Advantages of nanofiltration membranes

Release Time:

2022-02-17 09:24

Nanofiltration has many advantages. Here is a brief overview of them.

Nanofiltration is a relatively new membrane process that is commonly used with waters with low total dissolved solids (such as surface water and fresh groundwater) to soften (remove multivalent cations) and remove disinfection by-product precursors, Such as natural organic matter and synthetic organic matter.

Nanofiltration (NF) is one of four membrane technologies that use pressure to achieve the separation of contaminants in water streams. The other three are microfiltration, ultrafiltration and reverse osmosis (RO). All of these techniques utilize semi-permeable membranes that have the ability to block (repel) dissolved and/or suspended solids in water streams containing these contaminants.

Nanofiltration is also increasingly used in food processing applications, such as dairy products, for simultaneous concentration and partial (monovalent ion) desalination.

The mechanism depends on the valence of the salt ion in question. Salts are recognized as charged compounds of two or more ions. Valence is the number of charges on the ion that forms a particular salt, which is not always sodium chloride (NaCl); sodium and chloride are monovalent ions because they have only one charge, while calcium and sulfate ions are more Valence because they have more than one charge. The main feature of NF membranes is that they repel multivalent ions to a much greater extent than monovalent ions. The specific rejection of ions varies from one membrane manufacturer to another, but 95% multivalent ion rejection and 20% monovalent ion rejection are not uncommon for NF membranes.

Although capillary fiber nanofilters are expected to hit the market soon, most such membranes available today are only helically wound structures. Figure 1 illustrates the removal efficacy of NF as an example.

In many developing countries, access to clean drinking water is difficult, and nanotechnology offers a solution. Although nanofiltration is used to remove other substances from water sources, it is also commonly used for water desalination. Brackish water was tested using polymer nanofiltration combined with reverse osmosis, as seen in a recent study in South Africa. The tests produced drinking water, but as the researchers expected, reverse osmosis removed most of the solutes. This empties the water of all the essential nutrients (calcium, magnesium ions, etc.), keeping the nutrient levels below the standards required by the World Health Organization. This process may be a bit too much for drinking water production, as the researchers have to go back and add nutrients to bring solute levels up to standard levels for drinking water. On the other hand, providing nanofiltration methods to developing countries to increase their supply of clean water is a very cheap method compared to conventional methods. But questions remain about how these developing countries can incorporate this new technology into their economies without relying on foreign aid.

To dissolve the air for flotation, three types of pressurized systems are used. Full flow or full boost is used when the wastewater contains a lot of oily substances. Vigorous mixing that occurs in pressurized systems does not affect treatment. Diverted pressurization is used when moderate to low concentrations of oily substances are present. Likewise, vigorous mixing through the booster system does not significantly affect process efficiency. Circulating flow booster systems are used to handle solid or oily materials that would degrade due to intense mixing in other booster systems. This method can be used after chemical treatment of oil emulsions, or after they are used for clarification and thickening of flocculation suspensions.

In the schematic diagram of a dissolved air flotation system shown in the figure, the feed water mixture laden with solids or oil-water enters the flotation vessel, while the gas-solid mixture rises to the liquid surface. The specific gravity of the air-solid mixture is less than that of water. Solids with a specific gravity greater than water tend to settle to the bottom and are removed by the rotating scraper arm. Attached to the same shaft is a rotating skimmer blade that moves the float from the surface of the container into the skimmer hopper. Clean water passes under the skirt and must then exit the vessel through a chute located in the peripheral area.

Some typical applications of nanofiltration are:

· Demineralization of food, dairy and beverage products or by-products

Partial desalination of whey, UF permeate or retentate as required

· Desalting of dyes and optical brighteners

· Purification of spent clean-in-place (CIP) chemicals

· Color reduction or treatment of food

· Concentration of food, dairy and beverage products or by-products

· Fermentation by-product concentration.

Nanofiltration and softening: Water softening usually involves the removal of hardness ions, especially calcium and magnesium. Since these ions are multivalent ions, they are preferentially removed by the NF membrane.

The particular advantage of membrane technology in this application is that no chemical reagents are required, either for soda lime for municipal softening or water for table salt (sodium chloride), in the case of a typical household water softener regeneration to facilitate the removal of hard ions. Sodium ion exchange, a standard technology for residential softening for over 50 years, utilizes ion exchange resins (in the form of sodium) to absorb hardness ions from water flowing through a bed of such resins and release sodium ions for exchange. Because the technology requires sodium chloride or potassium chloride to regenerate the resin, these are released into the sewer (or septic tank) during each regeneration cycle.

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