The fundamental principle of nanofiltration membrane technology is the use of pressure to separate soluble ions from water through a semipermeable membrane. The membrane operates under a different hydraulic profile, which is also known as cross-flow filtration, unlike a dead-end filter.
WHAT IS FILTRATION?
Filtration is a process of removing particulate matter from water by forcing the water through a porous media. This porous media can be natural, in the case of sand, gravel and clay, or it can be a membrane wall made of various materials. Sometimes, large particles are settled before filtration; this is called sedimentation.
Nanofiltration in Water Treatment
Nanofiltration (NF), in water treatment, “bridges” a gap between ultrafiltration and reverse osmosis membranes. It is sometimes referred to as a loose or lower rejecting RO membrane. It could also be called a tight UF, as NF membrane pore size ranges from .01 to a little less than .001. Generally, an NF membrane system lets more salt pass than an RO membrane. In addition, an NF membrane element will produce the same quantity of product at 50 to 70% the applied pressure as an RO water treatment system.
Other applications of nanofiltration include:
* The removal of heavy metals from wastewater
* The removal of pesticides from groundwater
* Recycling of wastewater in laundries
* Water softening
* Nitrate removal
Nano chemical processing
Methods for the removal of persistent organic pollutants and xenobiotics in water
Since the implementation of the electrospinning process, filtration technology has achieved milestones. Nanofilters help to lower the solution’s ionic strength, reducing the organic pollutant hardness. Different natural and synthetic polymers are used to synthesize nanoporous membranes like polyvinyl fluoride cellulose acetate, polypropylene, polyacrylonitrile, etc. Nano-filtration work centered on a pressure-driven method that is efficient in removing components in low molecular weight with size ranges between 10 and 1 nm. It performs filtration based on hydrodynamics on the membrane surface and membrane nanopores. The effectiveness of the filtration depends highly on filter membrane charge, membrane porosity and the membrane surface concentration polarisation . Using electro-spinner technology for the development of nano-fibrous membranes, high-quality interconnected, 3D membranes are prepared. Nanofibers reject univalent, divalent and multivalent ions effectively, so the most useful approach is for treating arsenic components in potable water.
Nano-Filters for Arsenic and Pesticide Removal: Challenges and Mechanisms
Nano-filters prepared with highly charged polyamide facilitate the removal of arsenic compounds. Separation and elimination are typically accomplished through physicochemical interactions between membranes and pollutants, however, only nanofiltration is possible in the case of trace components such as pesticides. Nano-filters faced limitations for dissolved organic compounds and uncharged pesticides because of the spectrum of their molecular weights and the hydrophobicity nature of pesticide molecules. The nano-filter membrane was rejected by 11 different forms of pesticides, including aromatic, hydrophobic and phenol compounds, and the polarity of the pesticides affected membrane filtration capacity. The separation of charged pesticides is effective since it is relatively close to membranes.
What is a nanofiltration membrane?
Nanofiltration membranes are utilized for the filtration of water with low total dissolved solids (TDS) for the purposes of eradicating organic substances and softening water. Nanofiltration is usable in many water and wastewater treatment industries for the practical removal of ions and organic material. It has also been adopted as pretreatment to RO. Other than water purification, nanofiltration membranes are also used in the manufacturing production of food & beverages, and pharmaceuticals.
Nanofiltration Membranes
There are applications where a 75% rate of salt rejection is preferable to 95 to 99%, especially when it is achieved using only half of the energy. Unlike with RO membrane elements, the NF offerings of the major membrane manufacturers are quite different in performance from company to company. With RO membrane elements, any model with less than 99.5% sodium chloride rejection, especially with seawater membranes, is considered inferior. In the case of NF, there is a “place” for any membrane with a NaCl rejection rate of 40%, or more. Nanofiltration has also been called a “softening” membrane as while its salt rejection may be 80% or less, hardness rejection will often remain well over 90%.
What is the difference between nanofiltration and reverse osmosis?
While nanofiltration and reverse osmosis systems have similar designs and operation uses, there are some notable differences between the two. The primary distinction is that nanofiltration is not as rigid as reverse osmosis. It functions with less feed water pressure and is incapable of removing single charge ions from the water as definitely as reverse osmosis membranes.
While up to 99% of chlorides and sodium are removed by reverse osmosis systems, nanofiltration membranes typically remove only 50% to 80%. This percentage relies on the type of material and manufacturing of the membrane. However, due to its effectiveness in removing multiple charge ions, nanofiltration is the preferred choice for removing hardness from water while not affecting the total dissolved solids content than would reverse osmosis.
Conclusion
Most nanofiltration membranes used today are made of composite materials that are supported by a polymer substrate, offering durability and performance under varying operating conditions.
Unlike flat sheet or tubular designs, these membranes are typically manufactured in a spiral-wound configuration, which has become the dominant model for industrial applications due to its compact structure and efficiency.
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Reference
1-Ultrafiltration, Nanofiltration, and Reverse Osmosis
Ultrafiltration, Nanofiltration, and Reverse Osmosis — Safe Drinking Water Foundation
2-Nanofilters
https://www.sciencedirect.com/topics/engineering/nanofilters
