Reverse Osmosis
The term reverse osmosis comes from the process of osmosis, the natural movement
of solvent from an area of low solute concentration, through a membrane, to an
area of high solute concentration if no external pressure is applied.
In simple terms, reverse osmosis is the process of pushing a solution through a
filter that traps the solute on one side and allows the pure solvent to be
obtained from the other side. More formally, it is the process of forcing a
solvent from a region of high solute concentration through a membrane to a
region of low solute concentration by applying a pressure in excess of the
osmotic pressure. The membrane here is semipermeable, meaning it allows the
passage of solvent but not of solute.
The membranes used for reverse osmosis have no pores, the separation takes place
in a dense polymer layer of only microscopic thickness. In most cases the
membrane is designed to only allow water to pass through. The water goes into
solution in the polymer of which the membrane is manufactured, and crosses it by
diffusion. This process requires that a high pressure be exerted on the high
concentration side of the membrane, usually 5 MPa - 20 MPa (50 bar - 200 bar).
This process is best known for its use in desalination (removing the salt from
sea water to get fresh water) and has been used in this way since the early
1970s.
Method
When two solutions with different concentrations of a solute are mixed , the
total amount of solutes in the two solutions will be equally distributed in the
total amount of solvent from the two solutions. This is achieved by diffusion,
in which solutes will move from areas of higher concentration to areas of lower
concentrations until the concentration in all the different areas of the
resulting mixture are the same, a state called equilibrium.
Instead of mixing the two solutions together, they can be put in two
compartments where they are separated from each other by a semipermeable
membrane. The semipermeable membrane does not allow the solutes to move from one
compartment to the other, but allows the solvent to move. Since equilibrium
cannot be achieved by the movement of solutes from the compartment with high
solute concentration to the one with low solute concentration, it is instead,
achieved by the movement of the solvent from areas of low solute concentration
to areas of high solute concentration. When the solvent moves away from low
concentration areas, it causes these areas to become more concentrated. On the
other side, when the solvent moves into areas of high concentration, solute
concentration will decrease. This process is termed osmosis. The tendency for
solvent to flow through the membrane can be expressed as "osmotic pressure",
since it is analogous to flow caused by a pressure differential.
In reverse osmosis, in a similar setup as that in osmosis, pressure is applied
to the compartment with high concentration. In this case, there are two forces
to consider influencing the movement of water: the pressure caused by the
difference in solute concentration between the two compartments (the osmotic
pressure) and the externally applied pressure. In the same way as in
conventional osmosis, the solute cannot move from areas of high pressure to
areas of low pressure because the membrane is not permeable to it, only the
solvent can pass through the membrane. When the effect of the externally applied
pressure is greater than that of the concentration difference, net solvent
movement will be from areas of high solute concentration to low solute
concentration, and reverse osmosis occurs.
Reverse osmosis in use
Drinking water purification
In the United States, household drinking water purification systems, including a
reverse osmosis step, are commonly used for improving water for drinking and
cooking.
Such systems typically include four or five stages:
* a sediment filter to trap particles including rust and calcium carbonate
* optionally a second sediment filter with smaller pores
* an activated carbon filter to trap organic chemicals and chlorination
* a reverse osmosis filter with a thin film composite membrane (TFM or TFC)
* optionally a second carbon filter
In some systems, the carbon pre-filter is omitted and cellulose triacetate
membrane (CTA) is used. The CTA membrane is prone to rotting unless protected by
the chlorinated water, while the TFC membrane is prone to breaking down under
the influence of chlorine. In CTA systems, a carbon post-filter is needed to
eliminate the chlorine.
Portable reverse osmosis (RO) water processors are sold for personal water
purification in the home. These units are gravity powered (they need no water
pump), and need no electricity. The pressure of gravity pushes/drains the water
though the filters, much like a coffee-maker filter. A filter lasts for about
seven years before replacement is needed. RO water processors are used by people
who live in rural areas without clean water, far away from the city's water
pipes. Rural people filter river or ocean water themselves, as the device is
easy to use. Some travelers on long boating trips, fishing, island camping, or
in countries where the local water supply is polluted or substandard, use RO
water processors. In production of bottled mineral water, the water passes
through a RO water processor to remove pollutants and all microorganisms,
including the smallest microbe known, archaeobacteria. In water stores (located
in mini-malls usually), RO water processors change tap water into water, with
fewer or no traces of lead and fluoride remaining. Such water stores are more
popular in areas where the tap water is substandard. In developing nations where
people die from drinking bacteria-infested well water, water from mud holes, or
from ruptured water pipes, RO water processors may save lives.[citation needed]
In the deserts of Middle Eastern nations, fresh water is rare, but salt water
from the ocean is commonplace. This ocean water can be changed into fresh water
with RO water processors,[citation needed] to be used for drinking, bathing,
farming, etc. without the high energy cost of water desalination.
RO water used for drinking, generally has a post carbon filter for taste. RO
water has a flat taste that most do not like.[citation needed] Therefore the
post carbon filter, (usually walnut shells or coconut shell) gives the RO water
back its 'normal' flavour.
Wastewater Purification
Rain water collected from sewer drains is purified with reverse osmosis water
processors and used as tap water in Los Angeles and other cities, as a solution
to the problem of water shortages. In San Diego, waste water from sinks, baths,
showers, and toilet drains is also purified with RO, and then used as tap water.
In industry, reverse osmosis removes minerals from boiler water at power plants.
It is also used to clean effluent and brackish groundwater.
In July 2002, Singapore announced that a process named NEWater would be a
significant part of its future water plans. It involves using reverse osmosis to
treat domestic wastewater before discharging the NEWater back into the
reservoirs.
Application of reverse osmosis in food industry
In addition to desalination, reverse osmosis is a more economical unit operation
for concentrating food liquids, e.g. fruit juices, than conventional
heat-treatment processes. Research has been done on concentration of orange
juice and tomato juice. Its advantages include a low operating cost and the
ability to avoid heat treatment processes, which makes it suitable for
heat-sensitive substances like the protein and enzymes found in most food
products.
Although use of the process is frowned upon in the wine industry, it is
reportedly widely used. Château Léoville-Las Cases in Bordeaux, France, is one
of the few producers to acknowledge use of the process.
Maple syrup production
Starting in the 1970s, some maple syrup producers started using reverse osmosis
to remove water from sap before being further boiled down to syrup. The use of
reverse osmosis allows approximately 75 to 80 percent of the water to be removed
from the sap, reducing energy consumption and exposure of the syrup to high
temperatures. Microbial contamination and degradation of the membranes has to be
monitored.
History
Dennis Chancelor invented the reverse osmosis water processor in the 1970s. It
employed a brine seal between membranes, a pressure vessel, and 8 inch diameter
tubular membranes. In 2004, he invented an improved system that uses a
three-flow channel that allows more water to each membrane. Its tubular
membranes are 18 inches in diameter.
Information source:
Wikipedia
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