What Is an MBR (Membrane Bioreactor)?
It replaces the conventional secondary clarifier at the end of biological treatment with membrane modules, maintains high activated-sludge concentration in the bioreactor, raises the organic load of biological treatment, thereby reducing the footprint of wastewater facilities, and reduces excess sludge by keeping a low sludge load. It mainly uses membrane-separation equipment immersed in the aerobic basin to retain activated sludge and macromolecular organics in the tank. MLSS concentration in an MBR can reach 8,000–10,000 mg/L or higher; sludge age (SRT) can be extended beyond 30 days.
Because of its effective retention, an MBR preserves long-generation-cycle microorganisms, enabling deep purification of wastewater; nitrifiers multiply fully in the system, giving obvious nitrification and making deep phosphorus and nitrogen removal possible.
Wastewater treatment: China is a water-short country; wastewater treatment and reuse are effective measures to develop and utilize water resources. Reuse means treating urban and industrial sewage via MBR and similar equipment for non-potable purposes such as greening, flushing, and ornamental-water replenishment, while reserving clean water for drinking and other high-quality uses. Urban and industrial sewage is locally available, avoiding long-distance conveyance and enabling local treatment for full water-resource utilization; local treatment also prevents sewage leakage during long transport that would pollute groundwater. Wastewater reuse is widely adopted in many water-short regions and is regarded as the most practical wastewater technology of the 21st century.
Meanwhile, the related supporting processes and equipment are also continuously optimized and upgraded.
MBR is a highly efficient combination of membrane separation and biological treatment; it originated from using membrane separation to replace the secondary clarifier in the activated-sludge process for solid–liquid separation. This process not only effectively achieves sludge–water separation but also has advantages unmatched by conventional tertiary treatment:
(2) The membrane's high-efficiency retention keeps microorganisms entirely in the bioreactor, achieving complete separation of HRT and SRT, with flexible and stable operation control.
In addition, this technology also has broad application and practice in related fields.
The MBR is a new water-treatment technology combining membrane-separation and biological-treatment units; it uses membrane modules to replace the secondary clarifier, maintains high activated-sludge concentration, reduces footprint, and reduces sludge via low sludge load. Compared with conventional biochemical processes, MBR's main features are: high efficiency and good effluent quality; compact equipment and small footprint; easy automation and simple operation. Since the 1980s it has drawn increasing attention as a research hotspot. MBR is now applied in over ten countries including the USA, Germany, France, and Egypt, at scales from 6 to 13,000 m³/d.
China's MBR research is less than a decade old but has progressed rapidly, roughly in several directions: (1) exploring combinations of different biological processes with membrane units, extending from activated sludge to contact oxidation, biofilm, combined activated-sludge/biofilm, and two-phase anaerobic processes; (2) studying factors, mechanisms, and models of treatment performance and membrane fouling to find suitable conditions and parameters that minimize fouling and improve capacity and stability; (3) broadening MBR's scope from domestic sewage to high-concentration organic wastewater (food, brewery) and refractory industrial wastewater (petrochemical, dyeing), though domestic sewage remains primary.
In China, MBR research covers both domestic and industrial wastewater. Results show MBR achieves good removal of COD, NH₃-N, SS, and turbidity from various high-concentration organic and refractory wastewaters.
Under high MLSS and microfiltration, effluent quality is stable and high. At high volumetric load with short retention time, the MBR flow is simpler than conventional systems; footprint is reduced by fully replacing clarifiers and sand filters, saving about 30% area versus conventional methods, with no sludge-settling problems.
MLSS in the reactor can exceed 10,000 mg/L, giving strong shock-load resistance and effective treatment of high-concentration organic wastewater. Under MF, separation far outperforms conventional clarifiers and sand filters; effluent is good and stable with low SS and turbidity, and treated low-pollution municipal wastewater can be directly reused as mid-water or on-site reclaimed water. It favors retention, growth, and reproduction of slow-growing nitrifiers, raising nitrification efficiency; under A/O it efficiently removes nitrogen, and A/O and A²O effectively remove ammonia and phosphorus, especially suitable in water-quality-control zones. MF retains most bacteria, reducing disinfectant dose and yielding safe reuse water. Low energy and operating cost. Biomass is retained in-tank for long SRT operation, and via endogenous respiration sludge is reduced by over half. Low waste-sludge versus conventional activated sludge, long sludge-discharge cycle, high operational flexibility; the biofilm-tube system is an absolute filter with high MLSS, easily handling highly variable wastewater. PLC control design makes operation and maintenance easy and automation feasible; MLSS 6,000–10,000 mg/L reduces aerobic sludge-basin volume; enclosed design gives low nuisance, noise, and odor. Membrane separation greatly improves removal of macromolecular refractory substances. Standard mobile modular design allows fast, simple installation and easy phased expansion, suitable for retrofitting old WWTPs by merely adding MBR modules.
Original process + MBR process
In addition, this technology also has broad application and practice in related fields.
An internationally emerging water-treatment technology of the 1990s, it combines biological treatment with membrane separation: first biochemical degradation of organics and cultivation of dominant bacteria to block pathogens, then membrane filtration of suspended solids and water-soluble macromolecules to lower turbidity to discharge standards. MBR technology is widely used in wastewater treatment and reclaimed-water reuse.
i.e., the Continuous Cycle Aeration System, a continuous-feed SBR aeration system. The CCAS process is an improvement on SBR (Sequencing Batch Reactor). CCAS has low pretreatment requirements — only a 15 mm gap mechanical screen and grit chamber. Its biological core is the CCAS tank, where phosphorus/nitrogen removal, organic and suspended-solids degradation all occur, with compliant effluent.
(1) During aeration, CCAS sewage and sludge are in perfect complete mixing, ensuring BOD and COD removal up to 95%.
(2) The repeated 'aerobic–anoxic' and 'aerobic–anaerobic' cycles strengthen phosphorus uptake and nitrification–denitrification, giving N/P removal above 80% and compliant effluent.
(3) During settling, the whole CCAS tank is in ideal settling, giving very low effluent SS, and the low value also ensures phosphorus removal.
Microfiltration selectively filters and separates liquids, retaining components of a mixture under operating pressure to achieve separation, concentration, and purification. Continuous ultra/microfiltration is now widely used and mature. The company's polypropylene hollow-fiber elements have rich experience in potable, river, deep-well, and industrial-process-concentrate treatment. In the system raw water is outside the membrane, permeate inside, with high recirculation ratio and high velocity in the tubes, reducing fouling. Air–water backwash scours the surface, effectively protecting elements with good cleaning, removing bacteria, microbes, and suspended solids; effluent turbidity near zero....
Beyond this, multiple factors must be considered in practical engineering applications.
Conventional methods need complex treatment to meet RO/NF feed requirements, while CMF needs only one filtration step to produce high-quality pretreatment water directly as RO/NF feed, with yield above 95%.
Beyond this, multiple factors must be considered in practical engineering applications.
Mid-water is reclaimed water from tertiary-treated urban sewage, commonly used for irrigation, washing, sanitation, and landscaping (non-potable). A specialized fiber-membrane company supplies MBR hollow-fiber membrane systems and provides membrane-application experience. With years of MBR application experience, the company developed the 'TF-MBR-RT' series of self-controlled integrated mid-water reuse equipment, suitable for small/medium mid-water treatment and reuse, serving scattered users such as residential communities, hotels, resorts, schools, office buildings, and ships for domestic sewage treatment and recycling.
At the 2004 environmental protection annual conference held in Italy, MBR was universally recognized from all aspects as the ultimately viable mid-water reuse technology for municipal sewage.
In addition, this technology also has broad application and practice in related fields.
MBR membranes are of two types: organic and inorganic. MBRs have commonly used organic membranes, typically polyethylene and polypropylene. Separated MBRs usually use UF modules with molecular-weight cutoff generally 20,000–300,000. A larger cutoff gives larger initial flux, but not necessarily larger long-run flux.
Once the membrane is selected its physicochemical properties are fixed, so operating mode becomes the main factor in MBR fouling. Not only sludge concentration and mixed-liquor viscosity affect flux, but also the mixed liquor's filterability — e.g., sludge characteristics and biology affect flux decline. Studies show PAC and coagulant addition improve sludge–water separation, forming larger, less viscous flocs and reducing membrane clogging, but excessive coagulant limits sludge activity and hurts capacity and performance.
Also, from an industry-development perspective, market demand is driving technological progress.
MBR is now in large-scale practical application in water treatment and is classified by membrane module and principle. Let's look at MBR classifications.
Notably, the technologies and standards in this field are also continuously developing and improving.
Membrane Aeration Bioreactor (MABR): the membrane is used for gas transfer, usually supplying oxygen to aerobic processes, enabling bubbleless aeration and greatly raising oxygen-transfer efficiency.
Also, from an industry-development perspective, market demand is driving technological progress.
The immersed (integrated) system places membrane modules directly in the reactor; filtrate is obtained by suction, and the cross-flow needed for surface cleaning is generated by air scouring from diffusers set just below the membrane, with mixed liquor rising along the flow and creating shear at the membrane surface to reduce fouling. The immersed MBR process is an organic combination of biological treatment and membrane separation.
Aerobic MBR is generally used for municipal and industrial treatment; for municipal it usually aims at reusable effluent, while for industrial it mainly removes specific pollutants such as greasy/oily contaminants.
In an anaerobic MBR, high-efficiency membrane retention not only solves the problem of anaerobic sludge easily washing out and lowering effluent quality, but also strengthens the anaerobic reactor's structure and performance. Taking UASB + membrane as an example, the anaerobic MBR no longer needs a three-phase separator for solid–liquid–gas separation; for a two-phase anaerobic MBR, membrane separation raises acidogen concentration in the acidogenic reactor, improving hydrolysis–fermentation, while the membrane retains macromolecules in the acidogenic reactor for hydrolysis–fermentation, maintaining high acidification. Anaerobic MBR is used for high-concentration organic wastewater; because MBR lacks aeration, to keep anaerobic sludge suspended, high-concentration anaerobic MBRs all use the separated type.
Beyond this, multiple factors must be considered in practical engineering applications.
In a mid-water system, collected bath wastewater contains a little hair and fiber; if not removed they clog pumps and the MBR, lowering efficiency and possibly paralyzing the whole system, so a hair filter is needed in the mid-water system.
In the MBR tank, organic-pollutant degradation and sludge–water separation occur. As the core of the system, the tank contains microbial colonies, membrane modules, water-collection, effluent, and aeration systems.
Notably, the technologies and standards in this field are also continuously developing and improving.
(3) The membrane's mechanical retention prevents microorganism loss, keeping high sludge concentration in the bioreactor, raising volumetric load, lowering sludge load, and giving extremely strong shock resistance;
(5) Membrane retention extends SRT, creating an environment favorable to slow-growing microbes such as nitrifiers, raising nitrification capacity while improving treatment and thorough decomposition of refractory macromolecular organics;
(6) MBR aeration-tank activated sludge does not wash out with effluent; during operation sludge varies with incoming organic concentration and reaches dynamic equilibrium, giving stable effluent and shock-load resistance;
(7) Larger hydraulic circulation gives uniform mixing, so activated sludge is well dispersed, greatly raising its specific surface area. The high dispersion of MBR sludge is another reason for better treatment — something ordinary biochemical methods forming large zoogloea cannot match;
Notably, the technologies and standards in this field are also continuously developing and improving.
MBR is widely suitable for domestic sewage treatment and reuse in residential communities, hotels, resorts, schools, and offices, and for organic wastewater in brewery, tannery, food, and chemical industries. MBR product water is commonly used for irrigation, washing, sanitation, and landscaping (non-potable). It suits various industrial enterprises, hotels, schools, hospitals, residential areas, and laundries for sewage treatment, mid-water treatment, and upgrading old facilities. [2]
The MBR water-treatment technology is a highly efficient biochemical process combining biotechnology and membrane technology; it is an efficient wastewater technology combining membrane separation with conventional sludge processes. Because of membrane filtration, biomass is fully retained in the bioreactor, achieving complete separation of HRT and SRT and keeping high MLSS. It has strong nitrification and high pollutant-removal rates.
MBR is a new water-treatment technology efficiently combining membrane separation with the activated-sludge process. Hollow-fiber membranes replace the secondary clarifier for solid–liquid separation, effectively achieving sludge–water separation. Fully using the membrane's high retention, it retains nitrifiers entirely in the bioreactor, ensuring smooth nitrification and effective ammonia removal, avoiding sludge loss, and retaining refractory macromolecular organics to extend their residence time for maximum decomposition. After MBR, main pollutant removals reach: COD ≥ 93%, SS = 100%. Product water has near-zero SS and turbidity; the treated water is good and stable and can be directly reused, realizing wastewater resource recovery.
