Membrane filtration Process in Water Treatment Plant
The membrane filtration processes in water treatment process are reverse osmosis, ultrafiltration, microfiltration and nanofiltration. These processes have traditionally been applied to the production of water for industrial or pharmaceutical applications but are now being applied to the treatment of drinking water. Their characteristics are illustrated in Figure below
If two solutions are separated by a semipermeable membrane, i.e. a membrane that allows the passage of solvent but not of the solute, the solvent will pass from the lower concentration solution to the higher concentration solution. This process is known as osmosis. It is possible, however, to force the flow of solvent in the opposite direction, from the higher to the lower concentration, by increasing the pressure on the higher concentration solution. The required pressure differential is known as the osmotic pressure and the process as reverse osmosis.
Reverse osmosis results in the production of a treated water stream and a relatively concentrated waste stream. Typical operating pressures are in the range 15 to 50 bar depending on the application. Membrane pore sizes are less than 0.002µm. The most common application of reverse osmosis is desalination of sea water although the use of reverse osmosis for nitrate removal has also been proposed. In the case of private water supplies, reverse osmosis units sold as point of use devices are probably the most common type of membrane process encountered.
Ultrafiltration is similar in principle to reverse osmosis, but the membranes have much larger pore sizes (typically 0.002 to 0.03µm) and operate at lower pressures. Ultrafiltration membranes reject organic molecules of molecular weight above 800 and usually operate at pressures less than 5bar.
Microfiltration is a direct extension of conventional filtration into the sub-micron range. It is capable of sieving out particles greater than 0.05µm and will remove most bacteria and amoeboid cysts. It has been used for water treatment in combination with coagulation or powdered activated carbon (PAC) to remove viruses, bacteria, dissolved organic carbon and to improve permeate flux. Microfiltration membranes have pore sizes typically in the range 0.01 to 12µm and do not separate molecules but reject colloidal and suspended material at operating pressures of 1 to 2bar.
Nanofiltration uses a membrane with properties between those of reverse osmosis and ultrafiltration membranes; pore sizes are typically 0.001 to 0.01µm. Nanofiltration membranes allow monovalent ions such as sodium or potassium to pass but reject a high proportion of divalent ions such as calcium and magnesium and organic molecules of molecular weight greater than 200. Operating pressures are typically about 5bar. Nanofiltration may be effective for the removal of colour and organic compounds.
Membrane processes can provide adequate removals of pathogenic bacteria, Cryptosporidium, Giardia, and potentially, human viruses and bacteriophages. However, they should not be relied upon as the sole means of disinfection as in the case of the types of systems encountered in private supplies there is no simple means to check membrane integrity to warn of potential breakthrough of microorganisms
This is the Membrane filtration Process in water treatment plant.