Ultrafiltration (UF) is the separation of dissolved substances through a semipermeable membrane. Compared with the reverse osmosis system, ultrafiltration retains minerals in the water. The following article will introduce the working principle of ultrafiltration and other related issues.
What is the basic principle of ultrafiltration?
The basic principle of ultrafiltration is an advanced filtration method based on membrane separation technology. Its core is to use the special structure and performance of ultrafiltration membranes to achieve effective separation of molecules of different sizes in the solution. The ultrafiltration membrane filtration principle is widely used, not only in the field of water treatment, but also in many industries such as medicine, chemical industry, and food.
Reverse Osmosis vs Ultrafiltration
Many ultrafiltration systems use hollow fiber membranes to filter water from the inside out. Ultrafiltration has a larger pore size than reverse osmosis membranes, which can intercept larger molecules and suspended particles. Therefore, ultrafiltration can remove suspended particles, colloids, bacteria, and viruses, but it is less effective in removing dissolved salts.
Reverse osmosis water filtration systems filter through semipermeable membranes. The pore size of the reverse osmosis membrane is very small, allowing only water molecules to pass through, thereby removing dissolved salts, heavy metals, bacteria, viruses, organic matter, and other impurities in the water. Its filtration effect is very thorough.
Related: What Is a RO System and How Does It Work?
In general, reverse osmosis has a higher filtration accuracy than ultrafiltration. If you need highly pure drinking water, reverse osmosis is a better choice. If you only need to remove impurities such as suspended matter and bacteria, you can choose ultrafiltration.
Ultrafiltration vs. nanofiltration and microfiltration
Reverse osmosis, nanofiltration, ultrafiltration, and microfiltration are four common membrane separation technologies. Their differences lie in the structure of the membrane, the permeation mechanism of the membrane, the working pressure, etc. You can choose the appropriate filtration method according to your water quality filtration needs. You can learn more about it from the following points:
Different membrane structures
Generally, reverse osmosis membranes are high-density composite membranes with a pore size of about 0.0001 microns. Nanofiltration is also similar to RO membranes, but its pore size is slightly larger about 0.001 microns. Ultrafiltration is a porous membrane with a large water flux low salt removal rate and a pore size of about 0.01-0.1 microns. Microfiltration is a relatively loose porous membrane with a pore size of about 0.1-1 microns.
Different membrane permeation mechanisms
Reverse osmosis membranes are based on a dissolution-diffusion mechanism. When water molecules diffuse through the membrane, the solutes are completely excluded, and their selectivity is very high. It can almost intercept all solutes, such as ions, molecules, and microorganisms.
Nanofiltration is partly based on a dissolution-diffusion mechanism and partly belongs to a screening effect. It can pass through some small molecules and ions. Its selectivity is medium, because divalent and multivalent ions and most organic matter are intercepted, and monovalent ions are partially passed through.
Ultrafiltration is mainly based on sieving, and it can intercept macromolecules and microorganisms, but its selectivity is relatively low because it can intercept macromolecules, organic matter, colloids, bacteria viruses, etc., while small molecules can pass through.
Microfiltration is completely based on sieving, and it can intercept larger particles and bacteria. However, it has the lowest selectivity because it can only remove suspended particles and bacteria, while dissolved molecules and ions can pass freely.
Different working pressures
- Ultrafiltration (UF): 0.1-10 bar (1-150 psi)
- Reverse osmosis (RO): 40-80 bar (600-1200 psi)
- Nanofiltration (NF): 10-40 bar (150-600 psi)
- Microfiltration (MF): 1-5 bar (15-75 psi)
In general, this is just the approximate range of ultrafiltration, reverse osmosis, nanofiltration, and microfiltration. The actual working pressure needs to be adjusted according to the specific situation.
When to use ultrafiltration?
If you want to remove some contaminants from the water but need to retain the minerals in the water, then you can choose the ultrafiltration filtration method. Compared with RO systems, ultrafiltration wastes less water than reverse osmosis. In some parts of the United States that are subject to water control, such as California, they should choose ultrafiltration.
Reverse osmosis is used when all contaminants must be removed from the water. If you want to drink water without any contaminants, then the reverse osmosis system is your first choice. Generally speaking, reverse osmosis is more suitable for different water quality environments such as outdoor and aquariums. Nanofiltration can be used to soften water. Microfiltration can remove suspended solids such as algae and sediments.
Related: What is the best way to filter water?
What are the two types of ultrafiltration?
Ultrafiltration can be divided into two types based on the driving method, pressure ultrafiltration and concentration ultrafiltration.
Pressure ultrafiltration uses external pressure to remove large molecules in water, while small molecules pass through the membrane to achieve separation. It is generally used in water treatment, pharmaceuticals, food processing, etc.
Concentration ultrafiltration uses the gradient of molecules of different concentrations in the solution to drive molecules through the membrane for separation. It does not require external pressure but needs to establish a concentration gradient, usually requiring the use of dialysis membranes or other auxiliary equipment. It is generally used in biological separation, drug concentration, etc.
What can ultrafiltration remove?
UF membrane is an ultrafine filter that can remove relatively large particles and molecules in water. Generally speaking, the UF membrane can remove 90% to 100% of suspended solids, bacteria, and most viruses.
- Suspended matter (silt, sludge, algae)
- Bacteria
- Viruses
- Colloids (proteins, polysaccharides)
- Some organic matter (humic acid, pigments)
What ultrafiltration cannot remove
- Soluble salts: such as sodium chloride and magnesium sulfate.
- Small molecule organic matter: such as pesticide residues and disinfection by-products.
- Heavy metal ions: such as lead, mercury, and arsenic.
What is the life of the UF membrane?
In general, the life of UF membranes is about 3-5 years. It depends on many factors, such as water quality, operation, cleaning, and maintenance. If you operate and maintain well, the life of the UF membrane may be extended. The following factors affect UF membrane life:
Water quality
If the water you filter contains a large number of suspended solids, microorganisms, and organic pollutants, it will accelerate the pollution and blockage of the ultrafiltration membrane, thereby shortening the life of the UF membrane.
Operating condition
Operating pressure, flow rate, and other conditions will affect the UF membrane life. If you are using an ultrafiltration membrane, you need to maintain its operating range, as excessive pressure and flow rates can cause damage to the membrane.
Cleaning and maintenance
Regular cleaning of the ultrafiltration system is needed to effectively ensure that the life of the ultrafiltration membrane is extended. If you do not clean it for a long time, it may lead to serious membrane pollution, and may even affect the membrane can not recover its filtration effect, thereby shortening the service life of the membrane.
How to clean the UF membrane?
The cleaning method of the UF membrane depends on many factors. Here are several commonly used cleaning methods:
Backwashing: It can wash the membrane surface with reverse water flow to remove loose pollutants. It is also the most basic cleaning method.
Chemical cleaning: Acidic, alkaline, and other chemical cleaning agents can be used to effectively remove pollutants.
Ultrasonic cleaning: It mainly uses high-frequency sound waves to generate bubbles. When the bubbles burst, shock waves are generated to remove pollutants on the membrane surface. And its cleaning effect is very good.
Mechanical cleaning: You can use brushes, sponges, and other tools to remove pollutants on the membrane surface. However, when you use mechanical cleaning, you must be careful to avoid damaging the membrane surface.
Is ultrafiltration water safe to drink?
Ultrafiltration water is safe to drink. Ultrafiltration mainly removes particles, bacteria, and some viruses in water through a semipermeable membrane. At the same time, ultrafiltration does not remove dissolved salts and minerals in water.
Ultrafiltration cannot remove all viruses, chemical pollutants, and organic matter in water. To ensure the safety of drinking water, it is recommended to combine activated carbon and reverse osmosis for filtration, because these technologies can supplement the shortcomings of ultrafiltration, remove various pollutants in water, and ensure that the water quality meets drinking standards.
FAQs
Can ultrafiltration remove bacteria?
Yes, ultrafiltration can remove bacteria. It can not only remove bacteria from water but also remove viruses, macromolecular organic matter, and colloid substances from water.
Can ultrafiltration remove minerals?
Ultrafiltration cannot remove minerals. If you want to remove minerals from water, you can use reverse osmosis or ion exchange technology. Reverse osmosis can remove most dissolved substances in water, including mineral ions, through a semi-permeable membrane.
What is the difference between membrane filtration and ultrafiltration?
Filtration is a method of separation using filter media. Dirt and sediment particles are physically removed from the fluid, and the filter media can be quartz sand, activated carbon, ceramic, etc. The filter is usually able to remove particles as small as about 1 micron, but cannot remove some dissolved chemicals.
Ultrafiltration is a filtration method based on hollow fiber membranes. The pore size of ultrafiltration membranes is very small, which can block smaller particles and solids. At the same time, it can also filter most microorganisms, viruses, bacteria, etc.
Can ultrafiltration remove fluoride?
Ultrafiltration cannot remove fluoride. This is because fluoride is a small molecule ion, and the pore size of the ultrafiltration membrane is large, which cannot prevent it from passing through. If you want to filter fluoride from water, you can use reverse osmosis to filter it.
