Cleaning Methods for Microfiltration Membranes

There are four categories of cleaning methods for removing deposits from fouled membranes: physical cleaning, chemical cleaning, physical-chemical cleaning, and electro-cleaning. This section primarily introduces physical and chemical cleaning.

Physical Cleaning

Physical cleaning uses mechanical methods to remove contaminants from the membrane surface. This method is characterized by not introducing new contaminants and having a simple cleaning process. However, it is only effective for membranes in the early stages of fouling, and the cleaning effect is not long-lasting. Physical cleaning includes various methods, such as forward flushing, reverse flushing, permeate backpressure flushing, vibration, degassing and water filling, air jetting, automatic sponge ball cleaning, hydraulic methods, gas-liquid pulse, and circulating washing.

Backwashing

Backwashing refers to the method of removing contaminants from the membrane surface by blowing gas or liquid from the permeate side. It is important to note that backwashing should be performed at a relatively low operating pressure (around 132 kPa) to avoid membrane rupture. Backwashing time generally requires 20-30 minutes.

Static Immersion with Hydraulic Backwash

For membrane modules that experience decreased permeability and difficulty in regeneration due to long-term continuous operation, immersing them in pure water for at least 10 hours after shutdown, followed by hydraulic backwashing, is an effective method to improve flux.

Mechanical Scraping

For tubular modules, soft foam balls or sponge balls (slightly larger in diameter than the inner diameter of the membrane tube) can be used to clean the inner pressure membrane. Hydraulic pressure forces the foam or sponge balls repeatedly across the membrane surface, mechanically removing contaminants. This method can almost completely remove soft scale, but it is not only difficult to remove hard scale but also easily damages the membrane surface. Therefore, this method is particularly suitable for cleaning membrane surfaces fouled primarily by organic colloids.

Chemical Cleaning

Chemical cleaning essentially uses chemical reagents to react with deposits, dirt, corrosion products, and other contaminants that affect flux rate and permeate quality to remove contaminants from the membrane. These chemical reagents include acids, alkalis, chelating agents, oxidizing agents, and formulated products.

Acid and Alkali Solutions Acids are effective in removing calcium-based scale such as calcium carbonate and calcium phosphate, iron oxide, and metal sulfides. Alkaline cleaning solutions include phosphates, carbonates, and hydroxides. These solutions loosen, emulsify, and disperse precipitates. Surfactants are often added to enhance the descaling properties of alkaline cleaners in order to remove wet oils, greases, dirt, and biological material. When removing particularly difficult-to-remove deposits such as silicates, alkaline and acidic cleaners are used alternately.

Chelating Agents

Besides strong acids and alkalis, chelating agents are also used to remove deposits from fouling membranes. Commonly used chelating agents include ethylenediaminetetraacetic acid (EDTA), phosphoric acid, gluconic acid, and citric acid. Gluconic acid is generally effective in chelating iron ions in strong alkaline solutions, while EDTA is commonly used to dissolve alkaline earth metal sulfates.

Oxidizing Agents When NaOH or surfactants are ineffective, chlorine can be used for cleaning, with an optimal pH of 10-11.

Cleaning Examples

Cleaning microfiltration membranes is a major concern for membrane technicians, directly impacting production efficiency and economic benefits. Simple physical and chemical cleaning sometimes fails to achieve satisfactory results. In practice, a combined cleaning method is often used. Below is an example of cleaning a fouled membrane in a liquor microfiltration system: First, drain the remaining liquor from the outlet of the liquor filter containing the microfiltration membrane. Backwash with clean, impurity-free water for approximately 10-20 minutes. Then, clean with a 3-5% sodium hydroxide solution for approximately 10-20 minutes. Finally, backwash with clean, impurity-free water for approximately 10-20 minutes. For heavily fouled microfiltration membranes, acid-base circulation rinsing can be used. For older membranes used for extended periods, a longer soaking cleaning time can be employed. From an efficiency perspective, the optimal time for replacing the membrane should be carefully considered.

Microfiltration Membrane Maintenance

Cleaning Solution Requirements

The concentration of the cleaning agent must be appropriate to avoid chemical damage and corrosion to the microfiltration membrane. The cleaning water must be clean water free of impurities. Otherwise, impurities in the water will contaminate the microfiltration membrane and make it difficult to clean.

Microfiltration Membrane Shutdown and Storage

When the microfiltration device is not in operation, it must be thoroughly cleaned and then sealed for storage. For short-term shutdown, microfiltration devices processing spirits can be preserved by soaking in high-proof spirits. For long-term shutdown, the microfiltration membrane should be removed, dried, and sealed for storage. When restarting, it should be cleaned according to the membrane cleaning method before being put into use.

Conclusion

Membrane fouling is an unavoidable problem in microfiltration technology. Factors affecting membrane fouling are not only related to the characteristics of the membrane itself, but also to the membrane module structure and operating conditions. Therefore, a comprehensive consideration is necessary for specific applications.

Many factors need to be considered when effectively preventing and controlling microfiltration membrane fouling. Currently, optimizing membrane operating conditions and improving flow patterns on the membrane surface are the main means of preventing membrane fouling and concentration polarization. Although many new methods have been proposed, very few have been truly applied in practice. It is still necessary to continuously explore the exact fouling mechanisms and find control methods suitable for different systems to optimize membrane performance and extend membrane lifespan. Clearly, much work remains to be done in this area.

Due to the diverse nature of contaminants, cleaning microfiltration membranes is a complex issue. In short, selecting the most economical and effective cleaning agents and solutions is crucial. Furthermore, the maintenance of microfiltration membranes must be tailored to different applications.