Advantages of flat-sheet membranes in membrane bioreactors (MBRs)

The Membrane Bioreactor (MBR) is a biochemical reaction system that integrates membrane separation technology with the biodegradation process of a traditional bioreactor. Membrane separation, a widely used physical treatment technique in wastewater treatment, relies primarily on a "sieve effect." Depending on the pore size of the membrane surface, it can be categorized into processes such as microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF). In MBR systems, microfiltration and ultrafiltration membranes are typically employed, with their separation mechanism also based mainly on the sieve effect. However, due to variations in membrane structure, the retention mechanisms broadly include mechanical interception, adsorption-based retention, and bridging effects. The conventional activated sludge process involves several key steps: screening, grit removal, primary sedimentation, biological reactor, secondary sedimentation, and disinfection. Among these, the screening, grit removal, and primary settling stages are designed to eliminate impurities and larger particles from the incoming wastewater, creating optimal conditions for subsequent biological treatment. Meanwhile, the majority of organic pollutants—such as COD, BOD, nitrogen, and phosphorus—in the wastewater are efficiently consumed and degraded through the metabolic activities of microorganisms within the reactor. To ensure efficient operation of the biological reactor, specific requirements are placed on both the concentration and condition of the activated sludge. After exiting the biological reactor, the effluent still contains a high concentration of microorganisms, necessitating further treatment in a secondary sedimentation tank to separate the sludge from the treated water, resulting in clearer effluent. The settled sludge is then either returned to the reactor or properly disposed of. Finally, the clarified effluent undergoes disinfection before being discharged, meeting stringent environmental standards. An innovative aspect of the MBR system is its replacement of the traditional secondary sedimentation tank with an ultrafiltration or microfiltration membrane module, enabling direct sludge-water separation. This advanced process boasts several notable advantages: robust treatment capacity, exceptional solid-liquid separation efficiency, superior effluent quality, minimal land footprint, and straightforward operational management. Currently, MBR technology has demonstrated remarkable success in treating both domestic sewage and industrial wastewater. At the heart of the MBR system lies the membrane component, which remains the critical element for effective performance. In practical applications, two main types of membrane modules are predominantly utilized: hollow fiber membranes and flat-sheet membranes. While both membrane types offer distinct characteristics and application scopes, flat-sheet membranes stand out for their ease of hydraulic control, high permeate flux, strong resistance to fouling, and convenient cleaning and replacement procedures—features that allow them to maintain stable, high-flux operation even under elevated sludge concentrations. On the other hand, hollow fiber membranes are prized for their high packing density and cost-effectiveness. As of 2006, incomplete statistics indicated that approximately 2,259 MBR systems were operational worldwide, with flat-sheet membrane bioreactors accounting for 68% of these installations. However, in China, the adoption of flat-sheet membrane bioreactors in large-scale engineering applications remains relatively limited, lagging significantly behind the widespread use of hollow fiber membrane systems.

1. Compared to the conventional activated sludge process, MBR offers the following distinct advantages:

   (1) High pollutant removal rate, with excellent effluent water quality

  The MBR can be used for treating both high-concentration, difficult-to-degrade organic industrial wastewater, as well as for purifying domestic sewage and general industrial effluents. In MBR systems, the membrane components effectively retain microorganisms from the reactor—particularly nitrifying and denitrifying bacteria with longer generation cycles, as well as those residing in small sludge particles. Additionally, thanks to the presence of the membrane, the mixed liquor suspended solids (MLSS) concentration in MBR systems can reach as high as 8,000–15,000 mg/L, significantly exceeding the levels seen in conventional activated sludge processes (typically around 3,000–4,000 mg/L). This results in superior pollutant removal efficiency and excellent effluent quality. Not only does the MBR achieve high removal rates for suspended solids (SS) and organic matter, but it also ensures that SS and turbidity in the treated water are reduced to nearly zero. Moreover, the system is capable of eliminating harmful bacteria and viruses, making it an ideal technology for advanced wastewater treatment and resource recovery. Leveraging its highly efficient biological reactions combined with the membrane’s exceptional separation and retention capabilities, MBRs typically achieve COD, BOD, and SS removal rates of up to 95%, 98%, and 99%, respectively. As a result, the treated effluent from MBR systems can often be directly reused as reclaimed water.

   (2) Strong adaptability to load changes and excellent resistance to shock loads

  Thanks to the membrane's highly efficient retention capability, a membrane bioreactor can completely trap activated sludge, resulting in extremely high sludge concentrations within the reactor. This allows for a complete separation of hydraulic retention time (HRT) and sludge retention time (SRT), ensuring that even when influent flow suddenly increases, the biological characteristics inside the reactor remain remarkably stable. Additionally, the elevated sludge concentration enhances the adsorption capacity of the activated sludge. Moreover, thanks to the membrane's effective barrier, pollutants that aren’t readily biodegraded are prevented from being discharged along with the treated effluent. These advantages collectively enable more flexible and reliable operation and control of the entire reactor system. As a result, membrane bioreactor systems effectively address common challenges faced by conventional wastewater treatment processes—such as sludge bulking—that typically arise when hydraulic or organic loads fluctuate.

   (3) Low sludge discharge

  Membrane bioreactor water treatment technology, in addition to serving as a highly effective method for advanced wastewater treatment and resource recovery, also stands out as a crucial technique for sludge reduction—and a practical solution to the persistent challenge of managing large volumes of excess sludge generated by conventional wastewater plants. One of the key advantages of membrane bioreactors is their remarkably low sludge production, with some systems even achieving zero sludge discharge altogether. While sludge self-degradation and hydrolysis can reduce the efficiency of traditional water treatment systems, these processes are actually beneficial in membrane bioreactor setups. In contrast to conventional activated sludge processes, which typically rely on sludge nearing the end of its stabilization phase—just before entering the decay stage—membrane bioreactors maintain high sludge concentrations. This leads to extensive organic matter consumption, while a significant portion of microorganisms already in the decay phase continue metabolizing through intrinsic respiration. As a result, not only are effluent pollutant levels kept exceptionally low, but the system also efficiently consumes the excess sludge produced during normal growth cycles. Moreover, membrane separation ensures that large, recalcitrant molecules in the wastewater have ample retention time within the compact confines of the membrane bioreactor. This extended contact time significantly enhances the degradation efficiency of these difficult-to-break-down compounds. Operating under conditions of high volumetric loading, low sludge loading, and extended sludge age allows the reactor to function effectively for extended periods—often up to six months or longer—without requiring any sludge removal at all, or with minimal sludge discharge. In some cases, the system can even achieve near-zero sludge production entirely.

   (4) Short process flow, simple and compact system equipment, and minimal land use

  Since membrane bioreactors eliminate the need for floc formation in aerobic sludge systems—flocs that are typically required for subsequent sludge-water separation in secondary settling tanks—the sludge concentration within the bioreactor can be significantly higher than in conventional processes. Meanwhile, the rate of biochemical reactions is directly tied to the concentration of reactants: the higher the reactant concentration, the faster the reaction proceeds. As a result, membrane bioreactors can achieve volumetric loading rates as high as 5 kg COD/(m³·d), whereas traditional processes usually manage only 0.4–0.9 kg BOD/(m³·d). Moreover, when treating domestic wastewater, the hydraulic retention time (HRT) can be reduced to as little as 2 hours, allowing for a substantial reduction in the volume of the biological reactor itself. According to international research, for the same scale of wastewater treatment, the volume of the aerobic tank in an MBR system can be as small as one-third that of a conventional treatment process. In addition, MBR systems inherently simplify operations by eliminating the need for secondary settling tanks, filtration units, and complex sludge recirculation systems—often even obviating the requirement for dedicated sludge handling equipment and associated costs. Perhaps most notably, nearly all MBR processes demonstrate excellent efficiency in removing pathogens. In effluent streams, levels of enteric viruses, total coliforms, fecal streptococci, and fecal coliforms consistently fall below detection limits. This remarkable performance suggests that, if MBR-treated water is discharged directly into the environment or reused without residual chlorine disinfection, even the installation of additional disinfection facilities could be entirely unnecessary. Consequently, membrane bioreactors stand out for their compact, streamlined design and operational simplicity.

   (5) Easily adaptable to automated control, simple maintenance, and reduced labor requirements

  In the conventional activated sludge process, frequent operational fluctuations and instability often require significant investments of manpower, financial resources, and materials to ensure high-quality effluent. In contrast, membrane bioreactors utilize membrane separation technology to consistently deliver stable effluent quality while eliminating the need for separate sludge-water separation equipment. As a result, microcomputers can easily enable full-process automated control of the membrane bioreactor system—from influent intake to effluent discharge.

   (6) The system boots up quickly, allowing water quality to rapidly meet treatment requirements.

  Because it effectively maintains sludge concentration in the water, during the reactor startup phase, unlike conventional aeration tanks that rely on settling and decanting the supernatant to boost sludge levels, this system leverages the membrane’s complete retention of sludge particles. This allows for a rapid increase in sludge concentration within the system, driven by the combined effects of aeration and nutrient supply, enabling the entire membrane bioreactor system to start up swiftly—and ensuring that effluent quality quickly meets treatment requirements.

2. Compared to hollow-fiber MBRs, flat-plate MBRs offer the following distinct advantages:

   (1) Improved anti-pollution performance

  Compared to hollow-fiber membrane bioreactors, flat-sheet membrane bioreactors can maintain stable high flux operation even at significantly higher concentrations of activated sludge. In practical applications, despite the presence of pretreatment facilities equipped with devices like screens and hair removal systems, some materials—such as hair—inevitably find their way into the aeration tank. These filamentous substances tend to wrap around the membrane fibers, and when sludge concentrations reach a certain level, they form sludge clumps that cause more and more fibers to tangle together. This dramatically reduces the effective membrane surface area of the hollow fibers, leading to a sharp decline in membrane flux—and once this happens, the issue is often difficult to repair, typically requiring complete membrane replacement. In contrast, flat-sheet membrane bioreactors can handle much higher activated sludge concentrations (MLSS), ranging from 10,000 to 15,000 mg/L, far exceeding the typical range of around 6,000 mg/L for hollow-fiber membrane bioreactors. Moreover, the unique structural design of flat-sheet membranes allows for precisely controlled gaps between membrane sheets, facilitating efficient gas-liquid mixing that enables in-line cleaning of the membrane surfaces. This results in superior resistance to fouling and contamination. Additionally, by adjusting the aeration intensity at the bottom of the module, flat-sheet bioreactors can effectively dislodge contaminants from the membrane surface through the scouring action of the gas-water mixture. Even if fouling occurs unexpectedly on the membrane surface, the modules can be easily removed and cleaned using low-pressure water jets, ensuring long-term, reliable operation. In contrast, hollow-fiber membranes cannot be cleaned using this method.

   (2) Excellent mechanical stability, with no wire breakage.

  In actual operation, hollow fiber membrane modules inevitably experience filament breakage, which can be attributed to two main reasons: First, uneven wall thickness caused by defects during the spinning process—though this scenario is relatively rare and can be further minimized by purchasing high-quality products. Second, root fractures resulting from fatigue in the spinning material. As we know, due to aeration, hollow fibers constantly undergo significant vibrations while in service. Over time, this persistent motion leads to material fatigue at their roots. Once such fatigue-induced breakage occurs, it tends to happen on a large scale, which can be devastating for membrane bioreactors, severely compromising effluent quality. In contrast, flat-sheet membranes boast significantly higher material strength than hollow fibers, completely eliminating the risk of this type of failure and ensuring consistently superior effluent quality.

   (3) The cleaning method is more convenient, and the cleaning cycle is longer.

  The flat-sheet membrane bioreactor effectively cleans the membrane by controlling the aeration rate from the aeration system located at the bottom of the module, creating a powerful hydraulic scouring action that helps prevent fouling on the membrane surface during operation. Additionally, chemical cleaning (online cleaning) of the flat-sheet membrane module is much simpler—simply recirculate the prepared cleaning solution back into the membrane via the suction port and let it soak for a set period. In contrast, hollow-fiber membrane modules often require frequent removal of the entire unit for backwashing. Moreover, compared to hollow-fiber-based membrane bioreactors, flat-sheet membrane bioreactors enjoy significantly longer cleaning intervals—often exceeding 3 months. And if the operating pressure remains consistently low, these systems can even go without cleaning altogether. Lastly, flat-sheet membrane modules can be restored to their original flux levels through physical cleaning methods, a feat that is virtually impossible with hollow-fiber membranes.

   (4) Long lifespan, low operating costs

  According to incomplete statistics, the average lifespan of hollow-fiber membranes currently on the market is around 2 years, which means there’s a significant membrane replacement rate every two years. In contrast, flat-sheet membranes typically last between 5 and 7 years, reducing the frequency of replacements. This not only lowers operational costs considerably but also ensures consistently reliable performance. Flat-sheet membranes feature a highly robust support structure, resulting in minimal membrane damage and a much lower replacement rate. Additionally, flat-sheet membranes can be replaced individually, further cutting down on replacement costs.

   (5) The diaphragm replacement process is simple.

  Thanks to the unique design of the flat membrane module, individual membrane sheets can be replaced during maintenance without needing to replace the entire support structure. However, if a certain number of intermediate fiber filaments break, the entire module becomes unusable and must be replaced entirely—resulting in significantly higher costs.