The Reasons for Clogging of Reverse Osmosis Membranes and Solutions

1. The system is equipped with pre-treatment devices that are not suitable for the raw water quality and flow rate, or essential process devices and steps are not included in the system.

2. The pre-treatment devices are not functioning properly, meaning the existing pre-treatment equipment in the system has low removal capabilities for components such as SDI, turbidity, colloids, resulting in unsatisfactory pre-treatment effects.

3. Inappropriate selection of equipment or incorrect choice of equipment materials (pumps, piping, etc.) for the system.

4. Malfunction of chemical dosing equipment (acid, coagulant/ flocculant, scale inhibitor/ dispersant, reducing agent, etc.) in the system.

5. Intermittent operation of equipment or lack of proper protective measures after system shutdown.

6. Improper operation and utilization by operating personnel (recovery rate, product water flow, concentrate flow, pressure differentials, cleaning, etc.).

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

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

9. Considerable microbial contamination in the reverse osmosis membrane system.

Different Pollutants and Scaling Manifestations Blocking Membranes

1. Carbonate Scale

   Manifestation after scaling: Decreased permeate flow rate under standard conditions or reduced desalination rate.

   Reason: Increased concentration polarization on the membrane surface.

2. Iron/Manganese

   Manifestation after contamination: Increased standard pressure differential (mainly occurs in membrane elements at the front end of the unit), which may also lead to decreased permeate flow. Usually, manganese and iron are present simultaneously.

3. Sulfate Scale

   If deposition occurs, it primarily affects the membrane elements at the back of the system with the highest salt concentration, manifesting as a significant increase in two-stage pressure differential. Requires specialized cleaning agents.

4. Silica

   Granular silica: Blocks the water flow channels of membrane elements, leading to increased system pressure differential. Using 0.4% chlorine dioxide is effective against severe silica fouling.

   Colloidal silica: Similar to granular silica.

   Dissolved silica: Forms silicate precipitation, requiring chlorine dioxide cleaning.

5. Suspended Solids/Organic Matter

   Manifestation of fouling: Decreased permeate flow, significant increase in one-stage pressure differential. If feed water SDI is greater than 4 or turbidity exceeds 1, the likelihood of organic matter contamination is high.

6. Microorganisms

   Manifestation of fouling: Increased standard pressure differential or reduced standard permeate flow. Non-oxidizing biocides with alkali can be used for cleaning.

7. Iron Bacteria

   Manifestation of fouling: Increased standard pressure differential. EDTA sodium salt with alkali can be used for cleaning.

When to Clean the Membrane

1. A 10-15% decrease in system product water volume after standardization.

2. A 15% increase in operating pressure after standardization.

3. A 10-15% increase in salt permeation rate compared to the initial normal value after standardization.

4. A 15% increase in operating pressure differential compared to the initial operation.

Cleaning Methods for Reverse Osmosis Membranes

1. Negative Pressure Cleaning: Negative pressure cleaning involves creating pressure on the reverse osmosis membrane side through vacuum suction from the equipment, effectively removing contaminants on the membrane surface and inside.

2. Backwashing: Backwashing involves using strong gas or liquid to clean the membrane surface, effectively removing contaminants inside the membrane, restoring membrane cleanliness. Backwashing is currently a commonly used method for cleaning reverse osmosis membranes.

3. Chemical Cleaning: Chemical cleaning involves using chemical cleaning agents to clean the reverse osmosis membrane. Due to the targeted cleaning action of chemicals, this method can effectively clean the membrane and is commonly used for cleaning.