Bleaching and Dyeing
It is estimated that to dye 1 kg of cotton with reactive dyes, 0.6-0.8 kg of NaCl, 30-60 grams of dyestuff and 70-150 liter of water is required. Once the dyeing operation is over, the wastewater must be treated before reuse. Coagulation and Membrane Technique ( nanofilteration or reverse osmosis) are among the processes suggested for the treatment of water.
Coagulation and Flocculation
Natural and wastewater contain small particles. They are suspended in water in a form called as a colloid. These particles carry the same charges, and repulsion prevents them from combining into larger particles to settle. Thus, some chemical and physical techniques are applied to help them settle. The phenomenon is known as coagulation. A well known method is the addition of electrolyte. Charged particulates combine with ions neutralizing the charges. The neutral particulates combine to form larger particles, and finally settle down. Historically Alum is used for this purpose but it makes the pH of the solution slightly basic.
Another method is to use high-molecular-weight material to attract or trap the particulates and settle down together. Such a process is called flocculation. Starch and multiply charged ions are often used.
Here the basic advantage is that the dye molecules themselves are removed which is better than other methods where dye molecules are decomposed and produce harmful and toxic aromatic compounds.
The disadvantage is that in coagulation process, large amount of sludge is created which may become a pollutant itself and increase the treatment cost.
This method is useful for removing the insoluble dyes, but the cost of treating the sludge increases.
Ultrafilteration and Nanofilteration
Ultrafilteration filters substances with sizes less than than 10^-7 to 10^-8 m . It can effectively remove suspended organic solids. It can not remove multivalent ions. It needs low water pressure to operate.
Nano filteration filters substances with size less than 10^-8 to 10^-10 m. It can remove multivalent ions.
This method is useful for removing the insoluble dyes, but the cost of treating the sludge increases.
Ultrafilteration and Nanofilteration
Ultrafilteration filters substances with sizes less than than 10^-7 to 10^-8 m . It can effectively remove suspended organic solids. It can not remove multivalent ions. It needs low water pressure to operate.
Nano filteration filters substances with size less than 10^-8 to 10^-10 m. It can remove multivalent ions.
Reverse Osmosis
It can remove substances with size less than 10^-9 to 10^-11m. It can remove multivalent as well as monovalent ions.
When a compartment containing a dilute solution is connected to another compartment containing a concentrated solution by a semipermeable membrane, water molecules move from the dilute solution to concentrated solution. This phenomenon is called osmosis.
By applying pressure in the higher concentration solution, water molecules migrate from a high concentration solution to a low concentration solution. This method is called reverse osmosis water filter system. ( Source )
An excellent FAQ on Reverse Osmosis can be found here .
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