Desalination rate decreased, how to determine the problem with the reverse osmosis equipment?-Hezhong Environment (Beijing) Co., Ltd. was established in 2003

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Desalination rate decreased, how to determine the problem with the reverse osmosis equipment?

Published Time:

2022-10-11

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The decline in salt rejection rate and water production is the most common malfunction in reverse osmosis and nanofiltration systems. Once a performance decline is detected, the first step in troubleshooting is to determine the location of the problem and identify its cause. This can be achieved using run parameter logs or certain online measuring instruments.

If the system data is insufficient to determine the cause and take corrective measures, one or more membrane elements must be removed from the system for analysis to determine the location of leaks.

Step 1: Look for distribution patterns

To do this, TDS, conductivity, or other water quality-related values are measured separately on the product water side of all pressure vessels.

During sampling, precautions should be taken to prevent product water from other pressure vessels from mixing in and affecting the measurement results. Then, the concentration of dissolved solids TDS in all product water samples is measured.

In nanofiltration systems, analytical methods for determining sulfate ions or other relevant components must also be used.

The test results of product water samples from all pressure vessels in the same section should fall within the same range. Of course, it should be noted that from the first section to the second section, the average product water TDS or conductivity value should increase accordingly, because the feed water of the second section is the concentrate of the first section.

In order to determine the leakage rate of solutes in all pressure vessels, the feed water concentration of each section should also be measured.

Salt leakage rate is the percentage of product water concentration to feed water concentration. Thus, high salt leakage rate may occur in the first or second section, or in certain pressure vessels.

If a pressure vessel exhibits a higher product water concentration than other pressure vessels in the same section, the performance of the membrane elements in that pressure vessel should be investigated.

The detection method uses a 1/4-inch diameter plastic tube inserted into the product water center tube of the entire membrane assembly, as shown in the figure below. It requires disconnecting the product water pipe of the tested shell from the total product water pipe or removing the product water outlet plug at the other end of the pressure vessel.

When the RO/NF system is operating under normal operating conditions, the water sample initially diverted from the product water center tube of the pressure vessel is not representative. Several minutes should be waited to allow the probe water pipe to be flushed and the system to reach equilibrium. Then, the TDS value of the product water flowing from the probe tube can be measured by a handheld instrument and recorded. It reflects the TDS value of the product water at that location of the membrane element.

The probe tube should be pulled out 6 inches (depending on the membrane shell brand) to measure the conductivity of the product water at the adapter (commonly known as the grenade) between the pressure vessel end plate and the membrane element. Then, pull it out 8 inches to measure the conductivity of the product water there. Obtain the distribution pattern of product water conductivity at this interval as shown in the figure above. The sampling interval must be 8 inches (200 mm) so that the fifth product water sample in each group corresponds to the internal connector between the two components.

This measurement method can measure multiple data points for each component and simultaneously check all internal connectors and adapter "O"-rings.

Therefore, marks should be made on the sampling tube during measurement to quickly locate the required sampling position.

From the feed water end to the concentrate end of the container, the conductivity distribution of normal product water shows a balanced increasing trend.

If there is an abnormal deviation from this distribution pattern, the location of the high salt leakage fault can be determined. "O"-ring failure will generally reflect a sudden change in the conductivity change curve corresponding to the internal connector or adapter. A significant increase in conductivity at other locations indicates a fault in the corresponding membrane element.