The application technology of ceramic membranes prepared from various inorganic materials has gradually been integrated into industrial construction and daily life with the development of the times. Due to their advantages, such as high temperature resistance, good chemical stability, narrow pore size distribution, and high strength, ceramic membranes have attracted increasing attention. Ceramic membrane technology has become one of the fastest-growing and most promising technologies in the field of high-performance membrane materials in China, and has been widely used.
However, ceramic membranes are not perfect, and membrane fouling during application has always been one of the bottleneck factors affecting their application. Ceramic membrane fouling directly results in a decline in membrane flux, increases the operating cost of the ceramic membrane system, and affects the service life of the membrane. Usually, a ceramic membrane pretreatment system combined with membrane cleaning is adopted to ensure the normal operation of the membrane system and prolong the life of the membrane. Since ceramic membrane fouling is unavoidable, ceramic membrane cleaning technology is a powerful guarantee for the development and application of ceramic membranes.
1. Ceramic membrane fouling
Ceramic membrane fouling is similar to the fouling mechanism of organic membranes. It is the phenomenon that some components in the feed solution are adsorbed or deposited on the surface or pores of the membrane during the membrane separation process, resulting in a decrease in the permeate flux, including the increase of the filtration resistance caused by the blockage of the membrane pores by large molecules, the adsorption of solutes on the pore walls, and the formation of gel layers on the membrane surface, which increases the mass transfer resistance, etc. The adsorption and deposition that cause membrane fouling are the result of the interaction between the components of the ceramic membrane and the feed solution, as well as the components adsorbed on the ceramic membrane surface and other components in the feed solution. This interaction involves physical and chemical processes, as well as biological processes, and its extent is related to the concentration, charge, and pH value of the components.
2. Ceramic membrane cleaning technology
The development of ceramic membrane cleaning technology mainly includes physical cleaning, chemical cleaning, and biological cleaning technologies.
Physical cleaning technology mainly includes backwashing, low-pressure high-flow rate cleaning, negative pressure cleaning, flushing, soaking, mechanical scraping, etc. In addition, there are newly developed technologies such as electric cleaning and ultrasound cleaning. Among them, electric cleaning uses an electric field to charge the membrane, while ultrasound cleaning combines ultrasound technology with other cleaning technologies, which can enhance the cleaning effect and clean the dead corners and gaps of the contaminated ceramic membrane.
Chemical cleaning technology can remove pollutants that are difficult to remove by physical cleaning. Its principle is to use cleaning agents to react with pollutants. Commonly used cleaning agents can be selected according to the properties of the ceramic membrane, the characteristics of the feed solution, and the degree of pollution, such as acid-base solutions, chelating agents, oxidants, and surfactants.
Biological cleaning technology uses biological preparations such as enzymes to clean protein and other contaminants during the ceramic membrane filtration of food feed solutions, which has practical application value.
2.3 Cleaning Techniques for Ceramic Membranes in Wastewater Treatment
Compared to the application of organic membranes in wastewater treatment, ceramic membranes are mainly used in small volume wastewater with high oil content, alkalinity, or acidity. This type of wastewater is easily damaged by organic membranes. Ceramic membrane technology has been used for treating oily wastewater and has produced certain economic and social benefits.
Specific research includes:
① Chemical cleaning techniques for ceramic membranes used in oily wastewater treatment. The optimal cleaning combination was determined to be 1% C18H29O3SNa cleaning for 5 minutes, 1.5% NaOH cleaning for 15 minutes, and 2.0% HNO3 cleaning for 10 minutes.
② For cleaning ceramic ultrafiltration membranes contaminated with wastewater from a cold-rolling mill, the research concluded that using a 2% to 2.5% anionic surfactant, acid cleaning solution with a pH of 3, and alkaline cleaning solution with a pH of 11 can achieve ideal results.
③ In the application of ceramic membrane bioreactors for treating beer wastewater, the physical and chemical cleaning method of combining high-pressure water flushing, soaking in a 0.2% sodium hypochlorite solution for 4 hours, and soaking in a 0.1% nitric acid solution for 4 hours was found to be effective.
3 Summary
Ceramic membranes have gradually become integrated into industrial construction and daily applications due to their unique advantages in separating and concentrating complex liquid components. Ceramic membrane technology has become one of the fastest-growing and most promising technologies in China's high-performance membrane material field and has extensive applications.
However, ceramic membranes are not perfect, and membrane fouling during their application has always been one of the bottleneck factors affecting their use. Ceramic membrane fouling directly causes a decrease in membrane flux, leading to increased operating costs and shortened membrane lifespan. Typically, a ceramic membrane pretreatment system combined with membrane cleaning is used to ensure the normal operation of the membrane system and extend the life of the membrane. Since ceramic membrane fouling is inevitable, ceramic membrane cleaning technology provides strong support for the widespread development of ceramic membrane applications.
1 Ceramic Membrane Fouling
The mechanism of ceramic membrane fouling is similar to that of organic membrane fouling, both of which involve the adsorption or deposition of certain components in the feed solution on the membrane surface or in the membrane pores during the membrane separation process, resulting in a decrease in the permeate flux of the feed solution. This includes the blockage of membrane pores by large molecules, increased filtration resistance caused by solute adsorption on the pore walls, and the formation of gel layers on the membrane surface, increasing mass transfer resistance. The adsorption and deposition of components between ceramic membranes and feed solutions, as well as those adsorbed on the surface of ceramic membranes and interacting with other components in the feed solution, are the result of physical and chemical interactions, as well as possible biological effects. The degree of this effect is related to the concentration, charge, and pH of the components.
2 Ceramic Membrane Cleaning Techniques
Ceramic membrane cleaning techniques have developed to include physical cleaning, chemical cleaning, and biological cleaning techniques.
Physical cleaning techniques mainly include backwashing, low-pressure high-flow-rate cleaning, negative pressure cleaning, flushing, soaking, mechanical scraping, and new development techniques such as electro-cleaning and ultrasonic cleaning. Electro-cleaning is the use of an electric field to charge the membrane, and ultrasonic cleaning uses ultrasonic technology combined with other cleaning techniques to enhance cleaning efficiency, reaching the dead corners and gaps of the contaminated ceramic membrane.
Chemical cleaning techniques can remove contaminants that are difficult to remove with physical cleaning. The principle is to use cleaning agents to react with the contaminants. Commonly used cleaning agents for ceramic membranes based on the properties of the ceramic membrane, feed solution characteristics, and degree of contamination include acid-base solutions, chelating agents, oxidants, and surfactants.
Biological cleaning techniques use biological enzymes and other biological preparations to clean protein and other contaminants, which have practical value in the process of ceramic membrane filtration of food feed solutions.
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