Membrane Processes in Treatment of Drinking Water

The membrane processes have been in use for industrial or pharmaceutical applications, and now being applied to the treatment of drinking water
These do not provide residual disinfection hence a need to add small amount of persistent disinfectant like chlorine as a preservative during distribution.
The pore size of the membrane is smallest in RO and is in the following order:
RO < Nano filter < Ultra filter < Micro filter
The membrane processes of most significance in water treatment are:
High Pressure Processes
Reverse Osmosis
Nano-filtration
Low Pressure Processes
Ultrafiltration
Microfiltration

Reverse Osmosis
Principle: If two solutions are separated by a semipermeable membrane (i.e. a membrane that allows the passage of the solvent but not of the solute),
The solvent will naturally 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 is k/a ‘reverse osmosis’ (RO)
Therefore, RO creates two streams of water; one of treated water and another of waste water which is highly concentrated
RO rejects monovalent ions and organic with higher molecular weight
The most common application of RO is desalination of brackish water and sea water
Nano – filtration:
Uses a membrane with properties between that of RO and ultrafiltration
It allows monovalent ions such as sodium and potassium to pass but
Reject most of divalent ions such as calcium and magnesium and high molecular weight organics
Nano filtration may be effective for the removal of color forming organic compounds

Lower Pressure Membrane Processes

Ultrafiltration (operates at pressure < 5 bar)
It is similar in principle to RO but
The membranes have much larger pore sizes and
Hence operate at lower pressures
Ultrafiltration membranes reject organic molecules of molecular wt. above 800 Daltons

Microfiltration (operates at pressure 1 – 2 bar)
It is a direct extension of conventional filtration into the sub micrometer range
It sieves out particles > 0.05micrometer
Hence the membranes reject only the colloidal and suspended material but do not block molecules
Microfiltration has been used for water treatment on large scale but only
After coagulation to remove particulates and some dissolved organic carbon and
prior to RO and to improve permeate flux

super chlorination and break - point chlorination: http://www.ihatepsm.com/blog/super-chlorination-and-break-point-chlorina...

OT and OTA test: http://www.ihatepsm.com/blog/orthotolidine-ot-test-and-orthotolidine-%E2...

Membrane processes in water purification: http://www.ihatepsm.com/blog/membrane-processes-treatment-drinking-water

Super Chlorination and Break Point Chlorination: http://www.ihatepsm.com/blog/super-chlorination-and-break-point-chlorina...

Slow sand filter: http://www.ihatepsm.com/blog/slow-sand-filter-biological-filter

Rapid sand filter: http://www.ihatepsm.com/blog/rapid-sand-filter-mechanical-filter

Purification of water at a large scale: http://www.ihatepsm.com/blog/water-purification-large-scale-water-supply

Criteria for a Chemical to be Suitable as Water Disinfectant in Water Supplies: http://www.ihatepsm.com/blog/criteria-chemical-be-suitable-water-disinfe...

Water Supplies Disinfection with Chlorination: http://www.ihatepsm.com/blog/water-supplies-disinfection-chlorination

Principles of Chlorination: http://www.ihatepsm.com/blog/principles-chlorination

Methods of Chlorination in Water Supplies: http://www.ihatepsm.com/blog/methods-chlorination-water-supplies

Horrock's Apparatus: http://www.ihatepsm.com/blog/horrock%E2%80%99s-apparatus