Did you know. Damage to membranes and solutions when reverse osmosis equipment is out of service

I. Types of membrane damage caused by incorrect shutdown

1. Physical damage to the membrane element

Water Hammer Impact: The valve is closed too quickly during shutdown, resulting in a sudden drop or rise in pressure in the pipeline, forming a “water hammer effect”, which may cause the membrane element to physically rupture, sealing ring displacement, or diaphragm folds.

Back pressure damage: After shutdown, if the pressure on the concentrated water side is higher than that on the produced water side (e.g. the valve of the produced water pipeline is not opened), it may lead to reverse pressure on the membrane element, resulting in delamination or rupture of the membrane sheet.

2. Membrane contamination and microbial reproduction

Thick water retention: after shutdown without timely flushing of the membrane element, the high concentration of salts in the concentrated water, pollutants (such as calcium and magnesium ions, organic matter) will be deposited on the surface of the membrane, accelerating membrane contamination.

Microbial reproduction: equipment for a long time downtime, if the membrane element is not protected (such as sterilization, charging protective solution), the residual water in the microbial reproduction, the formation of biofilm, difficult to clean and irreversible damage to the membrane structure.

3. Chemical damage

pH value abnormal erosion: shutdown before not adjusting the pH value of the feed water, or flushing fluid water quality does not meet the requirements (such as containing strong oxidants), may lead to chemical degradation of membrane materials (such as polyamide membrane hydrolysis in the extreme pH environment)

Common mistakes shutdown operation

Wrong operation       Direct power off shutdown

Damage Principle        Not according to the process of depressurization, flushing, pumps suddenly shut down resulting in instantaneous fluctuations in pipeline pressure, resulting in water hammer impact.

Wrong operation   Close the inlet valve before stopping the pump.

Damage Principle      When the pump is still running, close the inlet valve, resulting in the membrane element being “pumped out”, and negative pressure is formed on the water side, which may flatten the membrane element.

Wrong operation   without low pressure flushing

Damage Principle        Salt is crystallized and deposited in the membrane when concentrated water is retained; microorganisms multiply rapidly in the high nutrient environment of concentrated water.

Wrong operation   Long-term shutdown without protection

Damage Principle        Membrane element is dry or contaminated by microorganisms, especially in the summer high temperature environment, biofilm breeding speed is accelerated.

Wrong operation   Shut down the machine when the production water valve is closed

Damage Principle       After shutting down the machine, the pressure of the concentrated water side is higher than that of the production water side, which forms back pressure and leads to reverse pressure on the membrane element.

Correct shutdown process and operation points

1. Normal shutdown process (short-term shutdown, <24 hours)

Step 1: Reduce system pressure

Slowly adjust the inlet valve and the thick water valve to reduce the system pressure to 0.2-0.3 MPa, maintain low-pressure operation for 3-5 minutes, so that the membrane element is gradually depressurized to avoid water hammer impact.

Step 2: Low-pressure flushing

Switch to flushing mode (or open the flushing valve), flush the membrane element with pre-treated water (or reverse osmosis water) for 5-10 minutes until the conductivity of the concentrated water is close to that of the feed water.

Purpose: To flush the membrane surface of the residual concentrated water, pollutants, reduce the risk of membrane contamination.

Step 3: Shut down pumps and valves

First stop the high pressure pump, then close the influent valve, the concentrated water valve, and finally open the produced water discharge valve (to prevent back pressure), to keep the produced water pipeline unobstructed.

Step 4: Keep the system moist

Make sure the membrane element is always filled with water after shutdown, avoid drying (drying will cause membrane element shrinkage and debonding).

2. Long-term shutdown protection (shutdown > 24 hours)

Step 1: Thorough Chemical Cleaning

If the membrane element is contaminated, it should be chemically cleaned (acid, alkali, or special cleaning agent depending on the type of contaminant) to remove the deposited impurities.

Step 2: Sterilization

Flush the membrane element with a solution containing 100-200 ppm sodium hypochlorite (or special biocide) to kill microorganisms and prevent biofilm growth.

Note: Polyamide membranes should be protected from strong oxidizing agents (e.g., free chlorine > 0.1 ppm), and non-oxidizing biocides (e.g., isothiazolinone) should be used in strictly controlled concentrations.

Step 3: Seal with Protective Fluid

For shutdowns longer than 7 days, it is recommended to fill the membrane housing with a 1-2% sodium bisulfite solution (or special protective fluid) and close all valves to prevent air from entering.

Operation Points: The protective fluid should completely fill the membrane element to avoid residual air that may cause the membrane to dry or oxidize. In the winter, pay attention to anti-freezing (can add propylene glycol and other antifreeze).

Step 4: Regular Maintenance

Check the pH value of the protective fluid every 15 days (should be maintained at 3-6), if the pH rises, the protective fluid needs to be replaced again; the checking period can be shortened to 7 days in summer when the temperature is high.

Key Notes

1. Avoid back pressure: the pressure on the produced water side should not be higher than that on the concentrated water side at any time, and make sure the produced water valve is open after shutting down the machine (or install a back pressure valve).

2. Control the shutdown environment: keep the ambient temperature at 5-45℃ during the shutdown period, and avoid direct sunlight (high temperature will accelerate the propagation of microorganisms and membrane aging).

3. Check before restarting: Before restarting, flush for 5-10 minutes at low pressure to discharge the protective liquid (if used) and test the quality of produced water to avoid the protective liquid from entering the subsequent system.

4. Train operators: Regularly train the equipment maintenance personnel to be familiar with the correct switching process and emergency measures (e.g. manual pressure relief in case of sudden power failure).

V. Saving measures after membrane damage

If the membrane has been damaged due to improper shutdown, try the following methods:

-Mild contamination: Restore part of the performance through chemical cleaning (e.g., alkaline washing to remove organic matter, acid washing to remove inorganic salts).

-Physical damage (e.g. rupture, delamination): membrane elements need to be replaced and cannot be repaired by cleaning.

-Bio-contamination: use special biofilm cleaning agent (such as enzyme-containing detergent) cycle cleaning, combined with sterilization treatment if necessary.

Summary

Correct shutdown operation is the key to extend the life of reverse osmosis membrane, the core principle is “slow pressure reduction, adequate flushing, prevent back pressure, regular protection”. By standardizing the process and maintenance management, we can effectively avoid damage to the membrane element due to improper operation and reduce the equipment failure rate and replacement costs.