First, the composition of MBR process
Membrane - bioreactor is mainly composed of membrane separation components and bioreactor two parts. Often referred to as membrane - bioreactor is actually a general term for three types of reactors: ① Aeration Membrane - Bioreactor (Aeration Membrane Bioreactor, AMBR); ② Extractive Membrane - Bioreactor (ExtractiveMembrane Bioreactor, EMBR); ③ Solid/Liquid SeparationMembrane - Bioreactor ( Solid/Liquid SeparationMembrane, EMBR). Solid/Liquid SeparationMembrane Bioreactor, SLSMBR, abbreviated MBR).
Second, the aeration membrane - bioreactor
Aeration membrane - bioreactor was first reported in Cote.P et al. 1988, the use of breathable dense membranes (such as silicone rubber membranes) or microporous membranes (such as hydrophobic polymerization membranes), to plate or hollow fiber type components, to keep the gas partial pressure is lower than the bubble point (Bubble Point), can be realized to the bioreactor bubble-free aeration. Bubble-free aeration of the reactor. The process is characterized by improved contact time and oxygen transfer efficiency, and is conducive to the control of the aeration process, independent of the factors of bubble size and residence time in conventional aeration. As shown in Figure [1].
Figure [1]
Third, Extractive Membrane - Bioreactor
Extractive Membrane - Bioreactor also known as EMBR (Extractive Membrane Bioreactor). Because of the existence of high acidity and alkalinity or toxic substances to the biological, some industrial wastewater is not suitable for direct contact with micro-organisms to deal with the method; when the wastewater contains volatile toxic substances, if the traditional aerobic biological treatment process, the pollutants are easy to volatilize the aeration airflow, the occurrence of gas lift phenomenon, not only is the treatment effect is very unstable, but also cause atmospheric pollution. In order to solve these technical problems, British scholar Livingston researched and developed EMB, whose process flow is shown in Figure 2. Its process flow is shown in Figure 2. wastewater and activated sludge are separated by the membrane, wastewater flow in the membrane, while the activated sludge containing some kind of specialized bacteria flow outside the membrane, wastewater and microorganisms do not come into direct contact with the organic pollutants can be selectively through the membrane by the other side of the microbial degradation. Due to the extraction of membrane on both sides of the bioreactor unit and wastewater recycling unit is independent of each other, the water flow of each unit has little impact on each other, the nutrients in the bioreactor and microbial living conditions are not affected by the quality of 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 to maintain the maximum rate of pollutant degradation.
Fourth, solid-liquid separation membrane - bioreactor
Solid-liquid separation membrane - bioreactor is the most widely researched in the field of water treatment in a class of membrane - bioreactor, is a membrane separation process to replace the traditional activated sludge method of secondary sedimentation tank water treatment technology. In the traditional wastewater biological treatment technology, mud-water separation is accomplished by gravity in the secondary sedimentation tank, and its separation efficiency depends on the settling performance of activated sludge, the better the settling, the higher the mud-water separation efficiency. The settling of sludge depends on the operating condition of the aeration tank, and the operating conditions of the aeration tank must be strictly controlled to improve the settling of sludge, which limits the scope of application of the 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, generally around 1.5~3.5g/L, which limits the biochemical reaction rate. Hydraulic retention time (HRT) and sludge age (SRT) are interdependent, and increasing the volumetric load and reducing the sludge load often form a contradiction. The system also produces a large amount of residual sludge during operation, and its disposal cost accounts for 25% to 40% of the operating cost of the wastewater treatment plant. The traditional activated sludge treatment system is also prone to sludge expansion, the effluent contains suspended solids, the effluent water quality deterioration. In view of the above problems, MBR combines the membrane separation technology in separation engineering with the traditional wastewater biological treatment technology, which greatly improves the solid-liquid separation efficiency, and improves the biochemical reaction rate due to the increased concentration of activated sludge in the aeration tank and the emergence of effective bacteria (especially the dominant group) in the sludge. At the same time, by reducing the F/M ratio to reduce the amount of residual sludge production (or even zero), thus basically solving many of the outstanding problems existing in the traditional activated sludge method.
V. Types of MBR Processes
The following discussions are all solid-liquid separation membrane - bioreactor. According to the combination of membrane module and bioreactor, the membrane - bioreactor can be divided into three basic types of split, integrated and composite. See Figure 3 for a diagram of a split and integrated MBR.
A split membrane-bioreactor has the membrane module and bioreactor set up separately, as shown in Figure 3. The mixed liquor in the bioreactor is pressurized by a circulation pump and then pumped to the filtration end of the membrane module. Under pressure, the liquid in the mixed liquor passes through the membrane and becomes the treated water of the system; solids and macromolecules are retained by the membrane, and the concentrated liquor flows back to the bioreactor. Displaced membrane - bioreactor is characterized by stable and reliable operation, easy to membrane cleaning, replacement and installation; and membrane flux is generally larger. However, under general conditions, in order to reduce the deposition of contaminants on the membrane surface and to extend the cleaning cycle of the membrane, it is necessary to use a circulation pump to provide a higher flow rate at the membrane surface, which results in a large amount of water circulation and a high cost of power (Yamamoto, 1989), and the high speed of the pump rotation generates a shear force that can inactivate some microbial organisms (Brockmann and Seyfried, 1997). ).
An integrated membrane-bioreactor is one in which the membrane module is placed inside the bioreactor, as shown in Figure 4. The influent water enters the membrane-bioreactor, where most of the contaminants are removed by the activated sludge in the mixture, and the water is filtered by the membrane under external pressure. This form of membrane - bioreactor due to the elimination of the mixture of liquid circulation system, and by pumping out the water, energy consumption is relatively low; occupies a more compact than the split type, in recent years in the field of water treatment has received special attention. But the general membrane flux is relatively low, easy to membrane contamination, membrane contamination is not easy to clean and replace.
Composite membrane - bioreactor in the form of a one-piece membrane - bioreactor, the difference is in the bioreactor with packing, thus forming a composite membrane - bioreactor, changing some of the characteristics of the reactor, as shown in Figure 5:
MBR process characteristics
Compared with many traditional biological water treatment processes, MBR has the following main features. Main features:
First, the quality and stability of the effluent water
Due to the efficient separation of the membrane, the separation effect is much better than the traditional sedimentation tank, the effluent water is extremely clear, the suspended solids and turbidity close to zero, bacteria and viruses are greatly removed, the effluent water quality is better than the water quality standards for miscellaneous water issued by the Ministry of Construction (CJ25.1-89), and it can be used directly as a municipal water for reuse for non-potable municipal miscellaneous water. It can be directly reused as non-potable municipal miscellaneous water.
At the same time, the membrane separation also makes the microorganisms are completely intercepted in the bioreactor, so that the system can maintain a high concentration of microorganisms, not only to improve the overall removal efficiency of pollutants in the reaction device to ensure a good quality of the effluent, and at the same time, the reactor has a very good adaptability to the load of the water inlet (water quality and water) of the changes in the load of the impact load, can be stabilized to obtain high-quality effluent water. Water quality.
Second, the residual sludge production
The process can be operated under high volumetric load and low sludge load, with low residual sludge production (theoretically zero sludge discharge), reducing the cost of sludge treatment.
Third, small footprint, not subject to set the occasion
The bioreactor can maintain a high concentration of microorganisms in the amount of high volumetric load of the treatment device, the footprint is greatly saved; the process is simple, compact structure, floor space, not subject to set the place of limitations, suitable for any occasion, can be made into a ground, semi-subterranean and underground type.
Fourth, the removal of ammonia and nitrogen and difficult to degrade organic matter
As the microorganisms are completely intercepted in the bioreactor, which is conducive to the slow proliferation of micro-organisms such as nitrifying bacteria to intercept the growth of the system nitrification efficiency can be improved. At the same time, can grow some difficult to degrade organic matter in the system of hydraulic residence time, is conducive to difficult to degrade organic matter degradation efficiency.
V. 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), the operation and control is more flexible and stable, and it is easy to realize the equipping of the new technology in the sewage treatment, and it can be realized by the automatic control of microcomputer, which makes the operation and management more convenient.
Six, easy to transform from the traditional process
The process can be used as a traditional wastewater treatment process of the depth of the treatment unit, in the city of secondary wastewater treatment plant effluent depth treatment (so as to achieve a large number of urban wastewater reuse) and other fields have broad prospects for application.
Membrane - bioreactor also has some shortcomings. The main performance in the following aspects:
o Membrane cost is high, so that the membrane - bioreactor infrastructure investment is higher than the traditional wastewater treatment process;
o Membrane contamination is prone to appear, to the operation and management of the inconvenience;
o Energy consumption is high: first of all, the MBR sludge-water separation process must be to maintain a certain degree of membrane driving pressure, and the second is the MBR tank MLSS concentration is very high, to maintain sufficient oxygen transfer rate, to maintain a high degree of oxygen transfer rate, to maintain a high degree of oxygen transfer rate, to maintain a high degree of oxygen transfer rate. Secondly, the MLSS concentration in the MBR tank is very high, in order to maintain sufficient oxygen transfer rate, must increase the aeration intensity, and in order to increase the membrane flux, reduce membrane contamination, must increase the flow rate, scouring the membrane surface, resulting in the energy consumption of MBR is higher than the traditional biological treatment process.
Membrane for MBR process
Membrane can be prepared from a variety of materials, can be liquid phase, solid phase or even gas phase. The vast majority of separation membranes currently in use are solid-phase membranes. According to different pore sizes can be divided into: microfiltration, ultrafiltration, nanofiltration and reverse osmosis membranes; according to different materials, can be divided into inorganic and organic membranes, inorganic membranes are mainly microfiltration level membrane. Membrane can be homogeneous or non-homogeneous, can be charged or electrically neutral. Widely used in wastewater treatment of the membrane is mainly prepared by organic polymer materials, solid-phase asymmetric membrane.
The classification of membranes is shown in the figure:
I. MBR Membrane Materials
1, polymer organic membrane materials: polyolefins, polyethylene, polyacrylonitrile, polysulfone, aromatic polyamides, fluorine-containing polymers and so on.
Organic membrane cost is relatively low, cheap, the membrane manufacturing process is more mature, the membrane aperture and the form is more diverse, widely used, but the operation process is easy to pollute, low strength, short service life.
2, inorganic membrane: a solid membrane, is made of inorganic materials, such as metals, metal oxides, ceramics, porous glass, zeolite, inorganic polymers made of semi-permeable membrane.
Currently used in MBR inorganic membranes are mostly ceramic membranes, the advantages are: it can be used in pH = 0~14, pressure P & lt; 10MPa, temperature & lt; 350 ℃ environment, its high flux, energy consumption is relatively low, in the treatment of high-concentration industrial wastewater has a great deal of competitiveness; the disadvantages are: costly, alkali intolerant, elasticity is small, the membrane processing and preparation of a certain degree of difficulty. The disadvantages are: expensive, not alkali resistant, small elasticity, membrane processing and preparation have certain difficulties.
Second, MBR membrane pore size
MBR process with the membrane is generally microfiltration (MF) and ultrafiltration (UF), most of the use of 0.1 ~ 0.4 μ m membrane pore size, which is sufficient for solid-liquid separation type of membrane reactor.
Microfiltration membrane commonly used polymer materials are: polycarbonate, cellulose ester, polyvinylidene fluoride, polysulfone, polytetrafluoroethylene, polyvinyl chloride, polyetherimide, polypropylene, polyetherether ketone, polyamide and so on.
Polymer materials commonly used in ultrafiltration are: polysulfone, polyethersulfone, polyamide, polyacrylonitrile (PAN), polyvinylidene fluoride, cellulose ester, polyetherether ketone, polyimide, polyether amide, and so on.
Third, MBR Membrane Module
In order to facilitate industrial production and installation, improve the efficiency of the membrane, in the unit volume to achieve the maximum area of the membrane, the membrane is usually assembled in a certain form in a basic unit of equipment, in a certain driving force, to complete the separation of the components of the mixture, this type of device is known as the membrane module (Module).
Five forms of membrane modules are commonly used in industry:
Plate and Frame (Plate and Frame Module), Spiral Wound (Spiral Wound Module), Tubular (TubularModule), Hollow Fiber (Hollow Fiber Module), and Capillary (Capillary Module). Capillary Module.) The first two use flat membranes and the last three use tubular membranes. Tubular membrane diameter >10mm; Capillary - 0.5~10.0mm; Hollow Fiber <0.5mm>.
Table: Characteristics of various membrane modules
Name/Item Hollow fiber type Capillary type Spiral coil type Flat plate type Round tube type
Price (yuan /m 3 ) 40~150 150~800 250~800 800~2500 400~1500
Filling density High Medium Medium Low Low
Cleaning Difficulty Easy Medium Easy Easy
Pressure drop High Medium Medium Low
Cleaning Difficulty Easy Medium Easy Easy
Pressure drop High Medium Medium Medium Medium Easy p>
Pressure drop High Medium Medium Medium Medium Low
Can be operated under high pressure Can be No Can be Difficult Difficult
Limitations of Membrane Module Forms Yes Yes No No No
Membrane module forms commonly used in the MBR process are: Plate and Frame, Tubular, and Hollow Fiber.
Plate and frame type:
It is one of the earliest forms of membrane module used in MBR process, and its shape is similar to common plate and frame filter press. Advantages: simple manufacturing and assembly, easy to operate, easy to maintain, clean, replace. Disadvantages are: more complex sealing, high pressure loss, small filling density.
Round tube type:
It is composed of membrane and membrane support body, there are two operation modes: internal pressure type and external pressure type. In practice, the internal pressure type is mostly used, that is, the inlet water flows in from the tube, and the permeate flows out from the outside of the tube. Membrane diameter between 6~24mm. Advantages of round tube membrane are: liquid can control turbulent flow, not easy to block, easy to clean, small pressure loss. Disadvantages: small filling density.
Hollow fiber type:
The assembly form is shown in the figure below:
[ Figure ]
The outer diameter is generally 40 ~ 250 μm, the inner diameter is 25 ~ 42 μm. Advantages: high compressive strength, not easy to deform. In MBR, the module is often put into the reactor directly, without pressure vessel, constituting a submerged membrane - bioreactor. Generally for the external pressure type membrane module. Advantages are: high packing density; relatively low cost; long life, can use stable physical and chemical properties, low water permeability nylon hollow fiber membrane; membrane pressure resistance is good, do not need support materials. Disadvantages are: sensitive to clogging, pollution and concentration polarization of the membrane separation performance has a great impact.
General requirements for MBR membrane module design:
o Provide sufficient mechanical support for the membrane, smooth flow path, no flow dead space and hydrostatic area;
o Low energy consumption, minimize concentration polarization, improve separation efficiency, reduce membrane contamination;
o As high as possible filling density, installation, cleaning, easy to replace;
o Sufficient mechanical strength, chemical and chemical properties of the membrane. Sufficient mechanical strength, chemical and thermal stability.
The selection of membrane modules should take into account the cost, packing density, application, system flow, membrane contamination and cleaning, service life.
MBR applications
Into the mid to late 90's, the membrane - bioreactor in foreign countries has entered the stage of practical application. Canada Zenon company first launched the ultrafiltration tube membrane - bioreactor, and its application in municipal wastewater treatment. In order to save energy, the company also developed immersed hollow fiber membrane module, and its developed membrane - bioreactor has been applied in more than ten places in the United States, Germany, France and Egypt, with the scale from 380m 3 /d to 7600m 3 /d. Mitsubishi Rayon Company of Japan is also the world's well-known provider of immersed hollow fiber membrane, which has accumulated many years of experience in the application of MBR, and has built a number of practical application of membrane - bioreactor in Japan as well as in other countries. Mitsubishi Rayon is also a well-known supplier of immersed hollow fiber membranes in the world. Kubota is another competitive company in the practical application of membrane-bioreactor, and its plate membrane is characterized by large flow capacity, pollution resistance and simple process. Some domestic researchers and enterprises are also trying to MBR practical application.
Now, the membrane - bioreactor has been applied to the following areas:
First, urban wastewater treatment and building water reuse
In 1967, the first MBR process wastewater treatment plant was built by the U.S. Dorr-Oliver company, the treatment plant to deal with 14m 3 / d wastewater. In 1977, a set of sewage reuse system in a high-rise building in Japan has been practically applied. In 1980, two MBR treatment plants with capacities of 10m 3 /d and 50m 3 /d were built in Japan. In the mid-90s, there were 39 such plants in operation in Japan, with a maximum capacity of 500m 3 /d, and there were more than 100 high-rise buildings using MBR to treat wastewater and reuse it in central waterways. In 1997, the British company Wessex built the world's largest MBR system in Porlock, England, with a capacity of 2,000 m 3 /d, and in 1999 built a 13,000m 3 /d MBR plant in Swanage, Dorset [14].
In May 1998, the integrated membrane-bioreactor pilot system carried out by Tsinghua University passed the national appraisal. In early 2000, Tsinghua University built a practical MBR system in Haidian Township Hospital in Beijing to treat hospital wastewater, which was completed and put into use in June 2000, and is now operating normally. In September 2000, Prof. Yang Zaoyan of Tianjin University and his leadership of the scientific research team built an MBR demonstration project in the Puchen Building of the Tianjin New Technology Industrial Park, the system treats 25 tons of wastewater per day, and all the treated wastewater is in a MBR plant [14]. The system treats 25 tons of sewage per day, and all the treated sewage is used for toilet flushing and green watering, covering an area of 10 square meters, and the energy consumption of treating each ton of sewage is 0.7kW - h. The MBR system can be operated on a daily basis.
II. Industrial wastewater treatment
Since the 90's, MBR treatment objects continue to broaden, in addition to water reuse, fecal wastewater treatment, MBR in industrial wastewater treatment applications have also received widespread attention, such as the treatment of food industry wastewater, aquatic processing wastewater, aquaculture wastewater, cosmetic wastewater, dyes wastewater, petrochemical wastewater, have obtained a good treatment effect. In the early 90's, the United States in Ohio built a set of industrial wastewater treatment of an automobile manufacturing plant MBR system, the scale of treatment for 151m 3 / d, the organic load of the system up to 6.3kgCOD/m 3 - d, COD removal rate of 94%, the vast majority of the oil and grease is degraded. In the Netherlands, a fat extraction processing plant using traditional oxidation ditch wastewater treatment technology to deal with its production of wastewater, due to the expansion of the scale of production, the results lead to sludge expansion, sludge is difficult to separate, and finally used Zenon's membrane modules instead of sedimentation tanks, the operating results are good.
III. Micro-pollution of drinking water purification
With the widespread use of nitrogen fertilizers and pesticides in agriculture, drinking water is also contaminated to varying degrees. LyonnaisedesEaux company in the mid-1990s developed a simultaneous biological nitrogen removal, adsorption of pesticides, turbidity removal function of the MBR process, in 1995, the company built in Douchy, France, the daily output of drinking water 400m 3 plant. The concentration of nitrogen in the effluent is less than 0.1mgNO2 /L, and the concentration of pesticides is less than 0.02 μ g/L.
IV. Fecal wastewater treatment
Fecal wastewater has a high content of organic matter, the traditional denitrification treatment method requires a high sludge concentration, solid-liquid separation is unstable, affecting the effect of tertiary treatment. The emergence of MBR is a good solution to this problem, and makes it possible to directly treat fecal wastewater without dilution.
Japan has developed a fecal urine treatment technology called the NS system, the core part of which is a flat-plate membrane device combined with an aerobic high-concentration activated sludge bioreactor system. The NS system was built in 1985 in Koshigaya City, Saitama Prefecture, Japan, with a production scale of 10kL/d, and new feces and urine treatment facilities were built in Nagasaki Prefecture and Kumamoto Prefecture in 1989. NS system in the flat membrane of each group of about 0.4m 2 *** dozens of groups installed side by side, made to automatically open the frame device, and can be automatically flushed. The membrane material is polysulfone ultrafiltration membrane with a MWCO of 20,000. The sludge concentration in the reactor is kept in the range of 15,000~18,000mg/L. The sludge concentration in the reactor is kept in the range of 15,000~18,000mg/L. By 1994, more than 1,200 MBR systems had been used in Japan to treat the fecal wastewater of more than 40 million people.