I. Composition of MBR process
Membrane bioreactor is mainly composed of membrane separation module and bioreactor. Generally, the membrane bioreactor is actually a general term for three types of reactors: ① Aerated membrane bioreactor (AMBR); ② Extraction membrane bioreactor (EMBR); ③ Solid/liquid separation membrane bioreactor (SLS MBR).
Two. Aerated membrane bioreactor
Cote first reported the aeration membrane bioreactor. P et al. in 1988. It adopts air-permeable dense membrane (such as silicone rubber membrane) or microporous membrane (such as hydrophobic polymer membrane), and adopts plate or hollow fiber module to realize bubble-free aeration of bioreactor under the condition of keeping the gas partial pressure below the bubble point. The process is characterized by improving contact time and oxygen transfer efficiency, which is beneficial to the control of aeration process and is not affected by bubble size and residence time in traditional aeration. As shown in figure [1].
Figure [1]
Three. Extraction membrane bioreactor
The extraction membrane bioreactor is also called EMBR (Extraction Membrane Bioreactor). Some industrial wastewater is not suitable for direct contact with microorganisms because of its high pH value or toxic substances to organisms. When the wastewater contains volatile toxic substances, if the traditional aerobic biological treatment process is adopted, the pollutants are easy to volatilize with the aeration flow, resulting in stripping phenomenon, which not only makes the treatment effect unstable, but also causes air pollution. In order to solve these technical problems, British scholar Livingston researched and developed EMB. The process is shown in Figure 2. The wastewater is separated from the activated sludge through the membrane, and the wastewater flows inside the membrane, while the activated sludge containing some specific bacteria flows outside the membrane. Wastewater is not in direct contact with microorganisms, and organic pollutants can be selectively degraded by microorganisms on the other side through the membrane. Because the bioreactor unit and wastewater circulation unit on both sides of the extraction membrane are independent, the water flow of each unit has little interaction, and the living conditions of nutrients and microorganisms in the bioreactor are not affected by the wastewater quality, so the water treatment effect is stable. The operating conditions of the system, such as hydraulic retention time and SRT, can be controlled in the optimal range to maintain the maximum degradation rate of pollutants.
4. Solid-liquid separation membrane bioreactor
Solid-liquid separation membrane bioreactor is the most widely studied membrane bioreactor in the field of water treatment, and it is a water treatment technology that uses membrane separation process to replace the secondary sedimentation tank in the traditional activated sludge process. In the traditional wastewater biological treatment technology, the separation of sludge and water is completed by gravity in the secondary sedimentation tank, and its separation efficiency depends on the sedimentation performance of activated sludge. The better the sedimentation, the higher the efficiency of mud-water separation. However, the sedimentation of sludge depends on the operation of aeration tank, and the operation conditions of aeration tank must be strictly controlled to improve the sedimentation of sludge, which limits the application scope of this method. Due to the requirement of solid-liquid separation in the secondary sedimentation tank, the sludge in the aeration tank cannot maintain a high concentration, which is generally around 1.5~3.5g/L, thus limiting the biochemical reaction rate. Hydraulic retention time (HRT) and sludge age (SRT) depend on each other, so increasing volume load and reducing sludge load are often contradictory. A large amount of excess sludge will be generated during the operation of the system, and its disposal cost accounts for 25% ~ 40% of the operating cost of the sewage treatment plant. The traditional activated sludge treatment system is also prone to sludge bulking, and the effluent contains suspended solids, which deteriorates the effluent quality. In view of the above problems, MBR organically combines membrane separation technology in separation engineering with traditional wastewater biological treatment technology, which greatly improves the solid-liquid separation efficiency, and improves the biochemical reaction rate due to the increase of activated sludge concentration in aeration tank and the appearance of special bacteria (especially dominant flora) in sludge. At the same time, by reducing the F/M ratio, the output of excess sludge is reduced (even zero), thus basically solving many outstanding problems existing in the traditional activated sludge process.
Verb (abbreviation of verb) MBR process type
The following are all solid-liquid separation membrane bioreactors. According to the combination mode of membrane module and bioreactor, membrane bioreactor can be divided into three basic types: separated, integrated and compound. Please refer to Figure 3 for separate and integrated MBR.
As shown in fig. 3, the separated membrane bioreactor separates the membrane module from the bioreactor. The mixed liquid in the bioreactor is pumped into the filtration end of the membrane module after being pressurized by the circulating pump, and the liquid in the mixed liquid permeates the membrane under pressure to become system treatment water; Solids, macromolecules, etc. Intercepted by the membrane and returned to the bioreactor together with the concentrated solution. The separated membrane bioreactor runs stably and reliably, and it is easy to clean, replace and add membranes. Moreover, the membrane flux is generally large. However, in general, in order to reduce the deposition of pollutants on the membrane surface and prolong the cleaning cycle of the membrane, it is necessary to use a circulating pump to provide a high cross-flow speed on the membrane surface, with a large amount of water circulation and high power cost (Yamamoto, 1989), and the shear force generated by the high-speed rotation of the pump will inactivate some microbial cells (Brockmann and Seyfried, 1997).
The integrated membrane bioreactor is to place the membrane module in the bioreactor, as shown in Figure 4. The influent enters the membrane bioreactor, most pollutants are removed by activated sludge in the mixed solution, and the effluent is filtered by membrane under external pressure. This type of membrane bioreactor saves the mixed liquid circulation system and pumping water, so the energy consumption is relatively low. The land occupation is more compact than the separation type, and it has received special attention in the field of water treatment in recent years. However, in general, the membrane flux is relatively low, which is prone to membrane pollution, and it is not easy to clean and replace after membrane pollution.
The composite membrane bioreactor also belongs to the integrated membrane bioreactor in form, but the difference is that filler is added to the bioreactor, thus forming a composite membrane bioreactor, which changes some characteristics of the reactor, as shown in Figure 5:
Characteristics of MBR process
Compared with many traditional biological water treatment processes, MBR has the following main characteristics:
First, the effluent quality is high and stable.
Because of the high separation efficiency of membrane, the separation effect is much better than that of traditional sedimentation tank. The treated water is extremely clear, the suspended matter and turbidity are close to zero, the bacteria and viruses are greatly removed, and the quality of the effluent is better than the Water Quality Standard for Domestic Miscellaneous Water (CJ25. 1-89) issued by the Ministry of Construction, which can be directly reused as non-drinking urban miscellaneous water.
At the same time, membrane separation makes microorganisms completely trapped in the bioreactor, so that the system maintains a high microbial concentration, which not only improves the overall removal efficiency of pollutants by the reaction device, but also ensures good effluent quality. At the same time, the reactor has good adaptability to various changes of influent load (water quality and quantity), and can resist impact load, thus stably obtaining high-quality effluent quality.
Second, the output of excess sludge is low.
The process can operate under high volume load and low sludge load, with low excess sludge output (zero sludge discharge can be realized theoretically), which reduces the sludge treatment cost.
Third, it occupies a small area and is not limited by setting occasions.
The bioreactor can maintain a high concentration of microbial biomass, and the volume load of the treatment device is high, which greatly saves the occupied area; The process flow is simple, the structure is compact, the floor space is saved, and it is not limited by the setting place. Suitable for any occasion, can be made into the ground, semi-underground and underground.
Fourthly, ammonia nitrogen and refractory organic matter can be removed.
Because microorganisms are completely trapped in the bioreactor, it is beneficial to the interception and growth of slow-growing microorganisms such as nitrifying bacteria, and the nitrification efficiency of the system is improved. At the same time, it can increase the hydraulic retention time of some refractory organic compounds in the system, which is beneficial to improve the degradation efficiency of refractory organic compounds.
5. Convenient operation and management, easy to realize automatic control.
The process realizes the complete separation of hydraulic retention time (HRT) and sludge retention time (SRT), and the operation control is more flexible and stable. It is a new technology that is easy to equip in sewage treatment, and can realize automatic control by microcomputer, thus making operation management more convenient.
Sixth, it is easy to transform from traditional technology.
This process can be used as the advanced treatment unit of traditional sewage treatment process, and has broad application prospects in the advanced treatment of effluent from urban secondary sewage treatment plants (thus realizing a large number of urban sewage reuse) and other fields.
Membrane bioreactor also has some disadvantages. Mainly in the following aspects:
O the high cost of membrane makes the capital investment of membrane bioreactor higher than that of traditional sewage treatment process;
O membrane pollution is easy to occur, which brings inconvenience to operation management;
O High energy consumption: firstly, a certain membrane driving pressure must be maintained in the mud-water separation process of MBR; Secondly, the concentration of MLSS in MBR tank is very high, so in order to maintain sufficient oxygen transfer rate, the aeration intensity must be increased; In order to increase membrane flux and reduce membrane pollution, it is necessary to increase the flow rate and clean the membrane surface, which leads to higher energy consumption of MBR than traditional biological treatment process.
Membrane for MBR process
The membrane can be made of many materials, which can be liquid, solid or even gas. At present, most separation membranes used are solid-phase membranes. According to different pore sizes, it can be divided into microfiltration membrane, ultrafiltration membrane, nanofiltration membrane and reverse osmosis membrane; According to different materials, it can be divided into inorganic membrane and organic membrane, and inorganic membrane is mainly microfiltration membrane. The film can be homogeneous or heterogeneous and can be charged or electrically neutral. The membranes widely used in wastewater treatment are mainly solid-phase asymmetric membranes made of organic polymer materials.
The classification of membranes is as shown in the figure:
First, MBR membrane materials
1. Polymer organic membrane materials: polyolefin, polyethylene, polyacrylonitrile, polysulfone, aromatic polyamide, fluoropolymer, etc.
The cost of organic membrane is relatively low, the manufacturing cost is cheap, the manufacturing process of membrane is mature, the pore size and form of membrane are also diverse, and it is widely used, but it is easy to be polluted during operation, with low strength and short service life.
2. Inorganic membrane: it is a solid membrane, which is a semi-permeable membrane made of inorganic materials, such as metals, metal oxides, ceramics, porous glass, zeolites, inorganic polymer materials, etc.
At present, most inorganic membranes used in MBR are ceramic membranes, which have the advantage that they can be used at pH = 0 ~ 14 and pressure P.
Second, the pore size of MBR membrane
Membranes used in MBR process are generally microfiltration membrane (MF) and ultrafiltration membrane (UF), and most of them adopt membrane pore size of 0. 1 ~ 0.4 micron, which is enough for membrane reactors with solid-liquid separation.
Polymer materials commonly used in microfiltration membranes are: polycarbonate, cellulose ester, polyvinylidene fluoride, polysulfone, polytetrafluoroethylene, polyvinyl chloride, polyetherimide, polypropylene, polyetheretherketone, polyamide and so on.
Macromolecular materials commonly used in ultrafiltration include polysulfone, polyethersulfone, polyamide, polyacrylonitrile (PAN), polyvinylidene fluoride, cellulose ester, polyetheretherketone, polyimide, polyetheramide and so on.
Three. MBR membrane module
In order to facilitate industrial production and installation, improve the working efficiency of the membrane and maximize the membrane area per unit volume, the membrane is usually assembled in a basic unit device in some form, and the components in the mixed solution are separated under a certain driving force. This device is called a membrane module.
There are five types of membrane modules commonly used in industry:
Plate and frame assembly, spiral winding assembly, tubular assembly, hollow fiber assembly and capillary assembly. The first two use flat membrane, and the last three use tubular membrane. Diameter of circular tube membrane >:10 mm; Capillary type -0.5 ~ 10.0 mm; Hollow fiber type
Table: Characteristics of various membrane modules
Name/Project Hollow Fiber Capillary Spiral Coil Flat Tube Type
Price (RMB/m3) 40 ~150150 ~ 800 250 ~ 800 800 ~ 2500 400 ~1500.
The packing density is high, medium and low, and medium and low.
Cleaning is difficult and easy, and cleaning is easy.
The pressure drop is high, medium and low.
Can you operate under high pressure? Can it be harder?
There is no limitation on the form of the film.
Membrane modules commonly used in MBR process are: plate-frame type, circular tube type and hollow fiber type.
Plate and frame type:
It is the earliest form of membrane module used in MBR process, and its appearance is similar to that of ordinary plate-and-frame filter press. Its advantages are simple manufacture and assembly, convenient operation, easy maintenance, cleaning and replacement. Disadvantages are: complex sealing, large pressure loss and low packing density.
Circular tube type:
It consists of membrane and membrane support, and has two operation modes: internal pressure mode and external pressure mode. In practice, the internal pressure type is mostly used, that is, water flows in from the pipeline and the permeate flows out from the outside. The diameter of the membrane is 6-24mm, and the advantages of the circular tube membrane are that it can control the turbulent flow of feed liquid, is not easy to block, is easy to clean and has small pressure loss. Disadvantages: low packing density.
Hollow fiber type:
The assembly form is shown in the following figure:
[figure]
Generally, the outer diameter is 40 ~ 250μ m, and the inner diameter is 25 ~ 42μ m. Advantages: high compressive strength, not easy to deform. In MBR, modules are usually put directly into the reactor without pressure vessel to form submerged membrane bioreactor. Generally, it is an external membrane pressing component. Advantages are: high packing density; The cost is relatively low; The service life is longer, and the nylon hollow fiber membrane with stable physical and chemical properties and low water permeability can be used; The film has good pressure resistance and does not need supporting materials. Disadvantages are: sensitive to clogging, pollution and concentration polarization have great influence on the separation performance of the membrane.
General requirements for MBR membrane module design:
O Provide sufficient mechanical support for the membrane, with smooth flow channel and no dead angle and stagnant water;
O Low energy consumption, minimizing concentration polarization, improving separation efficiency and reducing membrane pollution;
O The packing density is as high as possible, which is convenient for installation, cleaning and replacement;
O Sufficient mechanical strength, chemical and thermal stability.
The selection of membrane module should comprehensively consider factors such as cost, packing density, application situation, system flow, membrane pollution, cleaning and service life.
Application field of MBR
In the middle and late 1990s, membrane bioreactor has entered the practical application stage abroad. Zenon Company of Canada first introduced ultrafiltration tubular membrane bioreactor and applied it to municipal sewage treatment. In order to save energy, the company also developed submerged hollow fiber membrane modules, and the membrane bioreactors developed by it have been applied in more than ten places such as the United States, Germany, France, Egypt, etc., with scales ranging from 380m 3 /d to 7600m 3 /d d. Japan's Mitsubishi Liyang Company is also a world-renowned supplier of submerged hollow fiber membranes, and has accumulated many years of experience in MBR application, and built many practical MBR projects in Japan and other countries. Kubota Company of Japan is another competitive company in the practical application of membrane bioreactor, and its flat membrane has the characteristics of large circulation, anti-pollution and simple process. Some domestic researchers and enterprises are also trying to make MBR practical.
At present, membrane bioreactor has been applied in the following fields:
A, urban sewage treatment and building water reuse
1967 Dorr-Oliver Company of the United States built the first sewage treatment plant adopting MBR process to treat wastewater14m3/d. During the period of 1977, a high-rise building in Japan adopted a sewage reuse system. 1980, two MBR treatment plants were built in Japan, with the treatment capacities of 65438+1100m3/d and 50m 3 /d respectively. In the mid-1990s, there were 39 such factories in operation in Japan, with a maximum treatment capacity of 500m 3 /d, and more than 65,438+000 high-rise buildings used MBR to treat sewage and reuse it in the middle canal. 1997, Wessex established the largest MBR system in the world at that time in Pollock, England, with a daily processing capacity of 2,000 m3. 1999 also built a13,000m3/d MBR plant in Swanage, Dorset [14].
1May, 1998, the pilot system of integrated membrane bioreactor in Tsinghua University passed the national appraisal. At the beginning of 2000, Tsinghua University established a practical MBR system in Haidian Town Hospital to treat hospital wastewater. The project was completed and put into use in June 2000, and it is running normally at present. In September, 2000, Professor Yang Zaoyan of Tianjin University and his research team established the MBR demonstration project in Chen Pu Building, Tianjin New Technology Industrial Park. The system treats 25 tons of sewage every day, and all the treated sewage is used for toilet flushing and green space irrigation, covering an area of 10 square meter, and the energy consumption for treating each ton of sewage is 0.7 kW h.
Two. Industrial wastewater treatment
Since 1990s, the processing objects of MBR have been expanding. In addition to reclaimed water reuse and fecal sewage treatment, the application of MBR in industrial wastewater treatment has also been widely concerned, such as food industry wastewater, aquatic product processing wastewater, aquaculture wastewater, cosmetics production wastewater, dye wastewater and petrochemical wastewater, and achieved good treatment results. In the early 1990s, the United States built a MBR system in Ohio to treat industrial wastewater from an automobile factory. The treatment scale was 15 1m 3 /d, the organic load of the system reached 6.3 kg COD/m3 d, the cod removal rate was 94%, and most of the oil was degraded. A grease extraction and processing factory in the Netherlands used traditional oxidation ditch wastewater treatment technology to treat its production wastewater. Due to the expansion of production scale, sludge swells and it is difficult to separate. Finally, Zenon membrane module was used instead of sedimentation tank, and the operation effect was good.
Three. Purification of micro-polluted drinking water
With the wide application of nitrogen fertilizer and pesticides in agriculture, drinking water is polluted to varying degrees. In the mid-1990s, LyonnaisedesEaux developed an MBR process with the functions of biological nitrogen removal, pesticide adsorption and turbidity removal. 1995, Lyonnaise de Seaux built a factory with a daily drinking water capacity of 400m 3 in Douchy, France. The concentration of nitrogen in wastewater is lower than 0. 1.02 μ g/L, and the concentration of pesticide is lower than 0.02 μ g/L. ..
Four. Fecal sewage treatment
The organic matter content in fecal sewage is very high, and the traditional denitrification method needs high sludge concentration, and the solid-liquid separation is unstable, which affects the tertiary treatment effect. The appearance of MBR solved this problem well and made it possible to treat fecal sewage directly without dilution.
Japan has developed a fecal treatment technology called NS system, the core of which is a system combining flat membrane device with aerobic activated sludge bioreactor with high concentration. NS system was built in Yuegu City, saitama in 1985, and the production scale was10kl/d. In 1989, new excreta treatment facilities were built in Nagasaki Prefecture and Kumamoto Prefecture. In NS system, each group of flat membranes is about 0.4m2 * * and dozens of groups are installed in parallel, making it a frame device that can be automatically opened and cleaned. The membrane material is polysulfone ultrafiltration membrane with molecular weight cut-off of 20000. The sludge concentration in the reactor was kept within the range of15000 ~18000mg/L. By 1994, there were more than 1200 MBR systems in Japan to treat the fecal sewage of more than 40 million people.