The MBR device
Ⅰ、Introduction
In the field of sewage treatment and water resource reuse, MBR is also known as membrane bioreactor(Membrane Bio-Reactor)，It is a new water treatment technology which combines membrane separation unit and biological treatment unit。The membrane structure mainly consists of flat membrane and hollow fiber membrane, which can be divided into ultrafiltration technology according to the membrane pore size.
Ⅱ、Process
Membrane - bioreactor is mainly composed of membrane separation module and bioreactor. The membrane bioreactor commonly referred to is actually a general term for three types of reactors:
①Aeration Membrane Bioreactor, AMBR ;
② ExtractiveMembrane Bioreactor, EMBR;
③ Solid/Liquid SeparationMembrane Bioreactor, SLSMBR, MBR.
Aeration membrane
Aeration film -- Bioreactor (AMBR) was first reported in Cote.P et al., 1988. It was reported that the air-permeable tight film (such as silicone rubber film) or microporous film (such as hydrophobic polymerization film) was adopted to realize the bubble-free aeration to the bioreactor with plate or hollow fiber components, while keeping the gas fraction lower than the Bubble Point. The characteristics of this process are that the contact time and oxygen transfer efficiency are improved, which is conducive to the control of aeration process and is not affected by the factors of bubble size and residence time in traditional aeration.
Extraction of membrane
Extractive Membrane -- the Bioreactor is also known as the EMBR Membrane Bioreactor. Some industrial wastewater should not be treated by direct contact with microorganisms because of high ph or the presence of biotoxic substances. When the waste water contains volatile toxic substances, if the traditional aerobic biological treatment process is adopted, the pollutants are likely to be volatilized with aeration air flow, resulting in the phenomenon of gas extraction. The treatment effect is not only very stable, but also causes air pollution.
To solve these technical problems, Livingston, a British scholar, developed the EMB. Wastewater and activated sludge are separated by membrane, wastewater flows in the membrane, while activated sludge containing some obligate bacteria flows in the membrane, wastewater and microorganisms do not have direct contact, organic pollutants can be selectively through the membrane by the other side of the microbial degradation. As the bioreactor units and wastewater circulation units on both sides of the extraction membrane are independent, the water flow of each unit has little influence on each other, and the nutrients and living conditions of microorganisms in the bioreactor are not affected by the water quality of the wastewater, so that the water treatment effect is stable. The operating conditions of the system such as HRT and SRT can be controlled in the optimal range respectively to maintain the maximum pollutant degradation rate.
Solid-liquid separation membrane
Solid-liquid separation membrane bioreactor is one of the most widely and deeply studied membrane bioreactors in the field of water treatment. 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 sludge-water separation is completed by gravity in the secondary sedimentation tank, and its separation efficiency depends on the settling performance of activated sludge. The better the settling performance is, the higher the sludge-water separation efficiency will be. The sedimentation of sludge depends on the operational condition of aeration tank, so the operational condition of aeration tank must be strictly controlled to improve the sedimentation of sludge, which limits the application scope of this method. Due to the requirements of solid-liquid separation in the secondary sedimentation tank, the sludge in the aeration tank cannot maintain a high concentration, generally around 1.5~3.5g/L, which limits the biochemical reaction rate. Hydraulic retention time (HRT) and sludge age (SRT) depend on each other, and there is often a contradiction between increasing volume load and reducing sludge load. A large amount of residual sludge is produced 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, the effluent contains suspended solids, and the effluent quality deteriorates.
Aiming at 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. The increase of the concentration of activated sludge in the aeration tank and the emergence of specific bacteria (especially dominant bacteria) in the sludge increased the biochemical reaction rate. At the same time, by reducing the F/M ratio to reduce the amount of residual sludge (even 0), many outstanding problems existing in the traditional activated sludge method were basically solved.
Ⅲ、Process types
According to the combination mode of membrane module and bioreactor, membrane bioreactor can be divided into three basic types: split type, integrated type and compound type. (The following are all solid-liquid separation membrane bioreactors)
The metal
Separate the membrane module from the bioreactor. The mixture in the bioreactor is pressurized by the circulating pump and then pumped to the filter end of the membrane module. Under the action of pressure, the liquid in the mixture passes through the membrane and becomes the system treatment water. Solids and macromolecules are trapped by the membrane and returned to the bioreactor with the concentrate.
The characteristics of split membrane bioreactor are stable and reliable operation, easy to clean, replace and add the membrane. And the membrane flux is generally large. However, in general conditions, in order to reduce the deposition of pollutants on the membrane surface and prolong the membrane cleaning cycle, circulating pumps are required to provide a high cross-current velocity on the membrane surface, with a large flow cycle and high power cost (Yamamoto,1989). Moreover, the shear force generated by the high-speed rotation of the pump will cause inactivation of some microbial bacteria (Brockmann and Seyfried,1997).
one-piece
The membrane module is placed inside the bioreactor. The inlet water enters the membrane bioreactor, in which most of the pollutants are removed by the active sludge in the mixture, and then the effluent is filtered by the membrane under the action of external pressure.
This kind of membrane bioreactor saves the circulation system of mixed liquid and relies on pumping water, so the energy consumption is relatively low. It is more compact than split type and has received special attention in the field of water treatment. However, generally, the membrane flux is relatively low, which is prone to membrane fouling, and it is not easy to clean and replace after membrane fouling.
Compound
In form, it also belongs to the integrated membrane - bioreactor, but the difference is that a filler is added in the bioreactor to form the compound membrane - bioreactor, which changes some properties of the reactor.
Ⅳ、Process characteristics
Compared with many traditional biological water treatment processes, MBR has the following major advantages:
The quality of the effluent is stable
Due to the high efficiency of membrane separation, the separation effect is far better than the traditional sedimentation tank, the treated water is extremely clear, suspended matter and turbidity are close to zero, bacteria and viruses are greatly removed, the effluent water quality is better than the standard of domestic water quality issued by the Ministry of Construction (CJ25.1-89), can be directly used as non-drinking municipal water for reuse.
At the same time, membrane separation and microbial fully be captured within the bioreactor, enables the system to maintain high microorganism concentration, reaction device not only improves the overall removal efficiency of pollutants, to ensure the good water quality, and the reactor () of water quality and quantity of water load variations have good adaptability, resistance to shock loading, to be able to get high quality of effluent water quality stability.
The surplus sludge yield is low
The process can be operated under high volume load and low sludge load, with low surplus sludge yield (theoretically, zero sludge discharge can be achieved), which reduces the sludge treatment cost.
It covers a small area and is not limited by setting occasions
The bioreactor can maintain a high concentration of microorganisms, and the treatment unit has a high volume load, thus greatly saving the floor space. The process flow is simple, compact structure, the floor area is less, is not limited by the setting place, suitable for any occasion, can be made into surface type, semi-underground type and underground type.
Ammonia nitrogen and refractory organic matter can be removed
Since microorganisms are completely intercepted in the bioreactor, it is beneficial to the interception and growth of slow-proliferating microorganisms such as nitrifying bacteria, and the efficiency of system nitrification can be improved. At the same time, the hydraulic retention time of some refractory organic compounds in the system can be increased, which is conducive to the improvement of the degradation efficiency of refractory organic compounds.
Convenient operation and management, easy to realize automatic control
This process realizes the complete separation of hydraulic retention time (HRT) and sludge retention time (SRT), makes the operation control more flexible and stable, and is a new technology easy to realize the equipment in sewage treatment, and can realize the microcomputer automatic control, thus making the operation and management more convenient.
It is easy to transform from traditional technology
This process can be used as the deep treatment unit of the traditional sewage treatment process, and it has a broad application prospect in the fields such as the deep treatment of effluent from the urban secondary sewage treatment plant (so as to realize a large amount of reuse of urban sewage).
Membrane-bioreactor also has some shortcomings. It is mainly manifested in the following aspects:
(1)The high cost of membrane makes the construction investment of membrane bioreactor higher than that of traditional wastewater treatment technology;
(2)Membrane fouling is easy to occur, which brings inconvenience to operation and management;
(3)High energy consumption: First, the MBR mud-water separation process must maintain a certain membrane drive pressure; Secondly, the CONCENTRATION of MLSS in MBR pool is very high. To maintain sufficient oxygen transfer rate, the aeration strength must be increased. In addition, in order to increase the membrane flux and reduce membrane pollution, it is necessary to increase the flow rate and wash the membrane surface, which results in higher energy consumption of MBR than the traditional biological treatment process.
Ⅴ、Application field
In the middle and late 1990s, membrane bioreactor has entered the stage of practical application in foreign countries. Zenon, a Canadian firm, pioneered the bioreactor, an ultrafiltration membrane used in municipal wastewater treatment. To save energy, the company has developed immersive hollow fiber membrane modules, and its membrane-bioreactors, which range in size from 380m3 /d to 7,600m3 /d, have been used in more than a dozen locations in the United States, Germany, France and Egypt. Mitsubishi Rayon is also the world's leading provider of immersive hollow fiber membrane. It has accumulated many years of experience in the application of MBR, and has many practical MBR projects in Japan and other countries. Kubota corporation of Japan is another company that is competitive in the practical application of membrane bioreactor. The plate membrane produced by Kubota company has the characteristics of large flux, pollution resistance and simple process. Some domestic researchers and enterprises are also trying to make MBR practical.
Membrane bioreactors have been applied in the following fields:
Municipal sewage treatment and water reuse in buildings
In 1967 the first MBR wastewater treatment plant was built by dorr-oliver in the United States, which treated 14m3/d of wastewater. In 1977, a sewage reuse system was put into practice in a high-rise building in Japan. In 1980, Two MBR treatment plants with processing capacity of 10m3/d and 50m3/d were built in Japan. In the mid-1990s, Japan had 39 such plants in operation, with a maximum capacity of 500m3 /d, and more than 100 tall buildings using THE MBR to recycle their sewage back into waterways. In 1997, Wessex established the world's largest MBR system in Porlock, UK, with a daily processing capacity of 2000m3, and in 1999 built a 13000m3/d MBR factory in Swanage, Dorset.
In May 1998, the one-piece membrane bioreactor pilot test system carried out by Tsinghua University passed the national appraisal. At the beginning of 2000, Tsinghua University set up a practical MBR system to treat hospital wastewater in Haidian Township Hospital in Beijing. The project was completed and put into use in June 2000, and is running normally. In September 2000, tianjin university professor Yang Zao yan and his leadership team in tianjin new technology industrial park PuChen building built a demonstrative project of MBR, the sewage system, 25 t, all treated sewage used for toilet flushing and green land, labor, area of 10 square meters, with the energy consumption of per ton wastewater is 0.7 kW, h.
Industrial wastewater treatment
Since the 90 s, broadening the MBR process object, in addition to the water reuse, wastewater treatment, the MBR applications in industrial wastewater treatment has been widely attention, such as food industry wastewater treatment, aquatic products processing wastewater, aquaculture wastewater, cosmetics production wastewater, dye wastewater, oil chemical industry wastewater, has obtained the good processing effect. In the early 1990s, a set of MBR system was built in Ohio in the United States for the treatment of industrial wastewater from an automobile factory. The treatment scale was 151m3/d. The organic load of the system was up to 6.3kgCOD/m3•d, the COD removal rate was 94%, and most of the oil and grease were degraded. In the Netherlands, a fat extraction plant uses the traditional oxidation ditch wastewater treatment technology to treat its production wastewater. Due to the expansion of the production scale, the sludge bulges and is difficult to separate. Finally, the membrane module of Zenon is used to replace the sedimentation tank, which has a good operation effect.
Purification of micro - polluted drinking water
With the widespread use of nitrogen fertilizers and pesticides in agriculture, drinking water has become contaminated to varying degrees. LyonnaisedesEaux, a company that developed the MBR process in the mid-1990s, including biological nitrogen removal, pesticide absorption and turbidity removal, established a plant in Douchy, France, producing 400m3 of drinking water a day in 1995. Nitrogen concentration below 0.1mgNO2/L and pesticide concentration below 0.02 g/L in outlet water.
Faecal sewage treatment
The traditional denitrification treatment requires high sludge concentration and unstable solid-liquid separation, which affects the three-stage treatment effect. The emergence of MBR solves this problem well and makes it possible to treat fecal sewage directly without dilution.
The excrement and urine treatment technology known as NS system has been developed in Japan. The most important part is the system of combination of flat membrane device and aerobic high concentration activated sludge bioreactor. NS system was built in 1985 in Yuegu City, Saitama Prefecture, Japan, with a production scale of 10kL/d. In 1989, new excrement and urine treatment facilities were built successively in Nagasaki county and Kumamoto County. Each group of flat film in NS system is about 0.4m2 and dozens of groups are installed side by side to make the frame device which can open automatically and flush automatically. The membrane material is a polysulfone ultrafiltration membrane with 20000 mw closure. The sludge concentration in the reactor was kept within the range of 15000~18000mg/L. By 1994, Japan had more than 1,200 MBR systems to treat the excrement and sewage of more than 40 million people.
Landfill/fertilizer leachate treatment
Landfill/compost leachate contains high concentration of pollutants, and its water quality and quantity vary with climatic conditions and operating conditions. The MBR technology has been used by several sewage treatment plants before 1994. The combination of MBR and RO technology can not only remove SS, organic matter and nitrogen, but also remove salt and heavy metal effectively. Envirogen developed an MBR for landfill leachate treatment and built a 400,000 gallon a day facility in New Jersey that went into operation in late 2000. The MBR uses a naturally occurring mixture of bacteria to break down hydrocarbon and chlorine compounds in the leachate at concentrations 50 to 100 times that of conventional wastewater treatment plants. This effect is achieved because the MBR can retain efficient bacteria at a concentration of up to 50000g/ m2. In the field pilot test, the COD of the incoming liquid is from several hundred to 40000mg/L, and the removal rate of pollutants is above 90%.
