Nanofiltration refers to a specialty-membrane process that rejects dissolved solutes in the approximate size range of 1 nanometer (10 Angstroms) — hence the term “nanofiltration.”
With respect to the size and weight of solutes that nanofiltration membranes reject, NF operates in the realm between reverse osmosis (RO) and ultrafiltration (UF) : Organic molecules with molecular weights greater than 200 – 400 are rejected. Nanofiltration membranes can effectively reject, among other contaminants:
It also rejects certain soluble salts. Specifically, NF rejects dissolved salts in the range of 20 – 98 percent. Salts which have monovalent anions (e.g., sodium chloride or calcium chloride) have rejections of 20 – 80 percent, whereas salts with divalent anions (e.g., magnesium sulfate) have higher rejections of 90 – 98 percent. Transmembrane pressures are typically 50 – 225 psi (3.5 – 16 bar).
The ideal nanofiltration membrane has a very high water permeability, but the ideal permeability of solutes might be near zero or some higher value, depending on the solute and application. For example, an application may require near-zero permeability for pesticides and 50 percent permeability for calcium ions.
Typical applications of nanofiltration membrane systems include:
Like reverse osmosis membranes, nanofiltration membranes are used in separation systems employing applied pressure to effectively overcome the system’s osmotic pressure, reversing the flow of a solvent across a semipermeable membrane from an area of higher solute concentration to an area of lower concentration. This “reversed” flow, and the degree of permeability of the nanofiltration membrane, result in solutes too large to pass through the membrane remaining on the higher-concentration side of the membrane, while purer water that contains desired or acceptable solutes flows through.
Nanofiltration membranes are similar to RO membranes in another way: They are used in crossflow configurations. Crossflow helps to minimize fouling, or the accumulation of solutes that cannot pass through the semipermeable membrane against the membrane. Very simply stated, in crossflow a pressurized flow of feedwater forces lower-concentration water through the NF membrane, while the now-isolated flow of higher-concentration water moves across the surface of the membrane, carrying away the rejected salts and other impurities. The purified water is called the permeate, while the higher-concentration water is called the concentrate or reject.
Buying a new water purifier or reverse osmosis system for your home can be simplified if you get in prepared with the necessary knowledge. Before buying a new unit consider this:
1.Any RO system should comprise of at least three pre-filter stages – one sediment pre-filter rated at 5 microns, and two granular activated carbon filters. The granular activated carbon filters should ideally be catalytic carbon to remove chloramines.
2. The pre-filters are an important part of your Reverse Osmosis Filter, the sediment filter protects the carbon filters, and the carbon filters protect the reverse osmosis membrane, which is prone to oxidation damage from chlorine or chloramine. Chloramines are a combination of ammonia with chlorine. Unlike straight chlorine, which dissipates fairly quickly when exposed to air, chloramines remain in the water. Chloramines are very difficult to remove from your water supply.
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3.Ensure that your RO membrane is from a reputable company like DOW TFC membranes, and should be at least 50 GPD( gallons per day).
4.Be wary of any salesman that tells you that they have a pure water to waste water ratio of 1:1 . The ratio is more likely going to be 1:3 or 1:5 (pure:waste) . This ratio is necessary to ensure continuous high performance of your RO membrane, and helps flush contaminants from the tiny pores(0, microns) which otherwise can get clogged. This ratio is also dependent on a number of factors including water quality. High dissolved solid content like for instance hard water, will require a higher flush rate. A flow restrictor by-pass is also recommended as that will help clean out the membrane surface by initiating a high flow.
5.Residential RO membranes can only be used for town water supply i.e. chlorinated water with relatively low dissolved solid content. They should not be used for river, dam or rain water. Special membranes are required for brackish and seawater and they require significantly different configurations to suit the increased osmotic pressure caused by the increased salinity. Seawater which has a salt content of above 35,000 ppm would require at least 24 Bar just to overcome the osmotic pressure. Residential RO systems should be operated at pressures of 300 to 800 kPa( 3-8 Bar)- low water pressure means you would require a booster pump with your RO system, which is normally just a add-on.
6.The purified water from your reverse osmosis system is slightly acidic as the water is devoid of any minerals and the remaining CO2 gas in the water causes the slight acidic conditions. Hence we recommend a mineral or alkaline stage that does two things; it provides minerals back into the water (Calcium, Magnesium, etc) and also increases the ph to levels above 8. The added minerals will also help provide buffering capacity to the water.
7.Reverse osmosis systems are the best purifier to remove fluoride from water. The level of fluoride added is typically 0.5 ppm . Activated carbon does not remove fluoride, activated alumina and ion exchange resins with fluoride specific ions can, but require significant contact time. They also become saturated and to find out when that happens can be rather expensive. Measuring residual fluoride in water in the parts per billion range is very expensive- and the filter sales people know that. It is therefore better to rely on reverse osmosis filters, as the method of removal is by size exclusion. The size of the fluoride ion is larger than the tiny pores of a membrane which are only 0. microns.
8.There are three options for RO systems ; Tank option ,direct flow option and also units that automatically fill up a portable reservoir . A buffer tank with a bladder provides adequate flow when you want to fill up a glass. Ensure the tank is manufactured from food grade approved materials. Direct flow units are also popular but to be practical should produce at least 400 GPD of purified water. Automatic RO systems like the Astroboy, are becoming very popular as they allow the portable container to be taken anywhere, or positioned into a water dispenser.
9.A final stage carbon polishing filter is also recommended to ensure there is no residual taint as a result of water sitting in the line or the storage tank
10.Quick change cartridges are becoming very popular, as they require very little effort to replace and no tools. Remember be very diligent about changing cartridges- as that will ultimately ensure consistent good quality water and ensure your reverse osmosis membrane will last at least 3 years.
Nominal impurity removal characteristics of TFC RO membrane
Contact us to discuss your requirements of nanofiltration system. Our experienced sales team can help you identify the options that best suit your needs.