What is the difference between microfiltration, ultrafiltration, nanofiltration and reverse osmosis?

Microfiltration (MF):

The filtration accuracy is generally 0.1-50 microns. Common PP filter elements, activated carbon filter elements, ceramic filter elements, etc. all belong to the category of microfiltration. They are used for simple coarse filtration to filter large particles of impurities such as mud, rust, etc. in the water, but cannot remove harmful substances such as bacteria in the water. The filter element cannot usually be cleaned. It is a disposable filter material and needs to be replaced frequently. ① PP cotton core: Generally only used for coarse filtration with low requirements to remove large particles such as mud, rust, etc. in the water. ② Activated carbon: It can eliminate discoloration and odor in the water, but it cannot remove bacteria in the water, and the removal effect of mud, rust is also poor. ③ Ceramic filter element: The minimum filtration accuracy is only 0.1 micron, usually with a small flow rate and difficult to clean.

Ultrafiltration membrane (UF):

A microporous filtration membrane with consistent pore size specifications and a rated pore size range of 0.001-0.02 microns. The membrane filtration method using ultrafiltration membrane with pressure difference as the driving force is ultrafiltration membrane filtration. Ultrafiltration membranes are mostly made of acetate fiber or polymer materials with similar properties. It is suitable for the separation and concentration of solutes in the processing solution, and is also often used for the separation of colloidal suspensions that are difficult to complete with other separation technologies. Its application field is constantly expanding.

Membrane ultrafiltration, which uses pressure difference as the driving force, can be divided into three categories: ultrafiltration membrane filtration, microporous membrane filtration and reverse osmosis membrane filtration. They are distinguished based on the size or molecular weight of small particles that can be retained by the membrane layer. When the rated pore size range of the membrane is used as the distinction standard, the rated pore size range of the microporous membrane (MF) is 0.02-10μm; the ultrafiltration membrane (UF) is 0.001-0.02μm; the reverse osmosis membrane (RO) is 0.0001-0.001μm. There are many factors that control the pores. For example, ultrafiltration membranes with different pore sizes and pore size distributions can be obtained according to the type and concentration of the solution during membrane preparation, evaporation and condensation conditions, etc.

Nanofiltration (NF):

The filtration accuracy is between ultrafiltration and reverse osmosis, and the desalination rate is lower than that of reverse osmosis. There was a popular saying in the market: nanofiltration is a loose reverse osmosis. In fact, this is a technical misleading concept.

The real separation concept of nanofiltration is a filter membrane that satisfies the Donan effect and has selective retention of ions. It is a membrane whose sodium chloride permeability is proportional to the sodium chloride concentration and the ratio is greater than 0.4. It is mainly used for desalination and concentration of various feed liquids. The 0% NaCl retention rate is obtained by nanofiltration membrane testing under the condition of a mixture of 30,000 ppm NaCl and other types of ions. Under the condition of a pure 30,000 ppm NaCl solution. The retention rate of nanofiltration membrane for NaCl is 5% -15%. When the concentration of NaCl is lower than 30,000ppm or even lower, the retention rate of NaCl by nanofiltration membrane is above 15%. The actual rejection rate of nanofiltration membranes for salts such as NaCl depends mainly on the composition of the influent and the membrane properties (see figure).

Reverse Osmosis (RO):

The filtration accuracy is about 0.0001 micron. It is an ultra-high precision membrane separation technology using pressure difference developed in the United States in the early 1960s. It can filter out almost all impurities in water (including harmful and beneficial ones) and only allow water molecules to pass through. It is generally used in the production of pure water, industrial ultrapure water, and medical ultrapure water. Reverse osmosis technology requires pressurization and electricity.

The principle of reverse osmosis:

First, we need to understand the concept of "osmosis". Osmosis is a physical phenomenon. When two kinds of water containing different salts are separated by a semi-permeable membrane, it will be found that the water on the side with less salt will penetrate through the membrane into the water with higher salt content, while the salt contained in it will not penetrate. In this way, the salt concentration on both sides will gradually merge to an equal level. However, it takes a long time to complete this process, which is also called osmotic pressure.

But if a pressure is applied to the side with higher salt content, the above osmosis can also be stopped. The pressure at this time is called osmotic pressure. If the pressure is increased further, the osmosis can be made in the opposite direction, and the salt will remain. Therefore, the principle of reverse osmosis desalination is to apply a pressure greater than the natural osmotic pressure to the water with salt (such as raw water), so that the osmosis proceeds in the opposite direction, and the water molecules in the raw water are pressed to the other side of the membrane to become clean water, thereby achieving the purpose of removing impurities and salt in the water.