RO Membrane Maintenance And Cleaning Methods

Part 1: RO Membrane Maintenance Method

1. Storage before installation

   Make sure your ro membrane is vacuum-sealed in the plastic bags and stored in a dry and ventilated environment before installation. The storage temperature should be 20 to 35 ℃. Avoid contact with oxidizing gases or direct sunlight.

   
   

2.  Short-term storage of used RO membrane

   The short-term storage method is suitable for those reverse osmosis systems that have been stopped operation for more than 5 days and less than 30 days. At this time, the reverse osmosis membrane elements are still installed in the pressure vessel of the RO system. The specific steps are as follows:

   Step 1:  Flush the reverse osmosis system with feed water, and pay attention to completely remove the gas from the system.

   Step 2: Run for 1 to 2 hours a day, close relevant valves to prevent gas from entering the system.

3.  Long-term storage of used RO membrane

   It is suitable for reverse osmosis systems where the membrane elements are still installed in the pressure vessel after stopping operation for more than 30 days. The specific steps of the protection operation are as follows:

   Step 1: Cleaning the membrane in the system;

   Step 2: Prepare a sterilization solution mix with ro produce water (1% sodium bisulfite) and keep it in the system. Make sure that the system is completely fulfilled.

   Step 3: If the system temperature is lower than 27°C, the previous operation should be taken with fresh sterilizing liquid per 30 days; if the system temperature is higher than 27°C, the protection liquid (sterilizing liquid) should be replaced per 15 days.

   Step 4: Before the ro system is put into use again, flush the system with low-pressure water for one hour. Before the system returns to normal operation, check and confirm that the product water does not contain any fungicides.

Part 2: RO Membrane disinfection

1. The speed for sterilizing agent removes bacteria.(The condition is to remove 99.9999% spore bacteria at 20℃)

2. The Introduction of RO disinfectant

• Formaldehyde

Dosage: 0.1~1.0%

The agent is considered to be toxic in some countries

For the brand-new membrane, it must be used after 24 hours operation, otherwise it will cause irreversible water flux loss

Can be used as a disinfectant for long-term storage

• Isothiazolinone

Dosage: 15~25mg/L

“Kathon” from Rohm & Hass Company, or “Simicide C-68” from Betz Company

Can be used as a disinfectant for long-term storage

• Sodium hydrogensulfite

Dosage:  500mg/L， Operation 30~60 mins per time

1.0% solution can be used for long-term storage

• Hydrogen peroxide/peracetic acid

Dosage: 0.2% (Total content of the two compounds)

1.0% solution can be used for long-term storage

pH: 3~4 (Higher pH may cause membrane oxidation)

Temperature: 25℃ (maximum)

If there exist iron or transition metals, it may cause some models of ro membrane get damaged.

Recycle 20~30 mins, soak for 2~4 hours and flush

It may take 4 hours of contact time to destroy the biological mucosa

It is an effective and rapid oxidizing fungicide

More effective for destroying biological mucosa

This sterilizing liquid is not suitable for long-term storage

Part 3: RO Membrane Cleaning Method

Cleaning Condition

Consider that the pretreatment cannot completely remove pollutants such as inorganic salt scale, microorganisms, colloidal particles, insoluble organic matter and other pollutants, after operation for a while, the sheet of the ro membrane will be polluted under the action of reverse osmosis concentration, and the performance of ro membrane will be bad. When the following situations occur, the membranes need to be cleaned:

· The standardized product water flow rate is reduced by more than 10% compared to the last cleaning

· Standardized salt penetration rate increased by more than 5%

· The standardized pressure difference between the inlet water and the concentrated water is more than 15% higher than that after the last cleaning

· Before long-term shutdown

· As daily maintenance

7 Typical Pollutants and Cleaning Methods

Type 1: Carbonate sediments

Pollutants: Carbonate of metal ions such as Ca, Mg, Ba, Sr

The cause of pollution:

The scale inhibitor/dispersant addition system is malfunctioning.
The acid pH adjustment system fails.
Ion exchange resin regeneration is not timely.
The recovery rate is too high.

Symptoms:

The decrease in desalination rate and water yield.
The system pressure drop increases.
There are sediments on the concentrated water end of the membrane element.
The weight gain of the membrane element is more than 50%.

Cleaning solutions:

(1) 2% citric acid solution, ammonia solution with pH value of 4.0, temperature of 40℃;

(2) Can also be used pH2~3 of 2.0% hydrochloric acid solution cleaning

Cleaning procedures:
Step 1: The first step: It is best to use reverse osmosis product water to rinse, or use good filtered water (if the raw water contains special chemical substances, if it can react with the cleaning liquid, it cannot be used).

Step 2: Prepare a cleaning solution with ro product water, weigh it accurately and mix it evenly, and check whether the pH value of the cleaning solution and the agent content meet the requirements.

Step 3: Use normal cleaning flow and pressure of about 20 Psi to input cleaning liquid into the reverse osmosis system, and drain the initial return water to prevent the cleaning liquid from being diluted. Let the cleaning fluid circulate in the pipeline for 15 minutes. Observe the turbidity and pH value of the reflux liquid. If it becomes obviously turbid or the pH value changes more than 0.5, re-add chemicals to make the cleaning solution meet the cleaning requirements.

Step 4: Stop the cleaning pump cycle and observe the pollution of the components. All the membrane components are soaked in the cleaning solution for about 1 hour (the longest shall not exceed 2 hours). During this period, the circulating pump can be turned on intermittently to maintain a constant cleaning solution temperature (25-30°C).

Step 5: Increase the flow rate to 1.5 times the normal flow rate for cleaning. At this time, the pressure should not be too high, and the pressure should be limited to the system with no or slight water production. Cycle for 15 minutes.

Step 6: Rinse the system with pretreated qualified product water for 20-30 minutes at a pressure not higher than 20Psi. In order to prevent precipitation, the minimum flushing temperature is 10℃, and the cleaning solution is completely flushed out without residue. Turn on the system to run, check the cleaning effect, and drain the produced water.

Type 2: Sulfate precipitation

Pollutants: Insoluble sulfate of Ca, Mg, Ba, Sr and other metal ions

The cause of pollution:

The scale inhibitor/dispersant addition system is malfunctioning.
The acid pH adjustment system fails.
Ion exchange resin regeneration is not timely.
The recovery rate is too high.

Symptoms:

The decrease in desalination rate and water yield.
The system pressure drop increases.
There are sediments on the concentrated water end of the membrane element.
The weight gain of the membrane element is more than 50%.

Cleaning solutions: 

(1) 0.1% sodium hydroxide, 1.0% SODIUM EDTA, pH value of 12, temperature of 30℃.

(2) Can also be used 0.1% sodium hydroxide, 0.5% EDTA sodium tetrasodium, 0.5% sodium tripolyphosphate cleaning, pH value of 12, temperature of 30℃.

Cleaning procedures:

Step 1: It is best to use ro product water for washing, or use a good filter water (if the raw water contains special chemicals, if it can react with the cleaning liquid, it cannot be used).

Step 2: Prepare the cleaning solution with reverse osmosis product water, accurately weigh and mix evenly, check whether the pH value of the cleaning solution and the content of agents meet the requirements.

Step 3: Use normal cleaning flow and pressure of 20~40PSI to input cleaning fluid into the reverse osmosis system. The initial return water should be drained to prevent the cleaning fluid from being diluted. Let the cleaning solution circulate in the pipeline for 3~5 minutes. Observe the turbidity and pH value of the reflux liquid. If the turbidity is obviously turbid or the pH value changes more than 0.5, re-add the additive agent to make the cleaning solution meet the cleaning requirements.

Step 4: Pump the cleaning fluid into the system for circulating cleaning for 1 hour, then stop the cleaning pump cycle. If sulfate pollution occurs, all membrane modules should be soaked in the cleaning fluid for 24 hours. During this period, the circulating pump can be turned on intermittently to maintain a constant cleaning liquid temperature (25-30°C).

Step 5: After soaking for 24 hours, increase the flow rate to 1.5 times of the normal flow rate for cleaning. At this time, the pressure should not be too high, and it is limited by the pressure of no or slight water production in the system. Cycle for 30 to 60 minutes.

Step 6: Rinse the system with qualified pre-treatment water for 20~30 minutes, with the pressure not higher than 20 psi. In order to prevent precipitation, the minimum flushing temperature is 10℃, and the cleaning fluid is completely flushed out without any residue. Start the system operation, check the cleaning effect, and drain the production water.  When inorganic salt pollution occurs, it can be cleaned according to carbonate pollution first. If the performance of membrane elements does not change much before and after cleaning, it can be judged as sulfate or other inorganic salt pollution. Sulfate pollution belongs to the type of pollution which is difficult to clean. Generally, the performance of membrane elements will not recover obviously after cleaning.

Type 3: Other inorganic salt deposits

Pollutants: Insoluble salts such as Ca, Mg, Ba, Sr, etc., such as fluoride, phosphate, silicate, etc

The cause of pollution:

The scale inhibitor/dispersant addition system is malfunctioning.
The acid pH adjustment system fails.
Ion exchange resin regeneration is not timely.
The recovery rate is too high.

Symptoms: Refer to symptoms of Sulfate pollution

Cleaning solutions: Refer to cleaning solutions of Sulfate pollution

Cleaning procedures: Refer to cleaning procedures of Sulfate pollution

Notes: The cleaning effect of some inorganic salt pollution is not obvious, such as silicate, fluoride, fluorosilicate, etc., For influent water with high content of these substances, corresponding equipment must be designed in the pretreatment of the systemsuch as softening, adding scale inhibitor, etc., to prevent the membrane elements from being polluted.

Type 4: Metal oxide/hydroxide pollution

Pollutants: Oxides or hydroxides of Fe, Mn, Cu, Ni, Zn, etc.

The cause of pollution:

Corrosives in equipment pipelines and containers.

Oxidized metal ions, chlorine, ozone, potassium, permanganate in the air.

Iron or aluminum coagulant aids are used in the pretreatment filtration system.

Symptoms:

The decrease in desalination rate and water yield.
The system pressure drop increases.
There are red sediments at the water inlet and concentrated water end of the membrane element.
The weight gain of the membrane element is more than 30%.

Cleaning solutions: 

(1)  3% sodium bisulfite, use ammonia water to adjust pH value 4.0, temperature at 40℃.

(2) It can also be cleaned with 2.0% hydrochloric acid aqueous solution with pH 2~3.

Cleaning procedures: Refer to cleaning procedures of carbonate pollution

Type 5: Colloid pollution

Pollutants: Iron, aluminum, silicon, sulfur or organic matter.

The cause of pollution:

The pretreatment system failed to remove 100% of the colloids .

Symptoms:

No significant change in desalination rate.

Water flowrate dropped significantly.

There are viscous pollutants at the water inlet port.

The weight gain of the membrane element is more than 30%.

Cleaning solutions: 

(1) Adjust pH to 10.0 with sulfuric acid, 2% sodium tripolyphosphate solution, temperature 40℃.

(2) Sometimes it can also be cleaned with NaOH aqueous solution with pH less than 10 at a temperature of 40℃

Cleaning procedures: Refer to cleaning procedures of Sulfate pollution

Type 6: Colloid pollution

Pollutants: Humus, organic acid, etc.

The cause of pollution:

The decomposition of nutrients in surface water, deep well water, waste water and seawater.

Symptoms:

No significant change in desalination rate.

Water flowrate dropped significantly.

There are viscous pollutants at the water inlet port.

The weight gain of the membrane element is more than 20%.

Cleaning solutions: 

pH 10.0, 2% sodium tripolyphosphate solution, 0.8% tetrasodium EDTA (replaced with 0.25% sodium dodecylbenzene sulfonate in severe cases), temperature 40℃.

Cleaning procedures: Refer to cleaning procedures of Sulfate pollution.

Type 7: Oxidizing substance oxide membrane element

Pollutants: Sodium hypochlorite, residual chlorine, ozone, potassium permanganate, etc.

The cause of pollution:

The fungicide in the inlet water were not removed during the pretreatment.

Symptoms:

The desalination rate does not drop significantly.

Production water has increased significantly

No significant weight gain.

Cleaning solutions: 

None. The only solution is to replace the membrane.

Cleaning procedures: 

None. The only solution is to replace the membrane.

Notes: Oxidizing substance oxide situation is the most serious pollution phenomenon of reverse osmosis membranes. Because the oxidizing substances will directly damage the desalination layer on the surface of the ro membrane, making the amide bond fracture and the semi-permeable membrane failure. At the same time, the oxidation of the membrane element is an irreversible reaction. Once it occurs, it cannot be cleaned or its performance restored, and only a new membrane element can be replaced. Therefore, activated carbon must be used in the pretreatment to adsorb or add a reducing agent to remove various oxidizing substances in the water.