Selection of pre-treatment processes to prevent membrane fouling in reclaimed water reuse!

Membrane fouling refers to the phenomenon where particles, colloidal particles, or solute macromolecules in the feed solution in contact with the membrane adsorb and deposit substances on the surface or pores of the membrane due to physical, chemical, or biological interactions with the membrane, resulting in a decrease or blockage of the membrane pore size, a decrease in membrane flux, and a decrease in separation performance.

Due to membrane fouling, the permeation flux of the membrane is limited, which reduces the service life and separation efficiency of the membrane, seriously restricting the application and development of membrane technology. Therefore, research on membrane fouling in wastewater treatment by membrane method has always been of great concern.

To prevent and control membrane fouling, it is first necessary to understand the filtration mechanisms of various membrane processes.

Generally speaking, the micro/ultrafiltration process mainly follows the sieving mechanism, while the reverse osmosis process follows the dissolution diffusion mechanism. Accordingly, the pollution of reverse osmosis membrane is generally membrane surface pollution, while the pollution of micro/ultrafiltration membrane is generally membrane pollution caused by surface gel layer pollution, adsorption in pores and pore blockage. Surface fouling is generally reversible membrane fouling, while pore blockage and adsorption are generally irreversible membrane fouling.

After clarifying the filtration mechanisms of various membrane processes, membrane fouling can be prevented and controlled from the following aspects.

Selection of pretreatment process

Pre treatment refers to adding appropriate chemicals before filtering the raw material solution to change the properties of the material or solute, or flocculation and filtration of the material solution to remove large suspended particles or gelatinous substances, or adjusting the pH of the material solution to remove membrane pollutants, thereby reducing membrane load and pollution.

Before choosing a pre-treatment process, it is important to first clarify the purpose of the pre-treatment.

Pre treatment before membrane is mainly to prevent or reduce membrane fouling, and to minimize membrane fouling. Therefore, suitable pretreatment processes can be selected based on the needs of different membrane processes and influent requirements. Firstly, it is necessary to identify the key pollutants in various membrane processes in the laboratory and remove or reduce the key components that play a major role in membrane fouling. The purpose of pre-treatment is not to remove all pollutants, but to remove key pollutants that may contaminate or damage the membrane. Therefore, the treatment should be moderate, as excessive pre-treatment can actually cause new membrane fouling.

Selection of pre-treatment processes for membrane fouling caused by inorganic scaling substances

During the operation of the dual membrane system, scaling mainly occurs on the reverse osmosis membrane components. If the hardness and alkalinity of the ultrafiltration influent are too high, scaling is prone to occur in the reverse osmosis membrane section, causing a rapid decrease in the reverse osmosis water production flux. Therefore, in order to reduce fouling and pollution in the reverse osmosis membrane section, pre-treatment of the wastewater before the membrane is necessary. There are several pre-treatment methods to prevent scaling of reverse osmosis membranes, including adding scale inhibitors, adjusting acidity, and removing hardness.

Adding scale inhibitors can control carbonate scale, sulfate scale, and calcium fluoride scale, as well as inhibit silicon scale.

The added scale inhibitors can be divided into three categories: sodium hexametaphosphate, organic phosphates, and polyacrylates.

The advantages of adding scale inhibitors are simple operation, no treatment steps before the membrane, and the amount of additives can be precisely controlled; The disadvantage is that it will increase the COD of concentrated water, which may fail at high recovery rates. If the content of scale inhibitors is high or the type of scale inhibitor is not selected properly, it may still cause membrane blockage.

By adjusting the acidity, calcium carbonate can be maintained in a dissolved state. The advantage of acid regulation treatment is that it is easy to operate, and when the pH of wastewater is not high, the cost is lower than that of hard removal treatment; The disadvantage is that acid adjustment treatment has no removal effect on Ca and Mg ions, and the content of Ca and Mg ions on the concentrated water side is still high, and there is a high requirement for pipeline anti-corrosion. In addition, when using only acid addition to control calcium carbonate scaling, it is required that the LSI or S&DSI index in concentrated water must be negative.

Lime soda ash or sodium hydroxide soda ash can be used for de hardening treatment. Adding calcium hydroxide to water can remove carbonate hardness, while non carbonate hardness can be further reduced by adding sodium carbonate (soda ash). The advantage of lime in removing hardness is that it can simultaneously reduce hardness and alkalinity, which is lower in cost than sodium hydroxide and significantly reduces the LSI of concentrated water; The disadvantage is that the amount of sludge is large, and the sedimentation or filtration operation of sludge is cumbersome. The advantage of sodium hydroxide in removing hardness is that it has a small amount of sludge; The disadvantage is that the removal effect of alkalinity is not good, the LSI of concentrated water is still high, and the treatment cost is higher than that of lime method.

Selection of pre-treatment processes for membrane fouling caused by colloids

Colloidal pollution mainly occurs on ultrafiltration membrane components. If the content of iron, manganese, silicon, etc. in the reverse osmosis influent exceeds a certain value, it will also form fouling in the reverse osmosis membrane components. Therefore, it is necessary to control the content of iron, manganese, silicon, etc. in the influent.

The particle size range of colloids is 1 nm~1 μ m. Usually charged in water. Due to the electrostatic repulsion between colloidal particles, polymerization does not occur and it is difficult to settle naturally. Therefore, it is necessary to add specific chemical agents or use other methods to precipitate the colloidal substance in water based on its characteristics.

The main methods for removing colloids before ultrafiltration membranes include coagulation, flocculation (including electrocoagulation), coagulation, or air flotation. Coagulation is the addition of coagulants with positive ions to wastewater. The presence of a large number of positive ions between colloidal particles can eliminate electrostatic repulsion between colloidal particles, thereby causing particle aggregation. Commonly used coagulants include aluminum sulfate, ferrous sulfate, alum, ferric chloride, etc.

Flocculation is the process of adding a polymer coagulant to wastewater. After the polymer coagulant dissolves, it forms a polymer with a linear structure. One end of the line pulls a small particle, while the other end pulls another small particle. It acts as a bonding bridge between two particles that are far apart, gradually increasing the size of the particles and ultimately forming a large particle flocculent, accelerating particle settling.

Commonly used flocculants include polyacrylamide (PAM), polyiron (PE), etc.

The process of combining coagulation and flocculation is called coagulation process. Coagulation has a removal effect on impurities such as suspended solids, organic matter, and colloidal substances in raw water, with simple operation and low treatment cost. It is currently the most commonly used water treatment method both domestically and internationally. According to literature reports, general coagulation+filtration can remove 60% of colloidal silicon, while coagulation+clarification filtration can remove 90% of colloidal silicon.

Selection of pre-treatment processes for membrane fouling caused by organic matter

In the process of sewage treatment, the main goal is to remove and concentrate organic matter. During the operation of the dual membrane system, membrane fouling caused by organic matter is reflected in both the ultrafiltration and reverse osmosis membrane stages. Due to the relatively high amount of macromolecular organic matter in refinery wastewater, membrane surface pollution caused by macromolecular organic matter in the ultrafiltration membrane section may be more severe than that in the reverse osmosis membrane section. Organic matter is easily adsorbed on the membrane surface, causing a sharp decrease in membrane flux. When high molecular weight organic matter is hydrophobic or positively charged, this adsorption process is easier to carry out; When pH>9, both the membrane surface and organic matter exhibit negative charges. Therefore, a high pH is beneficial for preventing organic pollution. However, organic matter that appears in an emulsified state will form a thin layer of organic pollution on the surface of the membrane, causing serious degradation of membrane performance, which must be removed in the pre-treatment section.

At present, the methods for removing organic matter in pre membrane pretreatment can be divided into two categories: physicochemical methods and biological methods. The physical and chemical methods mainly include adsorption method, flocculant coagulation precipitation method, and advanced oxidation method; The biological method mainly involves deep biochemical treatment before the membrane, including contact oxidation, BAF, A/O, A/O/O, etc. Among them, the commonly used adsorbent in the adsorption method is activated carbon, but the adsorption cost of activated carbon is high, and regeneration is difficult, which is prone to secondary pollution. The flocculation method can effectively remove suspended substances in water and prevent colloidal fouling of the membrane. Advanced oxidation method can effectively remove difficult to degrade organic compounds in water. Biological methods such as contact oxidation and BAF can effectively reduce organic matter, ammonia nitrogen, and oil in water, and retain a large amount of suspended solids. BAF is particularly effective in removing ammonia nitrogen. Different pretreatment methods have their own advantages, and corresponding organic matter removal methods must be selected based on the actual wastewater quality.

Selection of pre-treatment processes for membrane fouling caused by microorganisms

Microbial contamination refers to the phenomenon where microorganisms accumulate and coagulate at the membrane water interface to form a biofilm, which affects the performance of the membrane system. The size of bacteria is generally 1-3 μ m. Microorganisms can be regarded as colloidal substances and can be removed by pre-treatment methods for colloidal contamination or in ultrafiltration membrane systems. However, microorganisms have a strong reproductive ability and can grow rapidly under suitable living conditions.

There are three reasons why membrane components are prone to microbial contamination:

Firstly, the membrane system has a larger membrane surface area, which increases the possibility of adhering bacteria;

The second is that membrane filtration will migrate bacteria to the surface of the membrane;

Thirdly, pre-treatment is also a source of biological pollution. Excessive amounts of flocculants, fungicides, or scale inhibitors added to pre-treatment can become a source of nutrients for microorganisms, and the dampness and darkness inside the membrane components provide an ideal environment for microbial growth.

If microorganisms are not killed before sewage enters the membrane system, these microorganisms will use the membrane as a carrier to reproduce and grow with the help of nutrient salts in the concentrated water section, seriously threatening the long-term stable operation of the membrane system. Therefore, sewage must undergo reasonable sterilization treatment before entering the membrane system, and effective sterilization methods are the key factors to ensure the stable operation of the membrane system.

Sterilization can be divided into chemical sterilization and physical sterilization according to their properties. Chemical sterilization mainly uses fungicides, and their modes of action can be divided into bactericidal and antibacterial effects. The commonly used fungicides in sewage treatment can be divided into inorganic and organic fungicides, as well as oxidizing and non oxidizing fungicides based on their chemical properties. Inorganic fungicides are mainly oxidizing, such as chlorine dioxide, liquid chlorine, ozone, etc., while organic fungicides are mainly cationic quaternary ammonium salts.

Oxidative fungicides are strong oxidizing agents that can oxidize enzymes that play a metabolic role in microorganisms and kill them.

The characteristics of oxidizing fungicides are fast sterilization speed and low cost, but there are certain safety hazards in the use of the agents; Non oxidizing fungicides cause the enzyme system of microorganisms to lose activity, disrupt cell metabolism, and damage cell walls, membranes, or other special parts due to the toxic effects of the agent. Their effects are not affected by reducing substances in water and are not sensitive to pH changes. Non oxidizing fungicides can compensate for the shortcomings of oxidizing fungicides.

The main physical sterilization methods include ultrasonic and magnetic field combination sterilization, variable frequency electric pulse sterilization, and ultraviolet sterilization.

The combination of ultrasound and magnetic field sterilization can automatically and regularly generate strong DC pulse electromagnetic waves of various frequencies, which directly penetrate the cell wall of bacteria and cause bacterial death. At the same time, under the action of this DC pulse electric field, sewage quickly undergoes a weak oxidation-reduction reaction, producing a certain amount of oxidizing substances near the anode area to interact with bacteria, disrupting the normal physiological function of bacteria, causing cell membrane peroxidation and death, thereby achieving the purpose of sterilization.

Ultraviolet sterilization is achieved through ultraviolet irradiation, which can cause photochemical harm to the nucleic acid of microorganisms. Ultraviolet light is absorbed by the nucleic acid of microorganisms, which can cause mutations in the nucleic acid, hinder its replication and transcription, and block protein synthesis; On the other hand, the generation of free radicals can cause photoionization, leading to cell death and achieving the goal of sterilization.

The advantages of UV sterilization are that there is no need to add chemicals to the water, low equipment maintenance requirements, and only regular cleaning or replacement of mercury vapor lamps are required; The disadvantage is that this method is only applicable to relatively clean water sources. Variable frequency electric pulse sterilization is the process of causing bacteria to be electrocuted, and an alternating electric field is applied to form a transmembrane potential that penetrates the cell membrane, leading to microbial death.

The sterilization rate of chemical fungicides is mainly influenced by the type of fungicide, the concentration of fungicides, and the duration of their action.

The surface of polyamide reverse osmosis membranes usually exhibits negative charge. In order to maintain membrane flux and desalination rate, it is advisable to choose fungicides with the same charge as the membrane surface.

Selection of membrane materials

Membrane materials are the core of membrane technology. The adsorption of pollutants on the membrane is the result of the interaction between the membrane, solvent, and pollutants, as well as factors such as membrane surface properties and pore size. Choosing appropriate anti fouling membrane materials based on the properties of pollutants can effectively reduce the adsorption of pollutants by the membrane.

The selection of membrane materials is mainly based on their hydrophilicity, mechanical strength, charge performance, chemical cleaning resistance, and surface roughness.

Many pollutants in the sewage system are charged. Metal hydroxide colloid is generally positively charged, and many polymer floc gel bodies are negatively charged. It is important to correctly judge the charge of main pollutants in sewage for membrane selection. If the charge carried by the ultrafiltration membrane is the same as that of the main pollutants in the wastewater, the membrane fouling will be reduced due to the repulsive effect. In addition, changing the pH of wastewater can alter the Zeta potential on the membrane surface, therefore, membrane fouling can also be slowed down by adjusting the pH of wastewater.

For the selection of hydrophilic and hydrophobic properties of membrane materials, generally speaking, hydrophilic membranes have better anti pollution ability in water treatment, because most of the pollutants in water are hydrophobic substances. However, hydrophilic membranes made from hydrophilic materials have slightly poorer chemical stability and a narrower pH range, while hydrophobic membranes made from hydrophobic polymer materials such as PVDF are widely used due to their more stable chemical properties and high mechanical strength.

To improve the anti fouling ability of hydrophobic membranes, membrane manufacturers generally use different chemical modification methods to modify the original hydrophobic membrane to a more hydrophilic one. At present, ultrafiltration mostly uses modified hydrophilic membranes, and the degree of hydrophilicity varies depending on the modification methods used by different manufacturers.

Selection of membrane pore size

A membrane with appropriate pore size plays an important role in preventing membrane fouling, so after determining the membrane material, it is also necessary to choose an appropriate membrane pore size. This requires first conducting a wastewater quality analysis to determine the main pollutants and their particle size distribution in the wastewater, and then selecting the membrane pore size based on the particle size and distribution of the main pollutants in the wastewater and the desired retention rate.

In theory, while ensuring the retention of required particles or macromolecular solutes, membranes with larger pore size or molecular weight should be selected as much as possible to achieve higher permeability. However, research has found that using membranes with larger pore sizes, although having a higher initial flux, has a higher fouling rate. Over time, the permeability decreases faster, which can easily lead to pore blockage and membrane fouling.

Therefore, the selection of membrane pore size should be smaller than the particle size of intercepted substances, which can not only achieve good treatment results but also reduce pollution caused by solute adsorption and blockage in membrane pores. But the smaller the aperture, the greater the fluid resistance and the smaller the flux.

There are research reports that the intercepted molecular weight (MWCO) of the membrane should be one order of magnitude smaller than the relative molecular weight of the particles to be separated. When the membrane pores are smaller than the size of particles or solutes, due to the effect of transverse flow, they are difficult to stay and aggregate on the membrane surface, making it difficult to block the pores. There are also studies indicating that membrane pore blockage is mainly related to the ratio of particle size to membrane pore size (dp/dm); When dp/dm<2.4, membrane pores formed by particulate matter on the membrane are mainly blocked.

Conclusion

In the process of membrane treatment, membrane fouling is an inevitable problem and also a key and difficult obstacle to the application of membrane technology. Membrane fouling limits the permeation flux of the membrane, shortens its service life, and leads to a decrease in membrane separation performance and efficiency. There are many factors that affect membrane fouling, which are not only related to the characteristics of the membrane itself, but also to membrane components, system operating conditions, and so on.