Semi-permeable membranes are the core components in the process of membrane separation. Depending on the characteristics of these membranes, they are used to separate selectively certain impurities in the water while allowing other impurities to remain in. The pore size in these membranes is very small and is measured in Angstrom units (1 Angstrom unit = 10 ¯¹º meter). The pore size of the membrane is selected according to the molecular sieve size of the dissolved impurities in the raw water. Three categories of membranes are available depending on the molecular size of the particle of the impurities in the water, namely:
- Ultra Filtration.
- Reverse Osmosis.
Each category has separate characteristics and requires different operating pressure during the separation process.
Ultra Filtration is employed to separate larger size impurities like colloidal matter and living organisms like bacteria, virus, pathogen etc. This operates at low pressures.
Nano-Filtration is employed for separating impurities having relatively smaller molecular sieve size particle and operates at medium pressures.
Reverse Osmosis is employed for impurities with particles of the smallest molecular sieve size and is operated at higher pressures.
The rejection rate of solute particles, the permeate separation rate, and the operating pressure for all the above categories depends on the characteristics of the membrane employed in the process.