The immobilized microbial technology is one kind of bio-enhancement technology. This paper reviews the application of microbial immobilization method and immobilization technology in wastewater treatment, and prospects the application prospect of immobilization technology and puts forward the problems that need to be solved in practical application. Keywords: immobilized microbial technology, wastewater treatment Preface: Immobilized microbial technology was developed on the basis of immobilized enzyme technology in the 1970s.
Immobilized microbial technology refers to the physical or chemical method of limiting or localizing free microbial cells, animal and plant cells, organelles or enzymes within a certain spatial range, retaining its inherent catalytic activity, and being able to be repeatedly and continuously used [ 1]. The essence of immobilized microbial technology is to immobilize, induce and acclimate specific bacteria groups with specificity of refractory organic substances by using bioactive polymer carriers, so that microorganisms can be arranged according to the degradation rate and order of organic substances, and the efficient removal of refractory organic substances can be realized. In addition, the influence of the polymer effect of the carrier creates a microenvironment suitable for the survival of microorganisms and improves the tolerance of microorganisms. The application of this technology provides a new technical approach for wastewater treatment and has broad application prospects. 1. Microbial Immobilization Methods There are many different methods for immobilizing microbial technology. At present, there is no uniform classification standard at home and abroad. The preparation methods of immobilized microorganisms can be roughly classified into an embedding method, an adsorption method, a covalent bonding method and a cross-linking method , and a newly developed unsupported immobilization method.
1.1 Embedding method is to confine the microorganisms to a limited space such as tiny lattices or microcapsules of the gel, and at the same time allow the matrix to penetrate and the product to diffuse. The network of gel polymers prevents the leakage of cells while allowing the substrate to penetrate and the product to diffuse out. The embedding method has little effect on microbial activity and high particle strength. It is the most commonly used and most widely studied immobilization method for preparing immobilized microorganisms . 1.2 Adsorption adsorption method is the earliest research, widely used and mature technology in the treatment of wastewater by immobilized microbial technology. In the early days of most biofilm reactors, the principles of adsorption were applied. The immobilized microbial methods can be divided into physical adsorption and ion adsorption . The method is simple in operation, and the microbial immobilization process has little effect on cell activity and mild conditions. However, this method combines a limited number of cells, and has poor reaction stability and repeatability. The number of microorganisms immobilized is limited by the type of carrier used and its surface area , and the adsorption strength between the microorganism and the carrier is not strong enough, so the carrier The choice is the key.
1.3 Covalent binding method The covalent binding method is to use the chemical covalent bond between the surface functional group of the microbial cell and the surface group of the solid phase carrier to fix the cell, so the binding is tight and the stability is good, but the reaction is intense when the group is combined. The operation is complicated and difficult to control. 1.4 Cross-linking method Cross-linking method is to achieve the purpose of immobilizing microorganisms by reacting microorganisms with reagents having two or more functional groups to interconnect microbial cells into a network structure . By using this method, the binding strength between the microbial cells is high, the stability is good, and the temperature and the pH value can withstand drastic changes. However, in the process of forming a covalent bond, the activity of the microbial cells is often greatly affected, and the cross-linking agent suitable for such immobilization is mostly expensive, and thus it is limited in application. 1.5 Carrier-free immobilization The carrier-free immobilization method, also known as self-immobilization, is a completely new concept. In the process of self-flocculation particle formation, the suitable ecological environment of microorganisms is formed at the same time, which is beneficial to the coordination between microbial metabolism or to facilitate the transfer of biological information between microorganisms . Compared with other immobilization methods, this method has the advantages of small mass transfer diffusion resistance, high activity of cell particles, and simple immobilization method, and will be widely used in the field of sewage treatment . This method generally does not require the use of artificial carriers or embedding agents, but requires a long immobilization time and is greatly affected by environmental factors. In the upflow anaerobic sludge blanket (UASB) reactor and the anaerobic baffled reactor (ABR), the formation of granular sludge belongs to the self-immobilization process of microorganisms .
2. Different immobilization methods for immobilized carriers have different requirements for immobilized carriers. Ideal immobilized carriers should have the characteristics of non-toxic to microbial cells, good mass transfer performance, stable properties, long service life and low price. The development of new immobilized carriers is also an important part of the research on immobilization technology. At present, commonly used carriers can be roughly classified into three types: inorganic carriers such as porous glass, diatomaceous earth , activated carbon, quartz sand, etc.; organic carriers such as agar, polyvinyl alcohol gel (PVA), and polyacrylamide (ACAM). a gel or the like; and a composite carrier comprising an organic carrier and an inorganic carrier. The properties of the material can be improved by combining their respective advantages. Composite carriers have obvious advantages in reducing costs and improving wastewater treatment effects.
3. Application of immobilized microbial technology in sewage treatment 3.1 Treatment of high-concentration ammonia-nitrogen wastewater With the improvement of people's living standards, the concentration of ammonia nitrogen in domestic sewage is getting higher and higher, and the traditional treatment process is difficult to make ammonia nitrogen reach the standard and use immobilization. Microbial technology can greatly increase the removal rate of ammonia nitrogen. Microbial removal of ammonia nitrogen requires two stages: aerobic nitrification and anaerobic denitrification. Cao Guomin et al  used PVA as a carrier to adopt a single-stage biological nitrogen removal technology, which uses an immobilized cell membrane on both sides and is in contact with aerobic ammonia nitrogen wastewater and anoxic ethanol carbon source to fix the membrane. The nitrifying bacteria in the process oxidize ammonia to nitrous oxide and nitrate nitrogen, and are subsequently reduced to nitrogen by denitrifying bacteria in the same membrane. The rate of ammoxidation in this method is twice that of nitrifying bacteria alone. Ye Zhengfang et al  used the functionalized macroporous carrier FPUFS to immobilize the high-efficiency microbial flora B350 by carrier binding method, and the obtained immobilized B350 was placed in an aeration tank to form an immobilized microorganism with an effective volume of 1500Lâ€”aerated organism Filter (I-BAF) sewage treatment system. The sewage with ammonia nitrogen of 451 mg/L was treated to 0.285 mg/L. 3.2 Treatment of wastewater containing refractory organic matter When treating wastewater containing refractory organic matter by conventional biological treatment methods, the treatment efficiency is low, mainly because the microorganisms that degrade such substances have a long generation period and are difficult to be used in conventional biological treatment structures. There are a lot of them. The immobilized microbial technology can be used to selectively select the dominant flora culture, fix it to the carrier, increase the concentration of microorganisms, and efficiently treat such substances . Jo-Shu Chang  immobilized Pseudomonas with polyacrylamide and calcium alginate as carriers, and decolorized the wastewater containing nitrogen dyes. The results showed that compared with the free cell system, long-term operation can be maintained. High reaction efficiency and good reusability, more suitable for practical industrial applications.
4. Conclusion The fixed microbial technology has attracted widespread attention in the field of sewage treatment with its unique advantages. It has been extensively researched and applied, but further research and solution is needed to realize its practical or industrialization. The development of a highly efficient biochemical reactor suitable for immobilizing microbial cells is also a problem to be solved; the contaminant contained in the sewage is a very complicated mixed system, and a single strain cannot achieve a satisfactory effect, so the mixed microorganism should be further strengthened. Research and development of immobilization systems, screening and constructing high-performance microorganisms with high efficiency, low cost and strong resistance, and establishing a micro-ecological environment composed of mixed micro-organisms in the reactor, so that various immobilized microorganisms can play a synergistic role and broaden Treatment of pollutant types and improvement of treatment efficiency; it can be seen that with the development of microbial technology and biotechnology, immobilized microbial technology will become the mainstream technology of sewage treatment.
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