Filtration and Membrane Separation for Water Purifcation

Water available from different sources is never pure and contains impurities, some suspended and others dissolved in it apart from some living organisms like bacteria, virus, pathogen etc. These impurities may adversely affect the purpose for which the water is used. The water is used for drinking, for cleaning, for processing in industry and well as on locations on a ship.

The impurities in water have to be removed, either partially or fully, in order to make it suitable for consumption or use in shipboard systems.

Filtration and membrane separation is the process most commonly used aboard ships to remove the suspended or dissolved impurities from the water. Filtration removes suspended impurities, whereas membrane separation is invariably used to remove suspended, dissolved impurities as well as living organisms.

A filter is nothing but a porous barrier in form of a sheet across which the water is allowed to flow in order to separate suspended impurities from the water. The size of the pores in the filter depends on the size of suspended impurities in the raw water. The raw water with bigger size impurities requires bigger pore size filter, and the process is called macro filtration. Water with smaller size impurities requires smaller pore size filter and the process is called micro filtration. The size of pores in micro filtration can be as small as one micron. Filters with pore size as low as 0.1 micron are also in use now.

The materials used for filter media can be paper, textile fibers, synthetic or metal wool, sintered metal, etc. The filtering material is installed in a casing with inlet and outlet pipe connections. The raw water enters the filter from the inlet and leaves the filter from the outlet minus any suspended impurities left behind in the filter casing. However, the filters also have some limitations, as very fine impurities in colloidal form cannot be separated by them.

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:

1. Ultra Filtration.
2. Nano-Filtration.
3. 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.