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Nanofiltration in Wastewater Treatment

Nanofiltration, A unusual technique that has grown in popularity in recent years. Nanofiltration is most commonly used in steps of the drinking water purification process, such as water softening, decoloring, and micropollutant removal. It’s mostly employed to remove two-valued ions as well as bigger monovalent ions such heavy metals.

Nano filtration is used in industrial operations to remove specific components such as coloring additives. Nanofiltration is a related procedure in which molecules are separated based on their size. Membranes are responsible for the separation. For univalent salts, nanofiltration membranes exhibit a moderate retention. The pores in a nanofiltration system are about 1nm in size. The retention of loaded and unloaded particles is how nanofiltration systems are classified. The ability to keep a Nanofiltration membrane in place can be determined via experimental filtration tests with pre-selected molecules.

A NF system is ion-selective as well. This is the capacity to discriminate between different ions. Because a nanofiltration system accumulates solid loaded groups in its membrane structure, electrostatic repulsion/attraction forces between the liquid components and the NF membrane surface may develop, resulting in ion selectivity. These ions are expected to diffuse across the membrane due to the sieve effect (pore size 1 nm) and the molecular size of chlorides (0.12 mm in size) and sulphates (0.23 mm in size).+

NANOFILTRATION SYSTEM IMPORTANCE

For liquid-phase separations, NF is the most recently developed pressure-driven membrane technique. Because of its reduced energy consumption and greater flux rates, NF has largely supplanted reverse osmosis (RO) in many applications. Non-porous RO systems (where transport is mediated by a solution-diffusion mechanism) and porous ultrafiltration (UF) membrane systems have similar features (where separation is usually assumed to be due to size exclusion and, in some cases, charge effects). Surface groups such as sulphurated or carboxyl acids dissociate in commercial NF systems, resulting in a fixed charge. As a result of the features of NF systems, ions can be separated using a combination of UF’s size and electrical effects, as well as RO’s ion contact mechanisms. The NF system is the most often used and is newly introduced technology in wastewater treatment system.

Because the pores in NF membranes are so small (about 1 nm), even small uncharged solutes are strongly rejected, but the surface electrostatic characteristics allow monovalent ions to get through rather easily while multivalent ions are primarily trapped. These features make the NF system ideal for fractionating and selectively removing solutes from complex process streams. Over the last few years, the development of NF technology as a viable process has resulted in a significant increase in its use in a variety of industries, including the treatment of pulp-bleaching effluents from the textile industry, the separation of pharmaceuticals from fermentation broths, demineralization in the dairy industry, metal recovery from wastewater, and virus removal. NF is a promising technology for the remediation of natural organic pollutants and inorganic pollutants in surface water.

Because the surface water has a low osmotic pressure, NF can operate at a low pressure. The NF method rejects a significant percentage of organic compounds, such as disinfection-byproducts precursors. Natural organic chemicals, which have relatively big molecules compared to membrane pore size, may be removed by sieving in the NF of surface waters, whereas inorganic salts might be removed by the charge effect of the ions.

Separation mechanisms in neurofibrillary tangles Because the features of the NF system are similar to those of ultrafiltration (UF) and reverse osmosis (RO), particle charge and size play a key role in the NF rejection process. It referred to NF as a charged UF system rather than a low-pressure RO system. NF, on the other hand, has a lower operating pressure than RO and a larger organic rejection than UF. Physical sieving would be the dominating rejection mechanism for colloids and big molecules, but solution diffusion and the charge effect of membranes play a prominent part in the separating process for ions and lower molecular weight substances.

The five steps of the NF rejection mechanism are as follows:

Wet surface water forms hydrogen bonds with the system, and the molecules that make the hydrogen bonds with the membrane can be transported.
Preferential sorption/Capillary rejection happens because the membrane is heterogeneous and micro porous, and electrostatic repulsion occurs because the solution and membrane have different electrostatic constants.
Solution diffusion — The NF system is homogeneous and nonporous, and the solute and solvent dissolve in the active layer of the membrane, with the solvent transported through diffusion.
Rejection is determined by a charged capillary — an electric double layer in the pores. Due to the streaming potential, ions with the same charge as the membrane are attracted while counter-ions are rejected.

NANOFILTRATION FOR TEXTILE EFFLUENT TREATMENT

Textile wastewater treatment for industrial reuse is still a difficult topic to solve for a variety of reasons. The primary difficulties are the Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Dissolved Solids (TDS) concentration of the wastewater, and the nonbiodegradable nature of organic dyestuffs present in the effluent. As a result, any treatment system that is selected, particularly for primary treatment, should be able to meet these concerns. Several technological advances have been made in order to tackle these issues. Filtration, ultraviolet radiation, chemical treatment, and desalination are examples of traditional water-treatment technologies, whereas Nano-enabled technologies employ a variety of membranes and filters. A comparison of traditional and Nano-enabled water treatment technologies was conducted. Nanofiltration membranes selectively reject items, allowing harmful contaminants to be removed while essential nutrients in water are retained. The reverse osmosis membranes eliminated nearly all of the solutes, but key elements including calcium and magnesium ions were lowered to levels below the standard water’s standards. Nanoscience’s ability to tackle technical issues linked with the removal of water contaminants has been discussed extensively in the literature. Separation membranes made of carbon nanotubes, nonporous ceramics, magnetic nanoparticles, and other nanomaterials with nanoscale structure can also be employed in low-cost separation.

At a lower operating pressure than reverse osmosis, nanofiltration membranes can produce sufficient permeate quality for some operations. Colored effluents from the textile sector have been treated using nanofiltration. Membranes in combination with physicochemical processes have the potential to produce water that can be reused from industry’s worldwide effluent. Textile dye effluents can be treated using a mix of adsorption and nanofiltration. Ultrafiltration and nanofiltration can also be combined to investigate the impact of ultrafiltration as a pre-treatment in a nanofiltration system. The results showed that the nanofiltration permeate flux rose significantly, while the COD concentration in the nanofiltration feed decreased. This preparation is necessary in order to avoid contamination.

The NF system uses physicochemical treatment to remove COD from textile wastewater, with a COD removal effectiveness of roughly 50%. Furthermore, the average colour removal by biological processes was only 70%, indicating the potential of adopting nanofiltration for postprocessing treatment. If modern technologies are used with them, the quality of the treated wastewater can be improved. The COD of physicochemically treated water cannot be greatly reduced with ultrafiltration. The COD concentration can be greatly lowered using nanofiltration membranes, and the permeate of the nanofiltration membrane can be reused in the industry. The combination of physicochemical treatment and nanofiltration achieves a COD elimination rate of about 100%. A comparison of the role of activated sludge treated wastewater in combination with nanofiltration and ozonation technologies was also investigated. The study’s findings revealed that nanofiltrations of biologically treated wastewater from the textile industry create permeates with negligible COD levels. Despite this, chlorides have a maximum retention of 90% while sulphates have a minimum retention of 90%. The shape of an NF membrane might be tubular, spiral, or flat. A spiral module is made up of layers of polyamide membrane twisted in a spiral pattern. The wound layers are sealed by a cover at the membrane’s edge. The wound module has a permeate collection tube in the centre. This tube catches all of the clean water that has flowed through the spiral winding.Despite this, chlorides have a maximum retention of 90% while sulphates have a minimum retention of 90%. The shape of an NF membrane might be tubular, spiral, or flat. A spiral module is made up of layers of polyamide membrane twisted in a spiral pattern. The wound layers are sealed by a cover at the membrane’s edge. The wound module has a permeate collection tube in the centre. This tube catches all of the clean water that has flowed through the spiral winding.

Nanofiltration and softening :

Water softening is the process of removing hardness ions from water, specifically calcium and magnesium. These ions are eliminated preferentially by NF membranes due to their multivalent nature.

In fact, NF has been utilised for municipal softening for a number of years, mainly in Florida. The advantage of NF over RO, another ion-rejecting membrane technology, is that it has a higher flux rate. This implies fewer membrane parts are needed, and the pump pressure is lower (in pounds per square inch (psi) or bars), resulting in cheaper running expenses.

The advantage of membrane technology in this application is that no chemicals, such as soda lime for municipal softening or common salt (sodium chloride) for regeneration of standard household water softeners, are required to allow the elimination of hardness ions. Sodium ion exchange, which has been the industry standard for domestic water softening for more than 50 years, works by adsorbing hardness ions from water passing through a bed of such resin and exchanging them for sodium ions. Because this technology necessitates the use of salt or potassium chloride for resin regeneration, these substances are discharged into the sewer (or septic tank) with each regeneration cycle.

Applications of NF Technology:

The NF process, which is widely used for water and wastewater treatment as well as other applications such as desalination, where its use is growing, plays a key role in partially replacing the Reverse Osmosis System RO, which saves energy and money.

Food, dairy, and beverage goods, as well as byproducts, are desalinated.
As needed, partial desalination of whey, UF permeate, or retentate
Dye desalination and optical brightening
Spent clean-in-place (CIP) chemicals are purified.
Food products with reduced colour or that have been manipulated in some way
Byproducts or concentrates of food, dairy, and beverage products
Concentration of fermentation byproducts