application Area:

    Major industries and applications where in RO is used are Textile industry, Pharmaceutical industry, Steel Mills, Vegetable oil Refinery and solvent extraction plants, Steam and process boilers, Power plants, Beverages and distilleries, Drinking water applications, Food industry, Hospitals for kidney dialysis, Sugar Mills, Automobile manufacturing, Agriculture, Biotechnology, Electronic industry, Fisheries.