Common Problems and Analysis of Reverse Osmosis Systems (Part 1)

Sediment and inorganic colloid contamination

I. Phenomena and Symptoms:

1. Sediment particles accumulate at the inlet end of the first few membrane elements in the first stage;

2. The system pressure will be higher, and the system desalination rate will be lower;

3. After the membrane element is dissected, visible pollutants are attached to the membrane surface and the inlet flow channel.

II. Causes:

1. Pretreatment failure or design defects;

2. Insufficient backwashing and fast washing of multi-media filters/activated carbon beds;

3. High SDI value of influent;

4. Hard sand particles mechanically scratch the membrane.

III. Cleaning or Solutions:

1. Difficult to restore by chemical cleaning, acid-base cleaning can be tried;

2. Strengthen the backwashing of the media filter/activated carbon bed;

3. Strengthen SDI value monitoring;

4. Strengthen the monitoring and replacement of security filters.

Pretreatment activated carbon breakage and leakage

I. Phenomena and Symptoms:

1. Black substances accumulate at the inlet end of the first few membrane elements in the first stage

2. The system pressure will be higher, the water production will be less, and the system desalination rate will be lower

3. After the membrane element is dissected, activated carbon particles are attached to the membrane surface and the inlet flow channel

II. Causes

1. Incorrect or insufficient backwashing and fast washing of the activated carbon bed

2. After long-term operation, the activated carbon is broken due to the reaction with chlorine or ozone, resulting in a decrease in mechanical strength.

3. Carbon particles mechanically scratch the membrane

III. Cleaning or Solutions:

1. Difficult to restore by chemical cleaning

2. Replace with new activated carbon; sufficient carbon bed backwashing; strengthen the monitoring and replacement of security filters.

Inorganic scaling - Calcium carbonate scale (CaCO3)

Scaling often occurs in the last stage and then gradually spreads to the previous stage. Raw water containing calcium, bicarbonate, or sulfate may block the membrane system within hours due to scaling.

Acid washing Effect of pH on calcium carbonate scale removal

I. Phenomena and Symptoms:

1. Low system water production, decreased desalination rate, increased pressure drop;

2. Membrane element becomes heavier.

II. Causes:

1. High three factors in RO influent (high hardness; high pH; high alkalinity);

2. High recovery rate of RO system.

III. Cleaning or Solutions:

1. The system can be cleaned and restored with acid;

2. The three high factors in the influent (high hardness; high pH; high alkalinity) and the recovery rate of the RO system should be reduced;

3. Adjust the amount of scale inhibitor added.

Inorganic scaling - Calcium sulfate scale (CaSO4)

I. Phenomena and Symptoms:

1. Low system water production, decreased desalination rate, increased pressure drop;

2. Membrane element becomes heavier.

II. Causes:

1. High calcium, barium, strontium, and sulfate content in RO influent;

2. High recovery rate of RO system leads to exceeding solubility limit;

3. Scale inhibitor failure.

III. Cleaning or Solutions:

1. The system is difficult to clean and restore;

2. The recovery rate of the RO system should be reduced;

3. Adjust the amount of scale inhibitor added or replace the scale inhibitor.

Microbial contamination

After the membrane element is dissected, viscous substances are attached to the membrane surface and the inlet flow channel, accompanied by odor.

Alkaline washing Effect of pH on biofouling removal

I. Phenomena and Symptoms:

1. Low system water production, decreased desalination rate, increased pressure drop;

2. Odor when the pressure vessel is opened or the membrane element is dissected.

II. Causes:

1. The influent is rich in nutrients, such as high TOC and COD;

2. Microorganisms in the influent or scale inhibitor;

3. Cleaning or solutions;

4. Clean and disinfect the entire system, including the pretreatment and membrane body parts. At the same time, it should be noted that if the cleaning and disinfection are not thorough, rapid recontamination will occur;

5. The system can be cleaned and restored with strong alkali, such as cleaning with a pH=12-13 alkaline solution;

6. Install or optimize pretreatment to deal with microbial contamination of raw water;

7. Eliminate the source of microorganisms;

8. Use anti-fouling membrane elements (FR series);

9. Use non-oxidizing bactericides.