Microorganisms include bacteria, algae, fungi, and viruses. Bacteria are extremely small, generally 1-3 μm, while viruses are even smaller, approximately 0.2-0.01 μm. Microbial contamination has at least two negative consequences for nanofiltration membrane systems: First, the massive reproduction and metabolism of microorganisms produce large amounts of colloidal substances, causing membrane blockage and a sharp decrease in membrane flux; second, it will increase the total number of bacteria in the product water. Microbial contamination of nanofiltration membranes is extremely detrimental to the long-term operation of the entire device, therefore, the microbial contamination of nanofiltration 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 carried out strictly in accordance with the technical manual requirements

(3) No sterilization of the influent or insufficient dosage of sterilant

(4) The influent water quality contains nutrients that easily breed microorganisms, leading to the massive proliferation of microorganisms;

(5) No regular sterilization and disinfection of pipelines. 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 proliferation of a large number of microorganisms in the pipeline and membrane elements. After chemical cleaning of the membrane system, due to the lack of sterilization and disinfection of the pipeline, when the system is started up, most of the microbial particles remaining in the pipeline enter the membrane end with the water flow, resulting in 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.

Membrane system failures caused by organic matter account for 60%-80% of all system failures. Organic matter in the influent adsorbed on the surface of the membrane element will cause a loss of flux, especially in the first stage. In many cases, the adsorption layer formed on the membrane surface acts like another separation barrier to dissolved salts in the water, blocking 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, a high concentration of organic matter in the water, and trace amounts of oil. Therefore, in nanofiltration membrane systems used in petrochemical wastewater advanced treatment devices, organic matter contamination is the most common type of contamination. Organic matter contamination of nanofiltration membranes can generally be judged by analyzing the oil and organic pollutant concentration in the influent. General organic contamination can be eliminated by regular chemical cleaning.

In the pretreatment process, in the shallow flotation treatment unit, by adding a certain amount of high-purity polyaluminum coagulant, colloids, large particulate impurities, and oil substances in the water are precipitated and removed.

The use of coagulants is mainly divided into inorganic and organic types. Inorganic types are generally polyferric and polyaluminum, and because inorganic coagulants are inexpensive, they are used more frequently. To avoid iron ion contamination of the membrane system, high-purity polyaluminum is generally selected as the coagulant in general membrane systems; organic coagulants are generally polyacrylamide and polyacrylate.

In the pretreatment units of some membrane systems, the combined use of inorganic and organic coagulants is more effective, but in actual use, the types and concentrations of coagulants used should be determined based on different system processes and water quality through actual screening. In actual operation, not all coagulants are flocculated into particles. No matter which type of coagulant, there will be a certain amount of residue in the water. After entering the subsequent treatment unit, under normal circumstances, the remaining coagulant will be discharged with the concentrate. However, if the coagulant dosage is too high and the residual amount in the membrane system influent is too high, secondary flocculation and precipitation will occur on the surface of the nanofiltration membrane, causing membrane fouling. Moreover, the contamination caused by excessive coagulant dosage is generally difficult to remove during cleaning, and may even lead to the need to replace the membrane in a short time.

Scaling is the precipitation of insoluble salts on the membrane surface, preventing scaling by ensuring that the insoluble salts do not exceed the saturation limit. The scales precipitated in nanofiltration systems 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, strontium sulfate, and some hydroxides. To prevent membrane scaling, an appropriate amount of membrane antiscalant is generally added before the security filter, and the amount added is generally controlled at 4-12 mg/L.

Sometimes, different chemicals added may interact to cause the precipitation of insoluble substances, thereby contaminating the membrane elements. For example, when polymeric organic antiscalants encounter multivalent cations such as aluminum or residual polymeric cationic flocculants, gel precipitation will form, severely contaminating the front-end membrane elements. This type of blockage is difficult to clean. Therefore, when adding multiple chemicals, the components of these chemicals should be noted. Based on water quality data, reverse osmosis design methods, and the selected membrane model, their compatibility should be confirmed through experiments, and the appropriate antiscalant type and dosage should be obtained.

Colloids are particles with a particle size of 1 nanometer (nm) to 1 micrometer (μm), similar to clay, that are difficult to naturally degrade and are usually negatively charged in water. Organic colloidal substances in wastewater, excessive coagulant dosage, and hydroxides formed by the hydrolysis of metal ions in wastewater are common causes of colloidal contamination. Common colloidal contaminants in wastewater include ferric hydroxide, aluminum hydroxide, and silica colloids.

For example, colloidal contamination can be caused by excessive chemical addition, pipeline corrosion, and high molecular weight organic matter entering the membrane system.

Remove most of the pollutants in the raw water during the pretreatment stage. Good pretreatment effects can effectively reduce the probability of various types of contamination in the nanofiltration system.

For example, regularly replace the security filter cartridges and check the security filter to prevent short-circuiting and the growth of biological slime within the filter, which could contaminate 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 achieved through shock sterilization or continuous sterilization, with different methods selected based on the system.

Pressure is the driving force for nanofiltration desalination. As pressure increases, the membrane module's water permeability 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 production requirements, which is extremely beneficial for the long-term operation of the system.

When the nanofiltration 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. Based on operating experience, a recovery rate of less than 75% is more suitable for nanofiltration systems.

Rinsing uses low-pressure, high-flow influent to rinse the membrane elements, removing contaminants 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.

The flow rate and pressure will fluctuate during system start-up and shutdown. Excessive or rapid fluctuations in flow rate and pressure may cause the system to experience extreme pressure drop phenomena, creating a water hammer effect that 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, a water hammer or hydraulic shock may occur 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 prevented during operation. If the membrane system needs to be shut down for a long time, protective liquid should be introduced into the system or water should be circulated regularly according to the technical manual requirements to ensure the normal standby of the membrane elements.

Even with a reasonable pretreatment system and good operational management, it only reduces the degree of membrane element contamination; it is impossible to completely eliminate membrane contamination. Therefore, after a nanofiltration membrane system has been running for some time, it may be contaminated by various pollutants, especially nanofiltration membrane systems used in wastewater advanced treatment devices, where contamination frequently occurs. Generally, when the standardized 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, chemical cleaning of the membrane elements is required.

During chemical cleaning, the type of contaminant must first be determined, and then a suitable cleaning formula and cleaning process should be selected based on the membrane's characteristics. During cleaning, the pH value, temperature, and flow rate of the cleaning solution should be controlled. To ensure the rinsing effect, segmented cleaning can be used if conditions permit. Currently, there are specialized membrane cleaning agents available for selection in both domestic and international markets. The cleaning effect can be confirmed by comparing the desalination rate, water production, and pressure drop performance of the device before and after cleaning.

For membrane systems used in petrochemical wastewater advanced treatment devices, chemical cleaning generally begins with sterilization, followed by alkaline washing to remove microbial, organic, and oil contamination, and then acid washing to remove scale and metal hydroxide contamination. The cleaning cycle is determined based on the actual operating conditions of the device.

If the membrane system cannot recover its performance after multiple online chemical cleanings, or if the membrane system is severely contaminated, offline chemical cleaning of the membrane elements is required. Severe contamination of the membrane elements refers to situations where the pressure difference of a single section after contamination is more than twice the initial pressure difference of a single section at the start of the system, the reverse osmosis system water production 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 contamination 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 performed using special equipment and instruments to determine the specific type of contaminant and the required cleaning formula. The disassembled membrane elements to be cleaned are cleaned in a dedicated offline cleaning device. After cleaning and passing inspection, they are reinstalled and put into use.