What are the common malfunctions of RO membrane reverse osmosis systems?

Malfunctions in reverse osmosis systems typically involve at least the following situations:

First, the water production rate decreases while the operating pressure and conductivity remain normal;

Second, the desalination rate decreases after standardization, which manifests as an increase in the conductivity of the produced water in the reverse osmosis system;

Third, the water production rate decreases after standardization, usually requiring an increase in operating pressure to maintain the rated water production rate, i.e., the operating pressure increases;

1. Decreased water production rate after standardization

If the RO system shows a decrease in water production rate after standardization, the cause can be found based on the following three situations:

（ 1) If the water production rate of the first stage of the RO system decreases, there is a deposition of particulate pollutants;

（ 2) If the water production rate of the last stage of the RO system decreases, there is scaling pollution;

（ 3) If the water production rate of all stages of the RO system decreases, there is blockage;

2. A decrease in water production rate after standardization is one of the most common system malfunctions, and the possible causes are:

（ 1) Reduction in membrane modules, i.e., operation according to the designed number of membrane modules;

（ 2) Low-pressure operation of the reverse osmosis membrane, i.e., operation below the designed baseline pressure (possibly a throttle valve);

（ 3) Membrane module compaction is found, i.e., when the reverse osmosis membrane operates at a pressure far exceeding the baseline pressure, membrane module compaction occurs and the membrane module must be replaced;

（ 4) Decrease in operating temperature; operate at the design temperature of 25 degrees;

（ 5) Operation under high recovery rate conditions, which increases the average feed water/concentrate TDS, thus increasing osmotic pressure. When operating under recovery rates above 75%, the amount of concentrate water decreases, increasing the concentration factor of water in the membrane module, resulting in a significant decrease in feed water quality. Due to the increase in osmotic pressure of this feed water, the permeate flow rate decreases. In severe cases, salt scaling will occur on the membrane surface, and water must be produced at the design recovery rate;

（ 6) Membrane fouling. Metal oxides or turbid substances adhere to the membrane surface, causing blockage of the reverse osmosis membrane (the main cause).

（ 7) Increase in pressure difference during operation of the reverse osmosis; improve the operation management of pretreatment, improve the water quality of reverse osmosis, and clean the reverse osmosis components with chemicals;

（ 8) Oil mixing; note that oil must not enter the feed water, as oil will contaminate the reverse osmosis membrane;

（ 9) Ensure that the filter cartridges are replaced regularly. Failure to replace them for a long time will lead to cartridge blockage, affecting the feed water volume of the reverse osmosis;

（ 10) Increase in feed water conductivity, which increases the osmotic pressure that the produced water must overcome when passing through the membrane;

4. Causes and solutions for decreased water production rate

（ 1) Whether the RO reverse osmosis membrane is contaminated or blocked;

（ 2) Increase in feed water conductivity;

（ 3) Membrane module compaction is found, i.e., when the reverse osmosis membrane operates at a pressure far exceeding the baseline pressure, membrane module compaction occurs and the membrane module must be replaced.

（ 4) Whether the operating pressure reaches the rated operating pressure of the reverse osmosis membrane. Determine the rated operating pressure according to the model of the reverse osmosis membrane. Appropriately increasing the feed water pressure can increase the water production rate.

5. The original water contains a certain concentration of suspended matter and dissolved substances.

Suspended matter mainly includes inorganic salts, colloids, and biological particles such as microorganisms and algae. Dissolved substances mainly include easily soluble and sparingly soluble salts, metal oxides, acids and bases, etc. During the reverse osmosis process, the volume of feed water decreases, and the concentration of suspended particles and dissolved substances increases. Suspended particles will deposit on the membrane, blocking the water channels and increasing frictional resistance (pressure drop). When the concentration of sparingly soluble salts exceeds their saturation limit, they will precipitate from the concentrate water, forming scaling on the membrane surface, reducing the flux of the RO membrane, increasing the operating pressure and pressure drop, and leading to a decrease in product water quality.

（ 6) Influence of water temperature.

The rated water production rate of the reverse osmosis membrane is based on a design temperature of 25℃. For every 1℃ decrease in temperature, the water production rate decreases by 2-3%.

All of the above factors cause the water production rate of the reverse osmosis membrane to gradually decrease, the salt passage rate to gradually increase, and the pure water quality to decrease. Generally, the service life of a reverse osmosis membrane is 2-3 years. Damaged reverse osmosis membranes should be replaced promptly; otherwise, not only will the water production rate be affected, but the water quality will also deteriorate.