1, Introduction to Membrane Fouling

　　When the reverse osmosis system is in operation, suspended substances, dissolved substances and microbial reproduction in the feed water can all cause membrane element contamination. The pretreatment of the reverse osmosis system should remove these pollutants as much as possible and minimize the possibility of membrane element contamination. The types of pollutants, their causes and treatment methods are shown in the table1. Generally, the main causes of membrane fouling are as follows:

　　1)The new device's pipeline contains oil substances, residues from welding the pipeline, and dust, which were not cleaned thoroughly before installing the membrane.

　　2)The design of the pretreatment device is unreasonable.

　　3) The amount of added chemicals is incorrect or the equipment malfunctions;

　　4)Human operational error;

　　5)When the operation was stopped, low-pressure flushing was not performed or the flushing conditions were not controlled correctly.

　　6)Changes occur in the water supply source or water quality.

　　The accumulation of pollutants can be known through parameters such as operating pressure, pressure difference increase, and changes in desalination rate recorded in daily data. When membrane elements are contaminated, their performance is often restored through cleaning. There are generally two ways of cleaning: physical cleaning (rinsing) and chemical cleaning (rinsing). Physical cleaning (rinsing) does not change the nature of the contaminants. It uses force to remove the contaminants from the membrane elements and restore their performance. Chemical cleaning involves the use of corresponding chemical agents to alter the composition or properties of contaminants and restore the performance of membrane elements. For particulate pollutants with low adsorption capacity, certain effects can be achieved through flushing (physical cleaning). However, for pollutants with strong adsorption capacity to the membrane, such as biological contamination, it is difficult to achieve the expected results by flushing. Chemical cleaning should be adopted for contaminants that are difficult to remove by flushing. To enhance the effectiveness of chemical cleaning, it is necessary to analyze the contamination status before cleaning to determine the type of contamination. When the types of pollutants are understood, choosing the appropriate cleaning agent can appropriately restore the performance of the membrane element.

　　2, Physical cleaning (rinsing

　　2.1 The function of rinsing

　　Flushing involves using low-pressure and high-flow feed water to flush the membrane elements, removing contaminants or deposits adhering to the membrane surface

　　2.2.1 Flushing flow rate

　　When the device is in operation, particulate contaminants gradually accumulate on the surface of the membrane. If the flow rate during flushing is the same as or slightly lower than that during water production, it will be very difficult to flush contaminants out of the membrane elements. Therefore, a higher flow rate should be used during flushing than during normal operation. Typically, the flushing flow rate within a single pressure vessel is:

　　1)8Inch membrane element7.2 - 12 m3/h;

　　2)4Inch membrane element1.8 - 2.5 m3/h.

　　2.2.2 Flushing pressure

　　During normal high-pressure operation, pollutants are pressed towards the membrane surface, causing contamination. So during the rinsing process, if the same high pressure is used, contaminants will still be pressed onto the membrane surface, and the cleaning effect will not be ideal. Therefore, during the flushing process, it is necessary to increase the horizontal shear force as much as possible through low pressure and high flow rate to flush out the contaminants from the membrane elements. Pressure is usually controlled at0.3 MPaBelow. If there0.3 MPaWhen it is difficult to achieve a certain flow rate, the inlet water pressure should be controlled as much as possible, with the standard of no water production or minimal water production. The general inlet water pressure should not be greater than0.4 MPa.

　　2.2.3 The frequency of flushing

　　When conditions permit, it is recommended to flush the system frequently. Increasing the number of rinses is more effective than conducting a single chemical cleaning. It is generally recommended that the frequency of rinsing be once a day. According to specific circumstances, users can control the frequency of flushing by themselves.

　　2.3 The steps of rinsing

　　① Stop the operation of the reverse osmosis system. Slowly reduce the operating pressure and stop the device. If the device is stopped quickly, the pressure will drop sharply, which may cause damage to the pipelines, pressure vessels and membrane elements.

　　② Regulating valve

　　- Fully open the concentrated water valve;

　　- Close the water inlet valve;

　　- Fully open the water production valve (if the water production valve is not fully open during operation).

　　If the production water valve is closed incorrectly, the membrane elements in the rear part of the pressure vessel may experience back pressure of the production water, causing damage to the membrane elements.

　　③ Flushing operation

　　- Start the low-pressure flushing pump;

　　- While slowly opening the inlet water pump, check the flow rate of the concentrated water flowmeter.

　　- Adjust the water inlet valve to regulate the flow rate and pressure to reach the standard values.

　　- 10 - 15After a few minutes, slowly close the water inlet valve and stop the water pump.

　　④ Resume normal operation. Start the system according to the daily startup procedure.

　　2.4 Precautions

　　① The inlet water pump needs to meet the inlet water flow rate during normal operation = Water production flow rate + The concentrated water flow rate should be taken into account, while also considering meeting the requirements of the flushing flow rate.

　　② Concentration...... Scaling is the solid precipitation of insoluble salts on the membrane surface. The way to prevent it is to ensure that the insoluble salts do not exceed the saturation limit