Renew and Enhance Reverse Osmosis Membranes by Efficient Cleaning
Reduced water production? Reduced desalination rate? Increased pressure difference between feedwater and concentrated water? It could be caused by membrane element fouling! The reverse osmosis (RO) membrane is trapped into "siege" of numerous pollutants including suspended solids, colloids, organics, microorganisms and other impurities in the feedwater during operation. Over time, this can lead to a decline in system performance. To restore its performance and maintain efficient operation, chemical cleaning is essential.
1. How to determine the timing of cleaning?
First, take preventive cleaning. To prevent pollutants from accumulating on the membrane surface, it is recommended to conduct chemical cleaning once every 6 months for long-running RO systems to ensure long-term stable operation of the membrane.
Second, perform standardized calculation for operating data. Chemical cleaning of the RO membrane system should be performed promptly in any of the following conditions occurring after data standardization:
● The water production of the system decreases by more than 15% from the initial value
● Salt passage rate increases by more than 10% from the initial value
● Pressure difference between feedwater and concentrated water increases by more than 15% from the initial value
Note, if the membrane system needs to be cleaned more than once a month, pretreatment shall be improved. If the cleaning is conducted once every 1-3 months, attention should be paid to adjusting and optimizing the operating parameters of the existing system.
2. What are the chemical cleaning methods?
Chemical cleaning refers to the use of chemical agents to dissolve or decompose pollutants on the membrane surface. Common chemical cleaning methods include:
Alkaline cleaning - use alkaline cleaning agents such as sodium hydroxide to remove organic and biological fouling from the membrane surface.
Acid cleaning - use acidic cleaning agents such as citric acid and hydrochloric acid to remove alkaline deposits from the membrane surface.
Chelating agent cleaning - use chelating agents such as ethylenediaminetetraacetic acid (EDTA) to remove metal deposits from the membrane surface.
Surfactant cleaning - use surfactants such as sodium dodecyl sulfate to remove grease and dirt from the membrane surface.
3. Membrane System Cleaning Steps
Step 1: Flush at low pressure
It is best to use RO produced water for flushing, or you can also use pretreated effluent (if the raw water contains special chemicals that can react with the cleaning solution, it can not be used).
Step 2: Prepare the cleaning solution
Prepare the cleaning solution with RO produced water, accurately weigh the chemicals and mix them evenly, and check whether the pH value, temperature (not lower than 25°C), and chemical content of the cleaning solution meet the requirements.
Step 3: Input and circulate the cleaning solution at low pressure and low flow rate
Input the cleaning solution into the RO system at 1/3 of the normal cleaning flow rate and 20-40 psi pressure. Discharge the initial return water to prevent dilution of the cleaning solution. Recycle the cleaning solution in the pipeline for 5-10 minutes. Observe the turbidity and pH value of the return liquid. If it becomes significantly turbid or the pH value changes by more than 0.5, add an appropriate amount of chemicals or re-prepare the cleaning solution and then repeat the above operation.
Step 4: Soak and perform an intermittent cycle
Stop the cleaning pump cycle in case that the cleaning solution flow out of the pressure vessel. You can close the inlet valve, concentrated water return valve, and produced water return valve for the cleaning solution.
Depending on the degree of component contamination, all membrane components are soaked into the cleaning solution for about 1 hour or longer (10-15 hours or overnight). During this period, you can intermittently turn on the recycle pump to maintain a constant cleaning solution temperature (25-30°C).
Step 5: Recycle at high flow
Increase the cleaning solution flow until it reaches 1.5 times the normal one and then recycle for 30-60 minutes. At this time, the pressure should not be too high, limited to that with no or slight product water production. The pressure difference of the membrane elements and the pressure vessel should not exceed the limit value.
Step 6: Flush
First, flush the system with produced water (minimum flushing temperature of 20°C) for about 5 minutes, then with pretreated qualified produced water for 20-30 minutes. To prevent precipitation, set the minimum flushing temperature at 20°C, and completely flush out the cleaning solution without any residue. Turn on the system, and check the cleaning effect, with produced water discharged. If the system is shut down, the components shall be properly preserved according to relevant methods.
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