Nanofiltration is a separation process characterized by organic, thin-film composite membranes with a pore size range of 0.1 to 10nm. Unlike reverse osmosis (RO) membranes, which reject all solutes, NF membranes can operate at lower pressures and offer selective solute rejection based on both size and charge.
Nanofiltration membranes are a relatively recent development, and offer greater selectivity of ions vs. reverse osmosis membranes that reject all ion species in a feed stream. This unique characteristic provides flexibility in separation process development that can greatly impact performance and profitability, especially for industrial applications.
Nanofiltration membranes are defined as having a pore size in the order of nanometers (nm) (1×10−9 m). As a comparison, the atomic radius of a sodium ion and a chlorine ion is about 0.97 nm (0.97×10−9 m) and 1.8 nm (1.8×10−9 m), respectively. This demonstrates that nanofiltration membranes are near the range to remove rather small ions.
However, the term nanofiltration is really a misnomer. As the nanofiltration membranes are charged, the removal mechanism is not purely filtration as with ultrafiltration membranes, but also osmotic. This makes them a true hybrid, bridging ultrafiltration and reverse osmosis membranes in the range of membrane treatment options.
In general, the primary factors that affect the performance of the membranes include the membrane material (charge of the membrane), concentration polarization at the membrane face (buildup of concentration at the membrane face), and fouling of the membrane to name a few. As such, pore size alone does not predict the removal of constituents. Adding more complication to the problem, every manufacturer’s membranes are slightly different, meaning there is no simple method for predicting removals. Pilot testing of the nanofiltration membranes is imperative when designing a system to target certain constituents.
Membrane manufacturers do discuss ranges for constituent removal based on atomic mass. Most indicate that nanofiltration membranes remove compounds/ions with a molecular weight greater than 300–400 g/mol. It should be noted that this number represents the size for complete (or nearly so) removal. For partial removals, the range extends down to less than 100 g/mol. This is evidenced by a figure published by Koch Membranes on their website that shows the range of nanofiltration membranes from 100 to 20,000 g/mol. In addition, testing completed by the Long Beach Water Department (Long Beach, California) confirms the smaller molecular weight as they have shown significant removal (up to 90 percent) of aqueous salts, which range in molecular weight from about 60 to 500 g/mol. With this range of removals, nanofiltration membranes provide engineers and scientists with a unique opportunity to produce customized water products.
With new breakthroughs in membrane technology, constituent specific membranes are a possibility. Membrane companies have developed nanofiltration membranes capable of 95 percent salt removal, increasing from the 20 percent that the early nanofiltration membranes had. In addition, research in the use of embedded “nanoparticles” in reverse osmosis membranes has shown promise in decreasing the transmembrane pressure, thus increasing the flux. This type of technology may provide similar improvements to nanofiltration flux and removal efficiencies.
NF membranes can reduce the ionic strength of the solution. Moreover, hardness, organics, and particulate contaminants can be removed by NF membranes. Many researchers have used NF to achieve those objectives.
Some researchers have investigated the softening of groundwater using NF systems. All of the results gave good data. Retentions higher than 90% were found for multivalent ions, whereas monovalent ions were about 60–70%. NF membranes with pellet softening and granular activated carbon for softening water were compared. Although all methods had good results, NF membranes had several advantages from the point of health and lower investment costs. The advantage of NF softening is that a smaller stream of water can be softened because NF membranes remove essentially all hardness cations. In municipal water treatment works, it is not necessary to reduce hardness to very low values, and therefore the use of NF membranes allows for side-stream treatment. This involves sending only a portion of the flow to be softened to the membranes, and permeate is then blended with the bulk flow stream to obtain a target blended value. In contrast, precipitated lime softening cannot reduce hardness to below approximately 50 mg/L CaCO3, and therefore side-stream treatment is generally not practical.
The primary application of NF membranes is desalting of saline, surface, or groundwater. Surface waters often have unsettled chemistry or composition owing to seasonal changes or after dilution with rain. NF is a reliable option for surface water treatment, although the focus is on removing organics rather than on softening.
Disinfection by-products (DBPs) are a significant regulatory concern. NF membranes are increasingly applied to remove DBP precursors such as natural organic matter (NOM), which can react with various disinfectants used in the water treatment process to form potential carcinogens. NOM removal is an important water treatment target for many utilities. NF as a standalone process has been shown in many cases to reduce total organic carbon (TOC) to less than 0.5 mg/L.
NF is also more effective than RO for lime softening, removing naturally occurring color and DBP precursors both consist primarily of organic carbon.
Semipermeable NF membranes are not porous; they have the ability to screen microorganisms and particulate matter in the feed water. This ability has been verified in a number of studies, such as one that demonstrated that NF membranes provide between 4 and 5 log removal of viruses.