Fouling and Cleaning Methods of Reverse Osmosis Membranes

Common Contaminants and Their Removal Methods:

Calcium Carbonate Scale: When the scale inhibitor addition system or acid addition system malfunctions, leading to an increase in feedwater pH, calcium carbonate may deposit. Early detection of calcium carbonate scale precipitation is crucial to prevent damage to the membrane surface from the growing crystals. Early detection can remove calcium carbonate scale by running the system with the feedwater pH lowered to 3.0–5.0 for 1–2 hours. For calcium carbonate scale with a longer settling time, citric acid cleaning solution should be used for circulating cleaning or overnight soaking.

Note: Ensure that the pH of any cleaning solution does not fall below 2.0, otherwise it may damage the RO membrane element, especially at higher temperatures. The maximum pH during alkaline cleaning should not exceed 11.0 (Dow membranes can reach 12 for a short time). Ammonia can be used to increase the pH, and sulfuric acid or hydrochloric acid can be used to decrease the pH.

Calcium Sulfate Scale: Cleaning solution 2 (see Table 2) is the best method for removing calcium sulfate scale from the RO membrane surface.

Metal Oxide Scale: Deposits of hydroxides (e.g., ferric hydroxide) can be easily removed using the methods described above for removing calcium carbonate scale.

Silica Scale: Silica scale that is not associated with metal oxides or organic matter generally requires specialized cleaning methods for removal. Please contact our company for detailed information.

Organic Deposits: Organic deposits (e.g., microbial slime or mold) can be removed using cleaning solution 3. To prevent regrowth, a bactericidal solution approved by our company can be circulated and used for soaking in the system. A longer soaking time is generally required for effectiveness. If the reverse osmosis unit is shut down for more than three days, disinfection is recommended. Please consult with our company to determine the appropriate bactericidal agent.

Reverse Osmosis Membrane Fouling and Cleaning Methods:

Cleaning Solution: The cleaning solutions listed in Table 2 are recommended for cleaning reverse osmosis membrane elements. Chemical analysis of contaminants is essential before determining the cleaning solution. Detailed comparison of the analysis results will help select the optimal cleaning agent and method. The cleaning method and results obtained for each cleaning should be recorded to provide a basis for identifying the optimal cleaning method under specific feed water conditions. For inorganic contaminants, cleaning solution 1 is recommended. For calcium sulfate and organic matter, cleaning solution 2 is recommended. For severe organic contamination, cleaning solution 3 is recommended. All cleaning solutions can be used at a maximum temperature of 104°F (40°C) for 60 minutes. The required dosage is calculated per 100 gallons (379 liters) as provided in Table 2. When preparing the cleaning solution, add the chemicals and cleaning water proportionally. Use reverse osmosis product water free of free chlorine to prepare the solution and mix thoroughly.

For further information or services, please contact the relevant technical service department.

Chemical Cleaning and Water Flushing of Reverse Osmosis Membrane Elements

During cleaning, the cleaning solution is circulated at low pressure and high flow rate on the high-pressure side of the membrane. The membrane element remains inside the pressure vessel, and this work requires a specialized cleaning device.

General steps for cleaning reverse osmosis membrane elements:

1. Pump clean, free chlorine-free reverse osmosis product water from the cleaning tank (or appropriate water source) into the pressure vessel and discharge for several minutes.

2. Prepare the cleaning solution in the cleaning tank using clean product water.

3. Circulate the cleaning solution in the pressure vessel for 1 hour or a pre-set time. For 8-inch or 8.5-inch pressure vessels, the flow rate should be 35 to 40 gallons/minute (133 to 151 liters/minute); for 6-inch pressure vessels, the flow rate should be 15 to 20 gallons/minute (57 to 76 liters/minute); and for 4-inch pressure vessels, the flow rate should be 9 to 10 gallons/minute (34 to 38 liters/minute).

4. After cleaning, drain the cleaning tank and rinse it. Then fill the cleaning tank with clean product water for the next rinsing step.

5. Pump clean, chlorine-free product water from the cleaning tank (or an appropriate water source) into the pressure vessel and discharge it for several minutes.

6. After rinsing the reverse osmosis system, run the reverse osmosis system with the product water drain valve open until the product water is clean and foam-free.