The causes and solutions for clogging of reverse osmosis membrane elements

1. 3 Warning Signs of Reverse Osmosis Membrane Blockage.

• The production volume has decreased by more than 10% compared to the initial value, and the pressure has significantly increased.
• The desalination rate decreased by 5%, and the total dissolved solids (TDS) value of the produced water rose sharply.
• When the pressure difference of the membrane module exceeds 0.07 MPa, the operating noise becomes louder.

2. 3 Common Types of Pollution.

• Inorganic pollution (the most common)
• Cause: Calcium and magnesium ions, carbonates, sulfates, iron and manganese ions, etc. present in the raw water deposit on the membrane surface as scale (such as calcium carbonate and barium sulfate), especially when the recovery rate is too high and the water temperature varies greatly, this phenomenon is prone to occur.
• Typical symptoms: The pressure difference of the membrane module rises rapidly, the production water volume decreases slowly, and the effect is obvious after cleaning with hydrochloric acid.
• High-risk scenarios: Groundwater treatment, high-hardness raw water systems.
• Organic pollution
• Cause: Organic substances such as humic acid, tannin, oil, and surfactants in the raw water adhere to the membrane surface, forming sticky dirt that clogs the membrane pore channels.
• Typical symptoms: The quality of the effluent water deteriorates (TDS increases), and yellow-brown or black sticky deposits appear on the membrane surface. These can be effectively removed after alkaline washing.
• High-risk scenarios: surface water treatment, municipal wastewater reuse systems.
• Biological contamination
• Cause: Microorganisms such as bacteria, fungi, and algae in the raw water multiply on the membrane surface, forming a biofilm. This not only clogs the membrane pores but also secretes metabolic products that corrode the membrane components.
• Typical symptoms: The membrane module emits an unpleasant odor, the water production volume drops sharply, and there is an excessive amount of microorganisms. In such cases, a disinfectant should be used in conjunction with cleaning.
• High-risk scenarios: Raw water systems without disinfection devices, damp and dim operating environments.

3. 4-step standard cleaning method

• Step 1: Pre-treatment —— Drain the old water and prepare the cleaning solution
• Shut down the reverse osmosis system, open the discharge valve for concentrated water, and completely drain the remaining raw water from the membrane module.
• Prepare to clean the water tank and prepare the cleaning solution in the prescribed ratio (it is recommended to use deionized water or distilled water to avoid introducing new impurities):
• Alkali cleaning solution: 0.1 - 0.5% sodium hydroxide (NaOH) + 0.05 - 0.1% surfactant (such as sodium dodecylbenzenesulfonate), with pH value controlled at 10 - 11;
• Pickling solution: 0.5 - 1% citric acid (or 0.2 - 0.5% hydrochloric acid), with pH value controlled at 2 - 3;

• Step 2: Alkali washing - Soaking + Circulation, to dissolve organic/biological contamination
• Inject the prepared alkaline cleaning solution into the cleaning water tank, and then pump it into the membrane module through a booster pump (with a pressure of ≤ 0.2 MPa) to ensure that the membrane elements are fully submerged.
• Close the discharge valve, perform a 30-60 minute cycle of cleaning. During this period, stop the machine every 10 minutes for a 5-minute soak to allow the cleaning solution to fully penetrate the dirt.
• After the cycle is completed, open the discharge valve and completely empty the alkaline cleaning solution. Rinse the membrane module with clean water 2-3 times until the effluent has no foam or odor.

• Step 3: Acid washing - Circulation + Neutralization, to remove inorganic scale
• Inject the prepared acid cleaning solution and perform a cyclic cleaning at a pressure of ≤ 0.2 MPa for 20 to 40 minutes. During this process, there should be a 10-minute soaking interval.
• Focus on cleaning the inlet end of the membrane module (the area with concentrated scale), and you can appropriately extend the circulation time.
• After the pickling process is completed, immediately rinse with clean water until the pH value of the water returns to neutral (6.5 - 7.5). Avoid leaving any acid residue that could corrode the membrane elements.

• Step 4: Post-processing - Pilot run, check cleaning effect
• Start the reverse osmosis system and conduct a low-pressure test run (pressure 0.3 - 0.5 MPa). Observe indicators such as production water volume and desalination rate:
• If the water production volume returns to more than 90% of the initial value and the desalination rate returns to normal, it indicates that the cleaning process has been successful.
• If the effect is not satisfactory, you can repeat the "alkali washing → acid washing" process once. Do not exceed 2 times (excessive washing will damage the membrane pore size).
• After a one-hour trial run, measure the TDS value, flow rate and pressure difference of the produced water, record the data and compare it with the data before cleaning. Then, establish a maintenance file.

Key notes:

• The use of cleaning solutions containing chlorine or oxidants (such as 84 disinfectant) is strictly prohibited throughout the process, as they may oxidize the film elements
• When cleaning, the pressure should not exceed 0.3MPa, and the flow rate should be controlled at 50-70% of the rated flow rate to avoid high-pressure impact on the membrane pores.
• If the biological contamination is severe, it can be soaked in 0.1% hydrogen peroxide (H₂O₂) or a special reverse osmosis membrane biocide for 10 minutes before alkaline washing, and then regular cleaning can be carried out.

4. 3 Tips to Reduce Membrane Blockage

• Strengthen the pre-treatment of raw water: Install pre-filters and activated carbon filters to remove large particles, organic substances and residual chlorine from the raw water.
• Control operating parameters: The recovery rate is recommended to be kept below 75%. Regularly flush the membrane modules (flush for 5 minutes before each startup and for 10 minutes after shutdown).
• Regular maintenance: Clean once every 3-6 months (more severely contaminated areas require cleaning once every 1-2 months). During long-term shutdown, immerse the membrane modules in a 1% sodium bisulfite solution to prevent the growth of microorganisms.

5. In these cases, it is recommended to replace the membrane elements directly!

• If any of the following situations occur, it indicates that the membrane element has suffered irreversible damage and cleaning cannot restore it. It is necessary to replace it promptly:
• After cleaning, the water output still remained below 70% of the initial value, and the desalination rate decreased by more than 10%.
• The membrane components have suffered damage or leakage (detected through pressure testing);
• The membrane elements have a service life exceeding 3 years (the conventional lifespan), but their performance continues to deteriorate.