Reverse osmosis membrane pollution and reverse osmosis membrane cleaning tips
1. Reverse osmosis membrane membrane pollution
After the system runs normally for a period of time, the reverse osmosis membrane elements will be polluted by suspended solids or insoluble salts that may exist in the feed water. The most common of these pollutions are calcium carbonate precipitation, calcium sulfate, barium sulfate, strontium sulfate precipitation, and metal (Iron, manganese, copper, nickel, aluminum, etc.) oxide precipitation, silicon deposits, inorganic or organic deposition mixtures, NOM natural organic substances, synthetic organic substances (such as scale inhibitors/dispersants, cationic polyelectrolytes), microorganisms ( Algae, molds, fungi), etc.
The nature of pollution and the rate of pollution depend on various factors, such as water supply quality and system recovery rate. Usually pollution develops gradually. If it is not controlled as soon as possible, the pollution will damage the membrane elements in a relatively short time. When the membrane element is confirmed to be contaminated, or before a long-term shutdown, or as a regular routine maintenance, it is recommended to clean the membrane element.
When the following problems occur in the reverse osmosis system (or device), chemical cleaning or physical flushing is required:
- Under normal water supply pressure conditions, the water production after temperature correction is 10-15% lower than the normal value;
- In order to maintain the normal water production, the feed water pressure after temperature correction has increased by 10-15%;
- The quality of produced water is reduced by 10-15%, and the salt permeability is increased by 10-15%;
- Water supply pressure increased by 10-15%;
- The pressure difference between the various sections of the system has increased significantly (there may be no instrument to monitor this parameter).
In the case that the operating data is not standardized, if the key parameters are not changed, the above cleaning principles can still be applied. The stable operating parameters mainly refer to the product water flow rate, product water back pressure, recovery rate, temperature and TDS. If these operating parameters fluctuate, it is strongly recommended to standardize the data to determine whether pollution occurs, or to confirm whether the reverse osmosis system is actually operating normally before the key operating parameters change. Regularly monitoring the overall performance of the system is the basic method to confirm whether the membrane element has been contaminated. The impact of pollution on membrane elements is gradual, and the extent of the impact depends on the nature of the pollution. The cleaning cycle of the contaminated reverse osmosis membrane depends on the actual situation on site. The normal cleaning cycle is once every 3-12 months. If it is cleaned more than once within a month, the reverse osmosis pretreatment system needs to be further adjusted and improved, such as additional investment or flushing for reverse osmosis system design.
When the membrane element is only slightly polluted, the membrane element should be cleaned in time. Severe pollution will hinder the penetration of chemicals into the polluted layer and affect the cleaning effect. If the performance of the membrane element is reduced to 30-50% of the normal value, it is impossible to completely restore the original performance of the membrane element when it leaves the factory.
In the design of the reverse osmosis system, reverse osmosis product water can be used to flush the contaminants in the system to reduce the frequency of cleaning. Soaking membrane elements with product water can help dissolve and fall off dirt and reduce the frequency of chemical cleaning.
What kind of pollutants to clean and how to clean should be carried out according to the pollution situation on site. For complex situations where several types of pollution coexist, the cleaning method is to alternately clean with low pH and high pH cleaning solutions.
When the membrane element is contaminated, the performance of the membrane element is often restored by cleaning. There are generally two cleaning methods, physical cleaning (cleaning) and chemical cleaning (chemical cleaning). Physical cleaning (cleaning) does not change the nature of the contaminants, and uses mechanical scouring of the contaminants in the membrane element to restore the performance of the membrane element.
Chemical cleaning is the use of corresponding chemical agents to change the composition or properties of pollutants, and then discharge the membrane element to restore the performance of the membrane element. Particulate pollutants with low adsorption properties can be washed (physical cleaning) to achieve a certain effect. It is difficult to achieve the expected effect by washing the pollutants with strong adsorption to the membrane like biological pollution. When it is difficult to remove contaminants by flushing, stop the device and use chemical cleaning. In order to improve the effect of chemical cleaning, before cleaning, it is necessary to analyze the pollution situation to determine the type of pollution (refer to Chapter 9 for details). On the basis of mastering the type, composition and quantity of pollution, choosing the right cleaning chemicals is the key factor for successful cleaning.
Chemical cleaning and physical cleaning are two cleaning methods that can cooperate with each other. In the face of light pollution, adding some chemicals when using physical cleaning can double the cleaning effect; also when using chemical cleaning for severe pollution, some physical strengthening methods can also be used to enhance the effect of chemical cleaning.
2. Special tips for reverse osmosis membrane cleaning
This content applies to composite polyamide reverse osmosis and nanofiltration membrane elements with diameters of 4, 6, 8, and 8.5 inches.
Polyamide reverse osmosis membrane elements should not be in contact with free chlorine under any circumstances, free chlorine oxidizing membrane elements will cause permanent damage.
Therefore, special attention should be paid to whether the feed water of the membrane system contains free chlorine after the sterilization of pipelines and equipment, the use of cleaning agents and storage protection agents. If you are not sure, you should carry out the corresponding test. If there is free chlorine residue, you can use sodium bisulfite (SBS) to reduce it, and meet the reaction time to ensure the full progress of the redox reaction. When free chlorine=1.0PPm, it is necessary to add sodium bisulfite (SBS) 1.8-3.0ppm.
- During the quality guarantee period of the reverse osmosis membrane element, it is recommended that each cleaning of the membrane element should first get in touch with Hyde Energy, and then proceed after consultation.
- In the cleaning solution, cationic surfactants and amphoteric surfactants should be avoided. The use of these chemicals in contact with the membrane element may cause an irreversible drop in the water production of the membrane element.