Causes and solutions for reverse osmosis membrane fouling

Reasons for and solutions to reverse osmosis membrane fouling

1. The pretreatment system is not suitable for the raw water quality and flow rate, or necessary processes and steps are not equipped in the system.

2. The pretreatment device is malfunctioning, that is, the existing pretreatment equipment in the system has low removal capacity for SDI components, turbidity, and colloids in the raw water, and the pretreatment effect is not ideal.

3. Inappropriate equipment or incorrect material selection of equipment (pumps, piping, etc.) was chosen for the system.

4. The system's chemical injection device malfunctions (acids, flocculants/coagulants, scale inhibitors/dispersants, reducing agents, etc.).

5. The equipment is intermittently operated or no appropriate protection measures are taken after the system is shut down.

6. Unreasonable equipment operation and use by operation and maintenance personnel (recovery rate, water production, concentrate water volume, pressure difference, cleaning, etc.).

7. Long-term accumulation of insoluble precipitates in the membrane system.

8. Significant changes in raw water composition or fundamental changes in water source characteristics.

9. Significant microbial contamination has occurred in the reverse osmosis membrane system.

Manifestations of scaling and membrane fouling by different pollutants

1. Carbonate scale

Manifestation after scaling: Standard permeate water flow rate decreases, or desalination rate decreases.

Reason: Increased concentration polarization on the membrane surface.

2. Iron/Manganese

Pollution manifestation: Standard pressure difference increases (mainly occurs in the membrane elements at the front end of the device), and may also cause a decrease in permeate flux. Manganese and iron usually coexist.

3. Sulfate scale

If deposition occurs, it will first affect the membrane elements at the end of the system with the highest salt concentration, manifested as a significant increase in the two-stage pressure difference. A special cleaning agent is required.

4. Silicon

Particulate silicon: Fouls the membrane element water flow channels, leading to an increase in system pressure difference. 0.4% dichloroamine is effective for severely contaminated dissolved silicon scale.

Colloidal silicon: Similar to particulate silicon.

Dissolved silicon: Forms silicate precipitation, dichloroamine cleaning should be used.

5. Suspended solids/organic matter

Fouling manifestation: Permeate flux decreases, and the first-stage pressure difference increases significantly. If the raw water SDI is greater than 4 or the turbidity is greater than 1, the possibility of organic matter pollution is high.

6. Microorganisms

Fouling manifestation: Standard pressure difference increases or standard permeate flux decreases. Non-oxidizing bactericides plus alkali can be used for cleaning.

7. Iron bacteria

Fouling manifestation: Standard pressure difference increases. EDTA sodium salt plus alkali can be used for cleaning.

The membrane should be cleaned when the following situations occur

1. The standardized equipment's water production has decreased by 10-15%;

2. The operating pressure of the standardized membrane system has increased by 15%;

3. The salt passage rate of the standardized membrane system has increased by 10-15% compared to the initial normal value;

4. The operating pressure difference has increased by 15% compared to the initial operation;

Cleaning methods for reverse osmosis membranes

1. Negative pressure cleaning: Negative pressure cleaning can be said to be through the vacuum suction of the equipment, forming pressure on the side of the reverse osmosis membrane, which can effectively remove pollutants on and inside the membrane surface;

2. Backwashing: Backwashing uses strong gas or liquid to clean the membrane surface, thereby cleaning the pollutants inside the membrane and restoring the membrane to cleanliness. Backwashing is currently one of the more commonly used methods for cleaning reverse osmosis membranes;

3. Chemical cleaning: Chemical cleaning uses chemical cleaning agents to clean the reverse osmosis membrane. Because it is a targeted cleaning with chemicals, it can also effectively clean the membrane. This method is also one of the commonly used cleaning methods.