MBR is a relatively new type of wastewater treatment and reuse technology, but it has encountered difficulties in popularization and application due to its own membrane pollution and membrane investment costs. Because of this, relevant research at home and abroad has not slacken more than 10 years. Sun Yat-sen University started from the source of pollution, established a new method of membrane fouling assessment, put forward an effective MBR backwashing method, developed a high efficiency denitrification function, low membrane fouling rate and Low-cost characteristics of the new MBR, and carried out the application and pilot studies.
Sun Yat - sen University develops new MBR technology to build a new method of membrane fouling assessment
MBR is a relatively new type of wastewater treatment and reuse technology that is formed by the coupling of activated sludge and membrane separation. Due to the solid-liquid separation of membranes, MBR has significant technical advantages such as less land occupation, high sludge concentration and good effluent quality. Therefore, MBR is widely used in the world. According to statistics, the market growth rate of the total volume of MBR wastewater in the world has been maintained at above 10%. As MBR technology matures and operational experience accumulates, more and more large-scale MBR processes (> 100,000 t / d) are being used for municipal wastewater treatment. For example, the largest MBR WWTP in the world will be commissioned in Stockholm, Sweden, with a design and treatment capacity of 860,000 tons / day. Obviously, MBR technology will play an important role in the field of sewage treatment.
Membrane fouling and membrane investment are problems that plague the promotion and application of MBR
As a new technology, MBR still has its own defects. During the operation of MBR, microorganisms and organic matter in the sludge mixture will be deposited on the membrane surface, causing membrane fouling, leading to greatly reduced water production and unstable operation of the process. At home and abroad, researchers and project managers conducted research on MBR membrane pollution for more than 10 years and made remarkable progress. However, there are still many scientific and technical problems in MBR fouling that have not yet been completely solved. Overall, "there are many sources of membrane pollutants (sewage, biological macromolecules, microorganisms), complex formation pathways (membrane entrapment, biogenesis, biodegradation, etc.)" and "immature membrane fouling control methods with unclear mechanisms" MBR membrane pollution identification, characterization and control of the key. In addition, excessive membrane investment costs will undoubtedly limit the MBR in the sewage (waste) water treatment applications. Therefore, the development of high-performance membrane materials or the use of low-cost filter media is expected to reduce the investment cost of the membrane. However, how to ensure the stable operation of the new membrane material and the low-cost filter medium is worth exploring.
Source Analysis of Membrane Pollutants
School of Environmental Science and Engineering, Sun Yat-sen Professor Meng Fan Gang research group has been engaged in membrane fouling and new MBR reactor research work. Extracellular polymeric substances (EPS) on the surface of microbial cells and the release of the resultant microbial products (SMP) have long been considered as the main source of membrane pollutants. The complexity of EPS and SMP components (polysaccharides, proteins and humic acids, etc.) and the mechanisms of formation (microbial secretion and matrix degradation, etc.) have led to a lack of understanding of the mechanisms of membrane fouling. The study found that SMPs with different molecular size range had significantly different membrane fouling behavior. SMP (composed mainly of polysaccharides) in the size range of 0.45 μm to 100000 Da deposit on the surface of the membrane and become the key membrane pollutant. SMPs smaller than 100,000 Da pass through the membrane and become the main component of membrane effluent organics . At the same time, with proteomics research, the study found that: SMP is an important contributor to protein in membrane pollutants during the initial stage of membrane fouling (before jump of TMP); while in severe membrane fouling stage (after jumping of TMP) Extracellular polymers (EPS) are gradually becoming the major source of proteins in membrane contaminants. In addition, biodegradation experiments and multi-component biodegradation models (G models) showed that polysaccharides in SMPs and polysaccharides in membrane contaminants (after TMP jump) had very similar biodegradability behavior; whereas the protein in EPS Proteins in membrane contaminants have similar degradation behavior. The above studies show that the EPS secreted by the membrane surface microorganisms and the SMP of the sludge supernatant, respectively, are the fundamental sources of proteins and polysaccharides in membrane pollutants (severe membrane fouling stage). These findings reveal the source and formation of membrane pollutants on the one hand and provide important theoretical basis for the optimization of MBR membrane pollution control methods (such as backwashing) on ​​the other hand.
Spectral Characterization of Membrane Pollutants
It is of great importance to establish a new method of in situ characterization of membrane pollutants for the prediction and control of membrane fouling. It was found that fluorescence spectroscopy (EEM) and UV-vis spectroscopy have potential advantages in characterizing dissolved organic matter (DOM) and its water environmental behavior. For example, EEM can effectively identify the interaction mechanism between different components of DOM (such as protein and humus components) and provide a theoretical basis for the analysis of membrane fouling mechanism of complex components (such as SMP or DOM) The characteristic spectral parameters (DSlope325-375, S275-295, SR) obtained by UV-vis scanning can not only characterize the molecular size changes of DOM aggregates in different aqueous environments (pH, calcium ions, aluminum ions) Pollution trend and membrane retention during membrane fouling. These studies provide theoretical and technical support for the in situ detection of SMP in MBR and the prediction of the trend of membrane fouling.
MBR In Situ Chemical Backwashing
Physical backwashing and chemical cleaning are essential to MBR operation. On the basis of traditional physical backwashing and traditional in situ chemical backwashing, the research group proposed to maintain the in situ chemical backwashing method with high frequency and low dose. Online lye backwashing not only reduces the fouling rate by about 50%, but also provides alkalinity for the nitrification of microorganisms in the membrane cell (aerobic cell) simultaneously, simplifying MBR process operation. Using low concentrations of sodium hypochlorite as a backwash agent also significantly extends the offline cleaning cycle of the membrane. Sodium hypochlorite chemical backwash can effectively prevent the deposition of filamentous bacteria (eg Thiothrix eikelboomi) on the membrane surface. Compared with the traditional in situ chemical backwashing method, this method can significantly reduce the dosage of sodium hypochlorite. Cleaning Agent Exposure Experiments show that oxidative agents and lye can destroy the physicochemical properties of proteins or polysaccharides in membrane contaminants (eg, reduced viscosity index or increased rheology, reduced molecular size, increased surface charge, etc.) and functional group structures (Such as the increase of carbonyl group and carboxyl group and the decrease of fatty acid chain, etc.), which to some extent enhanced the hydrophilicity of membrane pollutants and eventually changed the membrane fouling law (the filtration performance of membrane pollutants after exposure to chemical agents Significantly enhanced).
Low Cost Autotrophic Denitrification MBR Study
To reduce membrane investment costs and achieve efficient denitrification, the team developed a low-cost composite bioreactor (NWHBR) by combining the technical advantages of MBR and biofilm reactors. In NWHBR, the filter cake layer or biofilm has the main advantage of enhancing the mass transfer and biochemical processes of ammonia nitrogen, nitrate nitrogen and nitrite nitrogen in the biofilm in addition to enhanced interception of particulate matter and reduction of COD, thus enhancing nitrogen The removal rate. During the operation of NWHBR, the formation and escape of nitrogen in the biofilm help to ensure the biofilm water permeability and reduce the rate of membrane fouling. Physical backwash every 20-30 days maintains a stable water-permeable performance of the membrane module. The NWHBR reactor has more significant technical advantages in the startup and operation of the ANAMMOX process. The reactor has a denitrification efficiency 10-27% higher than traditional biofilm reactors. Although ANAMMOX bacteria are mainly attached to the membrane module in biofilm form, the membrane module remains uncontaminated (ie no TMP is detected) during more than 400 days of continuous operation. On the one hand, ANAMMOX has a faster reaction rate and the escape of nitrogen will ensure the water permeability of biofilm. On the other hand, the EPS of ANAMMOX bacteria is significantly different from EPS of common activated sludge, and its enriched α-helical protein II The structure helps to form ANAMMOX particles, thus ensuring the biofilm's water permeability. Therefore, NWHBR process not only has lower investment costs, but also is expected to achieve low pollution or pollution-free membrane module operation. On the basis of laboratory research, the research team gradually applied autotrophic denitrification technology to the treatment of high-ammonia-nitrogen wastewater such as aquatic processing wastewater and landfill leachate. Recently, Prof. Chen Guanghao, Prof. Hui Hui and Prof. Meng Fanang of the "Thousand Talents Program" of Sun Yat-sen University set up and run a pilot project of highly efficient nitrification SBR + anaerobic ammonium oxidation MBR for the efficient denitrification of landfill leachate.
Summary and Outlook
In recent years, the research group mainly in the analysis of the formation mechanism of membrane pollutants, the establishment of a new method of membrane pollution control and the development of new MBR process work in three areas. The source of membrane pollutants was elucidated on the molecular size and protein level. A new method of membrane fouling assessment based on UV-Vis spectral parameters was established. An effective MBR backwashing method was proposed. At the technical level, a new type of MBR with high efficiency denitrification, low membrane fouling rate and low cost has been developed and applied and pilot-scale studied.
In the future, the issue of membrane fouling remains an important research area in the field of MBR. With the deepening understanding of membrane fouling and the emergence of advanced analytical methods, researchers will pay more attention to the micro-mechanism of membrane fouling. For example, the molecular biological mechanism of biological macromolecule formation and degradation, the interaction mechanism between microorganisms or biopolymers and membrane materials, and the like. In terms of membrane pollution control, research and development of low-pollution membrane materials, optimization of aeration methods and regulation of microbial populations are undoubtedly important ways to reduce MBR membrane pollution and reduce energy consumption during operation. In engineering applications, MBR applications will also be more diverse (such as: municipal sewage anaerobic treatment).
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