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Pollution analysis of nanofiltration membrane in operation

Microbial contamination

Microorganisms include bacteria, algae, fungi and viruses The particle size of bacteria is very small, generally 1-3 BYM, and the virus is smaller, about 0.2-0.01 BYM. Microbial pollution causes at least two adverse consequences to the nanofiltration membrane system: first, the massive reproduction and metabolism of microorganisms produce a large amount of colloidal substances, resulting in the blockage of the membrane and the sharp decline of membrane flux; Second, it will increase the total number of bacteria in the water The microbial contamination of nanofiltration membrane is extremely unfavorable to the long-term operation of the whole device, so we should pay great attention to the microbial contamination of nanofiltration membrane

The causes of biological pollution generally include:

(l) The influent contains a high number of microorganisms;

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

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

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

(5) The pipeline was not sterilized and disinfected regularly The membrane surface polluted by microorganisms will be very greasy and often have a bad 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, resulting in a large number of microorganisms breeding in the pipeline and membrane elements. After the chemical cleaning of the membrane system, due to the failure to sterilize the pipeline, when the system is started, most of the microbial particles remaining in the pipeline enter the membrane end with the water flow, resulting in a serious decline in the water production rate of the system and a sharp rise in the pressure drop between membrane sections. The system finally passes offline Cleaning can eliminate pollution.)

Organic matter and mineral oil pollution

Membrane system failures caused by organic matter account for 60% - 80% of all system failures The organic matter in the influent adsorbed on the membrane element surface will cause the loss of flux, especially in the first stage. In many cases, the adsorption layer formed on the membrane surface is like another separation barrier layer to the dissolved salt in the water, blocking the channel of the membrane surface, resulting in the increase of desalination rate. Large amounts of organic matter with hydrophobic groups often cause this effect, such as trace oil droplets The membrane system will be polluted by organic substances with high molecular weight and difficult to degrade

(for example, the composition of petrochemical wastewater is complex, the concentration of organic matter in the water is high, and there is a trace of oil. Therefore, organic matter pollution is the most common type of pollution in the nanofiltration membrane system used in the advanced treatment unit of petrochemical wastewater. The organic matter pollution of nanofiltration membrane can be judged by the analysis of the concentration of oil and organic pollutants in the water. The general organic pollution can be determined by regular chemical cleaning Can be eliminated.)

Pollution caused by flocculant

In the pretreatment process of the system, a certain high-purity polyaluminium flocculant is added in the shallow flotation treatment unit to remove colloids, large particle impurity precipitation and oil substances in the water The use of flocculants is mainly divided into inorganic and organic. Inorganic flocculants are generally poly iron and poly aluminum. Inorganic flocculants are widely used because of their low price. In order to avoid iron ion pollution to the membrane system, high-purity poly aluminum is selected as flocculant in the general membrane system; Organic flocculants are generally polypropylene phthalocyanine amine and polypropylene salts In the pretreatment units of some membrane systems, inorganic and organic flocculants work well together, but in actual use, the types and concentrations of flocculants should be determined through actual screening according to different system processes and water quality In actual operation, not all flocculants will be flocculated into particles. No matter what kind of flocculant, there will be a certain residue in the water. After entering the subsequent treatment unit, the residual flocculant will be discharged with the concentrated water under normal circumstances. However, if the flocculant dosage concentration is too high and the residue in the influent of the membrane system is too much, secondary flocculation and sedimentation will be carried out on the surface of the nanofiltration membrane, Cause membrane pollution, and the pollution caused by too high dosage of flocculant is generally difficult to remove in cleaning, and even lead to the need to replace the membrane in a short time.

Pollution caused by scaling

Scaling is the solid precipitation of insoluble salts on the membrane surface. 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, mainly calcium carbonate. In addition to carbonate, many other inorganic salts also have low saturated solubility, such as calcium sulfate, barium sulfate, sulfuric acid trance and some hydroxides In order to prevent scaling on the membrane surface, an appropriate amount of membrane scale inhibitor should be added before the security filter, and the addition amount is generally controlled at 4-12 MGL/*

Sometimes, the interaction between different chemicals will lead to the precipitation of insoluble substances, which will pollute the membrane components For example, when the polymerized organic scale inhibitor meets with multivalent cations such as aluminum or residual polymerized cationic flocculant, it will form gel off precipitation and seriously pollute the front-end membrane components, which is difficult to clean Therefore, when adding a variety of agents, we should pay attention to the composition of these agents, confirm their compatibility through tests according to water quality data, reverse osmosis design method and selected membrane model, and obtain appropriate scale inhibitor type and dosage

Colloidal fouling

Colloids are particles with a particle size of 1 nm to 1 micron, which are difficult to degrade naturally like clay. They are usually negatively charged in water The common causes of colloidal pollution are the organic colloidal substances in sewage, the excess dosage of flocculant and the hydroxide colloid formed by the hydrolysis of metal ions in sewage Common colloidal pollutants in wastewater include iron hydroxide, aluminum hydroxide, silica colloid, etc

(for example, colloid pollution can be caused by excessive dosage, pipeline corrosion and membrane system of high molecular weight organic matter)

Long term operation experience of nanofiltration system

Keep the pretreatment effect stable

In the pretreatment stage, most pollutants in raw water are removed Good pretreatment effect can effectively reduce the probability of all kinds of pollution to the nanofiltration system

(for example, regularly replace the filter element of the security filter and check the security filter to prevent short flow and breeding of biological sludge in the filter from polluting the membrane element; strictly control the inlet turbidity and pollution index (SDI), control the inlet turbidity less than 0.5ntu and the pollution index less than 5; Disinfection and sterilization of pre membrane process and membrane system is an essential and key step to control microbial pollution Sterilization of the system is divided into impact sterilization and continuous sterilization, and different methods can be selected according to different systems)

Control low operating pressure and recovery

Pressure is the driving force of nanofiltration desalination. With the increase of pressure, the water permeability of 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. Too high pressure will aggravate the attenuation of the membrane and may damage the membrane module In order to prolong the service life of membrane module, when the desalination rate and water yield meet the production requirements, the operation with slightly lower pressure is usually adopted, which is of great benefit to the long-term operation of the system

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

Physical cleaning of membrane (product water flushing)

Flushing is to flush the membrane element with low-pressure and large flow water to flush 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 If conditions permit, it is recommended to wash the system frequently Increasing the number of flushing is more effective than a chemical cleaning

Standardize system startup and shutdown operation and shutdown protection measures

When the system starts and stops, the flow and pressure will fluctuate Excessive and rapid flow and pressure fluctuation may lead to extreme pressure drop in the system, forming water hammer, resulting in rupture of membrane elements. Therefore, it is necessary to slowly increase or reduce the pressure and flow during start-up and stop operation

Before startup and shutdown of the system, ensure that there is no vacuum in the pressure vessel, otherwise there will be water hammer or hydraulic impact at the moment of starting the membrane element again, and the above phenomenon will occur when the system that has leaked water is started at the initial startup or general operation

The system shall maintain a low back pressure (pressure on the produced water side), which is higher than that on the raw water side When the pressure is above 0.5MPa, the membrane element will be physically damaged Before starting and stopping the system, fully confirm the opening and closing of valves and the change of pressure, so as to prevent back pressure during operation If the membrane system needs to be shut down for a long time, it is necessary to fill the system with protective fluid or water regularly according to the requirements of the technical manual to ensure the normal standby of membrane components

Conduct on-line chemical cleaning of membrane components regularly

With reasonable pretreatment system and good operation management, it can only reduce the degree of membrane pollution, and it is impossible to completely eliminate membrane pollution Therefore, after the nanofiltration membrane system operates for a period of time, it may be polluted by a variety of pollutants, especially the nanofiltration membrane system used in the sewage advanced treatment unit. The pollution often occurs. Generally, the standardized water yield decreases by about 15%, the system pressure drop between inlet water and concentrated water increases to 1.5 times of the initial value, and the water quality decreases significantly, Chemical cleaning of membrane components is required

In chemical cleaning, we should first judge the types of pollutants, and then select the appropriate cleaning formula and cleaning process according to the characteristics of the membrane When cleaning, pay attention to control the pH value, temperature and flow of cleaning solution In order to ensure the flushing effect, chemical cleaning can be carried out by sectional cleaning if conditions permit At present, specialized membrane cleaning agents are available at home and abroad The cleaning effect can be confirmed by comparing the desalination rate, water production and pressure drop of the device before and after cleaning

For the membrane system of petrochemical wastewater advanced treatment unit, chemical cleaning is generally carried out first for sterilization, then for alkali washing to remove microbial pollution, organic pollution and oil pollution, and then for pickling to eliminate scale pollution and metal hydroxide pollution The cleaning cycle shall be determined according to the actual operation of the device

Off line chemical cleaning of membrane components

When the membrane system cannot recover its performance after multiple on-line chemical cleaning, or the membrane system is heavily polluted, off-line chemical cleaning of membrane components is required Severe pollution of membrane elements refers to the situation that the pressure difference of a single section after pollution is more than twice the pressure difference of a single section at the initial stage of system operation, the water production of the reverse osmosis system decreases by more than 30%, or the quality of a single reverse osmosis membrane element exceeds the normal value by more than 3kg

Judge the pollution type and cleaning process according to the user's full raw water analysis report, performance test results and known system information; If necessary, further verification shall be carried out through special equipment and appliances to determine the specific types of pollutants and the required cleaning formula Clean the removed membrane elements to be cleaned on the special off-line cleaning equipment, and reinstall them for use after passing the inspection