Look here, take you to understand the microbial contamination of reverse osmosis membrane and its prevention and control!

2021/02/22

     Reverse osmosis technology is the most mature and widely used membrane separation technology in the field of desalinated water treatment. For more than 30 years, membrane water treatment technology represented by reverse osmosis has developed vigorously in China, and my country has become the membrane water treatment technology. The world's largest market. In various industrial production processes, as long as separation, concentration, classification, and purification are involved, there will be opportunities for membrane technology applications.

     During the application of reverse osmosis membranes, due to the characteristics of reverse osmosis membranes, membrane pollution is prone to occur, including precipitation pollution (mainly some inorganic salt precipitation), adsorption pollution (mainly colloids, protein macromolecules, natural polymer organics, etc.) and Microbial contamination. Among them, the operating difficulties caused by microbial pollution to membrane equipment are the most serious, causing the reverse osmosis pressure difference to rise sharply, severely reducing the water production, and even damaging the equipment.

Microbial contamination of reverse osmosis membrane column

     The microorganisms in the reverse osmosis operation include bacteria, algae, fungi and their spores, spores and viruses. Bacterial particles are very small, generally 1-3 μm, while viruses are even smaller, about 0.01-0.2 μm, but the number is usually large. Microorganisms can be regarded as colloids with negative charges. Most of them can be removed by coagulation and filtration, but complete removal is very difficult and complicated. When the water flux is high or the concentration polarization occurs, these microorganisms multiply rapidly and increase in number by several times, which will produce a sticky substance, which has strong adhesion and is difficult to remove, so that the microorganisms are not subject to the shear force of the water The effect is washed away, forming microbial slime which has a great influence on the operation of the reverse osmosis membrane.

Characteristics of microbial contamination on reverse osmosis membrane

     Qualified feed water SDI does not guarantee the avoidance of microbial contamination. When the feed water SDI<4, or even SDI<3, a large amount of slime-like biofilm may still appear on the surface of the membrane, which depends on the size of the water flux of the bacteria in the water.

     Microbial contamination develops rapidly. Once bacterial communities appear on the membrane, especially in the case of concentration polarization, a mucous membrane will appear within one or two months or even within a few days, resulting in excessively high water supply pressure. In order to maintain the water production of reverse osmosis, the pressure of the feed water pump must be increased to increase power consumption.

     While biofilm (biological slime) causes membrane fouling, it also increases the water permeability of the membrane, reducing the water production and possibly reducing the product water quality.

     Biological slime is insoluble in acid, hardly soluble in alkali, and is hardly affected by the shear force of water flow. Even if it is washed frequently, it cannot be washed away.

     Control and prevention of microorganisms in reverse osmosis membrane

Prediction of microbial slime:

     1. Determine the total number of bacteria (TBC) in reverse osmosis pretreatment water, reverse osmosis water, concentrated water and product water, and calculate the bacterial change value. If the TBC in the concentrated water is found to increase significantly, it indicates that there may be slime formation on the reverse osmosis membrane.

     2. The organic matter in the influent water can not only form membrane pollution by itself, but also serve as nutrients for the growth of bacteria. Therefore, the organic matter can be monitored.

     3. Regularly and meticulously check the cleanliness of the filter element, the water supply pipe, and the inside of the concentrated water pipe before the reverse osmosis. If there is a sticky substance or odor, it is a sign of microbial slime.

Control of microbial contamination:

     Bactericide treatment can be added to the reverse osmosis water inlet pretreatment end, and oxidizing or non-oxidizing bactericide treatment can be used. If an oxidizing bactericide is used for treatment, a reducing agent must be added before the water inlet to prevent oxidation The bactericide enters the reverse osmosis membrane and endangers the operation of the reverse osmosis membrane. Specific dosing methods include continuous dosing and shock dosing.

Continuous dosing: Dosing should be continuous and even when dosing, which can minimize the bacteria and algae in the raw water, and the general bacteria and algae kill rate is above 99%. Dosing method: put the daily dosage of the medicine in the dosing tank, dilute it ten times with water, and do it evenly with the dosing pump.

     Shock dosing: Because there are still a small amount of living bacteria and microorganisms on the surface of the reverse osmosis membrane that are trapped on the surface of the membrane during continuous dosing, these bacteria and algae will multiply and grow within a certain period of time. Therefore, periodic impact dosing must be performed in a timely manner. Remove the bacteria and algae on the surface of the membrane. Shock dosing every week or two weeks to concentrate on killing bacteria and algae in the system. Dosing method: reduce the reverse osmosis pressure below 0.3Mpa, and the reagents used should be added within 2-4 hours.

     The oxidizing bactericides, reducing agents and non-oxidizing bactericides developed by Changlong Technology have fast and efficient bactericidal effects, strong antibacterial ability, a certain peeling effect on the slime formed by bacteria and are compatible with all membranes. Can significantly improve the operating cycle of the reverse osmosis membrane.

Application case-water recycling system of a circuit board factory in Huizhou

Daily treatment water volume: 12,000 cubic meters

     On-site problem: the biomembrane at the end of the reverse osmosis membrane is fouled, and the membrane flux drops by more than 50%.

     Solution: After the system is chemically cleaned in situ, use Boneng's non-oxidizing fungicide for sterilization:

     1. Add water to the cleaning tank and prepare the concentration of the fungicide at 400-800mg/L;

     2. Adjust the pH of the cleaning solution below 7;

     3. Circulate for 45 minutes;

     4. When the system is running normally, add the bactericide continuously at a concentration of 2-5mg/L.

     Products used: non-oxidizing bactericide, acid membrane cleaning agent, alkaline membrane cleaning agent.

     Operation status and effect: After the agent was added, the tracking was continued for one month, the membrane biological pollution was alleviated at the scene, and the water production rate increased by more than 20%.