There are currently four methods for producing pure water: distillation, electrodialysis, ion exchange, and reverse osmosis. Reverse osmosis is a high-efficiency and energy-saving technology for producing pure water. It rapidly developed as a water treatment process in the 1960s and has been used in various applications, such as producing pure water, city water supply, boiler feedwater, industrial wastewater treatment, seawater desalination, and solute separation in various solutions.
The phenomenon of osmosis was discovered in the 18th century through animal experimentation. However, animal membranes are not real semipermeable membranes and have many drawbacks, which make them unsuitable for industrial applications. Therefore, the development of reverse osmosis technology depends on the manufacturing process of semipermeable membranes.
A good semipermeable membrane should have the following characteristics:
1. high water permeability and desalination rate;
2. high mechanical strength;
3. resistance to acid, alkali, and microbial invasion;
4. long service life;
5. easy to manufacture and low in price.
Reverse Osmosis Cellulose Acetate Membrane
This practical artificial membrane was developed in 1960. With multiple improvements in the manufacturing process, the product has the advantages of high water permeability, high desalination rate, and low cost.
Reverse Osmosis Polyamide Membrane
Before 1970, mainly fat-based polyamide membranes, such as nylon-66 and nylon-6, were produced, but these membranes have poor water permeability. Later, aromatic polyamide membranes were developed with better water permeability, desalination rate, mechanical strength, and chemical stability. It can be used in the pH range of 4-10 (long-term use range is pH=5-9). Aromatic polyamide membranes are mainly made into hollow fibers.
Reverse Osmosis Composite Membrane
The reason why the aforementioned semipermeable membranes can exhibit osmotic properties is due to their activated surfaces. This activated layer only needs to be very thin. If it is too thick, it will not help the osmotic function. Instead, it will reduce the water permeability and accelerate the rate of flow decay over time. However, it is difficult to make the activated layer thinner than 0.0001 mm during membrane production. Therefore, a composite membrane was developed.
A composite membrane is a two-layer thin skin composite that is first made into a porous support layer on a fabric and then subjected to a polymerization reaction of the activated layer on its surface. The support layer material can be polycarbonate, and the activated layer can be polyurea. The performance of the composite membrane in terms of water permeability, desalination rate, and flow decay is superior. Its appearance greatly reduces the operating pressure of reverse osmosis, extends the life of the membrane, and improves the economic benefits of reverse osmosis.
Post time: Sep-12-2024