Wastewater Treatment Basic Terms: What Is a Nanofiltration Membrane
Nanofiltration membranes can retain substances at the nanometer scale (0.001 micron). The operating range of nanofiltration lies between ultrafiltration and reverse osmosis; its ability to retain dissolved salts is between 20% and 98%, with removal of soluble monovalent ions lower than that of multivalent ions. Nanofiltration is generally used to remove organics and color from surface water, and hardness and radium from groundwater, while partially removing dissolved salts; it is also used for extraction and concentration of valuable substances in food and pharmaceutical production. The operating pressure of nanofiltration membranes is generally 3.5-30 bar.
The key to the nanofiltration process is the nanofiltration membrane. Membrane materials must have good film-forming properties, thermal stability, chemical stability, high mechanical strength, resistance to acid/alkali and microbial erosion, resistance to chlorine and other oxidizing substances, high water flux and high salt rejection, and resistance to colloidal and suspended-solid fouling, at a low price. Most nanofiltration membranes in use are aromatic or polyamide composite membranes. A composite membrane is an asymmetric membrane consisting of two parts: a porous support layer that works by sieving, and a thinner dense layer that performs separation, which can be explained by the solution-diffusion theory. For composite membranes, the separating skin layer and the support layer can each be optimized in material and structure to obtain excellent performance. Membrane module types include hollow fiber, spiral wound, plate-and-frame and tubular. Hollow fiber and spiral wound modules have high packing density, low cost and good hydrodynamics inside the module; however, their manufacturing is technically demanding, sealing is difficult, fouling resistance in service is poor, and feed pretreatment requirements are high. Plate-and-frame and tubular modules are easy to clean and fouling-resistant, but have low packing density and high cost. Therefore, hollow fiber or spiral wound modules are mostly used in nanofiltration systems.
In China, theoretical research on nanofiltration started relatively early, but development of nanofiltration membranes is still at a preliminary stage. In the United States, Japan and other countries, nanofiltration membrane development has advanced greatly and reached commercialization - e.g., Filmtec's NF series in the US, Nitto Denko's NTR-7400 series and Toray's UTC series in Japan - all widely used commercial composite nanofiltration membranes in water treatment.
For ordinary reverse osmosis membranes, salt rejection is a key indicator of separation performance, but for nanofiltration membranes salt rejection alone cannot fully describe their separation performance. Sometimes a nanofiltration membrane retains larger-molecular-weight substances at a lower rate than smaller ones. The filtration mechanism of nanofiltration membranes is very complex. As nanofiltration is an emerging technology, mechanism research is still exploratory and the relevant literature is scarce. But since nanofiltration is a branch of reverse osmosis, many phenomena can be explained by reverse osmosis models. Membrane transport theories for reverse osmosis [2] include the solution-diffusion theory of Lonsdale, Merten et al.; the hydrogen-bond theory of Reid and Breton; Sherwood's diffusion-pore-flow theory; the preferential adsorption-pore-flow theory of Loeb and Sourirajan; and Glueckauf's pore theory.
The filtration performance of nanofiltration membranes is also related to membrane charge and the manufacturing process. Different nanofiltration membranes show different selective permeability to solutes; e.g., ordinary nanofiltration membranes retain divalent ions at higher rates than monovalent ions, and in multi-component mixtures the rejection of monovalent ions may even decrease. The actual separation performance also depends on the operating pressure, solution concentration and temperature of the nanofiltration process - e.g., permeate flux increases with operating pressure, while rejection decreases as solution concentration rises.
The nanofiltration membrane material is polyamide.
1. Concentration and purification take place at room temperature, with no phase change and no chemical reaction; no impurities are introduced and no decomposition or denaturation of the product occurs, making it especially suitable for heat-sensitive substances.
2. It can remove salts from the product, reduce ash content and improve product purity; compared with solvent desalting, product quality is better and yield can also improve.
3. The process has a high recovery rate and low losses. 4. Acids, alkalis, alcohols and other valuable substances in the solution can be recovered for resource recycling.
5. The equipment structure is simple and compact, with a small footprint and low energy consumption.
6. Operation is simple, automated, stable and easy to maintain.
It has a high removal rate of trace volatile organics in drinking water.
In drinking water treatment, besides softening, nanofiltration membranes are mostly used for decolorization, removal of natural and synthetic organics (such as pesticides), trihalomethane precursors, disinfection by-products (trihalomethanes and haloacetic acids) and their precursors, and volatile organics, ensuring the biological stability of drinking water.
Studies show that nanofiltration membranes can remove most toxic and harmful organics and Ames mutagens from water; the mutagenicity ratio (MR) of TA98 and TA100 strains at all tested doses was below 2, and Ames test results were negative. Further research will examine the rejection characteristics of nanofiltration for endocrine disruptors in drinking water, providing a basis for safe, high-quality drinking water.
Disinfection by-products mainly include trihalomethanes (THMs), haloacetic acids (HAAs) and possible trichloroacetaldehyde hydrate (CH). Foreign researchers have conducted extensive studies; the average rejection rates of nanofiltration membranes for the precursors of these three disinfection by-products are 97%, 94% and 86% respectively. Proper selection of nanofiltration membranes can bring drinking water quality up to higher safety standards. In addition, nanofiltration effluent is low-corrosivity, positively affecting the service life of drinking water pipe networks and the dissolution of metal ions from pipes, protecting all materials in the distribution system. Tests show that a nanofiltration system with the necessary post-treatment can reduce lead dissolution in pipe networks by 50%, while other dissolved metal ion concentrations meet drinking water standards.
Membrane softening exploits the selective permeability of nanofiltration membranes to ions of different valences to soften water. While removing hardness, membrane softening also removes turbidity, color and organics, with effluent quality clearly better than other softening processes. It needs no regeneration, generates no pollution, is simple to operate and saves space, offering obvious social and economic benefits. Membrane softening is already common in the US; over the past decade or so, new softening plants in Florida have adopted membrane softening instead of conventional lime softening and ion exchange. In recent years, as nanofiltration performance has improved and membrane module prices have fallen, membrane softening has become superior to or comparable with conventional methods in investment, operation and maintenance.
Nanofiltration can retain divalent and higher ions and other particles; only water molecules and some monovalent ions (such as sodium, potassium and chloride) pass through. Nanofiltration can produce direct drinking water that retains some ions, and can reduce treatment costs.
Nanofiltration membranes have a wide range of applications, mainly including the following aspects:
2. Removing organics and color from surface water
6. Separation and concentration of organic and inorganic liquids
7. Dye purification, concentration and desalting
Membrane technology. In rural China and small-town water plants, management is often lax, which can lead to bacteria in the effluent, so advanced treatment is also required.
