What Is a Membrane Bioreactor (MBR)? A Guide to Wastewater Treatment Terminology
Wastewater treatment for beverage plants, breweries, food processing plants, livestock and slaughterhouse operations, textile dyeing and finishing, leather tanning, pulp mills, pharmaceutical manufacturing, and other high-concentration organic wastewater; process improvement for existing wastewater treatment plants; and pretreatment for reverse osmosis systems.
In addition, this technology has also found wide application and practice in related fields.
The Membrane Bio-Reactor (MBR) is a new type of wastewater treatment system that organically combines membrane separation technology with biological treatment technology. By replacing the secondary sedimentation tank at the end of conventional biological treatment with membrane modules, a high concentration of activated sludge is maintained in the bioreactor, which increases the organic loading capacity of biological treatment, reduces the footprint of wastewater treatment facilities, and reduces excess sludge production through a low sludge loading. The membrane separation equipment is mainly used to retain activated sludge and macromolecular organic matter in the water. In an MBR system, the activated sludge (MLSS) concentration can be raised to 8,000-10,000 mg/L or even higher, and the sludge retention time (SRT) can be extended to more than 30 days.
Because of its effective retention, the membrane bioreactor can retain microorganisms with long generation cycles, achieving advanced purification of wastewater. Meanwhile, nitrifying bacteria can fully proliferate in the system, delivering a pronounced nitrification effect and making advanced phosphorus and nitrogen removal possible.
Wastewater treatment: China is a water-scarce country, and wastewater treatment and reuse are effective measures for developing and utilizing water resources. Wastewater reuse means that municipal and industrial wastewater, after treatment by equipment such as membrane bioreactors, is used for non-potable purposes such as landscaping, washing, and replenishing ornamental water bodies, while clean water is reserved for drinking and other high-quality-water uses. Municipal and industrial wastewater is available close at hand, which eliminates long-distance water conveyance and enables nearby treatment for full utilization of water resources. Nearby treatment also prevents sewage leakage during long-distance transport that could contaminate groundwater sources. Wastewater reuse has been widely adopted in many water-scarce regions around the world and is regarded as the most practical wastewater treatment technology of the 21st century.
Influent and effluent quality comparison: For typical municipal wastewater, the design influent quality is BOD5 < 300 mg/L; CODcr < 500 mg/L; SS < 300 mg/L; T-N < 45 mg/L. The effluent quality is BOD5 < 10 mg/L; NH4+-N < 10.0 mg/L; CODcr < 30 mg/L; turbidity < 5 NTU; total coliform count < 3 CFU/L. There are many types of membranes. Classified by separation mechanism, there are reactive membranes, ion-exchange membranes, and osmotic membranes; classified by membrane nature, there are natural membranes (biofilms) and synthetic membranes (organic and inorganic membranes); classified by membrane structure, there are flat-sheet, tubular, spiral-wound, and hollow-fiber types.
Building on this foundation, industry experts have also carried out extensive research and improvements.
In graywater treatment systems, the collected bath wastewater contains small amounts of hair and fibers. If not cleaned out thoroughly, they can clog the pumps and the membrane bioreactor, reduce treatment efficiency, and may ultimately paralyze the entire system. Therefore, a hair filter must be installed in graywater treatment systems.
In the MBR reaction tank, organic pollutants are degraded and sludge-water separation takes place. As the core of the treatment system, the reaction tank contains microbial colonies, membrane modules, a water collection system, an effluent system, and an aeration system.
Beyond that, multiple factors must also be taken into account in practical engineering applications.
The membrane bioreactor (MBR) is a new type of biological wastewater treatment device developed by Professor Yang Zaoyan and his research team over 10 years of work. This technology is known as "the water treatment technology of the 21st century." The project was listed as a key national science and technology project in both the Eighth and Ninth Five-Year Plans and was designated by the state as "a practical new technology for implementing China's Agenda 21 and sustainable development." This technology is at the leading level in China, with some indicators reaching the international leading level.
MBR is an efficient combination of membrane separation technology and biological treatment. It originated as a way to replace the secondary sedimentation tank in the activated sludge process with membrane separation for solid-liquid separation. This process not only achieves effective sludge-water separation but also offers advantages that conventional tertiary wastewater treatment processes cannot match:
2. The highly efficient retention of the membrane keeps microorganisms completely inside the bioreactor, achieving complete separation of the hydraulic retention time (HRT) and sludge retention time (SRT) in the reactor, with flexible and stable operational control.
Building on this foundation, industry experts have also carried out extensive research and improvements.
The membrane bioreactor (MBR) is a new water treatment technology that combines a membrane separation unit with a biological treatment unit. It uses membrane modules to replace the secondary sedimentation tank, maintains a high activated sludge concentration in the bioreactor, reduces the footprint of wastewater treatment facilities, and reduces sludge production through a low sludge loading. Compared with conventional biochemical water treatment technology, MBR has the following main features: high treatment efficiency and excellent effluent quality; compact equipment with a small footprint; easy automation with simple operation and management. Since the 1980s, the technology has attracted increasing attention and has become one of the research hotspots. Membrane bioreactors have already been applied in more than a dozen countries including the United States, Germany, France, and Egypt, with scales ranging from 6 m³/d to 13,000 m³/d.
China's research on MBR started less than a decade ago but has progressed very rapidly. Domestic MBR research can be roughly divided into several areas: (1) exploring combinations of different biological treatment processes with membrane separation units, with biological reaction processes expanding from the activated sludge process to contact oxidation, biofilm processes, hybrid processes combining activated sludge and biofilm, and two-phase anaerobic processes; (2) research on the factors, mechanisms, and mathematical models affecting treatment performance and membrane fouling, seeking appropriate operating conditions and process parameters to minimize membrane fouling and improve the treatment capacity and operational stability of membrane modules; (3) expanding the application scope of MBR, with research subjects extending from domestic sewage to high-concentration organic wastewater (food wastewater, brewery wastewater) and refractory industrial wastewater (petrochemical wastewater, printing and dyeing wastewater, etc.), while domestic sewage treatment remains the main focus.
In China, MBR is being studied for both domestic sewage and industrial wastewater treatment. All of these research results show that MBR achieves good removal of COD, NH3-N, SS, turbidity, and other indicators for various high-concentration organic wastewater and refractory wastewater streams.
As an emerging international water treatment technology of the 1990s, it combines biological wastewater treatment with membrane separation technology. First, biochemical technology is used to degrade organic matter in the water, cultivating dominant bacterial strains and blocking bacteria; then membrane technology is used to filter suspended solids and water-soluble macromolecular substances, reducing water turbidity to meet discharge standards. Membrane bioreactor technology can be widely used in wastewater treatment, graywater reuse, and other fields.
In addition, this technology has also found wide application and practice in related fields.
At the environmental protection annual conference held in Italy in 2004, MBR was widely recognized from all evaluation perspectives as the ultimate feasible graywater reuse technology for municipal wastewater.
The membrane bioreactor is one of the most common water treatment devices. A key focus of MBR research is to maximize membrane flux while ensuring effluent quality, thereby reducing the membrane area required and lowering the capital and operating costs of the membrane bioreactor. All of these are determined by membrane bioreactor parameters. Let us take a closer look at the parameters of the membrane bioreactor.
Membrane bioreactor materials fall into two categories: organic membranes and inorganic membranes. Organic membranes have been widely used in membrane bioreactors, with common membrane materials including polyethylene and polypropylene. Side-stream membrane bioreactors typically use ultrafiltration membrane modules with a molecular weight cut-off generally between 200,000 and 300,000. The larger the molecular weight cut-off, the higher the initial membrane flux, but the long-term membrane flux is not necessarily higher.
Once the membrane is selected, its physicochemical properties are fixed, so the operating mode becomes the main factor affecting membrane fouling in the membrane bioreactor. Not only do sludge concentration and mixed liquor viscosity affect membrane flux, but the filtration performance of the mixed liquor itself, such as activated sludge characteristics and biotic phase, also affects the decline of membrane flux in the membrane bioreactor. Studies have shown that the addition of powdered activated carbon (PAC) and flocculants helps improve sludge-water separation performance, forming larger and less viscous sludge flocs and reducing the chance of membrane clogging. However, excessive dosing of flocculants can limit sludge activity, affecting the treatment capacity and performance of the reactor.
As the membrane bioreactor has entered large-scale practical application in the water treatment industry, it can be classified in different ways according to membrane modules and operating principles. Let us take a look at the classification of membrane bioreactors.
Meanwhile, the associated supporting processes and equipment are also being continuously optimized and upgraded.
Membrane aeration bioreactor: In the membrane aeration bioreactor, the membrane is used for gas mass transfer, typically to supply oxygen for aerobic processes. It enables bubble-free aeration of the bioreactor, greatly improving the oxygen transfer efficiency of the reactor.
Extractive membrane bioreactor: The extractive membrane bioreactor is mainly used for treating priority pollutants in industry, with selectively permeable membranes used to extract specific pollutants.
Meanwhile, the associated supporting processes and equipment are also being continuously optimized and upgraded.
The integrated membrane bioreactor (also called the submerged membrane bioreactor) places the membrane modules directly inside the reactor, and the filtrate is obtained by suction. The cross-flow needed for membrane surface cleaning is generated by air agitation, arranged directly below the membrane; the mixed liquor flows upward with the air stream, producing shear force on the membrane surface to reduce membrane fouling. The integrated membrane bioreactor process is an organic combination of biological wastewater treatment technology and membrane separation technology.
Aerobic membrane bioreactors are generally used for municipal and industrial treatment. Aerobic MBR is used in municipal wastewater treatment usually to make the effluent suitable for reuse, while in industrial applications it is mainly used to remove specific pollutants, such as oil and grease contaminants.
In the anaerobic membrane bioreactor, the highly efficient retention of the membrane not only solves the problem of anaerobic sludge easily being washed out of the reactor and degrading effluent quality, but the membrane separation also strengthens the configuration and treatment performance of the anaerobic reactor. Taking the combination of UASB and a membrane unit as an example, the anaerobic membrane bioreactor no longer needs a specially designed three-phase separator to achieve solid-liquid-gas separation. For two-phase anaerobic MBR, membrane separation increases the concentration of acid-producing bacteria in the acidogenic reactor, improving hydrolytic fermentation capacity; at the same time, the membrane retains macromolecular organic matter in the acidogenic reactor for hydrolysis and fermentation, thus maintaining a high acidification rate. Anaerobic membrane bioreactors are commonly used for treating high-concentration organic wastewater. Since the membrane bioreactor lacks aeration, anaerobic membrane bioreactors for high-concentration organic wastewater all adopt the side-stream configuration to keep the anaerobic sludge in suspension.
(1) Excellent and stable effluent quality. Membrane filtration effluent gives the membrane bioreactor a much higher biological concentration than the conventional activated sludge process, greatly improving biodegradation capacity and resistance to shock loading. Meanwhile, the long sludge retention time allows slow-growing microorganisms such as refractory organic matter-decomposing bacteria and nitrifying bacteria to proliferate and enrich in the reactor, delivering ideal removal of refractory organics and ammonia nitrogen in particular. On the other hand, the retention by MBR membrane separation of organic macromolecules smaller than the membrane pore size ensures stable post-filtration effluent in terms of disinfection, suspended solids removal, and BOD reduction.
(2) Small footprint. The membrane bioreactor can maintain a high sludge concentration, typically MLSS of 8-20 g/L, which is 2.5 to 5 times that of conventional biological treatment. The system also eliminates the secondary sedimentation tank and sludge return equipment, saving floor space.
(3) Simple operation and maintenance. The membrane separation unit is simple in process; effluent and operation are not affected by factors such as sludge bulking. Operation and maintenance are simple and convenient, and automated control and management are easy to implement.
(4) Low sludge handling cost. The system has a high sludge concentration and long sludge age, which means less sludge is discharged; sludge production is only 30% of that of conventional processes, which is highly beneficial for subsequent sludge treatment.
Membrane bioreactor (MBR) technology is a new wastewater treatment technology that organically combines membrane separation technology with biotechnology. It uses membrane separation equipment to retain the activated sludge and macromolecular organic matter in the biochemical reaction tank, eliminating the secondary sedimentation tank. The membrane bioreactor process greatly enhances the function of the bioreactor through membrane separation, substantially increasing the activated sludge concentration, with the hydraulic retention time (HRT) and sludge retention time (SRT) controlled independently.
Building on this foundation, industry experts have also carried out extensive research and improvements.
Depending on the role of the membrane modules, MBR is usually divided into three types: solid-liquid separation MBR, membrane aeration MBR, and extractive MBR. In terms of research and application status, solid-liquid separation MBR holds an absolute advantage in wastewater treatment.
(1) Side-stream MBR (or external MBR) is the earliest type studied. The membrane modules are arranged separately from the bioreactor and operate relatively independently, making control and adjustment easy. Since the membrane modules are placed outside the reactor, membrane cleaning, replacement, and addition are easy. To reduce the deposition of suspended solids on the membrane surface, high-speed cross-flow filtration is adopted, which results in higher power consumption. Side-stream MBR is more suitable for treating wastewater that is difficult to biodegrade, has high organic concentration, or is toxic, and is widely used in industrial wastewater treatment.
(2) Integrated MBR (or submerged MBR) is simpler: the membrane modules are placed directly inside the bioreactor, and the effluent is pumped out to obtain the filtrate. Integrated MBR uses the strong agitation created by aeration to achieve cross-flow over the membrane surface; some systems use impeller stirring near the membrane modules or rotation of the modules themselves to achieve the cross-flow effect. The emergence of integrated MBR greatly reduced the energy consumption and footprint of the treatment system. To reduce membrane fouling and extend the operating cycle, the suction pump runs intermittently. Integrated MBR is highly competitive for municipal wastewater treatment plants. If packing media are added inside the bioreactor, it becomes a hybrid
integrated MBR. In this reactor, a large number of microorganisms grow attached to the packing media, ensuring high treatment performance and strong resistance to shock loading, while preventing the suspended solids concentration in the reactor from becoming too high and affecting membrane flux.
According to the oxygen requirements of microbial growth in the bioreactor, MBR is divided into two major categories: aerobic and anaerobic. Aerobic MBR is mainly aimed at municipal and domestic sewage treatment, while anaerobic MBR is mainly aimed at high-concentration organic wastewater treatment. The anaerobic process can improve the biodegradability of high-concentration organic wastewater and change the molecular structure of refractory organics, facilitating subsequent aerobic MBR treatment and significantly increasing organic removal rates.
Membrane bioreactor (MBR) water treatment technology is a highly efficient biochemical water treatment technology that combines biotechnology with membrane technology. The membrane bioreactor is an efficient wastewater treatment technology that combines membrane separation technology with the conventional sludge process. Due to membrane filtration, biomass is completely retained in the bioreactor, achieving complete separation of hydraulic retention time and sludge age and maintaining a high MLSS in the bioreactor. It has strong nitrification capacity and a high pollutant removal rate.
The membrane bioreactor is a new water treatment technology that combines highly efficient membrane separation technology with the activated sludge process. The application of hollow-fiber membranes replaces the secondary sedimentation tank in the activated sludge process for solid-liquid separation, effectively achieving sludge-water separation. Making full use of the highly efficient retention of the membrane, nitrifying bacteria can be effectively retained completely inside the bioreactor, ensuring smooth nitrification and effective removal of ammonia nitrogen, preventing sludge loss, and retaining macromolecular organics that are difficult to degrade at the moment, extending their residence time in the reactor so they are decomposed to the greatest extent. After applying MBR technology, the removal rates of major pollutants can reach COD ≥ 93% and SS = 100%. The permeate has near-zero suspended solids and turbidity, and the treated water quality is good and stable enough for direct reuse, realizing wastewater resource recovery.
In the membrane bioreactor, ultrafiltration membrane modules are used in place of the secondary sedimentation tank in the conventional activated sludge process, enabling highly efficient solid-liquid separation and overcoming the shortcomings of the conventional activated sludge process, such as unstable effluent quality and sludge bulking, thereby offering the following advantages:
Building on this foundation, industry experts have also carried out extensive research and improvements.
(5) The membrane bioreactor generally operates at a high volumetric loading and a low sludge loading, producing little excess sludge and reducing sludge treatment costs;
