Fouling and Cleaning Methods of Reverse Osmosis Membranes — Common Contaminants and Their Removal Methods

Common Contaminants and Their Removal Methods:

1.1 Calcium Carbonate Scale

When the scale inhibitor addition system malfunctions or the acid addition system causes 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. If calcium carbonate scale is detected early, it can be removed by lowering the feedwater pH to between 3.0 and 5.0 and running the system for 1-2 hours. For calcium carbonate scale with a longer settling time, RT-818A 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 should not exceed 11.0. Use ammonia to raise the pH and sulfuric acid or hydrochloric acid to lower the pH.

1.2 Calcium Sulfate Scale

RT-818B cleaning agent is the best method for removing calcium sulfate scale from the surface of reverse osmosis membranes.

1.3 Metal Oxide Scale

Deposited hydroxides (e.g., ferric hydroxide) can be easily removed using the methods described above for removing calcium carbonate scale.

1.4 Silica Scale

Silica scale that is not symbiotic with metal compounds or organic matter generally requires specialized cleaning methods for removal. Please contact the company for detailed methods.

1.5 Organic Deposits

Organic deposits (e.g., microbial slime or mold) can be removed using RT-818C cleaning agent. To prevent regrowth, a bactericidal solution approved by Hydranautics 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 three days, disinfection is recommended. Please consult with the company to determine the appropriate bactericidal agent.

1.6 Cleaning Fluid

RT-818 series RO membrane system cleaning agents are recommended for cleaning reverse osmosis membrane elements. Pre-cleaning chemical analysis of contaminants is crucial. Detailed analysis and comparison of the results ensure the selection of the optimal cleaning agent and method. The cleaning method and results achieved for each cleaning session should be recorded to provide a basis for identifying the best cleaning method under specific feedwater conditions.

For inorganic contaminants, RT-818A is recommended. For calcium sulfate and organic contamination, RT-818B is recommended. For severe organic contamination, RT-818C is recommended. All cleaning can be performed at a maximum temperature below 40°C for 60 minutes. The required dosage is calculated per 100 gallons (379 liters). 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.

During cleaning, circulate the cleaning solution 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, 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 preset time. For 8-inch or 8.5-inch pressure vessels, the flow rate is 35–40 gallons/minute (133–151 liters/minute); for 6-inch pressure vessels, the flow rate is 15–20 gallons/minute (57–76 liters/minute); and for 4-inch pressure vessels, the flow rate is 9–10 gallons/minute (34–38 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 appropriate water source) into the pressure vessel and discharge for several minutes.

6. After flushing the reverse osmosis system, run the system with the product water discharge valve open until the product water is clean, foam-free, or free of cleaning agent (usually 15-30 minutes).

Table 1. Characteristics and Treatment Methods of Reverse Osmosis Membrane Fouling (Note: The cause of fouling must be identified and the source eliminated. Contact the manufacturer for assistance.)