What Are the Differences and Correlations Between Ultrafiltration and Reverse Osmosis Membranes

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As the environmental protection industry grows, ultrafiltration and reverse osmosis membranes are becoming more widely used in various membrane elements for water treatment applications. They have their own unique advantages and are the most critical consumables in water treatment systems. So how to distinguish and understand ultrafiltration and reverse osmosis membranes?

What is an ultrafiltration membrane?

Ultrafiltration membrane is a microporous filtration membrane with uniform pore size and a nominal pore size range of 0.001~0.02 microns. It can screen solute molecules more minor than the pore size and separate particles with a molecular weight greater than 500 Daltons and particle size larger than the pore size when appropriate pressure is applied to one side of the membrane. Ultrafiltration membrane is one of the earliest polymer separation membranes developed, and the ultrafiltration device was industrialized as early as the 1960s.

Ultrafiltration membranes are divided into two categories according to membrane materials: organic and inorganic UF membranes. At present, the mainstream organic ultrafiltration membrane is generally made of PVDF or PTFE, while the ceramic membrane represents the inorganic ultrafiltration membrane. Ultrafiltration is a pressure-driven membrane separation technology with an operating differential static pressure of 0.1~0.5 MPa. Under the action of differential static pressure, the solvent and small solute particles in the feedstock flow from the high-pressure liquid side through the membrane to the low-pressure side, while the large particle components are blocked by the membrane, effectively retaining proteins, enzymes, viruses, colloids and other large molecular solutes in the screening separation process.

Compared to traditional water treatment technologies, ultrafiltration can effectively remove macromolecules, colloids, proteins, and particles in raw water. Suppose chemical or ultraviolet disinfection is not used. In that case, ultrafiltration still perfectly removes viruses, thereby reducing the amount of subsequent disinfection chlorination and the number of disinfection by-products generated. In other words, when used in drinking water treatment, ultrafiltration technology is the most effective technology to ensure the microbial safety of drinking water. It dramatically improves the chemical safety of drinking water. At the same time, it has the advantages of low pressure, large water production, and easy operation. Therefore, ultrafiltration technology is called the third generation of drinking water technology and has attracted the attention of water treatment researchers.

What is a reverse osmosis membrane?

A reverse osmosis membrane is an artificial semipermeable membrane with specific characteristics made by simulating biological semipermeable membranes. It is generally made of polymeric materials, such as cellulose acetate membrane, aromatic polyhydrazide membrane, aromatic polyamide membrane, etc. The diameter of surface micropores is usually between 0.5nm~10nm, and the size of permeability is related to the chemical structure of the membrane itself. For example, some polymeric materials have good salt resistance but poor water permeability. On the other hand, some polymer materials have more hydrophilic genes in their chemical structure and thus have a relatively fast water permeation rate. Therefore, a qualified reverse osmosis membrane should have an appropriate permeation or desalination rate.

Currently, low-pressure membranes, anti-pollution membranes, brackish water membranes, seawater desalination membranes, etc., are the common reverse osmosis membrane elements in the market. Different membrane manufacturers also have a variety of reverse osmosis membranes with various performances. These membranes can be flexibly selected according to the specific requirements of water treatment projects. Reverse osmosis (RO) is a membrane process in which the reverse osmosis membrane selectively separates ionic substances only by the solvent (usually water) and uses the static pressure difference on both sides of the membrane as the driving force to overcome the osmotic pressure of the solvent, allowing the solvent to pass through the RO membrane to separate the liquid mixture. The operating pressure difference is generally 1.5~10.5 MPa, and the size of retained components is 1~10 Å (1 Å = 10-10m) of small molecules of dissolved substances. In addition, it can remove all other suspended matter, dissolved matter, and colloids from the liquid mixture. Water can be separated for separation and purification.

Reverse osmosis water treatment technology has been successfully applied in various fields, mainly due to its simple and economical operation. Like many high-tech products, it has a high technical content, high added value of science and technology, and is easy to use. In application areas with strict water quality requirements, such as semiconductors and new energy sources, reverse osmosis equipment can be used as a pre-desalination device with unique features and advantages. Its use greatly extends the regeneration cycle of traditional ion exchange equipment, reduces acid and alkali emissions, and contributes to local environmental protection. At the same time, it dramatically reduces the labor intensity of operation and maintenance personnel and further improves the operation level and automation of the whole water treatment process.

What are the characteristics of an ultrafiltration membrane?

In general, ultrafiltration membrane has the following characteristics.

  1. Closed operation under mild conditions with no phase change at room temperature, simple process, and low energy consumption.
  2. Simple separation device, small footprint, high efficiency of single-stage separation.
  3. Simple process flow, high compatibility, and easy integration with other processes.
  4. No qualitative changes of substances in the membrane separation process, no by-products, suitable for separation, Concentration, or purification of substances sensitive to pH, temperature, and ionic strength.
  5. No reagent addition, no secondary pollution, green, clean, and efficient.
  6. Using ultrafiltration membranes with different retention molecular weights can classify or separate organic compounds.

What are the characteristics of reverse osmosis membranes?

The performance of membranes in membrane separation technology determines the performance of water treatment systems. In general, qualified reverse osmosis membranes must have the following characteristics.

  1. Have an efficient desalination rate at high flow rates.
  2. High mechanical strength and durability.
  3. The ability to operate at low operating pressures.
  4. Tolerant of chemical or biochemical effects.
  5. Less affected by PH, temperature, and other factors.
  6. Easy source of raw materials for membrane preparation, easy processing, and low cost.

What are the differences between UF and RO membranes?

In addition to the differences mentioned above in membrane material and surface micropore size, there are several other differences between UF and RO membranes.

Different operating pressures

The operating pressure of plants using ultrafiltration membranes should generally be maintained at 0.1 to 0.7 MPa, while the operating pressure of reverse osmosis membranes is much higher, generally between 1.2 to 7.2 MPa.

Different standards for bacteria detection

The qualification standard for an ultrafiltration membrane is 100 colonies per ml of water, while that for a reverse osmosis membrane is 20 colonies per ml of water. The test standard for reverse osmosis membrane is five times higher than that of ultrafiltration membrane.

Pore size varies widely

The pore size of a reverse osmosis membrane is only 1/100 of the pore size of an ultrafiltration membrane, so reverse osmosis membrane can remove smaller organic molecules in water, such as chemical organics, organic pesticide pollution, etc., while ultrafiltration membrane cannot. Reverse osmosis membranes also have the function of softening water, turning hard water into soft water.

Different application scope

Reverse osmosis membrane has different application scenarios according to different types. For example:

  1. Low-pressure membranes are usually used for industrial pure water, boiler make-up water, tap water purification, and desalination, and as a secondary treatment of reverse osmosis product water, which still has the performance of high desalination rate at lower operating pressure.
  2. Brackish water membranes are mainly used in the desalination of surface water, municipal water, industrial wastewater, water reuse treatment, etc. They can effectively reduce the frequency of chemical cleaning, reduce the operating cost, and at the same time, have the characteristics of a high desalination rate, high water production, and high chemical resistance.
  3. Anti-pollution membranes are usually used in highly polluted water sources, such as industrial wastewater with more severe pollution, which have better anti-pollution ability to organic matter and microorganisms in feed water and high chemical durability, which can effectively solve the pollution problem of membrane elements.
  4. Seawater desalination membranes are mainly used in seawater desalination, reverse osmosis concentrated water treatment, leachate, and other water sources with higher pollution levels.

On the other hand, the application of ultrafiltration membranes is mainly in the following areas.

  1. As reverse osmosis pre-treatment in pure water production and final ultrapure water treatment.
  2. Industrial water for removal of bacteria, heat sources, colloids, suspended solids, and macromolecular organics.
  3. Purification of drinking and mineral water and industrial and domestic wastewater purification and recycling.
  4. Concentration, purification, and clarification for fermentation, enzyme preparation, and pharmaceutical industries.
  5. Concentration and separation of fruit juices.
  6. Separation, concentration, and clarification of soybean, dairy, sugar industry, wine, tea, vinegar, etc.
  7. Recovery of electrophoretic paint, etc.

What are the correlations between UF and RO membranes?

Ultrafiltration and reverse osmosis membranes are neutral membranes that do not have ion exchange properties and are pressure-driven. The membrane elements can be made into rolled, tubular, and plate types.

Both ultrafiltration and reverse osmosis membranes are also common membrane separation elements. In pure water preparation, ultrafiltration is the basis of the reverse osmosis membrane. Ultrafiltration membrane is generally used in the front end to achieve stable operation of reverse osmosis membrane. Therefore, the position of the ultrafiltration membrane is also irreplaceable, which is the preliminary treatment of reverse osmosis technology. Meanwhile, ultrafiltration and reverse osmosis membranes are widely used in drinking water, mineral water purification, and wastewater recycling.

UF and RO membranes generally have their own strengths and weaknesses. However, both play an essential role in water treatment systems. Therefore, we can choose the right product according to our needs and the project’s actual situation.

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