Research shows that membrane technology is currently the most widely used technology in wastewater treatment and wastewater reuse, and membrane technology is the first choice for companies in the water treatment industry.
Extensive application of membrane technology in wastewater treatment and wastewater reuse
In urban sewage treatment and wastewater reuse, the membrane process is often used for advanced treatment after secondary treatment. Microfiltration (MF) and ultrafiltration (UF) are often used to replace the precipitation, filtration, adsorption, and sterilization in conventional advanced treatment. And other treatments, using nanofiltration membrane (NF), reverse osmosis (RO) for water softening and desalination. In wastewater reuse, the most commonly used membrane bioreactor is MF, UF and activated sludge.
The composition of industrial wastewater is different, the purpose of reuse is also different, and the application of membrane technology is also diverse. But no matter what kind of wastewater treatment, membrane technology must cooperate with other technologies to play its role. Because the composition of sewage is extremely complicated, different water reuse standards and treatment processes are required for different purposes of reuse. Any single water treatment technology is difficult to meet the water quality requirements of reuse water.
It is generally believed that MF can remove microorganisms, bacteria, eggs, and viruses in sewage, and UF can remove macromolecules in sewage, such as proteins, humic acids, and certain precursors and dyes that generate THM. Dissolved salts and most ions in MF and UF-treated wastewater are still difficult to remove, and these components must be removed through an RO membrane. The secondary effluent can reach the drinking water standard after RO treatment. Its desalination rate can reach more than 90%, the water recovery rate can be more than 75%, and the removal rate of COD and BOD is more than 85%. However, RO needs to operate at a higher pressure, and the cost is higher. In the past, MF and ultrafiltration membranes were more commonly used in sewage treatment. NF membranes developed in recent years can trap low-molecular and divalent and high-valent ions with a relative molecular mass greater than 200, and have a low retention rate for monovalent ions, so they can be operated at lower pressures, with large water permeability and low operating costs. Sewage treatment can remove all viruses, bacteria, pesticides, surfactants and chloroform precursors. Such materials will react with chlorine in water (such as adding chlorine during sterilization) to form carcinogens such as THM. Most of them are low-molecular organic compounds, which are difficult to remove with conventional water treatment techniques and MF and UF. NF can remove more than 90% of dissolved carbon and THM precursors in the secondary effluent, and 2/3 of the salinity, / 5 hardness, the effluent meets the requirements of the United States 1986 Safe Drinking Water Law for pollutants.
The electrodialysis of a charged membrane is a membrane process under the action of an electric field force. It is mostly used for industrial wastewater reuse and treatment, such as separation and reuse of charged ions such as metal ions in electroplating wastewater.
The liquid membrane technology can use chemical reactions to promote the mass transfer process, the mass transfer speed is fast, and the separation efficiency is high. It has been applied on a pilot scale in the treatment of phenol-containing wastewater.
The membrane bioreactor (MBR) that combines membrane separation (mainly MF and UF, also useful for NF) and biological reaction is most commonly used in wastewater treatment. The second settling tank, which is settled and separated by gravity in the sludge, has the advantages of compact equipment and good effluent water quality.
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