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
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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