Introduction to ultrafiltration membrane cleaning

Ultrafiltration membranes are the core components of ultrafiltration equipment and among the earliest developed polymer separation membranes. They are used in mid-range treatment devices in ultrapure water preparation. Because ultrafiltration membranes are porous materials that remove impurities from water by physical interception, they must be cleaned regularly to ensure membrane throughput and extend their service life.

The cleaning method for ultrafiltration membranes is generally determined based on the membrane properties and the nature of the treated liquid. Similar to reverse osmosis, it usually involves initial hydraulic cleaning, followed by chemical cleaning with different detergents depending on the situation. For example, ionic solubilizers can be used for electrophoretic coatings, and "bridge-bond" type solvents can be used for water-soluble organic coatings. In the food industry, protein precipitation can be achieved using protease solvents or alkaline detergents based on phosphates and silicates. Precipitates formed by inorganic salts on the membrane surface can be dissolved using chelating agents such as EDTA, or acids and bases. Ultrafiltration membrane cleaning is a seven-step process:

1. Prepare the cleaning solution

2. Low-flow input of cleaning solution

First, mix the cleaning solution using a cleaning water pump. When preheating the cleaning solution, use a low flow rate. Then, replace the original water in the component with the cleaning solution at the lowest possible pressure. The pressure only needs to be sufficient to compensate for the pressure loss from feed water to concentrate water; that is, the pressure must be low enough to avoid significant permeate production. Low-pressure replacement minimizes the redeposition of fouling on the membrane surface. Depending on the situation, some concentrate water should be discharged to prevent dilution of the cleaning solution.

3. Circulation

After the original water is replaced, the cleaning solution should appear in the concentrate water line. Circulate the cleaning solution back to the cleaning water tank and maintain a constant cleaning solution temperature.

4. Soaking

Stop the cleaning pump and allow the membrane components to fully soak in the cleaning solution. Sometimes, soaking for about 1 hour is sufficient, but for stubborn contaminants, the soaking time needs to be extended, such as soaking for 10-15 hours or overnight. To maintain the temperature during soaking, a very low circulation flow rate can be used.

5. High-flow pump circulation

High flow rates flush away contaminants removed by the cleaning solution. If the contamination is severe, using a flow rate 50% higher than that specified in Table 1 will help with cleaning. Under high flow conditions, excessive pressure drop may occur. The maximum allowable pressure drop for a single unit is 1 bar (15 psi), and the maximum allowable pressure drop for a multi-unit pressure vessel is 3.5 bar (50 psi). Do not exceed these limits.

6. Rinsing

Qualified pre-treated water can be used to rinse the cleaning solution from the system, unless corrosion issues exist (e.g., stagnant seawater will corrode stainless steel pipes). To prevent precipitation, the minimum rinsing temperature is 20°C. Note: During acid washing, the pH change of the cleaning solution should be checked regularly. When the acid is consumed in dissolving inorganic salt precipitates, if the pH increase exceeds 0.5 pH units, acid should be added to the cleaning tank. The total circulation time of the acidic cleaning solution should not exceed 20 minutes. After this time, the cleaning solution may become saturated with the cleaned inorganic salts, and contaminants will redeposit on the membrane surface. At this point, the first cleaning solution in the membrane system and cleaning system should be discharged using qualified pre-treated water, and the cleaning solution should be re-prepared for a second acid cleaning operation. If the system must be shut down for more than 24 hours, the components should be stored in a 1% (w/w) sodium bisulfite aqueous solution. Before cleaning a large system, it is recommended to remove a membrane component from the system to be cleaned and conduct a single-unit cleaning effectiveness test.

7. Cleaning multi-stage systems

In the rinsing and soaking steps of multi-stage systems, all stages of the entire system can be performed simultaneously. However, high-flow circulation must be performed section by section to ensure that the circulation flow rate is not too low for the first section and not too high for the last section. This can be achieved by using one pump to clean each section separately or by setting different cleaning pumps for each section's flow rate requirements.

Regular cleaning of ultrafiltration membranes maintains membrane permeability and lifespan, while proper cleaning methods save time and reduce membrane component damage during cleaning. Different cleaning methods can be used for different membrane modules. For example, tubular modules can be mechanically cleaned using sponge balls, and hollow fiber modules can be cleaned using backwashing. Membranes used in the food industry also require sterilization (using NaOH and H2O2, etc.).