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The membrane separation process is based on the presence of
semi permeable membranes. The principle is quite simple: the
membrane acts as a very specific filter that will let water
flow through, while it catches suspended solids and other
substances.
Membrane filtration can be divided up between micro and ultra
filtration on the one hand and nano filtration and Reverse
Osmosis (RO or hyper filtration) on the other hand. When membrane
filtration is used for the removal of larger particles, micro
filtration and ultra filtration are applied. Because of the
open character of the membranes the productivity is high while
the pressure differences are low. When salts need to be removed
from water, nano filtration and Reverse Osmosis are applied.
Nano filtration and RO membranes do not work according to
the principle of pores; separation takes place by 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.
Reverse Osmosis (RO): Reverse osmosis
(RO) 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 high dissolve solids concentration through a membrane to
a low dissolve solids 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 semi permeable,
meaning it allows the passage of solvent but not of solute.
Ultrafiltration (UF): UF is a pressure-driven
membrane-based separation process in which hydrostatic pressure
forces a liquid against a semipermeable membrane. Suspended
solids and solutes of high molecular weight are retained,
while water and low molecular weight solutes pass through
the membrane.
UF's main attraction is its ability to purify, separate, and
concentrate target macromolecules in continuous systems. UF
does this by pressurizing the solution flow. The solvent and
other dissolved components that pass through the membrane
are known as permeate. The components that do not pass through
are known as retentate. Depending on the Molecular Weight
Cut off (MWCO) of the membrane used, macromolecules may be
purified, separated, or concentrated in either fraction.
Microfiltration (MF): MF is a pressure-driven
membrane-based separation process in which particles and dissolved
macromolecules larger than 0.1 micrometers are rejected. The
retentate typically includes macromolecules and particulates
Microfiltration is not fundamentally different from reverse
osmosis, ultrafiltration or nanofiltration, except in terms
of the size of the molecules it retains.
Nanofiltration: Nanofiltration is a
pressure-driven membrane-based separation process in which
particles and dissolved molecules smaller than about 2 nm
are rejected. As with RO, retentate types include salts and
nonpolar solvents.
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