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The MBR device

Ⅰ.Introduction
In the field of sewage treatment and water resource reuse, MBR, also known as membrane bio-reactor, is a new water treatment technology combining membrane separation unit and biological treatment unit. The membrane structure used is mainly flat membrane and hollow fiber membrane, which can be divided into ultrafiltration technology according to the membrane pore size.

II. Process Composition
The membrane-bioreactor is mainly composed of membrane separation components and bioreactors. 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, abbreviated as MBR)

Aeration Membrane  

The Aeration Membrane—Bio-Reactor (AMBR) was first reported by Cote.P et al. in 1988. It employs gas-permeable dense membranes (such as silicone rubber membranes) or microporous membranes (such as hydrophobic polymer membranes) in plate or hollow fiber configurations. By maintaining the gas partial pressure below the bubble point, bubble-free aeration can be achieved in the bioreactor. This process is characterized by increased contact time and oxygen transfer efficiency, facilitating better control of the aeration process. It is not affected by factors such as bubble size and retention time, which are limitations in traditional aeration methods.  

Extractive Membrane  

The Extractive Membrane—Bio-Reactor, also known as EMBR (Extractive Membrane Bioreactor), addresses specific challenges in industrial wastewater treatment. Certain industrial wastewaters are unsuitable for direct contact with microorganisms due to high acidity, alkalinity, or the presence of toxic substances. When wastewater contains volatile toxic compounds, traditional aerobic biological treatment processes can lead to the volatilization of pollutants with the aeration airflow, causing stripping effects. This not only results in highly unstable treatment efficiency but also contributes to air pollution.  

To tackle these technical challenges, British researcher Livingston developed the EMBR. In this system, wastewater and activated sludge are separated by a membrane. The wastewater flows inside the membrane, while activated sludge containing specific bacteria flows outside. The wastewater and microorganisms do not come into direct contact, and organic pollutants can be degraded by microorganisms on the other side through a selectively permeable membrane. Since the bioreactor unit and the wastewater circulation unit on either side of the extractive membrane operate independently, the water flow in each unit has minimal mutual interference. The nutrient conditions and microbial survival environment in the bioreactor remain unaffected by the wastewater quality, ensuring stable treatment performance. System operating conditions, such as Hydraulic Retention Time (HRT) and Sludge Retention Time (SRT), can be independently optimized to maintain the maximum pollutant degradation rate.  

Solid-Liquid Separation Membrane  

The Solid-Liquid Separation Membrane—Bio-Reactor is one of the most extensively studied types of membrane bioreactors in the field of water treatment. It is a water treatment technology that replaces the secondary sedimentation tank in traditional activated sludge processes with a membrane separation process.  

In conventional biological wastewater treatment technologies, sludge-water separation is achieved in the secondary sedimentation tank through gravity. The separation efficiency depends on the settling properties of the activated sludge—the better the settling properties, the higher the separation efficiency. However, the settling properties of the sludge are influenced by the operating conditions of the aeration tank. Improving sludge settling requires strict control of the aeration tank's operating conditions, which limits the applicability of this method. Due to the solid-liquid separation requirements of the secondary sedimentation tank, the sludge concentration in the aeration tank cannot be maintained at a high level, typically ranging from 1.5 to 3.5 g/L. This restricts the rate of biochemical reactions. The Hydraulic Retention Time (HRT) and Sludge Retention Time (SRT) are interdependent, often creating a conflict between increasing volumetric loading and reducing sludge loading. Additionally, the system generates a significant amount of excess sludge during operation, with its disposal costs accounting for 25% to 40% of the operating costs of wastewater treatment plants. Traditional activated sludge systems are also prone to sludge bulking, leading to suspended solids in the effluent and deteriorating water quality.  

To address these issues, MBR integrates membrane separation technology from separation engineering with traditional biological wastewater treatment technology, significantly improving solid-liquid separation efficiency. Moreover, the increased concentration of activated sludge in the aeration tank and the presence of specialized bacteria (particularly dominant microbial populations) enhance the biochemical reaction rate. At the same time, by reducing the Food-to-Microorganism (F/M) ratio, the production of excess sludge is minimized (even to zero), effectively resolving many of the prominent issues associated with traditional activated sludge processes.  

III. Types of Processes
Based on the combination of membrane modules and bioreactors, membrane-bioreactors (MBRs) can be classified into three basic types: separate, integrated, and hybrid. (The following discussion focuses on solid-liquid separation MBRs.)

Separate MBR
In a separate MBR, the membrane module and bioreactor are set up separately. The mixed liquor in the bioreactor is pressurized by a circulation pump and sent to the filtration end of the membrane module. Under pressure, the liquid in the mixed liquor passes through the membrane, becoming the treated water of the system. Solids and macromolecules are retained by the membrane and returned to the bioreactor with the concentrated liquor. The separate MBR is characterized by its stable and reliable operation, ease of membrane cleaning, replacement, and addition. Additionally, it generally has a higher membrane flux. However, to reduce the deposition of pollutants on the membrane surface and extend the cleaning cycle of the membrane under normal conditions, a higher cross-flow velocity on the membrane surface needs to be provided by the circulation pump, resulting in a large water circulation volume and high energy costs (Yamamoto, 1989). Furthermore, the shear force generated by the high-speed rotation of the pump can cause inactivation of certain microbial cells (Brockmann and Seyfried, 1997).

Integrated MBR
In an integrated MBR, the membrane module is placed inside the bioreactor. Incoming water enters the MBR, where most pollutants are removed by the activated sludge in the mixed liquor, and then filtered by the membrane under external pressure to produce the outlet water. This type of MBR eliminates the need for a mixed liquor circulation system and relies on suction for water withdrawal, resulting in relatively low energy consumption. It also occupies a more compact space compared to the separate MBR and has received special attention in the field of water treatment. However, it generally has a lower membrane flux and is prone to membrane fouling, which is difficult to clean and replace once fouled.

Hybrid MBR
The hybrid MBR formally belongs to the category of integrated MBRs, but it differs in that it incorporates fillers within the bioreactor, thereby forming a hybrid MBR and altering certain characteristics of the reactor.

IV. Process Characteristics

Compared with many traditional biological water treatment processes, MBRs (Membrane Bioreactors) offer the following main advantages:

High-Quality and Stable Effluent Water
Due to the efficient separation capability of the membrane, the separation effect is far superior to traditional sedimentation tanks. The treated effluent water is extremely clear, with suspended solids and turbidity approaching zero. Bacteria and viruses are significantly removed, resulting in an effluent water quality that surpasses the Water Quality Standard for Domestic Miscellaneous Water Use issued by the Ministry of Construction (CJ25.1-89). This water can be directly reused as non-potable municipal miscellaneous water. Additionally, membrane separation completely retains microorganisms within the bioreactor, maintaining a high microbial concentration within the system. This not only enhances the overall removal efficiency of pollutants by the reaction unit, ensuring good effluent water quality, but also allows the reactor to adapt well to various changes in influent load (water quality and quantity). It is resistant to shock loads and can stably produce high-quality effluent water.

Low Production of Excess Sludge
This process can operate under high volumetric loads and low sludge loads, resulting in low production of excess sludge (potentially achieving zero sludge discharge theoretically), which reduces sludge treatment costs.

Small Footprint and No Setting Location Restrictions
High microbial concentrations can be maintained within the bioreactor, allowing for high volumetric loads in the treatment unit and significant savings in footprint. The process flow is simple, the structure is compact, and the footprint is small, with no setting location restrictions. It is suitable for any occasion and can be constructed as ground-level, semi-underground, or underground installations.

Removal of Ammonia Nitrogen and Refractory Organics
The complete retention of microorganisms within the bioreactor favors the retention and growth of slowly proliferating microorganisms such as nitrifying bacteria, improving the system's nitrification efficiency. At the same time, it increases the hydraulic retention time of some refractory organics in the system, which is conducive to improving the degradation efficiency of these organics.

Convenient Operation and Easy Automation
This process achieves a complete separation of hydraulic retention time (HRT) and sludge retention time (SRT), making operation control more flexible and stable. It is a new technology in wastewater treatment that is easy to equip and can be controlled automatically by microcomputers, making operation and management more convenient.

Easily upgraded from traditional methods
This process can serve as an advanced treatment unit for traditional wastewater treatment processes, and holds broad application prospects in areas such as advanced treatment of effluent from urban secondary wastewater treatment plants (thereby enabling large-scale reuse of urban wastewater).

There are also some drawbacks to membrane bioreactors, mainly manifested in the following aspects:

(1)The high cost of membranes results in higher capital investment for membrane bioreactors compared to traditional wastewater treatment processes.

(2)Membrane fouling occurs easily, causing inconvenience in operation and management.

(3)High energy consumption: Firstly, a certain membrane driving pressure must be maintained during the sludge-water separation process in MBRs. Secondly, the MLSS (Mixed Liquor Suspended Solids) concentration in MBR tanks is very high, and in order to maintain an adequate oxygen transfer rate, the aeration intensity must be increased. Additionally, in order to increase membrane flux and reduce membrane fouling, the flow rate must be increased to scour the membrane surface. All these factors contribute to higher energy consumption in MBRs compared to traditional biological treatment processes.

Ⅴ.Application Fields

In the mid-to-late 1990s, membrane bioreactors (MBRs) entered the practical application stage abroad. Zenon Environmental Inc. of Canada was the first to introduce the ultrafiltration tubular membrane bioreactor and applied it to municipal wastewater treatment. To save energy consumption, the company further developed submerged hollow fiber membrane modules. The MBRs developed by Zenon have been applied in over ten locations, including the United States, Germany, France, and Egypt, with capacities ranging from 380m³/d to 7600m³/d. Mitsubishi Rayon Co., Ltd. of Japan is also a well-known provider of submerged hollow fiber membranes worldwide and has accumulated years of experience in MBR applications, with multiple practical MBR projects built in Japan and other countries. Kubota Corporation of Japan is another competitive company in the practical application of MBRs, and its plate membranes feature high flux, resistance to fouling, and simple processing. Some domestic researchers and enterprises are also attempting to practicalize MBRs.

MBRs have been applied in the following fields:

Urban Wastewater Treatment and Reuse in Building Water Systems

In 1967, the first wastewater treatment plant utilizing MBR technology was built by Dorr-Oliver of the United States, with a treatment capacity of 14m³/d. In 1977, a wastewater reuse system was practically implemented in a high-rise building in Japan. By 1980, Japan had constructed two MBR treatment plants with capacities of 10m³/d and 50m³/d, respectively. By the mid-1990s, there were 39 such plants operating in Japan, with the largest having a treatment capacity of 500m³/d, and over 100 high-rise buildings utilizing MBRs for wastewater treatment and reuse in greywater systems. In 1997, Wessex Water of the UK established the world's largest MBR system at the time in Porlock, UK, with a daily treatment capacity of 2000m³. In 1999, they further built a 13,000m³/d MBR plant in Swanage, Dorset. In May 1998, Tsinghua University's pilot integrated membrane bioreactor system passed national evaluation. At the beginning of 2000, Tsinghua University constructed a practical MBR system at Haidian Township Hospital in Beijing for hospital wastewater treatment. The project was completed and put into use in June 2000, and has been operating normally. In September 2000, Professor Yang Zaoyan and his research team at Tianjin University completed an MBR demonstration project in Puchen Building, Tianjin Hi-Tech Industrial Park. The system treats 25 tons of wastewater per day, with the treated wastewater entirely used for toilet flushing and green space irrigation. It occupies an area of 10 square meters and consumes 0.7kW•h of energy per ton of wastewater treated.

Industrial Wastewater Treatment

Since the 1990s, the application scope of MBRs has continuously expanded. Besides reclaimed water reuse and fecal sewage treatment, MBRs have also garnered widespread attention in industrial wastewater treatment, achieving good treatment results in various industries such as food processing, aquatic product processing, aquaculture, cosmetics production, dye manufacturing, and petrochemicals. In the early 1990s, the United States constructed an MBR system in Ohio for treating industrial wastewater from an automobile manufacturing plant, with a treatment capacity of 151m³/d. The system's organic loading reached 6.3kgCOD/m³•d, and the COD removal rate was 94%, with most oils and greases being degraded. In the Netherlands, a fat extraction and processing factory initially used traditional oxidation ditch technology for its production wastewater treatment. However, due to an expansion in production scale, sludge bulking occurred, making sludge separation difficult. Eventually, Zenon's membrane modules were used to replace the sedimentation tanks, resulting in good operational performance.

Purification of Slightly Polluted Drinking Water

With the widespread use of nitrogen fertilizers and pesticides in agriculture, drinking water has also been contaminated to varying degrees. In the mid-1990s, Lyonnaise des Eaux developed an MBR process that simultaneously achieves biological nitrogen removal, pesticide adsorption, and turbidity removal. In 1995, the company constructed a drinking water production plant in Douchy, France, with a daily production capacity of 400m³. The nitrogen concentration in the effluent was below 0.1mgNO₂/L, and the pesticide concentration was below 0.02μg/L.

Fecal Sewage Treatment

Fecal sewage contains a high concentration of organics. Traditional denitrification treatment methods require a high sludge concentration, resulting in unstable solid-liquid separation and compromising the effectiveness of tertiary treatment. The emergence of MBR has effectively addressed this issue and made it possible to treat fecal sewage directly without dilution. Japan has developed a fecal and urine treatment technology known as the NS system, with its core component being a system combining flat-sheet membrane units with aerobic high-concentration activated sludge bioreactors. The NS system was first built in Koshigaya City, Saitama Prefecture, Japan, in 1985, with a production capacity of 10kL/d. In 1989, new fecal and urine treatment facilities were subsequently constructed in Nagasaki Prefecture and Kumamoto Prefecture. The flat-sheet membranes in the NS system, each with an area of about 0.4m², are installed side by side in dozens of groups, forming a framework device that can be automatically opened and flushed. The membrane material is polysulfone ultrafiltration membrane with a molecular weight cut-off of 20,000. The sludge concentration in the reactor is maintained within the range of 15,000 to 18,000 mg/L. By 1994, Japan had over 1,200 MBR systems treating fecal sewage for more than 40 million people.

Landfill/Compost Leachate Treatment

Landfill/compost leachate contains high concentrations of pollutants, with water quality and quantity varying depending on climatic and operational conditions. MBR technology was used by multiple wastewater treatment plants for the treatment of such wastewater before 1994. The combination of MBR and RO technology not only removes SS, organics, and nitrogen but also effectively removes salts and heavy metals. Envirogen, a U.S. company, has developed an MBR for landfill leachate treatment and constructed a plant in New Jersey with a daily treatment capacity of 400,000 gallons (approximately 1,500m³/d), which became operational at the end of 2000. This MBR uses a naturally occurring mixed bacterial culture to decompose hydrocarbons and chlorinated compounds in the leachate, with a pollutant concentration treated 50 to 100 times that of conventional wastewater treatment plants. The reason for achieving this treatment effect is that MBR can retain high-efficiency bacteria and achieve a bacterial concentration of 50,000g/. In pilot-scale field tests, with influent COD ranging from hundreds to 40,000 mg/L, the removal rate of pollutants exceeded 90%.

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