Phone:
0086 28 87573647
+86 19150187139
Membrane bioreactors (MBRs) are a novel water treatment technology that combines membrane separation units with biological treatment units. Replacing secondary sedimentation tanks with membrane modules maintains a high concentration of activated sludge in the bioreactor, reducing the footprint of wastewater treatment facilities and minimizing sludge volume by maintaining a low sludge load. Compared to traditional biological water treatment technologies, MBRs have the following main characteristics: high treatment efficiency and good effluent quality; compact equipment and small footprint; easy automation and simple operation and management. Since the 1980s, this technology has received increasing attention and become a research hotspot. Currently, membrane bioreactors are used in more than ten countries, including the United States, Germany, France, and Egypt, with scales ranging from 6 m³/d to 13,000 m³/d.
my country's research on MBRs is less than ten years old, but progress has been rapid. In October 1991, Cen Yunhua introduced the research status of MBRs in Japan. Around 1993, many universities and research institutes joined the development and research of MBRs, as detailed in Table 1. To gain a comprehensive understanding of the research status of membrane bioreactors (MBRs) in my country, this paper compiled statistics on publications related to MBRs by domestic researchers from 1991 to 2000 (only review and experimental articles were counted). A total of 104 papers were published over the ten years, and 10 doctoral students and more than 10 master's students obtained their degrees with MBRs as their research topic. As shown in Figure 1, the number of papers related to MBRs has shown a continuous upward trend in the last four years.
Domestic research on MBR can be roughly divided into several aspects: (1) exploring the combination of different biological treatment processes and membrane separation units, expanding biological reaction treatment processes from activated sludge to contact oxidation, biofilm, combined activated sludge and biofilm processes, and two-phase anaerobic processes; (2) researching the factors, mechanisms, and mathematical models affecting treatment efficiency and membrane fouling, exploring suitable operating conditions and process parameters, minimizing membrane fouling as much as possible, and improving the treatment capacity and operational stability of membrane modules; (3) expanding the application scope of MBR, expanding the research objects of MBR from domestic sewage to high-concentration organic wastewater (food wastewater, beer wastewater) and recalcitrant industrial wastewater (petrochemical wastewater, dyeing and printing wastewater, etc.), but mainly focusing on the treatment of domestic sewage. Therefore, the purpose of this paper is to review, analyze, and discuss the development of membrane bioreactors in wastewater treatment in my country.
Among the characteristics of MBR, good effluent quality and low sludge production rate are the most noteworthy.
Excellent Pollutant Removal Effect
Research on MBR in my country began in 1993. Comparative studies on separate MBR, suction-submersible MBR, gravity-submersible MBR, and traditional biological treatment processes in urban wastewater treatment have shown that the effluent quality of various MBRs is superior to that of traditional biological treatment processes. Tables 2 and 3 show the experimental parameters and treatment effects of MBR in treating domestic wastewater. Domestic wastewater treated by MBR has very low COD, BOD5, and turbidity. Most bacteria and viruses are retained, and the effluent quality meets or exceeds the Ministry of Construction's standards for domestic miscellaneous water use (CJ25.1-89), making it suitable for direct reuse as greywater in buildings, urban landscaping, sanitation, and fire fighting. Furthermore, the membrane's retention function prevents the loss of nitrifying bacteria, creating favorable conditions for maintaining a high concentration of nitrifying bacteria within the bioreactor, thus greatly improving nitrification efficiency. Research by Wang Chengwen and Zhang Jun on the nitrification characteristics of integrated MBR shows that MBR achieves a nitrogen removal efficiency of up to 97%. However, studies also show that the nitrogen removal efficiency of MBR is easily affected by factors such as temperature, shock load, and HRT.
In my country, MBR is being researched for the treatment of both domestic sewage and industrial wastewater. Tables 4 and 5 show the experimental parameters and treatment effects of MBR on various types of industrial wastewater, respectively. These research results all indicate that MBR achieves good removal effects on COD, NH3-N.SS, turbidity, etc., in various high-concentration organic wastewater and recalcitrant wastewater. In 1993, the Institute of Environmental Engineering at East China University of Science and Technology conducted a feasibility study on the treatment of synthetic wastewater and pharmaceutical wastewater using a separate ceramic membrane MBR. In 1995, Fan Yaobo applied MBR to the purification of petrochemical wastewater. Wang Lianjun used an inorganic membrane-bioreactor (IMBR) to treat beer wastewater. He Yiliang used a membrane-anaerobic bioreactor to treat high-concentration food wastewater: when the COD load was below 2 kg/(m³·d), the COD removal rate of the membrane effluent was over 90%. Ying Yuntao used a two-phase anaerobic MBR to treat artificially prepared starch wastewater: the COD load was 4-24 kg/(m³·d). When .d), the COD removal rate can reach over 95%; Gui Ping used MBR to treat three recalcitrant organic compounds—quinoline, EDTA, and polyethylene glycol—and found that MBR had higher COD removal rates, recalcitrant organic compound removal rates, and resistance to shock loads than the activated sludge process; Wu Zhichao's comparative study of MBR and conventional biological processes for treating brassic acid production wastewater also showed that MBR produces less sludge, has higher sludge activity, and can improve the removal rate of large-molecule recalcitrant organic compounds; Fan Yaobo and Zheng Xiang's small-scale and pilot-scale studies using MBR to treat wool textile dyeing wastewater showed that the wastewater treated by MBR can meet the standards for reuse as reclaimed water.
Low Sludge Yield
The activated sludge process is the most widely used biological treatment method for urban sewage and industrial wastewater. While converting organic pollutants in wastewater into organisms, CO2, and H2O, it produces a large amount of excess sludge. Currently, the treatment and disposal of excess sludge has become one of the limiting factors for the normal operation of wastewater treatment plants, accounting for 25%-40%, and even as high as 60%, of the total operating costs. Therefore, reducing sludge generation at the source is essential and crucial, and these factors have driven the development and research of MBR technology, which features low excess sludge production.
Theoretically, membrane bioreactors can completely retain sludge within the bioreactor, achieving zero sludge discharge. In 1991, in a small-scale test of MBR treatment of domestic wastewater, Chaize and Huyard first studied the impact of MBR on sludge production. At SRTs of 50 days and 100 days, sludge production was significantly reduced, which they attributed to a low F/M ratio and a longer sludge age. Muller's pilot-scale study of domestic wastewater treatment found that when the sludge concentration (MLSS) reached 4060 g L⁻¹ and the sludge retention SRT was ∞, almost no sludge was produced. Gui Ping, using an integrated MBR to treat domestic wastewater under different SRT (5-80 days) conditions, found that the theoretical yield coefficient YG and the decay coefficient b decreased with the extension of SRT. Liu Rui, using an integrated MBR to treat domestic wastewater, operated it for 280 days without sludge discharge and found that the apparent yield coefficient Yb showed a significant decreasing trend with the extension of operating time, decreasing from 0.248 kgVSS/kgCOD at the beginning of operation to 0.038 kgVSS/kgCOD. Zhang Shaoyuan, applying the principle of diminishing returns in the food chain, introduced metazoans—worms—into a two-stage MBR and found that the sludge yield was lower than that of conventional activated sludge wastewater treatment systems when worms were present; when the worm concentration was maintained above 100 worms/ml, the sludge yield was 0.1 kgSS/(kg COD removed), approximately 1/4 of that of the conventional activated sludge process. However, this method requires further research, such as maintaining the optimal number of worms within the system to bring the sludge production rate close to zero, thus achieving the goal of no sludge discharge.
Membrane bioreactors consist of membrane separation units and biological treatment units. Therefore, factors affecting the stable operation of MBRs include not only conventional biokinetic parameters such as volumetric loading, sludge concentration, and sludge load, but also membrane separation parameters such as the inherent properties of the membrane (membrane material, pore size, charge, etc.), the properties of the filtrate, the operating method, and the hydraulic conditions of the reactor. Among these, biokinetic parameters mainly affect the treatment effect of the MBR, while membrane separation parameters mainly affect the treatment capacity of the MBR.
Biokinetic Parameters Affecting the Stable Operation of MBRs
Organic Loading
Studies have shown that the effluent from aerobic MBRs is less affected by volumetric loading and hydraulic retention time (HRT), while the effluent from anaerobic MBRs is more significantly affected by shock loading and HRT. Li Hongbing used an MBR to treat domestic sewage. At hydraulic retention times of 1.5 and 5.8 hours, the treatment effect was basically the same at high loads (5.76 kg/(m³·d) and steady-state operation (0.8-1 kg/(m³·d)). The system achieved a COD removal rate of over 90% in both cases. Wu Zhichao used an aerobic MBR to treat brassic acid production wastewater and found that the effluent COD concentration did not change significantly at volumetric loading rates of 1.2, 2.4, 3.6, and 4.8 kg/m³·d; and the HRT had no significant impact on the effluent quality. However, He Yiliang used an anaerobic MBR to treat high-concentration food wastewater and found that when the volumetric loading rate increased from 2 kg/(m³·d) to 4.5 kg/(m³·d), the COD removal rate decreased from 90% to 70%; and the HRT had a significant impact on the treatment effect. Comparison of these studies revealed that in aerobic MBRs, sludge concentration increased rapidly with increasing volumetric loading, accelerating organic matter removal while maintaining a relatively constant sludge load, thus inhibiting effluent quality deterioration. In contrast, in anaerobic MBRs, sludge concentration increased slowly, and sludge load was almost positively correlated with volumetric loading. Therefore, effluent quality from anaerobic MBRs is easily affected by volumetric loading.
Li Hongbing and Gu Ping's research on MBR treatment of domestic wastewater showed that shock loading had no significant effect on organic matter removal, but NH3-N was significantly affected, with the degree of effluent NH3-N deterioration directly proportional to the magnitude of the shock load. This phenomenon may be because the membrane's interception effect does not contribute to NH3-N removal; therefore, the nitrogen removal efficiency of MBRs is easily affected by the treatment efficiency of the bioreactor. Gu Ping's research also found that under shock loading conditions, the membrane flux decline was tens of times greater than under normal COD loading. Analyzing the relationship between influent COD and MLSS during shock loading revealed a similar pattern in MLSS variation within the reactor to the decrease in maximum membrane flux. COD shock loading rapidly increased the activated sludge concentration and the viscosity of the mixed liquor, making liquid-solid separation difficult. Simultaneously, the sludge in its logarithmic growth phase exhibited high activity and contained a large amount of extracellular polymers, increasing membrane filtration resistance and leading to a decrease in maximum effluent flow.
Sludge Concentration
Sludge concentration is a crucial parameter in MBR systems, affecting not only organic matter removal capacity but also membrane flux. Numerous studies have shown that sludge concentration and dissolved microbial products are important parameters influencing membrane flux. Table 7 shows the impact of MLSS on membrane flux and filtration resistance. These findings indicate that under certain conditions, higher sludge concentration leads to lower membrane flux. Gu Ping's research on integrated MBR for treating domestic wastewater revealed that when the aeration intensity is sufficiently high (air-to-water ratio approximately 100:1), there is no significant correlation between MLSS and membrane flux when MLSS changes from 10 g/L to 35 g/L. However, reducing the aeration intensity may affect MLSS and membrane flux.
The degree to which sludge concentration affects membrane flux is closely related to aeration intensity, membrane surface circulation velocity, and hydraulic conditions. Gui Ping used orthogonal experiments to investigate the relationship between membrane fouling rate and sludge concentration, aeration rate, and membrane flux in an integrated MBR. The results showed that a critical membrane flux exists for different sludge concentrations, at which a large amount of sludge deposits on the membrane surface. When the membrane flux is less than the critical flux, membrane fouling is mainly caused by the deposition of dissolved organic matter on the membrane surface; when the membrane flux is greater than the critical flux, membrane fouling is mainly caused by the deposition of suspended sludge on the membrane surface. At low sludge concentrations, aeration intensity has little effect on membrane fouling. Under medium and high sludge concentrations, increasing the aeration intensity helps to mitigate membrane fouling. The critical membrane flux J is related to the sludge concentration MLSS and the aeration intensity QA as follows: QA/J = 8.34e0.07MLSS. However, the range of values for each variable in this experiment is relatively narrow. Based on Gui Ping's experiments, Liu Rui adopted a uniform design method to expand the range of values for each variable. Using the membrane filtration resistance rise rate K as a characterizing index of membrane fouling development speed, a membrane fouling development speed model was established: K = 8.933 * 10⁷ ΔP MLSS 0.532 J 0.376 ULr - 3.047. The membrane filtration resistance rise rate K increases with increasing membrane flux J and sludge concentration MLSS, and decreases with increasing intermembrane liquid upflow velocity ULr.
Membrane Separation Parameters
Under the premise of ensuring effluent quality, the membrane flux should be as large as possible. This reduces the membrane area used, lowering infrastructure and operating costs. Therefore, controlling membrane fouling and maintaining a high membrane flux are important aspects of MBR research.
Membrane Selection
Existing membrane materials can be divided into organic and inorganic membranes. Due to the high investment cost limiting the widespread application of inorganic membranes in my country, domestic MBRs have previously widely adopted organic membranes, with polyethylene and polypropylene being commonly used materials. Separate MBRs typically use ultrafiltration membrane modules, with a molecular weight cutoff generally between 200,000 and 300,000. A higher molecular weight cutoff results in a higher initial membrane flux, but this doesn't necessarily translate to higher long-term flux. Zhang Hongyu's flux decay experiments with inorganic membranes showed that 0.2 μm membranes are more suitable for MBRs than 0.8 μm membranes. He Yiliang's research on membrane flux decay using PES flat-sheet membrane modules found that under these experimental conditions, initial flux decay is mainly caused by concentration polarization; the smaller the molecular weight cutoff, the greater the flux decay rate. Long-term flux decay is mainly caused by membrane fouling; the larger the molecular weight cutoff, the greater the flux decay and the lower the chemical cleaning recovery rate.
For submerged MBRs, both ultrafiltration and microfiltration membranes can be used. Since the gel layer on the membrane surface also plays a filtering role, there is no significant difference in effluent quality between microfiltration and ultrafiltration membranes when treating domestic wastewater. Therefore, submerged MBRs often use 0.1–0.4 μm microfiltration membranes. 2.2.2 Optimization of Operation Methods
Once the membrane is selected, its true physicochemical properties are determined. Therefore, the operation method becomes the main factor affecting membrane fouling. To mitigate membrane fouling, backwashing is a crucial operation for maintaining stable operation of the separation-type MBR. Fan Yaobo derived a mathematical formula for determining the optimal backwashing cycle: f(t) = (Qf - Qb) / (tb + tf). This method avoids the need for trial-and-error experiments to determine the backwashing cycle, providing an important pathway for the automated control of the MBR system. For the suction-submerged MBR, Ymamoto proposed that intermittent suction can effectively mitigate membrane fouling. Gui Ping further pointed out through research that shortening the suction time or extending the stop time and increasing the aeration rate are both beneficial for mitigating membrane fouling. The suction time affects membrane resistance.
The increase in force has the greatest impact, followed by the aeration rate.
Not only sludge concentration and mixed liquor viscosity affect membrane flux, but the filtration performance of the mixed liquor itself, such as the activated sludge properties and biological phase, also affects the decline in membrane flux. Studies have shown that the addition of powdered activated carbon (PAC) and flocculants helps improve sludge-water separation performance, forming larger, less viscous sludge flocs and reducing the chance of membrane clogging. However, excessive addition of flocculants can limit sludge activity, affecting the reactor's treatment capacity and effect.
Improvement of Hydraulic Characteristics
Improving the hydrodynamic conditions of the feed solution near the membrane surface, such as increasing the influent flow rate, reducing concentration polarization, and ensuring that retained solutes are carried away promptly. Huang Xia and He Yiliang investigated the effect of sludge concentration on membrane flux at different membrane surface circulation velocities using PAN flat-plate ultrafiltration membranes and PAN/PS tubular membrane modules, respectively, and found that the degree of influence of MLSS on membrane flux is related to the membrane surface circulation velocity. Numerous experiments have shown that laminar flow of sludge through membranes is far more prone to clogging than turbulent flow. Therefore, theoretically determining the minimum membrane surface velocity (Vmin) for turbulent flow at different sludge concentrations is of great significance. Xing Chuanhong and Peng Yuelian's research both found a good linear relationship between the minimum membrane surface velocity and sludge concentration. However, their calculated critical membrane surface velocity may be too high because as sludge flows along the channel, water simultaneously permeates through the membrane, increasing turbulence in the vertical direction and thus lowering the lower critical Reynolds number (Rek) to some extent. He Yiliang's findings confirm this inference: the Rek for the transition from turbulent to laminar flow in a flat-sheet membrane module is 1083, and for an external pressure tubular membrane module it is 966, both lower than the lower critical Reynolds number of 2000 for typical Newtonian fluids.
In separated MBRs, cross-flow filtration is generally used; while integrated MBRs are essentially dead-end filtration. Compared to dead-end filtration, cross-flow filtration is more effective in preventing membrane fouling. Therefore, designing a reasonable flow channel structure to increase the upward flow velocity of the liquid between membranes, so that a larger amount of heating can achieve a cross-flow filtration effect on the membrane surface, is particularly important for submerged MBRs. Liu Rui, through uniform design experiments, obtained a suitable model for the upward flow of liquid between membranes suitable for activated sludge fluid, and proposed the influence of reactor structure on the upward flow velocity: under the same heating intensity, the taller the reactor, the narrower the upward flow channel, and the wider the downward flow channel and bottom channel, the greater the cross-flow velocity between membranes can be obtained. This model provides a theoretical basis for the design of integrated MBR reactor structures, but it needs to be verified in practice.
Energy Consumption
Energy consumption is an important evaluation indicator of wastewater treatment processes, directly related to the feasibility of the treatment method. Currently, the operating energy consumption of conventional separated MBRs is 3-4 kWh/m³, and that of submerged MBRs is 2 kWh/m³, far higher than the 0.3-0.4 kWh/m³ of activated sludge processes. High operating costs are a major problem encountered in the widespread application of MBRs. Many research results show that energy consumption is the main reason for the high operating costs of MBRs. Zhang Shaoyuan analyzed the energy consumption components of a separated MBR: pump heat loss, aeration energy consumption, pipeline resistance energy consumption, membrane module energy consumption, and return sludge head loss energy consumption. The energy consumption order was: membrane module > pump > aeration > pipeline > return sludge. Membrane module energy consumption accounted for 40-50% of the total energy consumption, with 80% of the energy used for membrane filtration being dissipated as heat. Gu Ping's energy consumption analysis of a submerged suction MBR showed that aeration energy consumption accounted for over 96% of the total energy consumption. Researchers generally consider energy reduction and membrane fouling control to be two independent topics in MBR research. However, Zhang Shaoguo and Zheng Xiang, using cross-flow and cross-current membrane modules in their separated MBR research, found that energy consumption increased with prolonged operating time and increased membrane fouling, rising from less than 0.5 kWh/m³ in the initial stage to 3 kWh/m³. This indicates that the energy consumption problem of separated membrane bioreactors is essentially a membrane fouling problem.
To further reduce energy consumption, Gu Ping applied a gravity-submerged MBR with differential-driven effluent and intermittent low-head operation. This effectively overcomes membrane fouling and clogging, allowing the membrane to maintain a high flux for extended periods. Furthermore, it eliminates the need for complex air-water backwashing equipment and reduces aeration requirements, enabling the energy consumption for treating domestic wastewater to be reduced to 1.0 kW·h/m³.
Economic Analysis of MBR in Reclaimed Water Reuse
The initial investment for an MBR process (scale 25-100 m³/d) applied to a reclaimed water reuse system is 3500-4000 RMB/m³, with membrane module costs accounting for approximately 25%. The depreciation cost over ten years (including construction and equipment material costs, excluding membrane modules) is 0.68-0.83 RMB/m³, membrane replacement costs (over two years) are 1.0 RMB/m³, and operating costs are 0.3-0.5 RMB/m³, resulting in a total operating cost of 2.0-2.3 RMB/m³. Since membrane prices have considerable room for reduction, experts estimate that within the next 3-5 years, membrane prices are expected to drop to 25-50% of current levels, significantly reducing the initial investment and replacement costs of MBR. With the decrease in membrane prices and the extension of service life, and the development of new, high-efficiency, low-energy-consumption MBRs, the total operating cost of MBRs is expected to decrease to 1.5 yuan/m³.
Taking the treatment cost of membrane bioreactor technology as an example, the output benefits of greywater are evident: greywater reuse is equivalent to saving an equivalent amount of fresh water, creating direct economic benefits. Taking Beijing's residential water price in 2000 as an example, it was 1.8 yuan/m³, while the water price for industries such as hotels, car washes, and bathhouses was 3.0-5.0 yuan/m³, higher than the price of tap water for general urban residents. The total operating cost of a membrane bioreactor was 2.3 yuan/m³, which, purely from an economic perspective, is currently economical for some industries. Given the current low tap water price, and the continued upward trend in tap water supply fees and sewage discharge fees, it is foreseeable that membrane bioreactors, as a wastewater reuse technology, will become increasingly competitive in terms of economy and technology.
Application Examples and Prospects of MBRs
In the past year or two, membrane bioreactors have entered the practical application stage in China. In 1998, Dalian Daqi Company's 200 m³/d greywater reuse system was put into operation in Dalian. Tianjin Deren Company pioneered the gravity-submerged MBR technology, which was applied to the greywater reuse system of Tianjin Pucheng Building in 2000, with a treatment capacity of 25 m³/d. This system occupies only 2.8 m² and has a treatment cost of 1.05 yuan/m³. Shanghai Ebara Company's PW system has been successfully applied to the treatment of high-concentration organic wastewater in dozens of industries, with capacities ranging from 5 m³/d to 700 m³/d. Hangzhou Hualu and Harbin Lubin companies are also competitive in MBR development and application. Table 8 lists some application examples and treatment effects of MBR in my country. In southern regions, MBR is currently mainly used for the treatment of high-concentration organic wastewater; while in northern regions like Tianjin and Dalian, which suffer from severe water shortages, MBR is mainly used as a greywater reuse technology.
my country's per capita water resources are only 2250 m³/person/year, less than one-quarter of the world average. Of my country's more than 600 cities, over 300 are water-scarce, with more than 100 experiencing severe water shortages, amounting to nearly 6 billion m³ annually, resulting in economic losses of approximately 200 billion RMB annually. In North China, per capita water resources are only 250-480 m³/person/year, less than one-fifth of the national average, and almost all cities in this region face water shortages. Therefore, wastewater reuse is one of the important measures to alleviate the water crisis in the North China Plain. Membrane bioreactor (MBR) technology, with its high-quality effluent, is considered a water-saving technology with good economic, social, and environmental benefits and has attracted much attention. Although high operating costs remain, with advancements in membrane manufacturing technology, improved membrane quality, and reduced manufacturing costs, MBR investment will decrease accordingly. For example, the development of polyethylene hollow fiber membranes and new ceramic membranes has significantly reduced their costs compared to the past. On the other hand, the development of various novel membrane bioreactors has significantly reduced actual operating costs. For example, gravity-submerged MBRs and anaerobic MBRs operating under low pressure have drastically lower operating costs compared to traditional aerobic pressurized membrane bioreactors. Therefore, from a long-term perspective, the application of membrane bioreactors in water treatment will inevitably become increasingly widespread. With increasingly stringent water environmental standards today, MBRs have demonstrated enormous development potential and will be a strong competitor to traditional wastewater treatment technologies in the new century.
Currently, there are few application examples of membrane bioreactors in wastewater treatment in my country. Further research is needed, taking into account China's economic development level and the characteristics of the MBR process, to promote its engineering application. It is expected that wastewater reuse will be the main direction for the promotion and application of MBRs in China.
