What Is MBR Technology? A Detailed Guide to the Membrane Bioreactor Process
In wastewater treatment and water reuse, MBR—short for Membrane Bio-Reactor—is a novel water-treatment technology that combines the activated-sludge method with membrane-separation technology. Membranes are highly diverse: by separation mechanism they include reaction membranes, ion-exchange membranes, and osmotic membranes; by membrane nature they include natural membranes (biomembranes) and synthetic membranes (organic and inorganic); and by structural type they include plate, tubular, spiral, and hollow-fiber forms.
Building on this, industry experts have carried out extensive research and improvement.
The aerated-membrane bioreactor was first reported by Cote et al. in 1988. Using gas-permeable dense membranes (e.g., silicone-rubber membranes) or microporous membranes (e.g., hydrophobic polymeric membranes) in plate or hollow-fiber modules, bubble-free aeration of the bioreactor can be achieved while keeping the gas partial pressure below the bubble point. This process improves contact time and oxygen-transfer efficiency, facilitates aeration control, and is unaffected by bubble size and residence time in conventional aeration, as shown in Figure [1].
The extractive membrane bioreactor, also called EMBR (Extractive Membrane Bioreactor), was developed because some industrial wastewaters contain high acidity/alkalinity or substances toxic to microorganisms and are therefore unsuitable for direct contact with microbes. When wastewater contains volatile toxic substances, conventional aerobic biological treatment lets pollutants volatilize with the aeration airflow (air stripping), yielding unstable performance and atmospheric pollution. To solve these problems, the British scholar Livingston developed the EMBR. Wastewater and activated sludge are separated by the membrane: wastewater flows inside the membrane while activated sludge containing specific bacteria flows outside, so the wastewater never contacts the microbes directly, and organic pollutants selectively pass through the membrane to be degraded by the microbes on the other side. Because the bioreactor unit and the wastewater circulation unit on either side of the extraction membrane are independent, the flows barely affect each other, and the nutrient and survival conditions of the microbes in the bioreactor are unaffected by wastewater quality, giving stable treatment. Operating conditions such as HRT and SRT can be controlled independently within optimal ranges to maintain the maximum pollutant-degradation rate.
The solid-liquid separation membrane bioreactor is the most extensively studied type in water treatment—a technology that replaces the secondary settling tank of the conventional activated-sludge process with membrane separation. In traditional biological wastewater treatment, sludge-water separation relies on gravity in the secondary clarifier, and its efficiency depends on the settleability of the activated sludge (better settleability → higher separation efficiency). Settleability depends on the operation of the aeration tank, and improving it requires strict control of operating conditions, which limits the method's applicability. Because the secondary clarifier demands solid-liquid separation, the aeration tank cannot maintain a high sludge concentration (typically about 1.5–3.5 g/L), limiting the biochemical reaction rate. Hydraulic retention time (HRT) and sludge retention time (SRT) are interdependent, so raising the volumetric load and lowering the sludge load are often in conflict. The system also generates large amounts of excess sludge whose disposal accounts for 25%–40% of a plant's operating cost. Conventional activated-sludge systems are also prone to sludge bulking, with suspended solids in the effluent degrading water quality. To address these issues, MBR integrates membrane separation with conventional biological treatment, decoupling SRT from HRT and greatly improving solid-liquid separation efficiency; the higher activated-sludge concentration in the aeration tank and the emergence of specialized (especially dominant) bacterial populations raise the biochemical reaction rate. Meanwhile, lowering the F/M ratio reduces excess-sludge production (even to zero), essentially resolving many prominent problems of the conventional activated-sludge method.
The split-type membrane bioreactor places the membrane module and the bioreactor separately, as shown in Figure 3. Mixed liquor from the bioreactor is pressurized by a circulation pump to the filtration side of the membrane module; under pressure, the liquid passes through the membrane to become the system's treated water, while solids and macromolecules are retained by the membrane and returned to the bioreactor with the concentrate. The split-type MBR is characterized by stable, reliable operation and easy membrane cleaning, replacement, and expansion, with generally high membrane flux. However, to reduce foulant deposition on the membrane surface and extend cleaning intervals, a circulation pump must provide high cross-flow velocity, resulting in large recirculation flows and high power costs (Yamamoto, 1989); moreover, the shear generated by the pump's high-speed rotation can inactivate some microbial cells (Brockmann and Seyfried, 1997).
The immersed (integrated) membrane bioreactor places the membrane module inside the bioreactor, as shown in Figure 4. Influent enters the MBR, most pollutants are removed by the activated sludge in the mixed liquor, and the effluent is then filtered through the membrane under external suction. Because it omits the mixed-liquor recirculation system and relies on suction, this form has relatively low energy consumption and a more compact footprint than the split type, and has attracted special attention in water treatment in recent years. However, its membrane flux is generally lower and membrane fouling is prone to occur, and once fouled the membrane is hard to clean or replace.
Furthermore, this technology has seen wide application and practice in related fields.
Thanks to the high-efficiency separation of the membrane, the separation effect is far better than that of a conventional settling tank: the effluent is extremely clear, with suspended solids and turbidity near zero, and bacteria and viruses largely removed. The effluent quality exceeds the Domestic Reclaimed-Water Quality Standard (CJ25.1-89) issued by the Ministry of Construction and can be directly reused as non-potable municipal reclaimed water.
At the same time, membrane separation fully retains microorganisms inside the bioreactor, allowing the system to maintain a high microbial concentration. This not only improves the overall pollutant-removal efficiency of the reactor and ensures good effluent quality, but also gives the reactor strong adaptability to variations in influent load (both quality and quantity), with good shock-load resistance and stable, high-quality effluent.
The process can operate at high volumetric load and low sludge load, with low excess-sludge production (theoretically achieving zero sludge discharge), reducing sludge-treatment costs.
The bioreactor can maintain a high microbial concentration, giving the treatment unit a high volumetric load and greatly saving floor area. The process is simple, compact, and space-saving, unrestricted by site, and suitable for any location—it can be built as an above-ground, semi-underground, or underground unit.
Because microorganisms are fully retained in the bioreactor, slow-growing microbes such as nitrifying bacteria are retained and allowed to grow, improving the system's nitrification efficiency. At the same time, the hydraulic retention time of some refractory organics in the system can be extended, improving the degradation efficiency of hard-to-degrade organics.
Moreover, from an industry-development perspective, market demand is also driving technological progress.
• High energy consumption: First, the MBR sludge-water separation process must maintain a certain membrane driving pressure; second, the MLSS concentration in the MBR tank is very high, so aeration intensity must be increased to maintain sufficient oxygen-transfer rate; and third, to raise membrane flux and mitigate fouling, flow velocity must be increased to scour the membrane surface—all of which make MBR's energy consumption higher than that of conventional biological treatment.
Building on this, industry experts have carried out extensive research and improvement.
This is one of the earliest membrane-module forms used in MBR; it resembles a conventional plate-and-frame filter press in appearance. Advantages: simple manufacture and assembly, easy operation, and easy maintenance, cleaning, and replacement. Disadvantages: more complex sealing, large pressure loss, and low packing density.
It is worth noting that the technologies and standards in this field are also constantly evolving and improving.
By the mid-to-late 1990s, MBR had entered the practical application stage abroad. Canada's Zenon Company first launched an ultrafiltration tubular MBR and applied it to municipal wastewater treatment. To save energy, the company then developed an immersed hollow-fiber membrane module; its MBR products have been applied in more than a dozen locations including the U.S., Germany, France, and Egypt, at scales from 380 m³/d to 7,600 m³/d. Japan's Mitsubishi Rayon is also a well-known global supplier of immersed hollow-fiber membranes and has accumulated years of MBR application experience, having built several actual MBR projects in Japan and other countries. Japan's Kubota Company is another competitive player in practical MBR application; its plate membranes feature large permeate flow, pollution resistance, and simple process. Some domestic researchers and companies are also exploring MBR commercialization.
In addition, many factors must be considered in practical engineering applications.
In 1967, the first wastewater treatment plant using MBR was built by the U.S. company Dorr-Oliver, treating 14 m³/d of wastewater. In 1977, a water-reuse system was put into practical use in a high-rise building in Japan. In 1980, Japan built two MBR plants with capacities of 10 m³/d and 50 m³/d. By the early 1990s, 39 such plants were operating in Japan with a maximum capacity of 500 m³/d, and more than 100 high-rises used MBR to treat wastewater for reuse as reclaimed (middle-water) systems. In 1997, the UK's Wessex Company built the world's largest MBR system at Porlock, UK, with a capacity of 2,000 m³/d; in 1999 it built a 13,000 m³/d MBR plant at Swanage, Dorset [14].
In May 1998, the integrated MBR pilot system developed by Tsinghua University passed national appraisal. In early 2000, Tsinghua University built a practical MBR system at Haidian Township Hospital in Beijing for hospital wastewater; the project was completed and put into operation in June 2000 and is running normally. In September 2000, Professor Yang Zaoyan and his research group at Tianjin University built an MBR demonstration project at Puchen Building in Tianjin New Technology Industrial Park; the system treats 25 tons of sewage per day, all of which is reused for toilet flushing and green-space irrigation, occupies 10 m², and consumes 0.7 kWh per ton of sewage treated.
Since the 1990s, MBR's treatment targets have kept expanding. Besides reclaimed-water reuse and fecal-sewage treatment, MBR's application in industrial wastewater treatment has gained wide attention—for example, in food-industry wastewater, aquaculture processing wastewater, livestock wastewater, cosmetics-production wastewater, dye wastewater, and petrochemical wastewater, all with good results. In the early 1990s, the U.S. built an MBR system in Ohio for the industrial wastewater of an automobile manufacturer, at a scale of 151 m³/d, with an organic load of 6.3 kgCOD/m³·d and a COD removal rate of 94%, degrading most oil and grease. In the Netherlands, a fat-extraction plant using conventional oxidation-ditch technology for its production wastewater suffered sludge bulking and poor separation as production expanded; finally it replaced the settling tank with Zenon's membrane module and achieved good results.
With the widespread use of nitrogen fertilizers and pesticides in agriculture, drinking water is also polluted to varying degrees. Lyonnaise des Eaux developed in the mid-1990s an MBR process with simultaneous biological nitrogen removal, pesticide adsorption, and turbidity removal; in 1995 the company built a plant in Douchy, France producing 400 m³/d of drinking water. The effluent had nitrogen concentration below 0.1 mgNO₂/L and pesticide concentration below 0.02 μg/L.
Fecal sewage has a very high organic content; conventional denitrification requires a high sludge concentration and gives unstable solid-liquid separation, impairing tertiary treatment. The advent of MBR solved this problem well and made it possible to treat fecal sewage directly without dilution.
Japan has developed a urine-feces treatment technology called the NS system, whose core is a combination of a flat-sheet membrane unit and an aerobic high-concentration activated-sludge bioreactor. The NS system was built in Koshigaya, Saitama Prefecture, Japan, in 1985 at a production scale of 10 kL/d; in 1989 new fecal-treatment facilities were built in Nagasaki and Kumamoto prefectures. In the NS system, flat-sheet membrane panels of about 0.4 m² each are installed in parallel by the dozens in a frame that can open automatically and self-rinse. The membrane material is a polysulfone ultrafiltration membrane with a molecular-weight cutoff of 20,000. The sludge concentration in the reactor is kept at 15,000–18,000 mg/L. By 1994, Japan had more than 1,200 MBR systems treating fecal sewage for over 40 million people.
Landfill/compost leachate contains high concentrations of pollutants, and its quality and quantity vary with climate and operating conditions. MBR technology was already used by several plants for this wastewater before 1994. By combining MBR with RO, not only SS, organics, and nitrogen are removed, but salts and heavy metals are also effectively removed. Recently, the U.S. company Envirogen developed an MBR for landfill leachate and built a 400,000-gallon/day (about 1,500 m³/d) unit in New Jersey, put into operation at the end of 2000. This MBR uses naturally occurring mixed bacteria to decompose hydrocarbons and chlorinated compounds in the leachate, treating pollutant concentrations 50–100 times those of conventional wastewater equipment. This is achievable because MBR retains highly efficient bacteria and reaches a bacterial concentration of 50,000 g/L. In field pilots, influent COD ranged from several hundred to 40,000 mg/L with pollutant removal above 90%.
Building on this, industry experts have carried out extensive research and improvement.
• Research on MBR process economics: determining the maximum economically viable flow rate for MBR in wastewater treatment under current domestic economic development, membrane-product supply, and standard design requirements.
• Development of new membrane bioreactors aimed at energy saving, treatment of special water qualities, simultaneous nitrogen and phosphorus removal, easy operation and maintenance, and long-term stable operation.
Used for advanced wastewater treatment, it can further reduce indicators such as COD, NH-N, and turbidity through a biological fluidized bed and a ceramic-membrane separation system on top of already compliant discharge, enabling direct reuse on one hand and serving as a pretreatment for RO desalination on the other—replacing the original lengthy filtration train of sand filtration, security filtration, and ultrafiltration. The reduced organic content greatly extends RO membrane life and lowers reuse-treatment cost. The inorganic ceramic membrane separation system is the world's first inorganic membrane separation system dedicated to wastewater treatment; compared with other organic and inorganic membranes, it features high flux, backwashability, and fully automatic operation.
