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Causes and Operation Analysis of Nanofiltration Membrane Pollution

Release Time:

2022-02-17 09:23

1. Analysis of pollution encountered in the operation of nanofiltration membranes

Microbial contamination

Microorganisms include bacteria, algae, fungi and viruses. Adverse consequences: First, the mass reproduction and metabolism of microorganisms will produce a large amount of colloidal substances, which will cause the membrane to be blocked and cause a sharp decrease in the membrane flux; second, it will cause an increase in the total number of bacteria in the produced water. It is extremely unfavorable for the long-term operation of the entire device, so great attention should be paid to the microbial contamination of the nanofiltration membrane.

Common causes of biological contamination are:

(l) Influent water contains a relatively high number of microorganisms;

(2) The deactivation, protection and flushing of the system are not carried out in strict accordance with the requirements of the technical manual

(3) The influent water is not sterilized or the dosage of the bactericide is too small

(4) The influent water quality contains nutrients that are easy to breed microorganisms, resulting in the proliferation of microorganisms;

(5) There is no regular sterilization and disinfection of the pipeline. The surface of the membrane contaminated by microorganisms will be very slippery and often have an unpleasant smell. The smell of burning biofilm samples is the same as burning hair.

(For example, the ammonia nitrogen index of the influent is seriously over-concentrated, which leads to the growth of a large number of microorganisms in the pipeline and in the membrane element. After the membrane system is chemically cleaned, since the pipeline is not sterilized, when the system is started, the remaining in the pipeline. Most of the microbial particles entered the membrane end with the water flow, resulting in a serious drop in the water production rate of the system and a sharp increase in the pressure drop between the membrane sections. The system was finally cleaned off-line to eliminate pollution.

Organic and Mineral Oil Pollution

Membrane system failures caused by organic matter account for 60% to 80% of all system failures. Organic matter in influent water is adsorbed on the surface of membrane elements, which will cause loss of flux, especially in the first stage. The adsorption layer formed on the surface acts as another separation barrier for the dissolved salts in the water, blocking the membrane surface channels, resulting in an increase in the salt rejection rate. Organic substances with large molecular weights and hydrophobic groups often cause this effect, such as trace amounts. Oil droplets, large molecular weight and refractory organics, etc., will cause the membrane system to be contaminated by organics.

(For example, petrochemical wastewater has complex composition, high concentration of organic matter and trace oil, so in the nanofiltration membrane system used in the advanced treatment device of petrochemical wastewater, organic pollution is one of the most common types of pollution. Organic pollution can generally be judged by analyzing the concentration of oil and organic pollutants in the influent water. General organic pollution can be eliminated by regular chemical cleaning.)

Contamination by flocculants

In the pretreatment process of the system, in the shallow flotation treatment unit, a certain high-purity polyaluminum flocculant is added to remove colloids, large particles of impurities and oily substances in the water. The use of flocculants is mainly divided into Inorganic and organic types, inorganic types are generally polyiron and polyaluminum. Due to the low price of inorganic flocculants, they are used more. In order to avoid iron ion pollution to the membrane system, high-purity polyaluminum is used in general membrane systems as flocculants. Organic flocculants are generally polypropylene phthalamide and polypropylene salts. In the pretreatment unit of some membrane systems, inorganic and organic flocculants work together well, but in actual use, According to the different system processes and water quality, the types and concentrations of flocculants to be used should be determined through actual screening. In actual operation, not all flocculants will be flocculated into granules, no matter what type of flocculants it is. , there will be a certain residue in the water. After entering the follow-up treatment unit, the residual flocculant will be discharged with the concentrated water under normal circumstances, but if the concentration of the flocculant is too high, the residual amount of the influent in the membrane system Too much will cause secondary flocculation and precipitation on the surface of the nanofiltration membrane, causing membrane pollution, and the pollution caused by the high dosage of flocculant is generally difficult to remove during cleaning, and may even lead to the need for a short period of time. Replace membrane.

fouling caused by fouling

Scaling is the solid precipitation of insoluble salts on the surface of the membrane. The method to prevent scaling is to ensure that the insoluble salts do not exceed the saturation limit. The scale precipitated in the nanofiltration system is mainly inorganic components, mainly calcium carbonate. , In addition to carbonate, many other inorganic salts also have low saturation solubility, such as calcium sulfate, barium sulfate, sulfuric acid and some hydroxides. To add an appropriate amount of scale inhibitor for membranes, the dosage is generally controlled at 4-12 mgl/*

Sometimes, the interaction of different chemicals added will lead to the precipitation of insoluble substances, which will contaminate the membrane elements. For example, when the polymerized organic antiscalant meets with multivalent cations such as aluminum or residual polymerized cationic flocculants, it will Formation of glue precipitation will seriously pollute the membrane elements at the front end. This kind of fouling is difficult to clean. Therefore, when adding a variety of chemicals, you should pay attention to the composition of these chemicals, according to the water quality data, reverse osmosis design method and the selected membrane type. , Confirm their compatibility through tests, and obtain the appropriate type and dosage of antiscalants.

colloidal contamination

Colloids are microparticles with a particle size of 1 nanometer (nm) to 1 micrometer (pin m), which are difficult to degrade naturally like clay, and are usually negatively charged in water. Organic colloidal substances in sewage and excess flocculant dosage , Hydroxide colloid formed by the hydrolysis of metal ions in sewage is a common cause of colloidal pollution. Common colloidal pollutants in wastewater include ferric hydroxide, aluminum hydroxide, silica colloid, etc.

(For example, colloid contamination can be caused by excessive dosing, corrosion of pipelines and membrane systems with large molecular weight organics)

Long-term operating experience of nanofiltration systems

Keep the preprocessing effect stable

In the pretreatment stage, most of the pollutants in the raw water are removed. The good pretreatment effect can effectively reduce the probability of various types of pollution in the nanofiltration system.

(For example, regularly replace the filter element of the security filter and check the security filter to prevent the short-flow phenomenon in the filter and the breeding of biological slime and cause pollution to the membrane elements; strictly control the turbidity and pollution index (SDI) of the influent water, and control the influent water. The turbidity is less than 0.5NTU, and the pollution index is less than 5; disinfection and sterilization of the pre-membrane process and membrane system, disinfection and sterilization are essential key steps to control microbial pollution. The sterilization of the system is divided into impact sterilization and continuous sterilization, Different methods can be selected according to different systems)

2. Long-term operating experience of nanofiltration system

Control lower operating pressures and recovery rates

Pressure is the driving force of nanofiltration desalination. When the pressure increases, the water permeability of the membrane module increases linearly, and the desalination rate increases at the beginning. When the pressure rises to a certain value, the desalination rate tends to be stable. Therefore, in actual operation, the pressure does not need to be too high. If the pressure is too high, the attenuation of the membrane will be aggravated, and the membrane module may be damaged. In order to prolong the service life of the membrane module, usually when the desalination rate and the water production volume meet the production requirements, a slightly lower pressure is used, which will not affect the system performance. There are great benefits to running for a long period of time.

When the nanofiltration system adopts a higher recovery rate, the salt content of the concentrated water increases accordingly, which not only easily produces concentration polarization on the concentrated water side, but also leads to an increase in the osmotic pressure of the system. In order to maintain the water production, the operating pressure must be Increase, the specific energy consumption of the produced water will also increase, the water quality of the produced water will become worse, the membrane fouling will increase, and the risk of scaling and microbial contamination will increase. According to operating experience, it is more appropriate to control the recovery rate of the nanofiltration system below 75%. .

Physical cleaning of the membrane (product water rinse)

Flushing is to use low-pressure and high-flow water to flush the membrane element to wash away the pollutants and deposits attached to the membrane surface. The low-pressure flushing of the membrane can reduce the depth difference and prevent the occurrence of membrane dehydration. When conditions permit, it is recommended to Flush the system frequently. Increasing the number of flushes is more effective than a chemical cleaning.

Standardize system start-stop operation and shutdown protection measures

When the system starts and stops, the flow and pressure will fluctuate. Excessive and fast flow and pressure fluctuations may cause the system to have a limit pressure drop phenomenon, resulting in a water hammer effect, which will cause the membrane element to rupture. Therefore, the start-stop operation is performed. It is necessary to slowly increase or decrease the pressure and flow.

Before starting the system and during the shutdown, it should be ensured that there is no vacuum in the pressure vessel, otherwise there will be water hammer or hydraulic shock when the membrane element is started again. The above phenomenon will occur.

The system should maintain a low back pressure (produced water side pressure), and the produced water side pressure is higher than the raw water side pressure. When the temperature is above .05MPa, the membrane element will be physically damaged. Before the system starts and stops running, it is necessary to fully confirm the opening and closing of the valve and the change of pressure to ensure that back pressure does not occur during the operation. If the membrane system needs to stop for a long time In order to ensure the normal standby of membrane elements, it is necessary to pass protective liquid or regular water into the system according to the requirements of the technical manual.

Regular online chemical cleaning of membrane elements

With a reasonable pretreatment system and good operation management, it can only reduce the degree of contamination of the membrane elements, and it is impossible to completely eliminate the contamination of the membrane. Therefore, after the nanofiltration membrane system runs for a period of time, the It may be polluted by a variety of pollutants, especially the nanofiltration membrane system used in the advanced sewage treatment device. The pollution occurs frequently. Under normal circumstances, the standardized water production will drop by about 15%, and the difference between the influent and concentrated water will be reduced. The pressure drop of the system increases to 1.5 times the initial value, and the quality of the permeate water decreases significantly, so it is necessary to chemically clean the membrane elements.

When chemical cleaning, first determine the type of pollutants, and then select the appropriate cleaning formula and cleaning process according to the characteristics of the membrane. When cleaning, pay attention to controlling the pH value, temperature and flow of the cleaning solution. In order to ensure the cleaning effect, conditions must be met. It can be chemically cleaned by the method of segmented cleaning. At present, there are professionally produced membrane cleaning agents for selection at home and abroad. The cleaning effect can be compared by comparing the performance of the device before and after cleaning, such as the desalination rate, water production and pressure drop. to confirm.

For membrane systems used in advanced petrochemical wastewater treatment units, chemical cleaning is generally first sterilized, followed by alkaline washing to remove microbial contamination, organic pollution and oil pollution, and then pickling to eliminate scale pollution and metal hydroxide pollution. The cleaning cycle is determined according to the actual operation of the device.

Offline chemical cleaning of membrane elements

When the membrane system cannot recover its performance after multiple online chemical cleaning, or the membrane system is heavily polluted, it is necessary to perform off-line chemical cleaning of the membrane element. Heavy pollution of the membrane element means that the single-stage pressure difference after pollution is greater than the system put into operation. The initial single-stage differential pressure value is more than 2 times, the water output of the reverse osmosis system drops by more than 30%, or the quality of a single reverse osmosis membrane element exceeds the normal value by more than 3kg.

According to the user's raw water full analysis report, performance test results and the system information they know, the pollution type and cleaning process are judged; if necessary, further verification is carried out through special equipment and appliances to determine the specific pollutant type and the required cleaning formula. The membrane elements to be cleaned are removed and cleaned on special offline cleaning equipment. After cleaning, they will be reassembled and put into use after passing the inspection.

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