How to deal with if the RO plant be polluted by Manganese

Appearance and harmfulness of Manganese pollution 

In recent years, in the process of troubleshooting, we have encountered serious manganese pollution in the RO plant of the reclaimed water treatment system, which caused the performance of the reverse osmosis system to decline. The main manifestations are the decrease of desalination rate and permeate water flow.

Through on-site investigation, the main appearance and performance characteristics of manganese pollution are as follows:

1. There are a lot of black substances in the cartridge filter element, and there are also different degrees of existence in the end face of the membrane element.

2. Black substances often coexist with a large amount of brown viscous substances;

3. The black material mainly exists in the front end of the membrane group, while the brown material exists from the front to the back. Analysis of pollutants: black pollutants are mainly manganese compounds and some organic components; brown substances are mainly organic components (should be bacterial substances);

4. The RO plant with manganese pollution is often accompanied by a large number of bacterial growth, which seriously fouls and blocks the membrane elements, resulting in a rapid increase in the system pressure difference and a significant reduction in the permeate water capacity. After chemical cleaning, the permeate water capacity can only be partially restored, while the system desalination rate is still greatly reduced.

Analysis on the causes of Manganese pollution 

Through on-site investigation and analysis of raw water, it is judged that manganese (divalent) in raw water exceeds the maximum requirement of RO plant inlet water (0.1ppm, it is recommended not to be higher than 0.05ppm), due to the continuous injection of reverse osmosis front-end ultrafiltration process The addition of sodium hypochlorite can lead to the oxidation of low-valent manganese ions to high-valent manganese (such as tetravalent) to form insoluble precipitation, but this cannot explain why there are often a lot of sticky substances (bacteria) when manganese pollutants appear. By consulting relevant information, we found that a certain type of bacteria (iron bacteria) existing in the biochemical process may be the culprit that causes manganese contamination and microbial contamination at the same time, and some types of iron bacteria (such as sheathed iron Bacteria) can oxidize Mn2+ on the surface of the pipeline to insoluble Mn4+ under the condition of neutral pH, so as to precipitate a large amount from the aqueous solution. It also grows during the oxidation process.

We made a speculation on the cause of manganese pollution, that is: low-valent manganese ions in raw water are oxidized to high-valent manganese (tetravalent black) under the combined action of oxidants and bacteria, and the precipitated manganese is adsorbed by iron bacteria to form The mixed form of manganese and bacteria will adhere to the pipes, filters and reverse osmosis membrane elements due to the growth and reproduction of bacteria. The mixed form of bacteria and manganese in the influent of the reverse osmosis system will cause fouling of the security filter of the reverse osmosis system. Due to the increased fouling or the leakage of the cartridge filter, the water inlet end of a section of the RO plant is polluted.

Solutions for RO plant Manganese pollution

At present, the RO plant with severe manganese pollution has a significant reduction in the desalination rate before and after chemical cleaning. This is due to the oxidation of the membrane element caused by the catalytic oxidation of manganese (especially in the case of high chloride ions or low pH in the water), and chemical cleaning. The manganese dioxide particles precipitated during the process may cause particle scratches to the film surface. Combining the above two points is easy to cause irreversible desalination rate of RO plant to drop significantly.

To prevent manganese pollution in RO plant, consider the following methods

1. Add iron and manganese filters or other types of filters before ultra-filtration to reduce the manganese content of reverse osmosis feed water to meet the feed requirements;

2. Add bactericide to each process section and water tank before entering the reverse osmosis security filter to inhibit the growth of iron bacteria and prevent them from oxidizing low-valent ions of iron and manganese;

3. Regularly check the pressure difference and retained substances of the security filter before reverse osmosis, and replace the filter element in time;

4. When it is found that a large amount of manganese-containing solid substances are precipitated on the reverse osmosis membrane, try to consider physical removal. If chemical cleaning is necessary, please try not to use hydrochloric acid (currently after some cleaning projects, hydrochloric acid cleaning can cause reverse osmosis system desalination If the rate decreases rapidly, it is recommended to consider using citric acid in combination with other pharmaceuticals, and you can also consult an experienced pharmaceutical manufacturer.)

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