Causes and Solutions of Reverse osmosis equipment Fouling

During the operation of the reverse osmosis equipment, the surface of the reverse osmosis membrane will be deposited due to the presence of mud, colloid, organic matter, microorganisms and other pollutants in the raw water and the concentration of insoluble substances during the membrane separation process, which will form the fouling on the reverse osmosis membrane.

Although the reverse osmosis equipment commonly be equipped with complete pretreatment equipment, it cannot completely avoid the pollution of the membrane during ooperation, so it is necessary to periodically remove the pollutants on the membrane system which we called CIP (Cleaning In Place).

After the reverse osmosis membrane is polluted, there will be a decline in membrane performance such as the decrease in permeate water capacity. However, since reverse osmosis equipment may be affected by other factors (changes in pressure, temperature, etc.), the phenomenon of membrane fouling may can not be found.

At present, most aromatic polyamide reverse osmosis composite membranes on the market have considerable stability and certain temperature resistance in a wide pH range, so users can clean the reverse osmosis equipment effectively. Years of engineering practice have shown that if the reverse osmosis equipment be fouled but not be cleaned timely, it is very difficult to completely remove the pollutants that have adhered to the surface of the membrane after a long time.

Performance of various pollutants scale

1) Carbonate scale

Performance after scaling: the standard permeate water flow rate decreases, or the desalination rate decreases.Reason: increased concentration polarization on the membrane surface

2) Iron/manganese

Performance after pollution: the standard pressure difference increases (mainly occurs in the membrane element at the front end of the device), which may also cause a decrease in water permeability. Usually manganese and iron are present together.

3) Sulfate scale

If deposition occurs, it will first affect the last membrane element of the system with the highest salt concentration, and the pressure difference in the second stage will increase significantly. A special cleaning agent is required.

4) Silicon

Granular silicon: Fouling blocks the water flow channel of the membrane element, resulting in an increase in the system pressure difference. The use of 0.4% dichloramine is effective for dissolving heavily polluted silica scale.

5) Suspended matter/organic matter

Pollution plugging performance: the water permeability decreases, and the pressure difference in a section increases significantly. If the SDI of the feed water is greater than 4 or the turbidity is greater than 1, the possibility of organic pollution is high.

6) Microorganisms

Fouling performance: the standard pressure difference increases or the standard water permeability decreases. It can be cleaned with non-oxidizing bactericide and alkali.

7) Iron bacteria

Fouling performance: the standard differential pressure rises. It can be cleaned with EDTA sodium salt and alkali.

Chemical Cleaning Process (CIP)

1) First use reverse osmosis product water (preferably use reverse osmosis product water, you can also use demineralized water or filtered water that meets the reverse osmosis inlet water standard) to flush the reverse osmosis membrane components and system pipes,

2) The reverse osmosis product water should be at least qualified demineralized water to prepare the cleaning solution, and ensure that the mixture is uniform; before cleaning, it is necessary to repeatedly confirm whether the pH value and temperature of the cleaning solution are appropriate.

3) First, use 1/2 of the normal cleaning flow rate and 40~60PSI operating pressure to inject cleaning solution into the reverse osmosis equipment, and remove the water retained in the membrane container. And drain the part of the cleaning solution that has just started to circulate to prevent the cleaning solution from being diluted.

In normal cleaning, the pressure control criterion of the cleaning system is to use the pressure that almost makes the system unable to produce pure water (that is, the supply pressure of the cleaning system is equal to the pressure difference between the raw water and the concentrated water). Because the proper cleaning operating pressure can minimize the possibility of re-accumulation of foreign substances on the surface of the reverse osmosis membrane.

4) When cleaning, first drain the water previously stored in the pressure vessel. Then circulate the concentrated water and produced water generated during the cleaning process to the cleaning tank, and keep the temperature of the cleaning solution stable. Before starting cyclic cleaning, first confirm whether the temperature and pH of the cleaning solution meet the standards. And confirm the intuitive conditions such as the turbidity of the backflow cleaning solution: if the backflow cleaning solution has changed color or turbidity, the cleaning solution should be prepared again; if the pH change of the backflow cleaning solution exceeds 0.5, it is best to readjust the pH value or Replace the washer fluid.

5) When performing chemical cleaning on the system, the general operation method is: first, use low flow (1/2 standard cleaning flow) to clean the pressure vessel to be cleaned in a circular manner for 5 to 15 minutes, and then use medium flow (2/3 standard cleaning flow) Cleaning flow) cycle cleaning for 10 to 15 minutes.

6) Then stop the pump and close the valve, so that the membrane element is soaked in the cleaning solution, and the soaking time is about 1 hour. If the membrane fouling is serious or the pollutants are difficult to remove, the soaking time of this process can be extended appropriately. In order to ensure the temperature of the cleaning solution during long-term immersion, a combination of repeated circulation and immersion can also be used. Generally speaking, the temperature of the cleaning solution should be kept at least above 20°C and below 40°C. A suitable temperature of the cleaning solution can enhance the cleaning effect; please note: if the temperature of the cleaning solution is too low, drug precipitation may occur during the cleaning process. When the temperature of the cleaning solution is too low, the cleaning should be arranged after the temperature of the cleaning solution is raised to a more suitable temperature.

7) During normal cleaning, after the immersion of the cleaning solution is completed, the cleaning can be completed by circulating the cleaning flow again for 20-60 minutes at the standard cleaning flow rate. Then use the same volume of reverse osmosis product water to flush the reverse osmosis membrane module, and discharge the flushing water into the sewer. After confirming that the flushing is clean, the reverse osmosis equipment can be re-run. We suggest that at least the product water produced within 15 minutes after re-running the system after chemical cleaning should be discharged. And after the results of careful chemical analysis of the on-site system product water quality are confirmed, the system product water obtained from the system operation should be drained. The effluent enters the product water tank. In addition, when using multiple chemicals for cleaning, in order to prevent chemical reactions between chemicals, it is best to drain the water discharged from the product water side before each cleaning.

8) If want to prevent the re-contamination of microorganisms, after cleaning the system can use the bactericidal solution allowed by the membrane manufacturer to sterilize and clean the membrane system, and the operation method is the same as before.

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