Ways To Purify Water

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 such as the ship.

Therefore, the impurities in the water have to be removed either partially or, fully in order to make it suitable for the purpose for which it is required.

Filtration & Membrane Separation is the process used to remove the suspended and / or dissolved impurities from the water. While Filtration removes suspended impurities, 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 separated suspended impurities from water. The size of 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 termed as Macro Filtration, while water with smaller size impurities requires smaller pore size filter and the process is termed as Micro Filtration. The size of pores in Micro Filtration could be as small as one micron. Filters with pore size as low as 0.1 micron are also in use now a days.

The materials used for filters could be paper, textile fibres, synthetic / 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 inlet and leaves the filter from outlet minus the suspended impurities left behind in the filter casing. However, the filters too have some limitations as very fine impurities in colloidal form can not be separated by filters which can be separated by Membrane Separation.

Semi-Permeable Membranes are the core components in the process of the Membrane Separation. Depending on the characteristics of these membranes, they are used to separate selectively, certain impurities in the water while leaving other impurities to remain in. The pore size in these membranes is very small measured in Angstrom units (1 Angstrom unit = 10 ¯¹º metre). 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 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 particle of smallest molecular sieve size and is operated at higher pressures.

Rejection rate of solute particle, the permeate separation rate and the operating pressure for all the above categories depends on the characteristics of the membrane employed in the process.