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Definition of Reverse Osmosis
The process of desalinate of water entails creation
of a flow through the membrane
- causing the water to leave the salty side of the membrane
and then flowing into the unsalted side. This can be achieved
only by creating pressure upon the water column on the
salt side of the membrane. This is done for two reasons.
Firstly, to remove the natural osmotic pressure and secondly,
to create extra pressure on the water column so as to
push the water through the membrane. The ideal pressure
required for desalination of seawater is 50-60 bars. There
are many processes and techniques that can be applied
for water desalination. Some of these water treatment are reverse osmosis,
electro dialysis, distillation and ion exchange
Of all the available processes, reverse osmosis
is the most economic process for desalination of brackish
water and seawater. When we compare reverse osmosis to
the traditional thermic process of distillation, the capital
investments and the energy used are much lower.
For those who are unaware and wish to be educated on the
processes of water filtration systems,
Membrane filtration can be considered as an alternative
for flocculation, sediment purification techniques, adsorption
(sand filters and active carbon filters, ion exchangers),
extraction and distillation.
There are two factors that are most relevant to determine
how effective a membrane filtration process is. these
two factors are selectivity and productivity. Selectivity
is expressed as a parameter called retention or separation
factor (expressed by the unit l/m2/h). Productivity is
expressed as a parameter called flux (expressed by the
unit l/m2/h). Both these factors i.e. Selectivity and
productivity are dependent upon the memberane.
Membrane
filtration can be divided into a few ranges. It is
divided between micro and ultra filtration on the one
hand and nano filtration and reverse Osmosis (also called
hyper filtration) on the other hand.
For the removal of larger particles through the process
of membrane filtration, usage is made of micro filtration
and ultra filtration. A low pressure difference ensures
high productivity owing to the open character of the membranes.
Nano filtration and reverse osmosis are utilized when
there is requirement to remove salts from water. Nano
filtration and reverse osmosis membranes do not work according
to the principle of pores. Instead, separation takes place
through diffusion through the membrane. The pressure that
is required to perform nano filtration and reverse osmosis
is much higher than the pressure required for micro and
ultra filtration, while productivity is much lower
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