Ultrafiltration, Nanofiltration and Reverse Osmosis

ULTRAFILTRATION, NANOFILTRATION AND REVERSE OSMOSIS FACT SHEET

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. For information on sedimentation and filtration, in general, see the Conventional Water Treatment: Coagulation and Filtration fact sheet.

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The size of materials that can be removed during filtration depends upon the size of the pores of the filter. The chart below summarizes the various separation processes relative to common materials that would be filtered out through each process. Particle filtration refers to conventional media filtration, while the other types are membrane filtrations. 

What is Ultrafiltration?

An ultrafiltration filter has a pore size around 0.01 micron. A microfiltration filter has a pore size around 0.1 micron, so when water undergoes microfiltration, many microorganisms are removed, but viruses remain in the water. Ultrafiltration would remove these larger particles, and may remove some viruses. Neither microfiltration nor ultrafiltration can remove dissolved substances unless they are first adsorbed (with activated carbon) or coagulated (with alum or iron salts).

What is Nanofiltration?

A nanofiltration filter has a pore size around 0.001 micron. Nanofiltration removes most organic molecules, nearly all viruses, most of the natural organic matter and a range of salts. Nanofiltration removes divalent ions, which make water hard, so nanofiltration is often used to soften hard water.

What is Reverse Osmosis?

Reverse osmosis filters have a pore size around 0. micron. After water passes through a reverse osmosis filter, it is essentially pure water. In addition to removing all organic molecules and viruses, reverse osmosis also removes most minerals that are present in the water. Reverse osmosis removes monovalent ions, which means that it desalinates the water. To understand how reverse osmosis works, it is helpful to understand osmosis.

Osmosis occurs when a semi-permeable membrane separates two salt solutions of different concentrations. The water will migrate from the weaker solution to the stronger solution, until the two solutions are of the same concentration, because the semi-permeable membrane allows the water to pass through, but not the salt. In the following diagram, (A) and (B) illustrate the process of osmosis.

In reverse osmosis, the two solutions are still separated by a semi-permeable membrane, but pressure is applied to reverse the natural flow of the water. This forces the water to move from the more concentrated solution to the weaker. Thus, the contaminants end up on one side of the semi-permeable membrane and the pure water is on the other side. In the diagram below, reverse osmosis is represented in (C).

What do These Three Processes Remove?

Ultrafiltration removes bacteria, protozoa and some viruses from the water. Nanofiltration removes these microbes, as well as most natural organic matter and some natural minerals, especially divalent ions which cause hard water. Nanofiltration, however, does not remove dissolved compounds. Reverse osmosis removes turbidity, including microbes and virtually all dissolved substances. However, while reverse osmosis removes many harmful minerals, such as salt and lead, it also removes some healthy minerals, such as calcium and magnesium. This is why water that is treated by reverse osmosis benefits by going through a magnesium and calcium mineral bed. This adds calcium and magnesium to the water, while also increasing the pH and decreasing the corrosive potential of the water. Corrosive water may leach lead and copper from distribution systems and household water pipes.

What Are the Advantages of Using Ultrafiltration, Nanofiltration or Reverse Osmosis to Treat Water?

All three of these membrane filtration processes are effective methods of treating water that cannot be treated using conventional treatment methods. Reverse osmosis, in particular, has been responsible for ending several nearly decade long Boil Water Advisories. For example, in , a reverse osmosis system, together with a biological treatment process, was set up to successfully treat drinking water for the Yellow Quill First Nation, which had been on a Boil Water Advisory since . The water in the First Nations community, which is located in Saskatchewan, contained high levels of organic matter, iron, manganese, ammonium and arsenic, to name a few. Besides the obvious benefit of providing safe drinking water to a community which had been under a Boil Water Advisory for approximately nine years, the reverse osmosis system (together with the biological treatment) allowed the community to treat their water using small quantities of chemicals.

A portable reverse osmosis unit was brought in to Kashechewan, a First Nations community in Ontario, in October of . The community had experienced water contamination issues for years, and in October , approximately 1,100 of the 1,900 residents were evacuated, after E. coli was found in their water. The reverse osmosis unit was brought in by the military, and could provide 50,000 litres of water each day for the residents of Kashechewan, until their water treatment plant could be fixed. The picture below compares the colour of the untreated water to that of the water after being treated with reverse osmosis.

What Are the Disadvantages of Using Ultrafiltration, Nanofiltration or Reverse Osmosis to Treat Water?

Compared with the benefits of using membrane filtration to treat water, there are very few disadvantages. If conventional water treatment processes can effectively treat the water, then constructing a reverse osmosis water treatment facility would be an unnecessary cost. But for the First Nations communities that have been on Boil Water Advisories for many years, a reverse osmosis treatment system can be a valuable investment that can provide safe drinking water for the residents.

Reverse osmosis removes a number of healthy minerals from water, in addition to the harmful minerals and particles. The removal of these minerals, including calcium and magnesium, can actually make water unhealthy, especially for people with inadequate diets and people who live in hot climates, as water can provide these necessary minerals. The addition of calcium and magnesium, as described above, can resolve these concerns.

The Safe Drinking Water Foundation has educational programs that can supplement the information found in this fact sheet. Operation Water Drop looks at the chemical contaminants that are found in water; it is designed for a science class. Operation Water Flow looks at how water is used, where it comes from and how much it costs; it has lessons that are designed for Social Studies, Math, Biology, Chemistry and Science classes. Operation Water Spirit presents a First Nations perspective of water and the surrounding issues; it is designed for Native Studies or Social Studies classes. Operation Water Health looks at common health issues surrounding drinking water in Canada and around the world and is designed for a Health, Science and Social Studies collaboration. Operation Water Pollution focuses on how water pollution occurs and how it is cleaned up and has been designed for a Science and Social Studies collaboration. To access more information on these and other educational activities, as well as additional fact sheets, visit the Safe Drinking Water Foundation website at www.safewater.org.

Did you know that our Operation Water Biology program teaches students about biological water treatment - a more effective and environmentally friendly way to treat drinking water? In the Operation Water Biology program students build a model of a biological water treatment plant and learn about chlorine, chloramine, ammonia, and iron in a hands-on manner. Please help us send more Operation Water Biology kits to schools! Please chip in $5 or donate $20 or more and receive an Official Donation Receipt for Income Tax Purposes - or donate $170 to provide a school with an Operation Water Biology kit.

Filtration method that uses nanometer sized pores in biological membranes

Nanofiltration is a membrane filtration process that uses nanometer sized pores through which particles smaller than about 1–10 nanometers pass through the membrane. Nanofiltration membranes have pore sizes of about 1–10 nanometers, smaller than those used in microfiltration and ultrafiltration, but a slightly bigger than those in reverse osmosis. Membranes used are predominantly polymer thin films.[1] It is used to soften, disinfect, and remove impurities from water, and to purify or separate chemicals such as pharmaceuticals.

Membranes

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Membrane materials that are commonly used are polymer thin films such as polyethylene terephthalate or metals such as aluminium.[2] Pore dimensions are controlled by pH, temperature and time during development with pore densities ranging from 1 to 106 pores per cm2. Membranes made from polyethylene terephthalate (PET) and other similar materials, are referred to as "track-etch" membranes, named after the way the pores on the membranes are made.[3] "Tracking" involves bombarding the polymer thin film with high energy particles. This results in making tracks that are chemically developed into the membrane, or "etched" into the membrane, which are the pores. Membranes created from metal such as alumina membranes, are made by electrochemically growing a thin layer of aluminum oxide from aluminum in an acidic medium.[citation needed]

Range of applications

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Historically, nanofiltration and other membrane technology used for molecular separation was applied entirely on aqueous systems. The original uses for nanofiltration were water treatment and in particular water softening.[4] Nanofilters "soften" water by retaining scale-forming divalent ions (e.g. Ca2+, Mg2+).[5][6]

Nanofiltration has been extended into other industries such as milk and juice production as well as pharmaceuticals, fine chemicals, and flavour and fragrance industries.[5]

Industry Uses Fine chemistry and Pharmaceuticals Non-thermal solvent recovery and management

Room temperature solvent exchange

Oil and Petroleum chemistry Removal of tar components in feed

Purification of gas condensates

Bulk Chemistry Product Polishing

Continuous recovery of homogeneous catalysts

Natural Essential Oils and similar products Fractionation of crude extracts

Enrichment of natural compounds Gentle Separations

Medicine Able to extract amino acids and lipids from blood and other cell culture.

Advantages and disadvantages

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One of the main advantages of nanofiltration as a method of softening water is that during the process of retaining calcium and magnesium ions while passing smaller hydrated monovalent ions, filtration is performed without adding extra sodium ions, as used in ion exchangers.[7] Many separation processes do not operate at room temperature (e.g. distillation), which greatly increases the cost of the process when continuous heating or cooling is applied. Performing gentle molecular separation is linked with nanofiltration that is often not included with other forms of separation processes (centrifugation). These are two of the main benefits that are associated with nanofiltration. Nanofiltration has a very favorable benefit of being able to process large volumes and continuously produce streams of products. Still, Nanofiltration is the least used method of membrane filtration in industry as the membrane pores sizes are limited to only a few nanometers. Anything smaller, reverse osmosis is used and anything larger is used for ultrafiltration. Ultrafiltration can also be used in cases where nanofiltration can be used, due to it being more conventional. A main disadvantage associated with nanotechnology, as with all membrane filter technology, is the cost and maintenance of the membranes used.[8] Nanofiltration membranes are an expensive part of the process. Repairs and replacement of membranes is dependent on total dissolved solids, flow rate and components of the feed. With nanofiltration being used across various industries, only an estimation of replacement frequency can be used. This causes nanofilters to be replaced a short time before or after their prime usage is complete.[citation needed]

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Additional resources:
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Design and operation

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When to Use Glass-Fused-to-Steel Tank？

Industrial applications of membranes require hundreds to thousands of square meters of membranes and therefore an efficient way to reduce the footprint by packing them is required. Membranes first became commercially viable when low cost methods of housing in 'modules' were achieved.[9] Membranes are not self-supporting. They need to be stayed by a porous support that can withstand the pressures required to operate the NF membrane without hindering the performance of the membrane. To do this effectively, the module needs to provide a channel to remove the membrane permeation and provide appropriate flow condition that reduces the phenomena of concentration polarisation. A good design minimises pressure losses on both the feed side and permeate side and thus energy requirements.[10]

Concentration polarisation

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Concentration polarization describes the accumulation of the species being retained close to the surface of the membrane which reduces separation capabilities. It occurs because the particles are convected towards the membrane with the solvent and its magnitude is the balance between this convection caused by solvent flux and the particle transport away from the membrane due to the concentration gradient (predominantly caused by diffusion.) Although concentration polarization is easily reversible, it can lead to fouling of the membrane.[10][11]

Spiral wound module

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Spiral wound modules are the most commonly used style of module and are 'standardized' design, available in a range of standard diameters (2.5", 4" and 8") to fit standard pressure vessel that can hold several modules in series connected by O-rings. The module uses flat sheets wrapped around a central tube. The membranes are glued along three edges over a permeate spacer to form 'leaves'. The permeate spacer supports the membrane and conducts the permeate to the central permeate tube. Between each leaf, a mesh like feed spacer is inserted.[11][12] The reason for the mesh like dimension of the spacer is to provide a hydrodynamic environment near the surface of the membrane that discourages concentration polarisation. Once the leaves have been wound around the central tube, the module is wrapped in a casing layer and caps placed on the end of the cylinder to prevent 'telescoping' that can occur in high flow rate and pressure conditions [13]

Tubular module

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Tubular modules look similar to shell and tube heat exchangers with bundles of tubes with the active surface of the membrane on the inside. Flow through the tubes is normally turbulent, ensuring low concentration polarisation but also increasing energy costs. The tubes can either be self-supporting or supported by insertion into perforated metal tubes. This module design is limited for nanofiltration by the pressure they can withstand before bursting, limiting the maximum flux possible.[9][10] Due to both the high energy operating costs of turbulent flow and the limiting burst pressure, tubular modules are more suited to 'dirty' applications where feeds have particulates such as filtering raw water to gain potable water in the Fyne process. The membranes can be easily cleaned through a 'pigging' technique with foam balls are squeezed through the tubes, scouring the caked deposits.[14]

Flux enhancing strategies

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These strategies work to reduce the magnitude of concentration polarisation and fouling. There is a range of techniques available however the most common is feed channel spacers as described in spiral wound modules. All of the strategies work by increasing eddies and generating a high shear in the flow near the membrane surface. Some of these strategies include vibrating the membrane, rotating the membrane, having a rotor disk above the membrane, pulsing the feed flow rate and introducing gas bubbling close to the surface of the membrane.[10][11][12]

Characterisation

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Performance parameters

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Retention of both charged and uncharged solutes and permeation measurements can be categorised into performance parameters since the performance under natural conditions of a membrane is based on the ratio of solute retained/ permeated through the membrane.[citation needed]

For charged solutes, the ionic distribution of salts near the membrane-solution interface plays an important role in determining the retention characteristic of a membrane. If the charge of the membrane and the composition and concentration of the solution to be filtered is known, the distribution of various salts can be found. This in turn can be combined with the known charge of the membrane and the Gibbs–Donnan effect to predict the retention characteristics for that membrane.[10]

Uncharged solutes cannot be characterised simply by Molecular Weight Cut Off (MWCO,) although in general an increase in molecular weight or solute size leads to an increase in retention. The charge and structure, pH of the solute, influence the retention characteristics.[1]

Morphology parameters

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The morphology of a membrane is usually established by microscopy. Atomic force microscopy (AFM) is one method used to characterise the surface roughness of a membrane by passing a small sharp tip (<100 Ă) across the surface of a membrane and measuring the resulting Van der Waals force between the atoms in the end of the tip and the surface.[10] This is useful as a direct correlation between surface roughness and colloidal fouling has been developed. Correlations also exist between fouling and other morphology parameters, such as hydrophobe, showing that the more hydrophobic a membrane is, the less prone to fouling it is. See membrane fouling for more information.[citation needed]

Methods to determine the porosity of porous membranes have also been found via permporometry, making use of differing vapour pressures to characterise the pore size and pore size distribution within the membrane. Initially all pores in the membrane are completely filled with a liquid and as such no permeation of a gas occurs, but after reducing the relative vapour pressure some gaps will start to form within the pores as dictated by the Kelvin equation. Polymeric (non-porous) membranes cannot be subjected to this methodology as the condensable vapour should have a negligible interaction within the membrane.[10]

Solute transport and rejection

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Unlike membranes with larger and smaller pore sizes, passage of solutes through nanofiltration is significantly more complex.[citation needed]

Because of the pore sizes, there are three modes of transport of solutes through the membrane. These include 1) diffusion (molecule travel due to concentration potential gradients, as seen through reverse osmosis membranes), 2) convection (travel with flow, like in larger pore size filtration such as microfiltration), and 3) electromigration (attraction or repulsion from charges within and near the membrane).[citation needed]

Additionally, the exclusion mechanisms in nanofiltration are more complex than in other forms of filtration. Most filtration systems operate solely by size (steric) exclusion, but at small length scales seen in nanofiltration, important effects include surface charge and hydration (solvation shell). The exclusion due to hydration is referred to as dielectric exclusion, a reference to the dielectric constants (energy) associated with a particles precense in solution versus within a membrane substrate. Solution pH strongly impacts surface charge,[15] providing a method to understand and better control rejection.

The transport and exclusion mechanisms are heavily influenced by membrane pore size, solvent viscosity, membrane thickness, solute diffusivity, solution temperature, solution pH, and membrane dielectric constant. The pore size distribution is also important. Modeling rejection accurately for NF is very challenging. It can be done with applications of the Nernst–Planck equation, although a heavy reliance on fitting parameters to experimental data is usually required.[1]

In general, charged solutes are much more effectively rejected in NF than uncharged solutes, and multivalent solutes like SO2−
4 (valence of 2) experience very high rejection.[citation needed]

Typical figures for industrial applications

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Keeping in mind that NF is usually part of a composite system for purification, a single unit is chosen based on the design specifications for the NF unit. For drinking water purification many commercial membranes exist, coming from chemical families having diverse structures, chemical tolerances and salt rejections.[citation needed]

NF units in drinking water purification range from extremely low salt rejection (<5% in A membranes) to almost complete rejection (99% in -TS80-TSA membranes.) Flow rates range from 25 to 60 m3/day for each unit, so commercial filtration requires multiple NF units in parallel to process large quantities of feed water. The pressures required in these units are generally between 4.5 and 7.5 bar.[10]

For seawater desalination using a NF-RO system a typical process is shown below.[citation needed]

Because NF permeate is rarely clean enough to be used as the final product for drinking water and other water purification, is it commonly used as a pre treatment step for reverse osmosis (RO)[8] as is shown above.

Post-treatment

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As with other membrane based separations such as ultrafiltration, microfiltration and reverse osmosis, post-treatment of either permeate or retentate flow streams (depending on the application) – is a necessary stage in industrial NF separation prior to commercial distribution of the product. The choice and order of unit operations employed in post-treatment is dependent on water quality regulations and the design of the NF system. Typical NF water purification post-treatment stages include aeration and disinfection & stabilisation.[citation needed]

Aeration

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A Polyvinyl chloride (PVC) or fibre-reinforced plastic (FRP) degasifier is used to remove dissolved gases such as carbon dioxide and hydrogen sulfide from the permeate stream.[16] This is achieved by blowing air in a countercurrent direction to the water falling through packing material in the degasifier. The air effectively strips the unwanted gases from the water.[citation needed]

Disinfection and stabilisation

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The permeate water from a NF separation is demineralised and may be disposed to large changes in pH, thus providing a substantial risk of corrosion in piping and other equipment components. To increase the stability of the water, chemical addition of alkaline solutions such as lime and caustic soda is employed. Furthermore, disinfectants such as chlorine or chloroamine are added to the permeate, as well as phosphate or fluoride corrosion inhibitors in some cases.[16]

Research trends

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Challenges in nanofiltration (NF) technology include minimising membrane fouling and reducing energy requirements. Thin film composite membranes (TFC), which consist of a number of extremely thin selective layers interfacially polymerized over a microporous substrate, have had commercial success in industrial membrane applications.[17] Electrospunnanofibrous membrane layers (ENMs) enhances permeate flux.[18] Energy-efficient alternatives to the commonly used spiral wound arrangement are hollow fibre membranes, which require less pre-treatment.[19] Titanium Dioxide nanoparticles have been used to minimize for membrane fouling.[20]

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See also

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• Applications of nanotechnology – Uses for technology on very small scales
• Nanomaterials – Materials whose granular size lies between 1 and 100 nm
• Nanotechnology – Technology with features near one nanometer
• Ultrafiltration – Filtration by force through a semipermeable membrane
• Reverse osmosis – Water purification process
• Mercury nano-trap water filtration – Method of eliminating pollution

References

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