Why does the reverse osmosis equipment scale when it just starts running?

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28 Jun 2024

Why does the reverse osmosis equipment scale when it just starts running?

During the daily reverse osmosis equipment operation, scaling is not uncommon in some areas. The main reasons are as follows:
1. The design water quality does not conduct a full water quality analysis, resulting in scaling particles in the inlet water during operation;
2. The water source changes during operation, but the corresponding scale inhibitor is not adjusted, resulting in scaling;
3. The scale inhibitor is incompatible with the system, resulting in membrane pollution or scaling;
4. The recovery rate is controlled too high during operation, resulting in scaling in the second stage;
The above are the general causes of scaling, but in the process of troubleshooting, we will also find scaling caused by some special reasons. Here I would like to share with you a scaling accident caused by the installation problems of the first and second stage membrane elements.

Fault description:
1. After the membrane group was installed, the operation time was short (the cumulative time did not exceed 20 hours), and the operation was intermittent;
2. During the operation, it was found that the desalination rate of the second stage decreased as a whole (the average conductivity of the first stage was 10us/cm, and the average conductivity of the second stage reached 268us/cm);
3. After removing the end cover for inspection, it was found that the two membrane elements after the second stage had obvious scaling, the end cover was white, and a large number of bubbles appeared when treated with hydrochloric acid (carbonate scaling);

4. When removing the second-stage reverse osmosis membrane element, it was found that there was a problem with the installation direction of the second-stage overall membrane element (that is, the concentrated brine sealing ring was not on the water inlet side, but on the concentrated water side, and its impact is shown in the figure below).

The recovery rate (R) of the designed membrane system is R = produced water/(produced water + concentrated water).

The consequence of incorrect installation is that part of the inlet water is directly discharged from the system as part of the concentrated water without being filtered by the membrane element, that is, the concentrated water flow at this time is membrane concentrated water + leaked water, and the actual amount of concentrated water produced by the membrane element is less than the amount of concentrated water detected by the system. When the produced water volume remains unchanged, our apparent recovery rate is still the system recovery rate R, but for the membrane element, the actual recovery rate is higher than the system recovery rate.

This means that in the case of scaling conditions, incorrect installation will cause the membrane element to scale before the correct installation method, and the scaling substances will be precipitated at the end of the second stage first.

When installing the membrane element, the concentrated brine sealing ring should be on the water inlet side. This should be paid enough attention when installing the new reverse osmosis equipment, otherwise abnormal phenomena will be found during operation (such as abnormal increase in the conductivity of produced water or damage to the membrane element, etc.). The operator should also consider on-site supervision and record when installing the system to facilitate on-site evidence collection.


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