Membrane bioreactor (MBR) unit

I. Overview of Process Composition and Types

A membrane bioreactor (MBR) primarily consists of two components: a membrane separation module and a bioreactor. The term "membrane bioreactor" commonly refers to three types of reactors:

1.Aeration Membrane Bioreactor (AMBR)

2.Extractive Membrane Bioreactor (EMBR)

3.Solid/Liquid Separation Membrane Bioreactor (SLSMBR, or simply MBR)

II. Detailed Explanation of Various Membrane Bioreactors

1. Aeration Membrane Bioreactor (AMBR)

The AMBR was first reported by Cote.P et al. in 1988. It employs gas-permeable dense membranes (e.g., silicone rubber membranes) or microporous membranes (e.g., hydrophobic polymeric membranes) in plate or hollow fiber configurations to achieve bubble-less aeration in the bioreactor while maintaining gas partial pressure below the bubble point. This process enhances contact time and oxygen transfer efficiency, facilitating better control of the aeration process, independent of bubble size and residence time factors in traditional aeration.

2. Extractive Membrane Bioreactor (EMBR)

The EMBR, also known as Extractive Membrane Bioreactor, addresses challenges in treating industrial wastewater containing high acidity/alkalinity or toxic substances to microorganisms. When wastewater contains volatile toxic substances, traditional aerobic biological treatment can lead to volatilization of pollutants via aeration airflow, causing atmospheric pollution and unstable treatment efficiency.

To overcome these issues, British scholar Livingston developed the EMBR, where wastewater and activated sludge are separated by a membrane. Wastewater flows inside the membrane, while activated sludge containing specific bacteria flows outside. Organic pollutants are degraded by microorganisms on the other side of the membrane through selective permeability. The independent operation of the bioreactor unit and wastewater circulation unit on either side of the extraction membrane ensures stable water treatment efficiency, unaffected by wastewater quality. Operational conditions such as HRT and SRT can be optimized to maintain maximum pollutant degradation rates.

3. Solid/Liquid Separation Membrane Bioreactor (SLSMBR or MBR)

The SLSMBR is the most extensively researched MBR in water treatment, replacing the secondary clarifier in conventional activated sludge processes with membrane separation.

In traditional wastewater biological treatment, solid-liquid separation occurs in secondary clarifiers via gravity, with separation efficiency dependent on activated sludge settling properties. However, sludge settling is influenced by aeration tank operating conditions, limiting the applicability of this method. Additionally, maintaining high sludge concentrations in aeration tanks (typically around 1.5 - 3.5 g/L) to improve biochemical reaction rates is constrained by the need for solid-liquid separation in secondary clarifiers. This leads to interdependence between hydraulic retention time (HRT) and sludge retention time (SRT), creating conflicts between increasing volumetric loading and reducing sludge loading. Traditional activated sludge systems also generate large amounts of excess sludge, with disposal costs accounting for 25% - 40% of sewage treatment plant operating expenses. Sludge bulking and poor effluent quality are also common issues.

The MBR addresses these problems by integrating membrane separation technology with traditional wastewater biological treatment, significantly improving solid-liquid separation efficiency. Increased activated sludge concentration and the presence of specialized bacteria (especially dominant microbial populations) in the aeration tank enhance biochemical reaction rates. Additionally, reducing the F/M ratio minimizes excess sludge production (even to zero), resolving many outstanding issues in conventional activated sludge processes.

III. Classification of Process Types

Based on the combination of membrane modules and bioreactors, MBRs can be classified into three basic types: separate, integrated, and hybrid (all discussions below pertain to solid/liquid separation MBRs):

1. Separate MBR

The membrane module and bioreactor are separated. Mixed liquor from the bioreactor is pressurized by a circulation pump and fed to the filtration end of the membrane module. Under pressure, liquid in the mixed liquor permeates through the membrane, becoming treated water, while solids and macromolecules are retained and returned to the bioreactor with the concentrate.

Advantages: stable and reliable operation, easy membrane cleaning, replacement, and addition; generally high membrane flux. However, high cross-flow velocities across the membrane surface are required to reduce foulant deposition, leading to high energy consumption due to large water circulation volumes and pump shear forces that may inactivate some microorganisms.

2. Integrated MBR

The membrane module is placed inside the bioreactor. Influent enters the MBR, where most pollutants are removed by activated sludge in the mixed liquor, and water is filtered out by the membrane under external pressure.

Advantages: energy-efficient due to the elimination of mixed liquor circulation and reliance on suction for water extraction; compact footprint, attracting significant attention in water treatment. However, membrane flux is generally lower, membrane fouling is prone to occur, and cleaning and replacement are challenging.

3. Hybrid MBR

This form also belongs to the integrated MBR category but differs by incorporating packing materials inside the bioreactor, forming a hybrid MBR that alters certain reactor characteristics.

IV. Process Characteristics

Compared to many conventional biological water treatment processes, MBR (Membrane Bioreactor) offers the following key advantages:

1. High-Quality and Stable Effluent

The membrane's highly efficient separation capability far exceeds that of traditional sedimentation tanks, producing exceptionally clear treated water with suspended solids and turbidity close to zero. Bacteria and viruses are significantly reduced, ensuring effluent quality surpasses the *Ministry of Construction¡¯s Water Quality Standards for Domestic Miscellaneous Water Use (CJ25.1-89)*. This allows direct reuse as non-potable municipal water. Additionally, membrane separation retains all microorganisms within the bioreactor, maintaining a high microbial concentration. This not only improves overall pollutant removal efficiency but also enhances system resilience against fluctuations in influent load (both quality and quantity), ensuring consistently high effluent quality.

2. Low Excess Sludge Production

The process operates at high volumetric loading and low sludge loading, resulting in minimal excess sludge production (theoretically achieving zero sludge discharge). This significantly reduces sludge treatment costs.

3. Compact Footprint and Flexible Installation

The high microbial concentration maintained in the bioreactor allows for a high volumetric loading rate, drastically reducing the required footprint. The process is simple, compact, and adaptable to any installation scenario¡ªwhether above-ground, semi-underground, or underground.

4. Effective Removal of Ammonia Nitrogen and Refractory Organics

Complete retention of microorganisms promotes the growth of slow-growing microbes such as nitrifying bacteria, improving nitrification efficiency. Additionally, the extended hydraulic retention time (HRT) enhances the degradation of refractory organic compounds.

5. Easy Operation and Automation

The process achieves complete separation of hydraulic retention time (HRT) and sludge retention time (SRT), enabling more flexible and stable operational control. As a modular technology, MBR is easily automated via computer control, simplifying operation and management.

6. Compatibility with Retrofitting Conventional Systems

MBR can serve as an advanced treatment unit in conventional wastewater treatment plants, making it highly promising for applications such as polishing secondary effluent to enable large-scale urban wastewater reuse.

Disadvantages of MBR:

Despite its advantages, MBR has some limitations:

(1) High membrane cost, resulting in higher capital investment compared to conventional treatment processes.

(2) Membrane fouling, which complicates operation and maintenance.

(3) High energy consumption, primarily due to:

The need for transmembrane pressure to drive separation.

Increased aeration intensity to maintain oxygen transfer efficiency in high-MLSS (mixed liquor suspended solids) conditions.

Higher flow velocities required to mitigate membrane fouling, further elevating energy demands.

V. Application Areas

In the mid-to-late 1990s, MBRs entered practical application stages abroad. Canadian company Zenon first introduced ultrafiltration tubular MBRs for municipal wastewater treatment. To save energy, the company developed immersed hollow fiber membrane modules, with MBRs applied in over ten locations, including the United States, Germany, France, and Egypt, with capacities ranging from 380 m3/d to 7600 m3/d. Japan's Mitsubishi Rayon is also a renowned provider of immersed hollow fiber membranes globally, with extensive MBR application experience and numerous practical MBR projects in Japan and other countries. Japan's Kubota Corporation is another competitive player in MBR applications, producing plate membranes with high flux, fouling resistance, and simple processes. Domestic researchers and enterprises are also exploring MBR practical applications.

MBR applications include:

1. Municipal Wastewater Treatment and Building Reclaimed Water Reuse

The first MBR wastewater treatment plant, with a capacity of 14 m3/d, was built by Dorr-Oliver in the United States in 1967. A wastewater reuse system was practically applied in a high-rise building in Japan in 1977. Two MBR treatment plants with capacities of 10 m3/d and 50 m3/d were built in Japan in 1980. By the mid-1990s, 39 such plants were operating in Japan, with a maximum capacity of 500 m3/d, and over 100 high-rise buildings reused MBR-treated water for reclaimed water systems. In 

1997, Wessex Water in the UK established the world's largest MBR system at the time in Porlock, with a daily treatment capacity of 2000 m3, and in 1999, built a 13000 m3/d MBR plant in Swanage, Dorset.

In May 1998, a pilot-scale integrated membrane bioreactor system at Tsinghua University passed national evaluation. In early 2000, Tsinghua University established a practical MBR system at the Haidian Township Hospital in Beijing to treat hospital wastewater, which was completed and put into operation in June 2000, operating normally. In September 2000, Professor Yang Zaoyan and his research team at Tianjin University built an MBR demonstration project at the Puchen Building in Tianjin New Technology Industrial Park, treating 25 tons of sewage daily, with all treated water reused for toilet flushing and green space irrigation. The system occupied 10 square meters and consumed 0.7 kW¡¤h of energy per ton of sewage treated.

2. Industrial Wastewater Treatment

Since the 1990s, MBR applications have expanded beyond reclaimed water reuse and fecal sewage treatment to include industrial wastewater treatment, achieving good treatment results for food industry wastewater, aquatic product processing wastewater, breeding wastewater, cosmetic production wastewater, dye wastewater, and petrochemical wastewater. In the early 1990s, the United States built an MBR system in Ohio to treat industrial wastewater from an automobile manufacturing plant, with a treatment capacity of 151 m3/d, an organic load of 6.3 kg COD/m3¡¤d, and a COD removal rate of 94%, with most oils and greases degraded. In the Netherlands, a fat extraction processing plant replaced its sedimentation tank with Zenon's membrane modules due to sludge bulking issues caused by production scale expansion, achieving good operational results.

3. Micro-Polluted Drinking Water Purification

With the widespread use of nitrogen fertilizers and pesticides in agriculture, drinking water is increasingly contaminated. Lyonnaise des Eaux developed an MBR process in the mid-1990s with simultaneous biological denitrification, pesticide adsorption, and turbidity removal functions. In 1995, the company built a plant in Douchy, France, producing 400 m3 of drinking water daily, with nitrogen concentrations below 0.1 mg NO₂/L and pesticide concentrations below 0.02 ¦Ìg/L in the effluent.

4. Fecal Sewage Treatment

Fecal sewage contains high organic matter concentrations, and traditional denitrification treatment methods require high sludge concentrations, with unstable solid-liquid separation affecting tertiary treatment efficiency. MBRs effectively address this issue, enabling direct treatment of undiluted fecal sewage.

Japan has developed the NS system for night soil treatment, which combines a flat membrane device with an aerobic high-concentration activated sludge bioreactor. The NS system was built in Koshigaya City, Saitama Prefecture, Japan, in 1985, with a production capacity of 10 kL/d, and new night soil treatment facilities were subsequently built in Nagasaki and Kumamoto prefectures in 1989. The NS system's flat membranes, each approximately 0.4 m2, are installed in parallel in groups of dozens, forming automatically opening frame devices capable of automatic flushing. The membrane material is a polysulfone ultrafiltration membrane with a molecular weight cutoff of 20,000. Sludge concentration in the reactor is maintained within the range of 15,000 - 18,000 mg/L. By 1994, over 1,200 MBR systems were treating fecal sewage for over 40 million people in Japan.

5. Landfill/Compost Leachate Treatment

Landfill/compost leachate contains high pollutant concentrations, with water quality and quantity varying with climate and operational conditions. MBR technology has been used in multiple sewage treatment plants for leachate treatment since 1994. Combining MBR with RO technology can remove not only SS, organics, and nitrogen but also salts and heavy metals effectively. Envirogen in the United States developed an MBR for landfill leachate treatment, building a 400,000-gallon-per-day (approximately 1500 m3/d) facility in New Jersey that became operational in late 2000. This MBR uses a naturally occurring mixed bacterial culture to decompose hydrocarbons and chlorinated compounds in leachate, with pollutant concentrations 50 - 100 times higher than those in conventional wastewater treatment units. This treatment efficiency is achieved because the MBR retains high-efficiency bacteria, reaching concentrations of 50,000 g/L. In field pilot tests, influent COD ranged from hundreds to 40,000 mg/L, with pollutant removal rates exceeding 90%.