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 components, turbidity, and colloids in the raw water, and the pretreatment effect is not ideal. SDI The pretreatment effect is not ideal.

3 Inappropriate equipment or incorrect material selection (pumps, piping, etc.).

4 Failure of the chemical injection device (acid, flocculant / coagulant, / scale inhibitor,

5 dispersant, reducing agent, etc.).

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 of the reverse osmosis membrane system.

Manifestations of scaling and fouling by different pollutants

1 Carbonate scale

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

Reason: Increased concentration polarization on the membrane surface.

2 Iron / Manganese

Manifestation after pollution: 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. Using 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 feed water SDI is greater than 4 or 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. Can be used EDTA sodium salt plus alkali for cleaning.

The membrane should be cleaned when the following situations occur:

1 The standardized equipment 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 compared to the initial normal value 10~15% ；

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

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. 1；

3 Chemical cleaning: Chemical cleaning uses chemical cleaning agents to clean the reverse osmosis membrane. Because it is a targeted chemical cleaning, it can also effectively clean the membrane. This method is also a commonly used cleaning method 1。