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 filter can be divided up between micro and ultra filter on the one hand and nano filtration and Reverse Osmosis (RO or hyper filter) on the other hand. When membrane filter is used for the removal of larger particles, micro filter 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 filter and Reverse Osmosis are applied. Nano filter 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 filter 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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