The use of reverse osmosis membranes in the water treatment industry is standardized practice, due to the requirement of filtration to take place in removing unwanted impurities from water. Membrane water treatment is done through the use of barriers that allow only specific substances to pass through the small pores of the membrane while not allowing other types of contaminants. There are a variety of membranes in use throughout the market which act as processes in treating groundwater, surface water, and wastewater to a level that is acceptable for industries and especially consumption. It is a rapidly growing industry, as a result of the need to combat low quality water sources such as rivers, lakes, wells, streams, and the ocean, in order to provide more alternative methods in supplying healthy, safe, steady, and accessible water to the masses. 

Membranes are the best filtration media for water. Membranes used for water purification can have their pore size so designed to give the best efficiency for the job in hand. For example to purifyriver water it is not neccessary to have very small pores like an RO membrane which requires high pressure and thus need a lot of energy to work. So for water with low content of salts like river water you need to remove only the germs and dirt in the water, so the ideal membrane for such an application would be the UF membrane which use very little energy and will filter out the smallest of viruses and dirt particles. The river water that comes out of a UF membrane is clean and safe to drink.

When there are salts in the water, like in seawater, then an RO membrane will be required to purify the water. So an RO membrane will purify water from almost all chemicals. What can pass through the RO membrane is water and molecules smaller than the water molecule, like CO2 gas. Smaller molecules than water is generally not harmful to health. The RO Water is very pure almost distilled water in quality. Bottled water manufacturers usually use Reverse Osmosis Water Filters to purify their water.

As per mentioned, there are different types of membranes which can be utilized for water treatment. The factors that necessitate which membrane would be more effective relies upon the types of contaminants existing in the water, and the desired water quality by the user.

The two most common type of membranes used in the industry are:

  • Membrane Filtration (ultrafiltration, microfiltration) works by eliminating particles (mainly suspended solids) from water through operating under pressure.
  • Reverse Osmosis/Nanofiltration Membranes that operates similarly in producing freshwater from the prevention of larger dissolved molecules from passing through the small pores of the permeable membrane, while allowing the pure water to flow through.

Asymmetric Membrane --- Cellulose Acetate(CA) Membrane

Historically, the asymmetric membrane is formed by casting a thin film acetone-based solution of cellulose acetate(CA) polymer.The resulting CA membrane has an asymmetric structure with a dense surface layer of about 0.1 - 0.2 μm which is responsible for the salt rejection property. The rest of the membrane, which is 100-200 μm thick and supports the thin surface layer mechanically, is spongy and porous, and has high water permeability. Salt rejection and water flux of a CA membrane can be controlled by variations in temperature and duration of the annealing step.

Thin Film Composite Membrane --- Polyamide(PA) Membrane

Thin film composite (TFC) polyamide membranes are consisted of a porous support layer and a thin film dense layer which is a cross linked membrane skin and is formed in situ on the porous support layer, usually made of polysulfone. The thin film dense layer is a cross-linked aromatic polyamide made from interfacial polymerization reaction of a polyfunctional amine such as m-phenylenediamine with a polyfunctional acid chloride such as tri-mesoyl chloride. This TFC manufacturing procedure enables independent optimization of the distinct properties of the support and salt rejecting skin. The TFC membrane is characterized by higher specific water flux and higher salt rejection than cellulose acetate membranes. 

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