At present, people's social activities and industrial production will produce a lot of waste water, the treatment and recovery of these waste water into drinkable domestic water is an important way to solve the problem of water resources.
Reverse osmosis membrane filtration technology is an advanced water treatment technology, the application of this technology does not need to carry out chemical reactions, will not produce excess harmful substances, friendly to the environment, has a broad application prospect.
Reverse osmosis membrane and its working principle
Reverse osmosis membrane is a kind of semi-permeable membrane with selectivity for material penetration, which allows the solvent in the mixed liquid to pass through the membrane in a specific direction, while stopping the solute on the other side of the membrane.
Contaminated water usually contains a variety of impurities or dissolved salts, if it is placed on one end of the reverse osmosis membrane, and the pressure is applied to it, you can promote the water in the solution through the reverse osmosis membrane to reach the other end, access to high-quality pure water, to achieve the purpose of purification.
The preparation of reverse osmosis membrane can be achieved by a variety of materials, and different materials have different properties.
Currently commonly used membrane making materials are:
Cellulose ester, cellulose diacetate, cellulose triacetate;
Polybenzimidazole, polybenzimidazolone, polyimide;
Polyaromatic amides;
Selection of membrane materials
The performance of reverse osmosis membrane directly determines the treatment effect of wastewater. Therefore, when applying reverse osmosis technology to filter sewage wastewater, the first issue to be considered is the selection of membrane materials.
Reverse osmosis membrane can be divided into inorganic and organic polymer materials according to the materials used. However, in view of the high application cost of inorganic polymer materials, most of the commonly used reverse osmosis membranes are made of organic polymer materials, and the main materials are aromatic polyamides and cellulose acetate.
When selecting membrane materials for actual wastewater treatment, the following factors can be comprehensively considered:
The composition of the wastewater, the temperature and pH value of the wastewater treated, the mechanical strength and chemical characteristics of the reverse osmosis membrane, etc.
Currently commonly used reverse osmosis membranes have the following types:
Acid fiber membrane. Acid fiber membrane is the only membrane resistant to chloride ions, and its desalination rate can exceed 95%. In addition, its pollution resistance is also strong, but it is not suitable to withstand the erosion of microorganisms, so the degradation rate is high, and the desalting effect will be significantly reduced after a period of use. When the acid alkalinity of the filtered wastewater exceeds a certain range, the acid fiber membrane will decompose. Its optimal application pH is between pH 5 and pH 6.
Polyamide membrane. The resistance of polyamide membrane to chlorine is very poor, and it is easy to be attacked by chlorine or other oxidants, which reduces the filtration performance of wastewater. However, this kind of membrane has good resistance to microorganisms and is not easy to be hydrolyzed, so its service life is relatively long. It is used in an environment where the pH of waste water is between pH 4 and pH 11, and the water temperature is within 40°C.
Composite membrane. The composite membrane is made of a variety of organic polymer materials, so its performance is more balanced and has a variety of properties. Specifically, it can be used in a low pressure environment of about 1.6MPa, and can inherit the chlorine resistance of acid fiber membrane and the microbial resistance of polyamide membrane. Although the performance is reduced in the longitudinal comparison, the performance is balanced in the horizontal comparison, which makes up for the shortcomings of reverse osmosis membrane using a certain material alone. It can be used in the pH range of 2 to 12, with a wide range of applications, but in long-term use, it matches pH values of 5 to 9.
Application of reverse osmosis membrane and related technology in water treatment
1. Wastewater treatment
Wastewater treatment is another core application direction of reverse osmosis membrane, through reverse osmosis technology can effectively treat power plants, printing and dyeing plants produced containing heavy metals or oily wastewater, can also treat municipal sewage, garbage leachate, etc., to achieve the recovery of water resources.
In the process of thermal power generation, a large number of circulating cooling water will be used, which usually contains a variety of impurities, and the water turbidity is high, which will pollute the environment if directly discharged. Combined with hollow fiber ultrafiltration membrane element and polyamide composite reverse osmosis membrane, the circulating cooling water can be treated into reusable water with turbidity less than 0.02 NTU and pollution index less than 0.7.
The wastewater produced by printing and dyeing plant contains a lot of salt ions, chrominance ions and other substances. The use of ultrafiltration and reverse osmosis double membrane technology can filter the organic matter and salt ions in the wastewater, and the chemical oxygen consumption of the water production is not higher than 10 mg/L, and the water treatment effect is obvious.
The traditional wastewater treatment process containing heavy metals needs to add chemical agents, and the treatment operation requirements are high. Using reverse osmosis membrane and multistage reverse osmosis technology to treat these wastewater can better remove the heavy metal elements contained in the wastewater.
The components contained in the leachate of municipal refuse are more complex. The treatment with polyamide reverse osmosis membrane can purify the landfill leachate to the target of discharge, and realize the treatment of landfill leachate.
2. Pure water production
Purified water has a wide range of applications in drinking water and industrial production and pharmaceuticals, such applications have high requirements for water, and the use of reverse osmosis membranes for water treatment needs to meet the necessary production standards.
Because the ultra-pure water production process is relatively mature, the corresponding permeable membrane components can be used in the ultra-pure water production. The reverse osmosis system can control the water flux up to 650 m³/h and produce ultra-pure water with a conductivity of 0.3 μS/cm. The problem of membrane contamination in the process of water treatment can be solved by chemical cleaning. At present, the more commonly used pure water treatment system is reverse osmosis - electric deion system.
In terms of industrial water production, in order to meet the needs of use, domestic researchers have proposed the use of ultra-low pressure composite membrane components to build a reverse osmosis system scheme. The water yield of the permeable membrane in the scheme can reach 41.4 cubic meters/day, and the retention rate is not less than 99.7%, which can fully meet the water demand of caustic soda industry.
In addition, the need for filtration of Ca² + Mg² + plasma in the water used in the chlor-alkali industrial production makes it possible to remove it using PVD reverse osmosis membranes while ensuring a low retention of sodium chloride and achieving the treatment and purification of water used in chlor-alkali industrial production.
In pharmaceutical production, the use of ultra-low pressure polyamide composite permeable membrane module ion separation system, the application of two-stage reverse osmosis method for the preparation of pure water required in the pharmaceutical production process can achieve better results. The recovery rate of this water treatment scheme after secondary reverse osmosis is not less than 80%, which brings obvious economic benefits to pharmaceutical companies.
Conclusion
Reverse osmosis membrane has good effect on water treatment, simple and convenient application, and little pollution to the environment. It is a kind of water treatment method with broad application prospect. With the deepening of reverse osmosis membrane technology research, reverse osmosis membrane components can obtain lower energy consumption, anti-pollution ability and high temperature and high pressure resistance, and the applicable field and scope will be further expanded.
