Reasons for manganese contamination of reverse osmosis system and disposal methods

I. Apparent phenomenon and harmfulness of manganese contamination

In recent years, in the process of dealing with faults, encountered a reclaimed water plant reverse osmosis membrane system appeared serious manganese contamination, and cause reverse osmosis system performance decline. Mainly manifested as a decline in desalination rate, reduced water production. Through on-site investigation, the main appearance and performance of manganese contamination are characterized as follows:

1. There is a large amount of black material in the security filter, and there are also different degrees of existence in the end face of the membrane element (see Figure 1);
2. Black substances are often present at the same time with a large number of brown sticky substances;
3. The black substance mainly exists in the front end of a section of the membrane group, while the brown substance exists from front to back. After pollutant analysis: black pollutants are mainly manganese compounds, some organic components; brown substances are mainly organic components (should be bacteria-like substances);
4. Manganese pollution exists in the reverse osmosis system, often accompanied by a large number of bacterial growth, serious fouling of the membrane element, resulting in a faster increase in differential pressure of the system, the ability to produce water is greatly reduced, chemical cleaning of water production can only be partially restored, and at the same time, the system desalination rate is greatly reduced.

                      
II. Summary of Manganese Pollution Project Site Investigation
By comparing several manganese contamination phenomena, the following characteristics are found:
1. Part of the raw water sampling and analysis found that the manganese and iron content in the feed water (0.1~0.4ppm) are more than the reverse osmosis feed water requirements (no more than 0.1ppm, no more than 0.05ppm is recommended).
2. Workshop processes are shorter, the process are: UF + RO, and ultrafiltration before the removal of iron and manganese filters or multi-media filters and other pre-treatment processes.
3. The water source are sewage (water for conventional sewage plant biochemical effluent), and the process are selected in the ultrafiltration continuous chlorination.



Three. Manganese pollution generation cause analysis 
Through the investigation of the site and the analysis of raw water, judged to be manganese in the raw water (divalent) beyond the maximum requirements of reverse osmosis system feed water (0.1ppm, it is recommended that no higher than 0.05ppm), due to the reverse osmosis front-end ultrafiltration process has a continuous addition of sodium hypochlorite, which can lead to the oxidation of low-priced manganese ions to high-priced manganese (such as tetravalent) to form insoluble precipitation, but this does not explain why manganese pollutants appear often However, this does not explain why manganese contaminants are often present in large quantities of sticky substances (bacteria). Through consulting related information, we found that some kind of bacteria (iron bacteria) in the biochemical process may be the culprit leading to the simultaneous occurrence of manganese contamination and microbial contamination (see Fig. 2), and some species of iron bacteria (e.g., ferric sheath bacteria) can oxidize Mn2+ on the surface of the pipeline into insoluble Mn4+ under the condition of pH neutrality and precipitate out of the aqueous solution in large quantities, and this is also the cause of the large amount of black color in the reverse osmosis inlet end that we have found. This may also be the source of the large amount of black material we found at the inlet end of the reverse osmosis, while bacteria will also grow during the oxidation process. We made a speculation on the formation of manganese pollution, namely: the low valence manganese ions in the raw water are oxidized into high valence manganese (tetravalent black) precipitation under the joint action of oxidant and bacteria, the precipitated manganese is adsorbed by iron bacteria to form a mixture of manganese and bacteria, and due to the growth of bacterial reproduction adhesion to the pipeline, filters and reverse osmosis membrane elements, the reverse osmosis system feed water in the mixture of bacteria and manganese to reverse osmosis system security filter fouling blockage, and due to the growth of bacteria and manganese to reverse osmosis system security filter fouling blockage, and due to the growth of bacterial propagation. The mixed form of bacteria and manganese in the feed water of the reverse osmosis system causes fouling of the security filter of the reverse osmosis system.



IV. Prevention and treatment program 
At present, serious manganese contamination of reverse osmosis system before and after the chemical cleaning desalination rate is greatly reduced, this is due to manganese's catalytic oxidation of the membrane element caused by oxidation (especially in the case of high chloride ions in the water or low pH), and at the same time, chemical cleaning process of precipitation of manganese dioxide particles may be caused by particles of scratches on the surface of the membrane. Combined the above two points is easy to reverse osmosis system caused by irreversible desalination rate decreased significantly.
In order to prevent the reverse osmosis system from manganese pollution, the following methods can be considered: 1. In the ultrafiltration before the addition of iron and manganese filters or other types of filters to reduce the manganese content of reverse osmosis water to meet the requirements of the water; 2. into the reverse osmosis security filters before the process sections and water tanks are injected with bactericides to inhibit the growth of iron bacteria, so that it can not oxidize the iron and manganese type of low-valent ions; 3. regular check of the security filters before the reverse osmosis pressure difference Regularly check the pressure difference of the security filter before reverse osmosis and the condition of retained substances, and replace the filter element in time; 4. When a large amount of manganese-containing solid substances precipitate on the reverse osmosis membrane, try to consider physical removal, such as the need to use chemical cleaning, please try not to use hydrochloric acid (at present, after the cleaning of some projects to see, hydrochloric acid cleaning can cause the reverse osmosis system desalination rate to decrease rapidly, it is recommended to consider using citric acid with other chemicals, and you can consult with the experienced pharmaceutical manufacturers). (Please try not to use hydrochloric acid.)