Detailed Explanation of Wastewater Treatment Processes: MBR Technology
In the fields of wastewater treatment and water reuse, MBR, also known as Membrane Bio-Reactor, is a new type of water treatment technology that combines the activated sludge method with membrane separation technology. There are many kinds of membranes: classified by separation mechanism, there are reaction membranes, ion-exchange membranes, osmotic membranes, etc.; classified by membrane nature, there are natural membranes (biofilms) and synthetic membranes (organic and inorganic membranes); and classified by membrane configuration, there are plate, tubular, spiral, and hollow-fiber types.
Moreover, from the perspective of industrial development, market demand is also driving technological progress.
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 polymer 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. The characteristics of this process are improved contact time and oxygen-transfer efficiency, easier aeration control, and independence from the bubble size and residence-time factors of conventional aeration. As shown in Figure [1].
The extractive membrane bioreactor, also known as EMBR (Extractive Membrane Bioreactor). Because of high acidity/alkalinity or the presence of substances toxic to organisms, some industrial wastewaters are unsuitable for treatment by direct contact with microorganisms; when the wastewater contains volatile toxic substances, the conventional aerobic biological process tends to volatilize the pollutants with the aeration airflow, causing stripping, which makes the treatment effect unstable and also causes air pollution. To solve these technical problems, the British scholar Livingston developed the EMBR. Wastewater and activated sludge are separated by the membrane: the wastewater flows inside the membrane while the activated sludge containing specific bacteria flows outside; the wastewater does not directly contact the microorganisms, and organic pollutants selectively pass through the membrane to be degraded by the microorganisms on the other side. Since the bioreactor unit and the wastewater circulation unit on the two sides of the extraction membrane are independent, the water flows of the units hardly affect each other, and the nutrient and survival conditions of the microorganisms in the bioreactor are not affected by the wastewater quality, giving stable treatment. Operating conditions such as HRT and SRT can each be controlled within an optimal range to maintain maximum pollutant degradation rates.
The solid-liquid separation membrane bioreactor is the most extensively studied type of MBR in water treatment - a technology that replaces the secondary sedimentation tank in the conventional activated sludge process with a membrane separation process. In conventional biological wastewater treatment, sludge-water separation is accomplished in the secondary clarifier by gravity, and its efficiency depends on the settleability of the activated sludge; the better the settleability, the higher the separation efficiency. But sludge settleability depends on the operating conditions of the aeration tank, and improving it requires strict control of those conditions, which limits the method's applicability. Because of the secondary clarifier's solid-liquid separation requirement, the sludge concentration in the aeration tank cannot be maintained high, generally around 1.5-3.5 g/L, limiting the biochemical reaction rate. Hydraulic retention time (HRT) and sludge retention time (SRT) are interdependent, and increasing the volumetric load often conflicts with reducing the sludge load. The system also generates large amounts of excess sludge, whose disposal cost accounts for 25-40% of a wastewater treatment plant's operating cost. Conventional activated sludge systems are also prone to sludge bulking, with suspended solids in the effluent and worsened effluent quality. To address these problems, the MBR organically combines membrane separation with conventional biological treatment; the MBR separates sludge retention time from hydraulic retention time, greatly improving solid-liquid separation efficiency, and because of the increased activated-sludge concentration in the aeration tank and the emergence of highly effective bacteria (especially dominant flora) in the sludge, the biochemical reaction rate is raised. At the same time, by lowering the F/M ratio the amount of excess sludge is reduced (even to zero), basically solving many prominent problems of the conventional activated sludge process.
The split-type membrane bioreactor separates the membrane module from the bioreactor, as shown in Figure 3. The mixed liquor in the bioreactor is pressurized by a circulation pump and delivered to the filtration side of the membrane module; under pressure, the liquid in the mixed liquor passes through the membrane to become the system's treated water, while solids, macromolecules, etc. are retained by the membrane and return to the bioreactor with the concentrate. The split-type MBR is characterized by stable and reliable operation and easy membrane cleaning, replacement, and addition, and its membrane flux is generally large. But under normal conditions, to reduce pollutant deposition on the membrane surface and extend the cleaning cycle, a higher cross-flow velocity at the membrane surface must be provided by the circulation pump, leading to large water circulation and high power cost (Yamamoto, 1989), and the shear force from the pump's high-speed rotation can inactivate some microbial cells (Brockmann and Seyfried, 1997).
The submerged membrane bioreactor places the membrane module inside the bioreactor, as shown in Figure 4. Influent enters the membrane bioreactor, most pollutants are removed by the activated sludge in the mixed liquor, and then the water is filtered out by the membrane under external pressure. This form of MBR saves the mixed-liquor circulation system and relies on suction for effluent, so its energy consumption is relatively low; it occupies less space than the split type and has received special attention in water treatment in recent years. However, its membrane flux is generally low, it is prone to membrane fouling, and fouled membranes are not easy to clean or replace.
In addition, this technology has also been widely applied and practiced in related fields.
Thanks to the membrane's efficient separation, the separation effect is far better than that of a conventional sedimentation tank: the treated effluent is extremely clear, suspended solids and turbidity are close to zero, and bacteria and viruses are largely removed. The effluent quality is better than the 'Water Quality Standard for Non-potable Urban Reuse Water' (CJ25.1-89) issued by the Ministry of Construction, and it can be directly reused as non-potable municipal reclaimed water.
At the same time, membrane separation also completely retains the microorganisms inside the bioreactor, allowing the system to maintain a high microbial concentration, which not only improves the overall pollutant removal efficiency of the reactor and ensures good effluent quality, but also gives the reactor good adaptability to various changes in influent load (quality and quantity), with shock-load resistance and stable production of high-quality effluent.
This process can operate at high volumetric load and low sludge load, with low excess sludge production (theoretically zero sludge discharge), reducing sludge treatment costs.
The bioreactor can maintain a high microbial concentration, the treatment unit has a high volumetric load, and the footprint is greatly reduced; the process flow is simple, compact, and space-saving, is not limited by the installation site, is suitable for any occasion, and can be built as above-ground, semi-underground, or underground.
Because the microorganisms are completely retained in the bioreactor, the growth of slowly proliferating microorganisms such as nitrifying bacteria is favored, and the system's nitrification efficiency is improved. At the same time, the hydraulic retention time of some refractory organic matter in the system can be extended, which helps improve the degradation efficiency of refractory organics.
In addition, this technology has also been widely applied and practiced in related fields.
o 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 to maintain sufficient oxygen-transfer rate the aeration intensity must be increased; and to increase membrane flux and mitigate membrane fouling the flow velocity must be raised to scour the membrane surface - all of which makes the MBR's energy consumption higher than that of conventional biological treatment processes.
In addition, this technology has also been widely applied and practiced in related fields.
It is the earliest applied membrane module form of the MBR process, with an appearance similar to an ordinary plate-and-frame filter press. Advantages: simple manufacture and assembly, easy operation, and easy maintenance, cleaning, and replacement. Disadvantages: complex sealing, large pressure loss, and low packing density.
Moreover, from the perspective of industrial development, market demand is also driving technological progress.
In the mid-to-late 1990s, membrane bioreactors entered the practical application stage abroad. Canada's Zenon Environmental first launched an ultrafiltration tubular MBR and applied it to municipal wastewater treatment. To save energy, the company also developed a submerged hollow-fiber membrane module; its MBR has been applied in more than ten locations including the United States, Germany, France, and Egypt, at scales from 380 m3/d to 7,600 m3/d. Japan's Mitsubishi Rayon is also a well-known provider of submerged hollow-fiber membranes, with years of MBR application experience and several actual MBR projects in Japan and other countries. Japan's Kubota Corporation is another competitive company in MBR practical application; its plate membranes feature high flux, pollution resistance, and simple process. Some domestic researchers and enterprises are also trying to put MBR into practical use.
In addition, multiple factors must be considered in actual engineering applications.
In 1967, the first wastewater treatment plant using the MBR process 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 in operation in Japan, with a maximum capacity of 500 m3/d, and more than 100 high-rise buildings used MBR to treat wastewater for reuse in miscellaneous water systems. In 1997, the British company Wessex Water built in Porlock, UK, what was then the world's largest MBR system, treating 2,000 m3/d; in 1999 it built a 13,000 m3/d MBR plant in Swanage, Dorset [14].
In May 1998, the submerged MBR pilot system developed by Tsinghua University passed national appraisal. In early 2000, Tsinghua University built a practical MBR system at the Haidian Township Hospital in Beijing to treat hospital wastewater; the project was completed and put into use in June 2000 and is currently operating normally. In September 2000, Professor Yang Zaoyan and his research team at Tianjin University built an MBR demonstration project in the Puchen Building of the Tianjin New Technology Industrial Park; the system treats 25 tons of sewage per day, all of which is used for toilet flushing and green-space irrigation, occupies 10 square meters, and consumes 0.7 kWh per ton of sewage treated.
Since the 1990s, the application scope of MBR has continuously broadened. Besides reclaimed-water reuse and nightsoil sewage treatment, MBR's application in industrial wastewater treatment has also received wide attention, such as treatment of 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 United States built an MBR system in Ohio to treat the industrial wastewater of an automobile manufacturer, at a scale of 151 m3/d; the system's organic loading reached 6.3 kgCOD/m3.d, with a COD removal rate of 94% and most oils and greases degraded. In the Netherlands, a fat-extraction plant used the conventional oxidation-ditch process for its production wastewater; due to expanded production scale it suffered sludge bulking and difficult sludge separation, and finally replaced the sedimentation tank with Zenon's membrane module, with good operating results.
With the widespread use of nitrogen fertilizer and pesticides in agriculture, drinking water is also polluted to varying degrees. Lyonnaise des Eaux developed in the mid-1990s an MBR process with the combined functions of biological nitrogen removal, pesticide adsorption, and turbidity removal; in 1995 the company built in Douchy, France, a plant producing 400 m3/day of drinking water. The effluent nitrogen concentration was below 0.1 mgNO2/L and the pesticide concentration below 0.02 ug/L.
Nightsoil sewage has a very high organic content; conventional denitrification treatment requires a high sludge concentration and unstable solid-liquid separation, affecting tertiary treatment. The advent of MBR well solved this problem and made it possible to treat nightsoil sewage directly without dilution.
Japan has developed a nightsoil treatment technology called the NS system, the core of which is a system combining a plate-membrane device with an aerobic high-concentration activated-sludge bioreactor. The NS system was built in 1985 in Koshigaya, Saitama Prefecture, Japan, at a production scale of 10 kL/d; in 1989 new nightsoil treatment facilities were built in Nagasaki and Kumamoto prefectures. In the NS system, dozens of plate-membrane modules of about 0.4 m2 each are installed in parallel in frames that can open automatically and self-flush. The membrane material is a polysulfone ultrafiltration membrane with a molecular-weight cutoff of 20,000. The sludge concentration in the reactor is kept within 15,000-18,000 mg/L. By 1994, Japan had more than 1,200 MBR systems treating the nightsoil sewage of over 40 million people.
Landfill/compost leachate contains high concentrations of pollutants, and its quality and quantity vary with climate and operating conditions. Before 1994, MBR technology was already used by several wastewater treatment plants for this kind of sewage. By combining MBR with RO technology, not only SS, organics, and nitrogen can be removed, but salts and heavy metals can also be effectively removed. Recently, the U.S. company Envirogen developed an MBR for landfill leachate treatment and built a unit in New Jersey with a capacity of 400,000 gallons/day (about 1,500 m3/d), 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 treatment units. The reason it achieves this effect is that the MBR can retain highly efficient bacteria and bring the bacterial concentration to 50,000 g/L. In field pilot tests, the influent COD ranged from several hundred to 40,000 mg/L, with a pollutant removal rate above 90%.
In addition, this technology has also been widely applied and practiced in related fields.
o Research on MBR process economics. Under current domestic economic development level, membrane product supply situation, and standard design requirements, determining the maximum economic flow for MBR used in wastewater treatment.
o Developing new membrane bioreactors aimed at energy saving, treatment of special water qualities, combined nitrogen and phosphorus removal, simple operation and maintenance, and long-term stable operation.
Used for advanced wastewater treatment, on the basis of the original compliant discharge, it can further reduce COD, NH-N, turbidity, and other indicators through a biological fluidized bed and a ceramic membrane separation system. On one hand it can be directly reused, and on the other it can serve as a pretreatment process for RO desalination, replacing the original lengthy filtration train of sand filtration, security filtration, and ultrafiltration; meanwhile, the reduced organic content greatly extends RO membrane life and lowers reuse-water treatment cost. The inorganic ceramic membrane separation system is the world's first inorganic membrane separation system dedicated to wastewater treatment, and compared with other organic and inorganic membranes it offers advantages such as high membrane flux, backwashability, and fully automatic operation.
