Nanofiltration is a liquid separation technique that uses membrane technology and is currently considered profitable for various applications. It is a pressure-driven separation method used to separate particles from a solution that have a molecular weight of 200-1000 gmol-1. This molecular weight range is mostly used for water treatment applications or other aqueous solution separation processes. It is normally carried out at pressures between the range of 5-20 bar, depending on the optimal processing conditions.
Nanofiltration processes have several similarities with the Reverse Osmosis (RO) processes, which works by blocking and rejecting dissolved particles in water. Nanofiltration, however, also offers the rejection of multivalent charged ions, such as calcium and magnesium, but is less efficient in rejecting monovalent ions such as sodium and chlorides.
Nowadays, the most interesting aspect of this process for researchers is the separation of organic solvents. This newly developed technique is referred to as Organic Solvent Nanofiltration (OSN). However, the most unique advantage that nanofiltration technology offers is its flexibility in low-temperature ranges as it is usually carried out in ambient or near ambient temperatures. In contrast, other conventional temperature-assisted separation techniques have the limitation of not functioning in an ambient temperature range and therefore require high energy consumptions to function, for example, a vacuum or steam distillation. So, these separation techniques are obviously not suitable for temperature-sensitive processing and heat sensitive products.
In recent years, nanofiltration has become popular due to it its flexible nature. The applications of nanofiltration are mainly connected to purifying drinking water and other primary utility purposes. Healthy and clean drinking water is an essential right for all human beings and freshwater resources are depleted day by day due to increased consumption and rising pollution rates. A recent WHO report showed that 780 million people globally lack access to pure and healthy water. Water pollution is therefore one of the most significant problems for countries all around the world. In a different study, the WHO estimated that around 100 million people have lost their lives because of polluted and contaminated drinking water.
In many industrial processes, nanofiltration is used to remove or separate certain substances from water, for example pigments, dyes, impurities, metals, and salts. The table below lists some of the main application areas.
|Main application areas of nanofiltration technologies:|
|Demineralization of water|
|Purification of contaminated ground water|
|Production of ultra-pure water|
|Heavy metal removal|
|Wastewater treatment in textile production|
|Wastewater treatment in paper and pulp production|
|Agricultural water treatment|
|Removal of dyes and pigments|
|Purification of drinking water|
|Cleaning the wastewater of laundries|
|Removal of nitrates|
|Water treatment in the food & beverages industries|
|Removal of pollutants and the purification of industrial wastewater|
|Purification of industrial wastewater|
The separation process depends entirely upon the molecular or pore size of the surface membrane which, in essence, stops certain particles or substances from passing through it. The pore size of the nanofiltration membrane is usually between 1-10 nanometres, which is smaller than those used in micro and ultrafiltration processes, but larger than the membranes used in reverse osmosis. Below we cover some of the most useful applications of nanofiltration technologies, specifically those related to water treatment and purification.
Nanofiltration can be applied to separate the amoxicillin present in wastewater produced by the pharmaceutical industries. As amoxicillin is widely used for treating various types of infections, obtaining it through nanofiltration has several advantages, including avoiding the potential resistance against treatments for bacterial infections and preventing harmful long-term effects on human health. The special type of nanofiltration membranes used enables the effective and efficient separation of the amoxicillin from pharmaceutical wastewater. Recent research has shown that the separation of amoxicillin through nanofiltration membranes can be as high as 97% for the permeation rate of 1.5 litres per minute. This is a much higher rate of separation than a similar technique uses, called the Chemical Oxygen Demand (COD) method, whose maximum separation rate is around 40%.
The Total Dissolved Solids (TDS) scale of pure and clean water recommends that the concentration of minerals in water should range from 100 to 500 parts per million (ppm) for healthy drinking water. It outlines that the taste, ability to quench thirst and corrosiveness are the best criteria for outlining the amount of minerals present in freshwater and that this range (100-500 ppm) is used as a guide for nanofiltration membranes. Furthermore, nanofiltration membranes are preferable in the treatment of drinking-water as they separate and reject particles more efficiently, which is why nanofiltration is considered a better alternative to surface water purification. Nanofiltration is also successful in ‘softening’ water (removing the ‘hardness’ of the water) through the separation of multivalent ions, which can be separated efficiently at low energy levels or low pressure, compared to reverse osmosis, which has already been mentioned. Another advantage that it has in this field is that it is very efficient in separating corrosive, unestablished, and dilute mixtures, and is also considered the most suitable technique for removing organic and ionic compounds. In addition to these points, nanofiltration has also proven successful in the removal of microbes in water, making it an ideal choice for eliminating fluoride, heavy metals, inorganic carbon, Disinfectant By-Product (DBP) precursors, pesticides, arsenic, oxyanions, and many other organic impurities from raw water.
Some other advantages of using this technique over others include the efficient removal of organic matter, low requirements of chemicals, less formation of sludge and slurry, efficient chemical storage, and small carbon footprints. If the treatment of highly coloured water is required, the nanofiltration membrane produces water at a lower price compared to other techniques that use ozonation, Granular Activated Carbon (GAC), and lime soda.
One negative aspect of nanofiltration in this industry, however, is that due to the high material and maintenance cost, nanofiltration is not always considered the most ideal option from an economic point of view. Yet recent research studies have shown that for very small-scale water treatment systems, nanofiltration can be cost-effective, as the smaller systems are cheaper and, because of the smaller volumes of water passing through the membranes, require less maintenance.
Due to nanofiltration’s restrictions on eliminating monovalent ions, the nanofiltration desalination market is currently minimal. Nanofiltration has been considered as a hybrid or stand-alone process for some applications in the desalination industry, as incorporating nanofiltration technology into the desalination processes could minimize the waste of reverse osmosis desalination by around 30 %. The recovery of desalination systems could be significantly improved by using nanofiltration technology, but, at the same time, the economic benefits are only marginal, where energy recovery devices are being used. However, an important application of nanofiltration technology could be in the removal of sulphates in seawater, as nanofiltration membranes would be able to selectively eliminate the sulphates present in seawater. This would be a huge advantage in the oilfield industry as it would prevent scaling in oilfield waterflood systems.
Dairy Products (Partial Desalination of Whey)
Nanofiltration plays an important role in the control of minerals and the concentration of dairy products. In the case of lactose, for example, nano filtration technology is used to regulate the amount of minerals and the concentration of the product simultaneously, according to the purity required. By-products and wastewater from dairy production produce high amounts of nutritious, bio-active, and functional compounds that can be reused for food processing or other purposes. Due to its unique characteristics, nanofiltration plays a vital role in the recovery of certain compounds, all the while enabling the reuse of its main component, water.
Nanofiltration membranes with specialized properties and pore sizes are used to extract whey from milk, and its specific components. Through this technique, lactose free milk can also be obtained. In addition to these processes, nanofiltration offers 3 other valuable uses: the removal of colorants from the milk, the demineralization of milk and the desalination of milk, creating a pure and salt-free milky product. After post-processing of this milky-product, milk, whey or cheese can be obtained as a direct result of nanofiltration. Due to its vital role and high efficiency in water treatment, nanofiltration has become a leading treatment process in dairy production.
Textile Wastewater Treatment
The industrial reuse of treated textile wastewater continues to be a complex problem due to various reasons. Among them, Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD), Total Dissolved Solids (TDS) content in wastewater, and the non-biodegradable nature of organic dyestuffs are the key complications.
In the textile industry, nanofiltration functions by passing salts and water through the nanofiltration membranes, which is a more appropriate alternative for the textile industry than other options. This is because the membranes used in nanofiltration can separate large concentrations of dyes, including fluorescent ones, which require a higher level of separation than normal. Also, this process can separate the optical brightening agents from the wastewater, which are extensively used to improve the lightening effect in textiles, allowing these agents to be recovered and reused in future processes. Furthermore, it can also be used to purify wastewater that has already been released into the sea or river, purifying this contaminated water in order to reuse it as fresh feed water later. To achieve this, the contaminated sea or river water is treated in an activated sludge system and passes through a sand filter, which is used as a pre-treatment filter. This is done to protect the lifespan of the nanofiltration membrane which can be susceptible to fouling (the result of when biological matter builds up on a surface, preventing the transfer of heat) when working with mixtures of this type. The sand filter therefore removes all solids found in the water (floating and settled) from the raw mixture to prevent membrane damage, allowing the membrane to function normally when the filtered mixture passes through it.
Industrial Processes and Wastewater Treatment
In several industrial processes, nanofiltration is used to remove dissolved sulphates due to the corrosion that they can cause to oilfield and oil-refinery equipment. It also removes Natural Organic Matter (NOM) from surface water in order to obtain NOM supplemented water in several industrial operations. It can also remove and recover brine from sulphate-rich sodium chloride solutions, as the pores of the membrane only allow sodium and chloride ions to pass through them, allowing the sulphate content to be recovered. As a lower energy alternative, nanofiltration can be used to treat certain landfill ‘leachate’ (polluted water that has run-off a landfill) by eliminating pollutants, reducing the Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) levels, and lowering the number of suspended solids in the water and, in effect, lowering its turbidity.
The overwhelming issue that industrial wastewater presents is related to cyanobacteria metabolites. These metabolites increase the toxicity of the water and alter its taste and odour, creating filthier and more impure water. The larger molecules or compounds present in this wastewater can, theoretically, be removed through nanofiltration membranes, following the theory of ‘size exclusion’.
Finally, there are various applications of nanofiltration for the recovery of many waste compounds and components, especially metals in industrial processes. All these applications have a common factor of water treatment. Some of the more significant uses include:
- phosphoric and nitric acid purification
- chromium reclamation through acid waste
- spent sulphuric acid purification
- boric acid recovery from radioactive solutions
- copper sulphate recovery through acid rinse water
Despite its many advantages in various industries, nanofiltration is often avoided due to the potential damage that can be done to the membrane pores, which causes inconsistency in the next filtration cycle and therefore decreases the overall efficiency of the process. This damage is dependent upon the solution that is being processed, especially solute-solute and membrane-solute interactions, and is unfortunately irreversible.
A further major disadvantage associated with nanofiltration, especially for water treatment processes, is the cost of the materials and their maintenance. From an industrial point of view, they are considered an expensive part of the whole operation. Maintenance and replacement costs are principally associated with the amount of water or liquid being filtered, the content of impurities, and the feeding fluid’s flow rate.
Despite these disadvantages, however, nanofiltration offers a high level of efficiency and is more successful than similar methods in the processing of larger molecules and the production of continuous stream outlets. Overall, nanofiltration is an effective membrane separation technique, especially in the fields of water purification and treatment, and also in the removal of dissolved solids in ground and surface water, allowing it to provide clean and healthy water.