Causes and operational analysis of reverse osmosis membrane pollution

Causes of reverse osmosis membrane fouling  
　　1. Microbial contamination
　　Microorganisms include bacteria, algae, fungi, and viruses. Bacteria are extremely small particles; the diameter of cocci is generally 0.5～1.0 micrometers; bacilli are 1 micrometer wide and 2 micrometers long. Viruses are even smaller; the largest currently known is the poxvirus, with a diameter of approximately 300 nanometers, while the smallest is the circovirus, with a diameter of 17 nanometers. Microbial contamination has at least two negative consequences for reverse osmosis membrane systems: First, the massive reproduction and metabolism of microorganisms produce large amounts of colloidal substances, causing membrane blockage and a sharp drop in membrane flux; second, it will increase the total number of bacteria in the product water. Microbial contamination of reverse osmosis membranes is extremely detrimental to the long-term operation of the entire device, so the microbial contamination of reverse osmosis membranes must be given high priority.
　　The causes of biological contamination generally include:  
　　(1) The influent contains a high number of microorganisms;
　　(2) System shutdown, protection, and flushing have not been strictly performed according to the technical manual;
　　(3) The influent has not been sterilized, or the amount of disinfectant added is too small;
　　(4) The influent water quality contains nutrients that easily breed microorganisms, leading to the massive proliferation of microorganisms;
　　(5) The pipelines have not been regularly sterilized and disinfected. The surface of a microbially contaminated membrane will be very slippery and often have an unpleasant odor; the smell of burning biofilm samples is the same as burning hair.
　　(For example, the influent ammonia nitrogen index severely exceeds the concentration, leading to the massive proliferation of microorganisms in the pipeline and membrane elements. After chemical cleaning of the membrane system, because the pipeline has not been sterilized and disinfected, when the system is started up, most of the microbial particles remaining in the pipeline enter the membrane end with the water flow, causing a serious decrease in the system water production rate and a sharp increase in the pressure drop between membrane sections. The system was eventually cleaned offline to eliminate the contamination.)

　　2. Organic matter and mineral oil contamination
　　Membrane system failures caused by organic matter account for 60%—80% of all system failures. Organic matter in the influent adsorbs to the surface of the membrane elements, causing flux loss, especially in the first stage. In many cases, the adsorption layer formed on the membrane surface acts like another separation barrier to the dissolved salts in the water, blocking the membrane surface channels, leading to an increase in desalination rate. High-molecular-weight organic matter with hydrophobic groups often causes this effect, such as trace amounts of oil droplets and high-molecular-weight, difficult-to-degrade organic matter, which can lead to organic matter contamination of the membrane system.
　　(For example, petrochemical wastewater has a complex composition, with a high concentration of organic matter in the water and trace amounts of oil. Therefore, in reverse osmosis membrane systems used in petrochemical wastewater advanced treatment devices, organic matter contamination is the most common type of contamination. Organic matter contamination of reverse osmosis membranes can generally be judged by analyzing the oil and organic pollutant concentration in the influent. General organic contamination can be eliminated through regular chemical cleaning.)

　　3. Fouling caused by flocculants
　　In the pretreatment process of the reverse osmosis system, such as the air flotation or coagulation sedimentation treatment unit, a certain amount of high-purity polyaluminum flocculant is added to remove colloids, particulate impurities, and oily substances in the water.
　　The use of flocculants is mainly divided into inorganic and organic types. Inorganic types are generally polyferric and polyaluminum, and because inorganic flocculants are inexpensive, they are used more frequently. To avoid iron ion contamination of the membrane system, high-purity polyaluminum is generally used as a flocculant in general membrane systems; organic flocculants are generally polyacrylamide and polyacrylate. In the pretreatment units of some membrane systems, the combined use of inorganic and organic flocculants is more effective, but in actual use, the type and concentration of flocculant used should be determined based on the different system processes and water quality through actual screening. In actual operation, not all flocculants are flocculated into particles; regardless of the type of flocculant, there will be a certain amount of residue in the water. After entering the subsequent treatment unit, under normal circumstances, the residual flocculant will be discharged with the concentrate. However, if the flocculant dosage is too high and the residual amount in the membrane system influent is too high, secondary flocculation and sedimentation will occur on the surface of the reverse osmosis membrane, causing membrane fouling. Moreover, the fouling caused by excessive flocculant dosage is generally difficult to remove during cleaning, and it can even lead to the need to replace the membrane in a short time.

　　4. Fouling caused by scaling
　　Scaling is the precipitation of insoluble salts on the membrane surface, preventing the formation of solid precipitates. The method to prevent scaling is to ensure that the insoluble salts do not exceed the saturation limit. The scales precipitated in the reverse osmosis system are mainly inorganic components, mainly calcium carbonate. In addition to carbonates, many other inorganic salts also have low saturation solubility, such as calcium sulfate, barium sulfate, magnesium sulfate, and some hydroxides. To prevent membrane scaling, an appropriate amount of membrane-use antiscalant is generally added before the security filter. Sometimes, different chemicals added may interact to cause the precipitation of insoluble substances, thereby fouling the membrane elements. For example, when a polymeric organic antiscalant encounters multivalent cations such as aluminum or residual polymeric cationic flocculants, a precipitation reaction may occur, such as aluminum or iron. The resulting colloidal reaction products are very difficult to remove from the membrane surface. Therefore, when adding multiple chemicals, the components of these chemicals should be noted, and their compatibility should be confirmed through experiments based on water quality data and the selected membrane model, and the appropriate type and dosage of antiscalant should be obtained.

　　5. Colloidal fouling
　　Colloids have a particle size of 1 nanometer (nm) to 1 micrometer, are difficult to naturally degrade like clay, and are usually negatively charged in water. Organic colloidal substances in wastewater, excessive flocculant dosage, and hydroxides formed by the hydrolysis of metal ions in wastewater are common causes of colloidal fouling. Common colloidal pollutants in wastewater include ferric hydroxide, aluminum hydroxide, and silica colloids.

Long-term operating experience of reverse osmosis systems
　　1. Maintain the stability of pretreatment effects
　　Remove most pollutants from the raw water during the pretreatment stage. Good pretreatment can effectively reduce the chance of various pollutants affecting the reverse osmosis system.
　　(For example, regularly replace the security filter cartridges and check the security filters to prevent short-circuiting and the growth of biological sludge in the filters, which could pollute the membrane elements; strictly control the influent turbidity and pollution index (SDI), keeping the influent turbidity below 0.5 NTU and the pollution index below 5; disinfect and sterilize the pre-membrane process and membrane system. Disinfection and sterilization are essential steps in controlling microbial contamination. System sterilization can be shock sterilization or continuous sterilization, and different methods can be chosen depending on the system.)

　　2. Control lower operating pressure and recovery rate
　　Pressure is the driving force for reverse osmosis desalination. As pressure increases, the membrane module permeate flux increases linearly, and the desalination rate initially increases. When the pressure reaches a certain value, the desalination rate tends to stabilize. Therefore, in actual operation, the pressure does not need to be too high. Excessive pressure will accelerate membrane degradation and may damage the membrane module. To extend the service life of the membrane module, slightly lower pressure is usually used when the desalination rate and water production meet the production requirements, which is extremely beneficial for the long-term operation of the system.  
　　When the reverse osmosis system uses a higher recovery rate, the salt content of the concentrate increases accordingly. This not only easily causes concentration polarization on the concentrate side, but also leads to an increase in the system osmotic pressure. To maintain water production, the operating pressure must be increased, and the specific energy consumption of water production will also increase, resulting in poorer water quality, increased membrane fouling, and increased risk of scaling and microbial contamination. According to operating experience, the recovery rate of the reverse osmosis system is controlled at 75% is more appropriate.

　　3. Physical cleaning of the membrane (product water rinsing)
　　Rinsing uses low-pressure, high-flow influent to rinse the membrane elements, removing pollutants and deposits attached to the membrane surface. Low-pressure rinsing of the membrane can reduce the depth difference and prevent membrane dehydration. If conditions permit, it is recommended to rinse the system frequently. Increasing the number of rinses is more effective than performing a single chemical cleaning.

　　4. Standardized system start-up and shutdown operations and shutdown protection measures
　　During system start-up and shutdown, the flow rate and pressure will fluctuate. Excessive or rapid fluctuations in flow rate and pressure may cause the system to experience extreme pressure drop, resulting in water hammer, which can cause membrane element rupture. Therefore, when performing start-up and shutdown operations, the pressure and flow rate should be increased or decreased slowly.  
　　Before starting up and shutting down the system, ensure that there is no vacuum in the pressure vessel. Otherwise, water hammer or hydraulic shock may occur at the moment the membrane element is restarted. This phenomenon will occur when the system, which has already lost water, is initially started or generally started during operation.  
　　The system should maintain a low backpressure (product water side pressure). When the product water side pressure is more than 0.05 MPa higher than the raw water side pressure, the membrane element will be physically damaged. Before starting and stopping the system, fully confirm the opening and closing of the valves and the pressure changes to ensure that backpressure is avoided during operation. If the membrane system needs to be shut down for a long time, it is necessary to introduce a protective liquid into the system or regularly flush the system according to the technical manual requirements to ensure the normal standby of the membrane elements.

　　5. Regularly perform online chemical cleaning of the membrane elements
　　With a reasonable pretreatment system and good operation management, it only reduces the degree of membrane element contamination. It is impossible to completely eliminate membrane contamination. Therefore, after a period of operation, the reverse osmosis membrane system may be contaminated by various pollutants, especially the reverse osmosis membrane system used in wastewater advanced treatment devices, where contamination occurs frequently. Generally, after standardization, the water production decreases by about 15%, the system pressure drop between the influent and concentrate increases to 1.5 times the initial value, and the product water quality shows a significant decline, then chemical cleaning of the membrane elements is required.
　　During chemical cleaning, first determine the type of pollutant, and then select the appropriate cleaning formula and process according to the membrane characteristics. During cleaning, pay attention to controlling the pH value, temperature, and flow rate of the cleaning solution. To ensure the rinsing effect, segmented cleaning can be used if conditions permit. Currently, there are specialized membrane cleaning agents available for selection in China and internationally. The cleaning effect can be confirmed by comparing the desalination rate, water production, and pressure drop performance of the device before and after cleaning. The cleaning effect can be confirmed by comparing the desalination rate, water production, and pressure drop performance of the device before and after cleaning.

　　6. Offline chemical cleaning of membrane elements
　　When the membrane system cannot recover its performance after multiple online chemical cleanings, or when the membrane system is severely contaminated, offline chemical cleaning of the membrane elements is required. Severe contamination of the membrane elements refers to the situation where the pressure difference of a single section after contamination is more than 2 times the pressure difference of a single section at the beginning of the system operation, the water production of the reverse osmosis system decreases by more than 30%, or the mass of a single reverse osmosis membrane element exceeds the normal value by more than 3 kg.
　　Determine the type of pollution and the cleaning process based on the user's raw water full analysis report, performance test results, and the system information understood; if necessary, further verification can be carried out using special equipment and instruments to determine the specific type of pollutant and the required cleaning formula. The disassembled membrane elements to be cleaned are cleaned in a dedicated offline cleaning equipment, and after passing the inspection, they are reinstalled and put into use.