Nanofiltration Plants

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AES Arabia Ltd.

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Softening (removing polyvalent cations) and disinfecting by-product precursors, like natural and synthetic organic matter, in water with low total dissolved solids, like fresh groundwater and surfacewater, are the most popular uses for nanofiltration, a relatively new membrane technique. Nanofiltration (NF) is one of four membrane technologies that extract contaminants from water streams using pressure. The other three are microfiltration, reverse osmosis (RO), and ultrafiltration. All of these technologies use semi-permeable membranes that can hold (reject) suspended and/or dissolved solids from a water stream that contains these contaminants. In food processing applications like dairy, nanofiltration is also becoming more popular because it allows for partial (monovalent ion) demineralization and simultaneous concentration.
This mechanism depends upon the valence of the salt ion in question. Recognize that a salt is a compound of two or more ions with an ionic charge. Valence is the number of charges on the ions that form the specific salt, which is not always sodium chloride (NaCl); sodium and chloride are monovalent ions because they have only one charge, whereas ions such as calcium and sulfate are multivalent. After all, they have more than one charge. A defining characteristic of NF membranes is that they reject multivalent ions to a significantly greater degree than monovalent ions. The specific rejection of ions varies from one membrane manufacturer to another, but a multivalent ion rejection of 95 percent with a monovalent ion rejection of only 20 percent isn’t unusual for NF membranes. Most of these membranes available today are in spiral wound construction only, although it’s expected that capillary fiber nanofilters will soon be on the market. Figure 1 illustrates NF in terms of its removal efficacy.
In much of the developing world, clean drinking water is hard to come by, and nanotechnology provides one solution. While nanofiltration is used for the removal of other substances from a water source, it is also commonly used for the desalination of water. As seen in a recent study in South Africa, tests were run using polymeric nanofiltration in conjunction with reverse osmosis to treat brackish groundwater. These tests produced potable water, but as the researchers expected, the reverse osmosis removed a large majority of solutes. This left the water void of any essential nutrients (calcium, magnesium ions, etc.), placing the nutrient levels below that of the required World Health Organization standards. This process was probably a little too much for the production of potable water, as researchers had to go back and add nutrients to bring solute levels to the standard levels for drinking water. On another note, providing nanofiltration methods to developing countries to increase their supply of clean water is a very inexpensive method compared to conventional ones. However, there remain issues as to how these developing countries will be able to incorporate this new technology into their economy without creating a dependency on foreign assistance.
For flotation, three different kinds of pressurized systems are used to dissolve air. When wastewater contains a lot of oily material, full-flow or total pressurization is used. The results of the treatment are unaffected by the vigorous mixing that takes place in the pressurization system. Partial-flow pressurization is used when moderate to low levels of oily material are present. Again, severe mixing during passage through the pressurization systems does not significantly affect treatment efficacy. The recycle-flow pressurization system is used to treat solids or oily materials that would decompose as a result of the intense mixing found in other pressurization systems. This technique is applied after chemically treating oil emulsions or to thicken and clarify flocculent suspensions.
In the schematic drawing of dissolved-air flotation system shown in the figure, The solids-laden or oily-water influent mixture enters the flotation vessel, and the air-solids mixture rises to the liquid surface. The air-solids mixture has a specific gravity less than water. Solids having a specific gravity greater than water tend to settle to the bottom and are removed by a rotating scraper arm. Attached to the same shaft is a rotating skimmer blade that removes the floating matter from the surface of the vessel into a skimming hopper. Clean water passes underneath a skirt and then must leave the vessel through a launder, which is located in the peripheral region.
Nanofiltration combined with softening:
During the water softening process, hardness ions—especially calcium and magnesium—are usually removed. Because these ions are multivalent, they are preferentially removed by NF membranes. Actually, for many years, particularly in Florida, NF has been used for municipal softening. NF is superior to RO, another ion-rejecting membrane technology, due to its higher flux rate. Because it uses fewer membrane elements and operates at a lower pump pressure—either pounds per square inch (psi) or bars—it lowers operating costs.
In this application, membrane technology offers the unique advantage of removing hardness ions without the need for chemicals, such as regular salt (sodium chloride) for residential water softener regeneration or soda lime for municipal softening. Sodium ion exchange has been the industry standard for softening residential water for more than 50 years. It functions by utilizing ion exchange resin (in the sodium form), which releases sodium ions in exchange for absorbing hardness ions from water as it flows through a bed of it. With each regeneration cycle, sodium or potassium chloride is discharged into the sewer (or septic tank) because this technology needs it to regenerate the resin.
Recent legislation has been passed to limit these discharges due to concerns ranging from excessive chlorides to contamination from total dissolved solids (TDS), and it appears that more and more communities will eventually prohibit the installation of traditional residential water softeners.

Some typical applications for Nanofiltration are

Food, dairy, and beverage products or byproducts that have been desalinated
When necessary, partially desalinate whey, UF permeate, or retentate.
Desalination of optical brighteners and dyes
Clean-in-place (CIP) chemical purification and disposal
Food products that have had their color reduced or altered
Food, dairy, and beverage products or byproducts concentrated in one area
Byproduct concentration of fermentation.

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