1、 What is ultrafiltration membrane

Ultrafiltration membrane is one of the earliest developed polymer membranes, and it is a microporous filter membrane with a rated pore size range of 0.001-0.02 μ M. The solvent in the solution and some of the solutes with low molecular weight penetrate the other side of the membrane from the micro pores of the ultrafiltration membrane, while the solute with higher molecular weight or some emulsion micelles are intercepted, thus achieving the filtration and separation effect.

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In the field of water treatment, the filtration efficiency of impurities is higher than other filtration technologies, and the filtration precision can reach 99.99%, which can effectively remove most of the harmful substances in water; And the use of few or no chemicals, effectively avoid secondary pollution of water quality, so the treated water quality is better. From the operation level, the filtration system based on ultrafiltration membrane technology has high automation, simple and reliable operation, only on and off. Because of the strong chemical stability, acid-base corrosion and high temperature resistance of the ultrafiltration membrane, it can be sterilized at high temperature and has wide applicability.

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1. Ultrafiltration membrane technology and its characteristics

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The technical principle of ultrafiltration membrane

Ultrafiltration membrane technology is a membrane through separation technology, whose filtration capacity is between nanofiltration and microfiltration. Its working principle is as follows:

When the solution passes through a semi permeable membrane, under the action of pressure, the solvent and small molecular matter in the solute can reach the other side of the membrane through the filter membrane. However, the macromolecular substances and colloids in the solute are intercepted because they cannot pass through the pore of the filter membrane. With the solution not flowing, more and more substances are intercepted on the membrane, Therefore, to achieve ultrafiltration, it is necessary to exert greater pressure on the solvent. At the same time, the substances formed on the surface of the membrane also show certain chemical properties, and also have the function of interception and decomposition for some pollutants, so as to realize water purification.

With the increasing concentration of macromolecule matter, the filtration speed on the membrane surface is decreasing, and the phenomenon of "concentration polarization" appears. In order to make ultrafiltration can be carried out continuously and effectively, the phenomenon of "concentration polarization" is often eliminated by using the stirrer filter device in practice.

Characteristics of ultrafiltration membrane technology

Compared with other water treatment technologies, ultrafiltration membrane technology has many incomparable advantages

First, the ultrafiltration membrane has high chemical stability, high temperature, acid and alkali resistance, so the requirements for water quality of the water are not high and the universality is strong;

Secondly, the ultrafiltration membrane technology is simple in principle, easy to realize automatic operation, saving labor, easy to operate, easy to maintain, and safe and stable;

Thirdly, ultrafiltration membrane technology belongs to physical method, and no chemical agent is needed in water treatment process, so it can effectively prevent secondary pollution of water body;

Fourth, the technology efficiency of ultrafiltration membrane is high, and the water treatment is large, especially for the drinking water treatment in cities with less pollution, which shows high efficiency;

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Application of ultrafiltration membrane technology in water treatment of environmental protection engineering

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Purification of drinking water in cities

With the development of society, people have higher and higher requirements for drinking water safety. But at the same time, the pollution of urban water source is becoming more and more serious. The water quality of direct water intake is increasingly unable to meet the standard of drinking water. Therefore, it is necessary to purify the drinking water in cities.

Urban drinking water mainly comes from groundwater and surface water. The pollution mechanism of the two sources is different. The source of drinking water is mainly based on the two types of groundwater and surface water. The pollution mechanism of the two sources is different, but the pollutants are mainly asexual, bacterial, fungi, viruses, suspended substances, etc.

Traditional drinking water purification method can realize the inactivation purification of microorganisms and fungi, and the purification of micron suspended particles. On this basis, ultrafiltration membrane technology can also effectively remove nano particles. Therefore, the effluent quality is higher and it is of great significance for the health of urban residents.

Seawater desalination

Non renewable resources, the freshwater resources available for human consumption on the earth are increasingly exhausted, and the shortage of water resources has become one of the most urgent problems facing contemporary human beings. Desalination is considered to be an effective way to solve the drinking water crisis. At present, the technology of seawater conversions which are studied more and more in the world is electrodialysis technology. Although electrodialysis is considered to be an effective method to desalinate seawater, it has high operating cost and low recovery rate. With the development of Technology, ultrafiltration membrane technology has been used in reverse osmosis desalination, Its excellent separation and physicochemical properties make the desalination efficiency further improved, and the energy consumption will be greatly reduced.

Treatment of electroplating wastewater

The wastewater produced by the electric industry is huge, and it contains a large number of heavy metals such as hexavalent chromium, copper, nickel, etc., which is very harmful and has very low biochemical properties. In practice, iron oxidation electrolysis is often used. However, iron oxidation will produce a large amount of sludge, which needs further treatment: Although electrolysis can treat the electric wastewater well, it has a high operating cost, Not suitable for wide-scale promotion. The combination of ultrafiltration purchase technology and reverse osmosis technology is considered as an effective method for the treatment of electric wastewater. The use of two membrane technologies can remove most of heavy metals, organic carbon and nitrate in electroplating wastewater, and the use of ultrafiltration membrane also reduces the pollution of the membrane and improves the service life.

Treatment of oily wastewater

The main sources of oil-bearing wastewater include crude oil leakage, slaughterhouse wastewater and domestic wastewater. The main components of the wastewater are oil floating, dispersed oil, emulsifying oil and heavy oil. The commonly used oil-bearing wastewater treatment device is oil separation tank, but it can not treat emulsion oil, so it is often used for axial treatment by air flotation. Because the emulsification oil molecules are generally large, the ultrafiltration membrane technology can be used to make the oil-bearing wastewater pass through the ultrafiltration membrane under the pressure condition, and the emulsion oil and other macromolecular pollutants will be intercepted and the removal efficiency is high.

Reuse of urban sewage

The reuse of urban sewage is an important measure to relieve the pressure of urban water. After the treatment of urban domestic sewage reaches the reuse standard, it is used in urban green water and urban water system. The ultrafiltration membrane technology can quickly treat the municipal sewage to the standard. Because of the good biochemical property of a unit of urban sewage, in order to improve the effluent quality, the periodic circulating activated sludge method (CASS) and ultrafiltration membrane technology are often used simultaneously. Under the condition of 12 hours of hydraulic action, the COD removal rate of this method is over 86%, ammonia nitrogen removal rate is over 90%, and the pH value of effluent is 7.25-7.89, which meets the standard of urban water reuse.

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Food industry wastewater recovery

In addition to improving the quality of effluent, ultrafiltration membrane technology can concentrate and recycle a large number of useful solid substances. The most typical application is in the field of food industry. The waste water produced by food industry contains a lot of fat, protein, starch, yeast, etc. if these substances are discharged into the external environment, they will not only cause environmental pollution, but also a lot of waste. Therefore, the ultrafiltration model technology is used to cut off the useful components in the wastewater, and the BOD and COD in the water are also separated from the water, The recovery of the separated peripheral materials can bring great economic benefits to the enterprises.

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Closing remarks

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In conclusion, ultrafiltration membrane technology is an important technology of environmental protection engineering water treatment, which has a very broad application prospect in urban sewage treatment, various industrial wastewater treatment and useful material recovery. At present, the research direction of ultrafiltration membrane technology is to invent more efficient ultrafiltration device, and the other is to combine with other water treatment technologies according to the characteristics of water quality of water inlet to improve the quality of effluent.

2、 Operation mode of ultrafiltration

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1.1.2 cross flow filtration

When the suspended matter and turbidity of ultrafiltration water is high, such as sewage or sewage reuse treatment application, ultrafiltration can operate in the mode of cross flow filtration. The water enters the ultrafiltration membrane assembly, and part of it becomes water production through the membrane surface, while the other part is discharged into concentrated water by containing impurities such as suspended substances. After the discharged concentrated water is pressurized again, it will be recycled back to the membrane assembly, so as to maintain the shear force generated by the high velocity on the membrane surface, and take away the suspended substances and other impurities intercepted on the membrane surface, Thus, the pollution layer of ultrafiltration membrane module is kept at a thin level.

1.1.3 concentrated water discharge filtration

When the content of suspended matter in the influent of ultrafiltration is low, ultrafiltration can be operated according to the filtration mode of concentrated water discharge. The water enters the ultrafiltration membrane module, and the membrane module is discharged with low concentration water. Generally, 5-10% of the water inflow is generated by most of the water passing through the membrane surface.

The operation of concentrated water discharge filtration and cross flow filtration mode also requires regular water backwash, chemical strengthening backwash and regular chemical cleaning to restore the filtration performance of ultrafiltration membrane. The energy consumption and operating pressure of the full flow filtration mode are low, so the operation cost is lower; The cross flow filtration mode can deal with the higher suspended matter content of water. The selection of specific models needs to be determined according to the suspended matter content, turbidity and COD in the inflow.

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2. Ultrafiltration membrane pollution

2.1 detection in operation of ultrafiltration membrane

In order to check the operation effect of ultrafiltration device and the possibility of membrane fouling, some key parameters need to be monitored during the operation of ultrafiltration device.

2.1.1 turbidity: refers to the suspended substances and colloidal substances such as mud sand, dust, fine organic matter, plankton and other suspended substances in water, which will cause the water quality to become turbid and present a certain degree of turbidity. Usually these suspended substances and colloidal substances will also parasitize bacteria and viruses. For example, the turbidity of drinking water should not exceed 1 NTU, and the turbidity of effluent of ultrafiltration membrane should not exceed 0.1 NTU.

2.1.2 TSS (total suspended solid): it refers to the water sample passing through the pore diameter of 0.45 μ The membrane of M is retained on the filter membrane and dried to constant weight solid matter at 103 ℃ ~ 105 ℃. Total suspended solids are one of the important indicators to measure the pollution degree of water quality. Generally, the parameters are more accurate than turbidity (turbidity usually cannot detect very fine particles).

2.1.3 SDI (siltation density index): it is one of the important water quality index parameters of reverse osmosis water treatment system. SDI value represents the content of particles, colloids and other substances that can block various water purification equipment. Generally, this parameter is used to judge the possibility of blocking various water purification equipment by particles and colloids in water (see the figure below).

SDI was determined at a diameter of 47mm and a diameter of 0.45 μ The measured water samples with a certain pressure (30psi, equivalent to 2.1kg/cm) are added to the membrane of M. the time Ti (seconds) required for filtering 500ml of water and the time TF (seconds) required for filtering 500ml of water after continuous filtration for 15 minutes (T) are recorded. The SDI value is calculated by formula; Generally, SDI value of RO inlet (i.e. ultrafiltration water production) shall not exceed 5.

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2.1.4 TOC: this parameter is most commonly used to determine the content of organic matter in water, which refers to the total amount of carbon in dissolved and suspended organic matter in water, including natural organic matter and synthetic organic matter. Total organic carbon is generally used to evaluate the possibility and trend of organic and biological fouling plugging in membrane caused by ultrafiltration. When TOC of the filter membrane is more than 2 mg / L, it is very likely that the biofiltration membrane surface will be blocked by biological pollution.

2.1.5 doc (dissolved organic carbon): the part of total organic carbon (TOC) that can be dissolved in water, generally refers to the organic carbon which can pass through a 0.45 micron filter membrane and can not evaporate during the analysis. The proportion of dissolved organic carbon (DOC) in most natural water bodies to total organic carbon (TOC) is about 80-95% except for polluted water.

2.1.6 iron and manganese: the oxidation morphology of iron and manganese can be intercepted by ultrafiltration membrane system, but it will also cause fouling blocking of the membrane. Iron ions generally exist naturally (such as groundwater), or are caused by corrosion of the pipeline or equipment before ultrafiltration, or the flocculant residue is added to the coagulation and clarification equipment of ultrafiltration pretreatment.

2.1.7 calcium and magnesium: the hardness of water mainly comes from calcium and magnesium ions. According to the hardness, the water can be soft water (the maximum value is not more than 60mg/l in CaCO3), hard water (the maximum is 180mg/l in CaCO3) and extremely hard water (more than 180mg/l in CaCO3). Hardness is not harmful to human health, but the hardness of water is too high, which will cause scaling on the surface of pipes, equipment or membrane during water treatment.

2.1.8 conductivity: the conductivity of water is linear with total dissolved solids (TDS), indicating the conductivity of water.

2.1.9 pH value: used to indicate the acid-base value of water. If the pH value is less than 7, it is acidic and the pH value is more than 7, it is alkaline. The pH value of pure water is 7 neutral. High pH will cause bitterness of water and easily cause scaling of water pipes and equipment, and low pH water will corrode or dissolve metals and other equipment.

2.1.10 silica: it can be divided into active silica (dissolved silicon) or inactive silica (colloidal silicon). Generally, colloidal silicon will accelerate the fouling plugging of ultrafiltration membrane.

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2.3 types of ultrafiltration membrane pollution

2.3.1 colloid pollution: colloids mainly exist in surface water, especially with the change of seasons, there are a lot of suspended substances such as clay and silt in the water, which is very harmful to the ultrafiltration membrane. Because during the filtration process, a large number of colloidal particles flow to the surface of the membrane with the passing film. The particles left behind by the membrane are easy to form a gel layer, and some particles which are equivalent to the pore size of the membrane and smaller than the pore size of the membrane will permeate into the membrane hole and block the flow channel and produce irreversible change. In addition, iron and manganese in water and colloid formed by adding iron or Aluminum Coagulant in ultrafiltration pretreatment may form a gel layer on the surface of the membrane.

2.3.2 organic pollution: some organic substances in water are added manually during water treatment, such as surfactant, cleaner and polymer flocculant, and others are in natural water; These substances can also be adsorbed on the surface of the membrane and damage the performance of the membrane.

2.3.3 microbial pollution: microbial pollution is also a risk factor for the safe operation of ultrafiltration membrane. Some nutrients are trapped by the membrane and accumulate on the membrane surface. Bacteria propagate rapidly in this environment. The living bacteria, together with their excretion substances, form microbial mucus and adhere to the membrane surface. These mucus are combined with other precipitates to form a complex covering layer. The results not only affect the permeability of the membrane, but also make the membrane produce irreversible pollution blocking.

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3.1 bubble observation

Fill the membrane assembly with the liquid used for the test, make the membrane wire completely infiltrate, and all the holes of the membrane wire are filled with liquid. Slowly put oil-free compressed air into the inlet side of membrane assembly, and gradually increase the inlet pressure. Meanwhile, observe whether there is bubble overflow on the water producing side (the water producing valve is in open state). Normally, the pressure of air is from 0bar to 1.5bar. If there is continuous bubble generation at 1.5bar, it indicates that the membrane assembly is defective.

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3.2 pressure attenuation method

Fill the membrane module with the liquid used for the test to completely infiltrate the membrane wire, and all holes of the membrane wire are filled with liquid. Slowly inject oil-free compressed air into the water inlet side of the membrane module, and gradually increase the inlet pressure to the set value (the water production valve is open). For the external pressure ultrafiltration membrane module, the set value of the test pressure is 2.0bar.

Initially, the liquid on the inlet side will enter the water production side through the membrane wire under pressure, so a certain amount of liquid will be discharged (lasting for about 2 minutes). When the pressure is stable at the set value, stop the air inlet (the valve on the water production side is open), seal the air inlet side, maintain the test pressure, and keep the pressure stationary for 10 minutes.

At this time, the water inlet side of the membrane module is filled with pressurized air and isolated from the outside world; The water production side is filled with liquid and connected with the atmosphere. If the pressure drop at the inlet side is not greater than 0.2bar after maintaining the pressure test for 10 minutes, it indicates that the membrane module is complete; If the pressure drop is greater than 0.2bar, it indicates that the membrane module is defective (broken wire or leakage, etc.).

The pressure attenuation test can be carried out either for a single membrane module or for a whole membrane device. It is a simple and easy method on site.

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4. Conclusion

4.1 operation management of ultrafiltration system

4.1.1 monitor and record the COD, turbidity and iron / manganese metal of inlet and outlet water of ultrafiltration pretreatment at least weekly;

4.1.2 check the consumption of flocculant, coagulant aid and other chemicals used for pretreatment once a week;

4.1.3 calibrate various instruments at least every three months;

4.2 maintenance of ultrafiltration system

4.2.1 the ultrafiltration system can be shut down for 1-2 days for 30-60 minutes every day or conduct a separate backwash;

4.2.2 shut down the ultrafiltration system for 2-7 days. After thorough backwashing, close the inlet and outlet valves for storage; The inlet and outlet of hso3 protection solution can be kept for 30 minutes, or the inlet and outlet of hso3 protection solution can be injected for 30 minutes every day;

4.2.3 the ultrafiltration system is out of service for more than 7 days. Before shutdown, strengthen backwashing, inject protective solution (0.5-1.0% NaHSO3 solution), and close the inlet and outlet valves for storage. Check the pH value of the protective solution once a month. If the pH is less than 3, replace the protective solution in time;

When the 4.2.4 ultrafiltration system is put into operation again after a long time shutdown, the ultrafiltration device should be continuously washed to the foam without water, and the protective fluid will be rinsed clean.

4.3 precautions for chemical cleaning

Chemical cleaning is the most effective method to solve the problem of membrane pollution. For specific pollution, only corresponding cleaning methods can achieve good results. If you choose cleaning chemicals and methods incorrectly, sometimes the situation will worsen. Therefore, it is necessary to determine the type of dirt on the membrane surface before cleaning.

During daily operation, the operation performance of ultrafiltration system must be strictly monitored, including operating differential pressure and produced water flow. With the pollution of ultrafiltration membrane, the differential pressure will increase and the produced water flow will decrease. When the standardized water yield decreases by 25% or the standardized transmembrane pressure difference increases by 1.0-2.0bar, chemical cleaning must be carried out to restore its performance; However, it should be noted that if the inlet water temperature decreases, the water production flow of the ultrafiltration membrane will also decrease. This is a normal phenomenon, not caused by membrane pollution. At this time, the ultrafiltration membrane may not need to be cleaned.

In short, ultrafiltration device is the most effective reverse osmosis pretreatment technology so far. Only when it is reasonably controlled in the actual operation process to curb the occurrence of membrane fouling, can the service life of ultrafiltration membrane be effectively guaranteed.