The difference in function and usage between membrane bioreactor (MBR) and submerged ultrafiltration. What should be used in what situation?
MBR is placed in an aeration tank or a secondary sedimentation tank, with a large amount of activated sludge in the influent. Immersion ultrafiltration is relative to pressure ultrafiltration, which is placed in a membrane tank and requires a wider range of influent requirements and stronger anti pollution capabilities. Generally speaking, if ultrafiltration filtration is directly used without further treatment after biochemical methods, MBR is used. If further treatment is needed (mainly to remove COD), immersion ultrafiltration is used in the final step.
Advantages: MBR process is simple, investment is low, submerged ultrafiltration has a large operating flux, high recovery rate, and good water quality
Disadvantages: MBR has low operating flux and requires more membranes for the same amount of water production; The immersion ultrafiltration process is complex and requires multiple peripheral supporting equipment.
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MBR process
In the field of sewage treatment and water resource reuse, MBR, also known as membrane bioreactor, is a new water treatment technology that combines activated sludge process and membrane separation technology.
brief introduction
In the field of sewage treatment and water resource reuse, MBR, also known as Membrane Bio Reactor, is a new water treatment technology that combines activated sludge process with membrane separation technology. There are various types of membranes, classified according to their separation mechanisms, including reaction membranes, ion exchange membranes, permeable membranes, etc; According to the properties of membranes, there are natural membranes (biofilms) and synthetic membranes (organic and inorganic membranes); According to the structural types of membranes, there are flat plate type, tube type, spiral type, and hollow fiber type.
Process composition
The membrane bioreactor is mainly composed of membrane separation components and a bioreactor. The commonly mentioned membrane bioreactor is actually a general term for three types of reactors: ① Aeration Membrane Bioreactor (AMBR); ② Extractive Membrane Bioreactor (EMBR); ③ Solid/Liquid Separation Membrane Bioreactor (SLSMBR).
Aeration membrane
Aeration membrane bioreactor was first seen in Cote P et al. reported in 1988 that the use of breathable dense membranes (such as silicone rubber membranes) or microporous membranes (such as hydrophobic polymer membranes) in plate or hollow fiber modules can achieve bubble free aeration into bioreactors while maintaining gas partial pressure below the bubble point. The characteristic of this process is to improve the contact time and oxygen transfer efficiency, 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. As shown in Figure [1].
Extraction membrane
The extraction membrane bioreactor, also known as EMBR (Extractive Membrane Bioreactor). Due to high acidity or the presence of toxic substances to organisms, some industrial wastewater should not be treated by direct contact with microorganisms; When volatile toxic substances are present in wastewater, if traditional aerobic biological treatment processes are used, pollutants are prone to evaporate with the aeration airflow, resulting in gas stripping. This not only leads to unstable treatment effects but also causes air pollution. To address these technical challenges, British scholar Livingston researched and developed EMB. Wastewater and activated sludge are separated by a membrane, and wastewater flows inside the membrane, while activated sludge containing certain specialized bacteria flows outside the membrane. Wastewater does not directly contact microorganisms, and organic pollutants can selectively pass through the membrane and be degraded by microorganisms on the other side. Due to the independent nature of the bioreactor units and wastewater circulation units on both sides of the extraction membrane, the water flow of each unit has little influence on each other. Nutrients and microbial living conditions in the bioreactor are not affected by the quality of the wastewater, resulting in stable water treatment efficiency. The operating conditions of the system, such as HRT and SRT, can be controlled within the optimal range to maintain the maximum pollutant degradation rate.
Solid-liquid separation membrane
Solid liquid separation membrane bioreactor is the most widely and deeply studied type of membrane bioreactor in the field of water treatment. It is a water treatment technology that uses membrane separation process to replace the secondary sedimentation tank in traditional activated sludge process. In traditional wastewater biological treatment technology, 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, the higher the sludge water separation efficiency. The settling property of sludge depends on the operating conditions of the aeration tank, and improving the settling property of sludge requires strict control of the operating conditions of the aeration tank, which limits the applicability 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, generally around 1.5-3.5 mg/L, which limits the biochemical reaction rate.
The hydraulic retention time (HRT) and sludge age (SRT) are interdependent, and increasing volumetric load and reducing sludge load often create a contradiction. The system also generates a large amount of residual sludge during operation, and its disposal cost accounts for 25% to 40% of the operating cost of the sewage treatment plant. Traditional activated sludge treatment systems are also prone to sludge expansion, resulting in suspended solids in the effluent and deteriorating water quality.
In response to the above issues, MBR combines membrane separation technology with traditional biological treatment technology. MBR achieves the separation of sludge retention time and hydraulic retention time, greatly improving solid-liquid separation efficiency. Moreover, due to the increase in activated sludge concentration in the aeration tank and the emergence of specific bacteria (especially dominant bacterial groups) in the sludge, the biochemical reaction rate is increased. At the same time, by reducing the F/M ratio to reduce the amount of excess sludge generated (even to zero), many prominent problems existing in traditional activated sludge processes have been basically solved.
The activated sludge is removed and then filtered out through a membrane under external pressure. This form of membrane bioreactor eliminates the need for a mixed liquid circulation system and relies on water suction, resulting in relatively low energy consumption; It occupies more space and is more compact than a separate type, and has received special attention in the field of water treatment in recent years. However, the membrane flux is generally relatively low, making it prone to membrane fouling and difficult to clean and replace after fouling.
The composite membrane bioreactor also belongs to the integrated membrane bioreactor in form, with the difference being the addition of fillers inside the bioreactor to form a composite membrane bioreactor, which changes certain characteristics of the reactor.
Process characteristics
Compared with many traditional biological water treatment processes, MBR has the following main characteristics:
1、 High quality and stable effluent water quality
Due to the efficient separation effect of the membrane, the separation efficiency is much better than that of traditional sedimentation tanks. The treated effluent is extremely clear, with suspended solids and turbidity close to zero. Bacteria and viruses are greatly removed, and the effluent quality is better than the domestic miscellaneous water quality standard issued by the Ministry of Construction (CJ25.1-89). It can be directly reused as non potable municipal miscellaneous water.
At the same time, membrane separation also completely intercepts microorganisms in the bioreactor, allowing the system to maintain a high concentration of microorganisms. This 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 in inlet load (water quality and quantity), is resistant to shock loads, and can stably obtain high-quality effluent quality.
2、 Low production of surplus sludge
This process can operate under high volume load and low sludge load, with low residual sludge production (theoretically achieving zero sludge discharge), reducing sludge treatment costs.
3、 Small footprint, not limited by the setting location
The bioreactor can maintain a high concentration of microbial biomass, with a high volumetric load on the treatment device and a large footprint, resulting in significant cost savings; This process is simple, compact in structure, and occupies a small area. It is not limited by the installation location and is suitable for any occasion. It can be made into ground, semi underground, and underground types.
4、 Can remove ammonia nitrogen and difficult to degrade organic matter
Due to the complete interception of microorganisms in the bioreactor, it facilitates the retention and growth of slow proliferating microorganisms such as nitrifying bacteria, thereby improving the nitrification efficiency of the system. At the same time, it can increase the hydraulic retention time of some recalcitrant organic compounds in the system, which is beneficial for improving the degradation efficiency of recalcitrant organic compounds.
5、 Convenient operation and management, easy to achieve automatic control
This process achieves complete separation of hydraulic retention time (HRT) and sludge retention time (SRT), making operation control more flexible and stable. It is a new technology that is easy to implement in wastewater treatment and can achieve microcomputer automatic control, making operation management more convenient.
6、 Easy to transform from traditional craftsmanship
This process can serve as a deep treatment unit for traditional sewage treatment processes, and has broad application prospects in areas such as deep treatment of effluent from urban secondary sewage treatment plants (thereby achieving large-scale reuse of urban sewage).
Membrane bioreactors also have some shortcomings. Mainly manifested in the following aspects:
The high cost of membranes results in higher infrastructure investment for membrane bioreactors compared to traditional wastewater treatment processes;
Membrane fouling is prone to occur, which brings inconvenience to operation and management;
High energy consumption: Firstly, the MBR sludge water separation process must maintain a certain membrane driving pressure. Secondly, the MLSS concentration in the MBR tank is very high. To maintain sufficient oxygen transfer rate, it is necessary to increase aeration intensity. In order to increase membrane flux and reduce membrane fouling, it is necessary to increase flow rate and flush the membrane surface, resulting in higher energy consumption of MBR compared to traditional biological treatment processes.
Process film
Membrane can be prepared from various materials, including liquid phase, solid phase, and even gas phase. The vast majority of separation membranes currently in use are solid-phase membranes. According to different pore sizes, it can be divided into microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes; According to different materials, it can be divided into inorganic membranes and organic membranes. Inorganic membranes are mainly microfiltration grade membranes. The membrane can be homogeneous or heterogeneous, and can be charged or electrically neutral. The membranes widely used in wastewater treatment are mainly solid-state asymmetric membranes prepared from organic polymer materials.
Classification criteria and classification of membranes:
1、 MBR membrane material
1. Polymer organic film materials: polyolefin, polyethylene, polyacrylonitrile, polysulfone, aromatic polyamide, fluoropolymer, etc.
Organic membranes have relatively low costs, are inexpensive, have mature manufacturing processes, diverse pore sizes and forms, and are widely used. However, they are prone to pollution during operation, have low strength, and have a short service life.
2. Inorganic membrane: It is a type of solid-state membrane that is a semi permeable membrane made of inorganic materials such as metals, metal oxides, ceramics, porous glass, zeolites, inorganic polymer materials, etc.
The inorganic membranes currently used in MBR are mostly ceramic membranes, which have the advantages of being able to be used in environments with pH=0-14, pressure P<10MPa, and temperature<350 ℃. They have high flux and relatively low energy consumption, making them highly competitive in the treatment of high concentration industrial wastewater; The disadvantages are: high cost, alkali resistance, low elasticity, and difficulty in processing and preparing the film.
2、 MBR membrane pore size
The membranes commonly used in MBR technology are microfiltration membranes (MF) and ultrafiltration membranes (UF), mostly with a pore size of 0.1-0.4 μ m, which is sufficient for solid-liquid separation type membrane reactors.
The commonly used polymer materials for microfiltration membranes include polycarbonate, cellulose ester, polyvinylidene fluoride, polysulfone, polytetrafluoroethylene, polyvinyl chloride, polyetherimide, polypropylene, polyetheretherketone, polyamide, etc.
Common polymer materials for ultrafiltration include polysulfone, polyethersulfone, polyamide, polyacrylonitrile (PAN), polyvinylidene fluoride, cellulose ester, polyetheretherketone, polyimide, polyetheramide, etc.
3、 MBR membrane module
In order to facilitate industrial production and installation, improve membrane efficiency, and achieve maximum membrane area per unit volume, the membrane is usually assembled in a basic unit equipment in some form, and under a certain driving force, the separation of various components in the mixed liquid is completed. This type of device is called a membrane module.
There are five commonly used forms of membrane components in industry:
Plate and Frame Module, Spiral Wound Module, TubularModule, Hollow Fiber Module, and Capillary Module. The first two use flat film, while the latter three use tubular film. Circular tube membrane diameter>10mm; Capillary type -0.5~10.0mm; Hollow fiber type<0.5mm>.
Table: Characteristics of Various Membrane Components
The commonly used membrane module forms in MBR process include plate frame type, circular tube type, and hollow fiber type. Plate and frame type:
It is one of the earliest membrane module forms used in MBR technology, with an appearance similar to a regular plate and frame filter press. The advantages are: simple manufacturing and assembly, convenient operation, easy maintenance, cleaning, and replacement. The disadvantages are: complex sealing, high pressure loss, and low packing density.
Round tube type:
It is composed of a membrane and a membrane support, and has two operating modes: internal pressure type and external pressure type. In practice, the internal pressure type is often used, where the inlet water flows in from inside the pipe and the permeate flows out from outside the pipe. The membrane diameter is between 6-24mm. The advantages of circular tube membrane are: the feed liquid can control turbulent flow, is not easy to block, is easy to clean, and has low pressure loss. The disadvantage is that the packing density is low.
Hollow fiber type:
The outer diameter is generally 40-250 μ m, and the inner diameter is 25-42 μ m. The advantages are high compressive strength and resistance to deformation. In MBR, components are often directly placed into the reactor without the need for pressure vessels, forming an immersed membrane bioreactor. Generally, it is an external pressure membrane component. The advantages are: high packing density; Relatively low cost; Long lifespan, nylon hollow fiber membranes with stable physical and chemical properties and low permeability can be used; The membrane has good pressure resistance and does not require supporting materials. The disadvantage is that it is sensitive to blockage, and pollution and concentration polarization have a significant impact on the separation performance of the membrane.
General requirements for MBR membrane module design:
Provide sufficient mechanical support for the membrane, ensure smooth flow channels, and eliminate dead corners and stagnant water areas;
Low energy consumption, minimize concentration polarization, improve separation efficiency, and reduce membrane fouling;
The highest possible packing density, easy installation, cleaning, and replacement;
O Has sufficient mechanical strength, chemical and thermal stability.
The selection of membrane components should comprehensively consider their cost, packing density, application scenarios, system processes, membrane fouling and cleaning, service life, etc.
application area
In the mid to late 1990s, membrane bioreactors had entered the practical application stage abroad. Zenon, a Canadian company, was the first to launch an ultrafiltration tubular membrane bioreactor and applied it to urban sewage treatment. In order to save energy consumption, the company has also developed immersed hollow fiber membrane modules. The membrane bioreactor developed by the company has been applied in more than ten places including the United States, Germany, France, and Egypt, with a scale ranging from 380m3/d to 7600m3/d. Mitsubishi Rayon is also a well-known provider of immersed hollow fiber membranes in the world, and has accumulated years of experience in the application of MBR. It has built multiple actual MBR projects in Japan and other countries. Kubota Corporation in Japan is another competitive company in the practical application of membrane bioreactors, producing plate membranes with characteristics such as high flow rate, pollution resistance, and simple process. Some domestic researchers and enterprises are also making attempts in the practical application of MBR.
Nowadays, membrane bioreactors have been applied in the following fields:
1、 Urban sewage treatment and building water reuse
The first wastewater treatment plant using MBR technology was built by the American company Dorr Oliver in 1967, which treated 14m3/d of wastewater. In 1977, a sewage reuse system was put into practical use in a high-rise building in Japan. In 1980, Japan built two MBR treatment plants with processing capacities of 10m3/d and 50m3/d, respectively. In the early 1990s, there were 39 such factories in operation in Japan, with a maximum processing capacity of 500m3/d, and more than 100 high-rise buildings used MBR to treat wastewater and reuse it in intermediate waterways. In 1997, Wessex established the world's largest MBR system in Porlock, UK, with a daily processing capacity of 2000 m3. In 1999, Wessex also built a 13000 m3/d MBR plant in Swanage, Dorset.
In May 1998, the integrated membrane bioreactor pilot system conducted by Tsinghua University passed national certification. In early 2000, Tsinghua University built a practical MBR system at Haidian Township Hospital in Beijing to treat hospital wastewater. The project was completed and put into use in June 2000, and is currently operating normally. In September 2000, Professor Yang Zaoyan and her research team from Tianjin University completed an MBR demonstration project at Puchen Building in Tianjin New Technology Industrial Park. The system treats 25 tons of sewage per day, all of which is used for flushing toilets and sprinkling green spaces. The system covers an area of 10 square meters and consumes 0.7 kW · h of energy per ton of sewage.
2、 Industrial wastewater treatment
Since the 1990s, the treatment objects of MBR have been continuously expanded. In addition to reclaimed water reuse and fecal wastewater treatment, MBR has also received widespread attention in industrial wastewater treatment, such as treating food industry wastewater, aquatic processing wastewater, aquaculture wastewater, cosmetics production wastewater, dye wastewater, and petrochemical wastewater, all of which have achieved good treatment effects. In the early 1990s, the United States built an MBR system in Ohio to treat industrial wastewater from a certain automobile manufacturing plant. The treatment capacity was 151m3/d, and the organic load of the system reached 6.3kgCOD/m3 · d. The COD removal rate was 94%, and the vast majority of oil and grease were degraded. In the Netherlands, a fat extraction and processing plant uses traditional oxidation ditch wastewater treatment technology to treat its production wastewater. Due to the expansion of production scale, the sludge swells and is difficult to separate. Finally, Zenon membrane modules are used instead of sedimentation tanks, and the operation effect is good.
3、 Micro polluted drinking water purification
With the widespread use of nitrogen fertilizers and insecticides in agriculture, drinking water has also been polluted to varying degrees. Lyonnaise des Eaux developed the MBR process in the mid-1990s, which has the functions of biological denitrification, insecticide adsorption, and turbidity removal. In 1995, the company built a factory in Douchy, France with a daily production capacity of 400m3 of drinking water. The nitrogen concentration in the effluent is below 0.1mg/L NO2, and the insecticide concentration is below 0.02 μ g/L.
4、 Fecal wastewater treatment
The organic matter content in fecal wastewater is high, and traditional denitrification treatment methods require high sludge concentration. The solid-liquid separation is unstable, which affects the effectiveness of tertiary treatment. The emergence of MBR has effectively solved this problem and made it possible to directly treat fecal wastewater without dilution.
Japan has developed a feces and urine treatment technology known as the NS system, with the core component being a combination of a flat membrane device and an aerobic high concentration activated sludge bioreactor. The NS system was built in Echigo City, Saitama Prefecture, Japan in 1985, with a production capacity of 10kL/d. In 1989, new sewage treatment facilities were built in Nagasaki Prefecture and Kumamoto Prefecture. The flat film in the NS system is installed in parallel with dozens of groups, each with an area of about 0.4m2, to create a frame device that can automatically open and flush. The membrane material is a polysulfone ultrafiltration membrane with a cutoff molecular weight of 20000. The sludge concentration in the reactor is maintained within the range of 15000-18000mg/L. By 1994, Japan had over 1200 MBR systems used to treat fecal wastewater from more than 40 million people.
5、 Landfill/compost leachate treatment
Landfill/compost leachate contains high concentrations of pollutants, and its water quality and quantity vary with climate and operational conditions. MBR technology was used by multiple sewage treatment plants for the treatment of this type of wastewater before 1994. The combination of MBR and RO technology can not only remove SS, organic matter, and nitrogen, but also effectively remove salts and heavy metals. Recently, Envirogen Corporation in the United States developed an MBR for the treatment of leachate from landfills and built a device with a daily processing capacity of 400000 gallons (approximately 1500m3/d) in New Jersey, which was put into operation at the end of 2000. This MBR uses a naturally occurring mixed bacteria to decompose hydrocarbons and chlorinated compounds in the leachate, and its concentration of pollutants treated is 50-100 times that of conventional wastewater treatment devices. The reason for achieving this treatment effect is that MBR can retain efficient bacteria and achieve a bacterial concentration of 50000mg/L. In the on-site pilot test, the influent COD ranged from several hundred to 40000, and the removal rate of pollutants reached over 90%.
Main application areas and corresponding percentage rates of MBR at home and abroad:
Percentage rate of sewage types (%)
Industrial wastewater 27 Urban wastewater 12
Construction wastewater 24 garbage 9
Household sewage 27
