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Reverse
osmosis is a separation process that uses pressure to
force a solvent through a membrane that retains the solute
on one side and allows the pure solvent to pass to 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.
This is the reverse of the normal osmosis process, which
is the natural movement of solvent from an area of low
solute concentration,
through a membrane, to an area of high solute concentration
when no external pressure is applied. The membrane here
is semipermeable, meaning it allows the passage of solvent
but not of solute. |
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The membranes used for reverse osmosis
have a dense barrier layer in the polymer matrix where
most separation occurs. In most cases the membrane is
designed to allow only water to pass through this dense
layer while preventing the passage of solutes (such as
salt ions). This process requires that a high pressure
be exerted on the high concentration side of the membrane,
usually 2–17 bar (30–250 psi) for fresh and
brackish water, and 40–70 bar (600–1000 psi)
for seawater, which has around 24 bar (350 psi) natural
osmotic pressure which must be overcome. |
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This process is best known for its
use in desalination (removing the salt from sea water
to get fresh water), but has also purified naturally
occurring freshwater for medical, industrial process
and rinsing applications since the early 1970s |
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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. |
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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. |
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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
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. |
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| Drinking
waterpurification |
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systems typically include four or five stages: |
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- a sediment filter to trap particles including
rust and calcium carbonate
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- optionally a second sediment filter with smaller
pores
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- an activated carbon filter to trap organic chemicals,
and chlorine which will attack and degrade TFC
reverse osmosis
membranes
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- a reverse osmosis (RO) filter which is a thin
film composite membrane ( TFM or TFC)
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- optionally a second carbon filter to capture
those chemicals not removed by the RO membrane.
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- optionally an ultra-violet lamp is used for disinfection
of any microbes that may escape filtering by the
reverse osmosis membrane.
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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 remove chlorine from the final
product water. |
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Portable reverse osmosis
(RO) water processors are sold for personal water purification
in various locations. To work effectively, the water
feeding to these units should best be under some pressure
(40psi or over is the norm). Portable RO water processors
can be 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 (Saline water may need
special membranes). 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 coupled with one or more UV
sterilizers. RO systems are also now extensively used
by marine aquarium enthusiasts, as the domestic water
supply contains substances that are extremely toxic
to most species of saltwater fish. In the production
of bottled mineral water, the water passes through
a RO water processor to remove pollutants and microorganisms.
In European countries, though, such processing of Natural
Mineral Water (as defined by a European Directive)
is not allowed under European law.(In practice, a fraction
of the living bacteria can and do pass through RO membranes
through minor imperfections, or bypass the membrane
entirely through tiny leaks in surrounding seals. Thus,
complete RO systems may include additional water treatment
stages that use ultraviolet light or ozone to prevent
microbiological contamination.) |
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In
the water treatment industry there is a chart of
types of contaminants, their sizes and which ones
pass through the various types of membranes. [1]
Membrane pore sizes can vary from 1 to 50,000 angstroms
depending on filter type. "Particle
filtration" removes particles of 10,000 angstroms or
larger. Microfiltration removes particles of 500 angstroms
or larger. "Ultrafiltration" removes particles of roughly
30 angstroms or larger. "Nanofiltration" removes particles
of 10 angstroms or larger. Reverse osmosis is in the
final category of membrane filtration, "Hyperfiltration," and
removes particles larger than 1 angstrom. |
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