Environmental protection is the basic national policy of China. The practice of world economic development proves that to achieve sustained and stable economic development, the contradiction between development and environmental protection must be resolved. With the rapid development of China's social economy, urban environmental pollution, especially water pollution, has become increasingly serious. The discharge of urban domestic sewage is increasing year by year. In 2002, the total discharge of industrial and urban domestic sewage in China was 43.95 billion tons, an increase of 65.438 0.5% over the previous year. Among them, the industrial wastewater discharge was 20.72 billion tons, an increase of 2.3% over the previous year; The discharge of urban domestic sewage was 23.23 billion tons, up by 0.9% over the previous year, of which only 10% was treated. [1] Domestic sewage contains high nutrients such as nitrogen and phosphorus, and it is directly discharged into rivers, lakes and seas without treatment, which is the main cause of eutrophication pollution in water areas. The monitoring data in 2002 showed that the water systems of Liaohe and Haihe rivers were seriously polluted, and the water bodies below Grade V accounted for more than 60%. The water quality in the main stream of Huaihe River is mainly Class III-V water, while the water quality in tributaries and provincial boundaries is still poor. The overall water quality of the Yellow River system is poor. The water quality of the main stream is mainly Class III-IV water bodies, and the pollution of tributaries is widespread and serious. The Songhua River system is dominated by III-IV water bodies; The water quality of the Pearl River system is generally good, mainly class II water; The water quality of the main stream and main first-class tributaries of the Yangtze River is good, mainly class II water bodies. The shortage of water resources caused by "pollution" has become a prominent problem that seriously restricts the sustainable development of China's social economy and needs to be solved urgently. At present, the focus of water pollution control in China has gradually changed from industrial point source to urban sewage pollution. According to the forecast [2], by 20 10, the total urban sewage discharge in China will be10.5 billion m3, and the urban sewage treatment rate will reach 50%. It is estimated that more than 0/000 sewage treatment plants will be built, and the main factor that determines the investment and operation cost of urban sewage treatment plants is the choice of sewage treatment technology, so it is necessary to develop sewage treatment plants with high efficiency and low consumption, which are suitable for China's national conditions.

Second, the research and application of domestic sewage treatment technology is equally worrying.

For a long time, the secondary biological treatment of municipal domestic sewage mostly adopts activated sludge process, which is the most widely used secondary biological treatment process in the world at present, and has the advantages of high treatment capacity and good effluent quality. However, there are many problems such as high capital construction cost, high operation cost, high energy consumption, complex management, easy sludge bulking and sludge floating, and the inability to remove inorganic nutrients such as nitrogen and phosphorus. For China, a developing country with insufficient resources and a large population, it is not suitable for China's national conditions from the perspective of sustainable development. Because sewage treatment is a project that pays attention to environmental and social benefits, it is often limited by funds in the process of construction and actual operation, which makes the treatment technology and funds become the bottleneck of water pollution control in China. To sum up, at present, in the field of research and application of urban domestic sewage treatment, the common problems are as follows:

(1) The traditional activated sludge process often has high capital and operating costs, high energy consumption, complicated management and easy sludge bulking; Process equipment can not meet the requirements of high efficiency and low consumption.

(2) With the increasingly strict sewage discharge standards, the discharge requirements of nitrogen, phosphorus and other nutrients in sewage are higher. The traditional sewage treatment process with nitrogen and phosphorus removal function is mainly activated sludge process, which often needs to connect multiple anaerobic and aerobic reaction tanks in series to form multi-stage reaction tanks. The purpose of nitrogen and phosphorus removal is achieved by increasing internal circulation, which will inevitably increase the cost and energy consumption of capital investment and make the operation management more complicated.

(3) At present, the treatment of urban sewage is mainly centralized treatment, and the huge investment in sewage collection system far exceeds the sewage treatment plant itself. Therefore, from the point of view of sewage recycling, it is not necessarily the only desirable scheme to build a large sewage treatment plant to treat domestic sewage centrally.

Therefore, how to make the urban sewage treatment process develop towards the sustainable direction of low energy consumption, high efficiency, less excess sludge, the most convenient operation and management, and the realization of phosphorus recovery and treated water reuse. This has become a common concern in the research and application of water treatment technology, which requires that sewage treatment should not only meet the single water quality improvement, but also consider the resource utilization and energy utilization of sewage and pollutants. The adopted technology must be based on low energy consumption and less resource loss.

Third, the application research and development of biofilm process in domestic sewage treatment

In the development and application of wastewater biological treatment, activated sludge process and biofilm process have always occupied a dominant position. With the development of new fillers and the continuous improvement of supporting technologies, biofilm process developed in parallel with activated sludge process has developed rapidly in recent years. Biofilm method has the advantages of high treatment efficiency, good impact load resistance, low sludge output, small floor space and convenient operation and management, and it is very competitive in treatment.

1. Mechanism of sewage purification by biofilm process

There are many kinds of organic pollutants in sewage with complex components. But for domestic sewage, its organic components mainly include: protein (40%-60%), carbohydrates (25%-50%) and oil (65,438+00%), in addition to a certain amount of urea [3]. Biofilm method relies on microbial membrane fixed on the surface of carrier to degrade organic matter. Because microbial cells can firmly attach, grow and reproduce on almost any suitable carrier surface in water environment, extracellular polymers extending from the inside of cells make microbial cells form fibrous entangled structures, so biofilm usually has porous structure and strong adsorption performance.

Biofilm adheres to the surface of carrier, which is a highly hydrophilic substance. In the case of continuous flow of sewage, there is always an attached water layer outside it. Biofilm is also a highly microbial-intensive substance. A large number of microorganisms and micro-animals grow and breed on the surface of the membrane and inside this depth, forming a food chain composed of organic pollutants → bacteria → protozoa (metazoa). Biofilm is composed of bacteria, fungi, algae, protozoa, metazoa and other visible biological communities. Among them, bacteria are generally Pseudomonas, Blastomyces, Alcaligenes, Kiwifruit and Gymnosperms, and protozoa are mostly bell worms, orphans, isopods, fibroworms and so on. Metazoa only appear when the dissolved oxygen is very sufficient. They are mainly nematodes. When the sewage flows over the surface of the carrier, the organic pollutants in the sewage are adsorbed by the microorganisms in the biofilm and diffused into the biofilm through oxygen, and biological oxidation occurs in the membrane, thus completing the degradation of organic matter. Aerobic and facultative microorganisms grow on the surface of biofilm, while the microorganisms in the inner layer of biofilm are often in anaerobic state. When the biofilm gradually thickens and the thickness of anaerobic layer exceeds that of aerobic layer, the biofilm will fall off, and a new biofilm will regenerate on the surface of the carrier, and the normal operation of the biofilm reactor will be maintained through the periodic renewal of biofilm.

Biofilm method realizes the separation of microbial residence time and hydraulic residence time by fixing microbial cells on the carrier in the reactor. The existence of carrier filler plays a role in forced turbulence of water flow, and at the same time can promote the full contact between pollutants in water and microbial cells, which essentially strengthens the mass transfer process. Biofilm method overcomes the problems of sludge bulking and sludge floating in activated sludge method. In many cases, it can not only replace the activated sludge process for secondary biological treatment of municipal sewage, but also has the advantages of stable operation, strong impact load, more economy and energy saving, certain nitrification and denitrification function, closed operation to prevent odor and so on.

Biofilm reactor is formed by artificially strengthening the introduction of biofilm into sewage treatment reactor. In recent years, biofilm reactor has developed rapidly, from single to complex, aerobic to anaerobic, and gradually formed a relatively complete biological treatment system.

Packing is one of the cores of biofilm technology, and its performance is directly related to the efficiency, energy consumption, stability and reliability of wastewater treatment process.

2. Research progress in the application of anaerobic biofilm process in domestic sewage treatment.

(1), anaerobic degradation stage of composite materials.

In the process of anaerobic treatment of wastewater, the organic matter in wastewater is finally transformed into methane, carbon dioxide, water, hydrogen sulfide and ammonia through the interaction of a large number of microorganisms. In this process, the metabolic processes of different microorganisms interact and restrict each other, forming a complex ecosystem. The description of anaerobic process of complex materials is helpful for us to understand the basic content of this process. The so-called complex substances are those macromolecular organic substances, which exist in the form of suspended solids or colloids in wastewater.

The anaerobic degradation process of complex materials can be divided into four stages.

Hydrolysis stage: high molecular weight organic matter can't penetrate cell membrane, so it can't be directly used by bacteria. So they are broken down into small molecules by bacterial extracellular enzymes in the first stage. For example, cellulose is hydrolyzed into cellobiose and glucose by cellulase, starch is decomposed into maltose and glucose by amylase, and protein is hydrolyzed into short peptides and amino acids by protease. Hydrolysates of these small molecules can be dissolved in water and used by bacteria through cell membranes.

Fermentation (or acidification) stage: at this stage, the above-mentioned small molecular compounds are transformed into simpler compounds in the cells of fermentation bacteria (acidification bacteria) and secreted outside the cells. The main products in this stage are volatile fatty acids (VFA), alcohols, lactic acid, carbon dioxide, hydrogen, ammonia and hydrogen sulfide. At the same time, acidification bacteria also use some substances to synthesize new cellular substances, so more excess sludge will be produced when anaerobic treatment of non-acidification wastewater.

Acetic acid production stage: At this stage, the products of the previous stage are further converted into acetic acid, hydrogen, carbonic acid and new cellular substances.

Methane production stage: At this stage, acetic acid, hydrogen, carbonic acid, formic acid and methanol are converted into methane, carbon dioxide and new cellular substances.

In the above stages, there are the following processes: a, the hydrolysis stage includes protein hydrolysis, carbohydrate hydrolysis and lipid hydrolysis; B, the fermentation acidification stage includes anaerobic oxidation of amino acids and sugars and anaerobic oxidation of higher fatty acids and alcohols; C, in the acetic acid production stage, the intermediate products form acetic acid and hydrogen, and hydrogen and carbon monoxide form acetic acid; The methanation stage includes methane formation from acetic acid and methane formation from hydrogen and carbon dioxide. In addition to these processes, when the wastewater contains sulfate, there will be sulfate reduction process. The anaerobic degradation of complex compounds can be described graphically (see figure 1).

(2) Research progress in the application of anaerobic biofilm process.

A. anaerobic filter

Anaerobic filter is the first high-speed anaerobic reactor developed and established by McCarty and others in the United States on the basis of Coulter's research in the late 1960s. The general volume load of traditional aerobic biological system is 2KgCOD/(m3? D) below. Before AF was invented, the general volumetric load of anaerobic reactor was 4-5kgCOD/(m3? D) below. But the load of AF can be as high as 10- 15 kgCOD/(m3? D) test. [4] Therefore, the development of AF greatly improves the treatment rate of anaerobic reactor and greatly reduces the reactor volume.

The establishment of AF as a high-speed anaerobic reactor also lies in its biological immobilization technology, which greatly prolongs the residence time (SRT) of sludge in the reactor. McCarty found that increasing SRT can greatly shorten the hydraulic retention time (HRT) of wastewater, thus reducing the reactor volume, or increasing the treated water volume at the same reactor volume. This idea of expanding SRT by biological immobilization and treating SRT and HRT separately promoted the development of a new generation of high-speed anaerobic reactors.

The expansion of SRT is essentially to maintain the high concentration of sludge in the reactor. In AF, the concentration of anaerobic sludge can reach10-20g VSS/L. The retention of anaerobic sludge in AF can be accomplished in two ways: one is that bacteria form biofilm on the surface of the filler fixed in AF (including the inner wall of the reactor); The second is that bacteria form aggregates between fillers. The accumulation of high concentration anaerobic sludge in the reactor is the biological basis of the rapid reaction performance of AF. Under a certain sludge specific methanogenic activity, the load of anaerobic reactor is directly proportional to the sludge concentration. At the same time, compared with the anaerobic contact process, the anaerobic sludge formed in AF has higher density and better settling performance, so it is not difficult to separate the remaining sludge in its effluent. Because high concentration of sludge can be retained in AF, there is no need for sludge backflow.

In AF, because the filler is fixed, wastewater enters the reactor, which is gradually hydrolyzed and acidified by bacteria and converted into acetic acid and methane. The composition of wastewater changes gradually at different reactor heights. Therefore, the distribution of microbial population also presents regularity. At the bottom (water inlet), fermentation bacteria and acid-producing bacteria account for the largest proportion. With the increase of reactor height, acetic acid-producing bacteria and methanogenic bacteria gradually increase and dominate. The types of bacteria are related to the composition of wastewater. In acidified wastewater, the concentration of fermentation and acid-producing bacteria will not be too large.

Another characteristic of bacteria distribution in the reactor is that the bacteria at the entrance of the reactor (such as the inside of upflow AF) get the most nutrition, so the sludge concentration is the highest, and the sludge concentration decreases rapidly with the height.

This distribution of sludge gives AF some technical characteristics. First of all, the removal of organic matter in wastewater by AF is mainly carried out at the bottom of AF (referring to upflow AF). According to the report of Young and Dahab [4], the COD removal rate of AF reactor hardly increased above 1m, but most of the COD was removed within 0.3m Therefore, researchers think that shallow AF reactor can have better treatment efficiency than deep reactor under a certain volume load. Secondly, due to the extremely high sludge concentration at the bottom of the reactor, it is easy to cause reactor blockage. Blockage is one of the most important problems affecting the application of AF. It is reported that the sludge concentration at the bottom of upflow AF can be as high as 60g/L. The regular distribution of anaerobic sludge in AF also makes the reactor more adaptable to toxic substances, and the concentration of biodegradable toxic substances in the reactor also changes regularly. In addition, anaerobic biofilm forms a good system of various flora, so it is easy to cultivate anaerobic sludge adapted to toxic substances in AF. For example, in the treatment of chloroform and formaldehyde wastewater, it is found that the sludge in AF reactor has good adaptability, and the removal effect of these toxic substances and the allowable influent concentration are gradually improved. AF also has a strong impact load resistance. It is generally believed that under the same temperature conditions, the load of AF can be 2~3 times higher than that of anaerobic contact process, and at the same time, it will have a higher COD removal rate.

In addition to the blockage and channeling caused by local blockage, another problem of AF application is that it needs a lot of fillers, and the use of fillers increases its cost. Due to the above problems, AF system is not widely used in foreign production scale. According to Le-ttinga's estimation in 1993, there are only about 30~40 AF systems in foreign production scale. [4]

Anaerobic membrane bed is an innovative technology of upflow anaerobic filter. SAFB's special anaerobic film bed replaces the traditional small particle packing with larger particles and porosity, which effectively solves the blockage problem of the reactor. Anaerobic membrane bed has the following characteristics:

Effectively overcome the shortcomings of anaerobic filter, such as easy blockage and poor effluent quality;

High concentration of biosolids can obtain higher organic load;

In anaerobic membrane bed, microorganisms form biofilm by adhering to the surface of filler and form bacterial aggregates by suspending in the pores of filler, so high biomass can be maintained in anaerobic membrane bed. Therefore, the hydraulic retention time can be shortened, and the impact load resistance is strong;

The startup time is short, and it is easier to restart after stopping operation;

There is no need to return sludge, which is convenient for operation and management;

The impact resistance is good under the condition that the water quantity and load change greatly.

B. Anaerobic fluidized bed reactor (AFBR)

In fluidized bed system, anaerobic sludge is trapped by biofilm formed on the surface of inert filler particles, and the mixing of liquid and sludge and the transfer of substances are realized by fluidization of these particles and biofilm.

The main features of the fluidized bed reactor can be summarized as follows:

Fluidization can make anaerobic sludge contact with treated wastewater to the maximum extent;

Due to the high relative movement speed between particles and fluid, low liquid film diffusion resistance, thin biofilm, strong mass transfer effect and faster biochemical process, the hydraulic retention time of wastewater in the reactor is short;

Overcome that problems of bloc and channeling of anaerobic filters;

High reactor volume load can reduce the reactor volume, and at the same time, because the height-diameter ratio is larger than other anaerobic reactors, it can reduce the occupied area.

However, there are still several problems in the anaerobic fluidized bed reactor. First of all, in order to achieve good fluidization and prevent sludge and filler from losing from the reactor, it is necessary to keep the shape, size and density of biofilm particles uniform, but this is almost impossible, so it is difficult to ensure stable fluidization. [5] Secondly, some relatively new studies think that the fluidized bed reactor needs a separate pre-acidification reactor. At the same time, in order to obtain a higher upflow velocity to ensure fluidization, the fluidized bed reactor needs a lot of reflux water, which leads to an increase in energy consumption and cost. For the above reasons, the fluidized bed reactor has no production-scale equipment. Some people think that its future application prospect is not great. [5]

C. anaerobic attached film expanded bed reactor

Anaerobic attached film expanded bed is a sewage treatment process developed by Jewel et al. in 1974. Compared with the biological fluidized bed, the difference lies in the expansion degree of the carrier. Based on the height of the packed bed, the expansion rate of the expanded bed is about 10%~20%, and the particles still keep in contact, while the expansion rate of the fluidized bed is 20%~70%. Bruce J.Alderman and others [6] compared the economy of anaerobic expanded bed, trickling filter tower and activated sludge process, and found that the design of trickling filter tower after anaerobic expanded bed was the most economical choice for small sewage treatment plants, with less energy consumption and low sludge output. But at present, this process is still mainly in the stage of small-scale and pilot-scale research.

To sum up, as the core method of domestic sewage treatment, anaerobic treatment technology with anaerobic biofilm reactor as the main body is technically mature and has some unique advantages compared with other methods. However, anaerobic method has little effect in concentrating nutrients (nitrogen and phosphorus) and can only remove some pathogenic microorganisms. In addition, residual BOD, suspended solids or reducing substances will also affect the quality of effluent. Therefore, if anaerobic biofilm reactor is to become a complete environmental treatment technology, suitable post-treatment means are essential.

3. Aerobic biofilm treatment technology-biological contact oxidation

Biological contact oxidation is developed from biological filter and contact aeration oxidation tank. As early as the 1930s, production facilities appeared in the United States. At that time, the biological contact oxidation tank was filled with sand and gravel, bamboo and wood products and metal products, which were mainly used to treat sewage with low concentration and low organic load. It overcomes the shortcoming that the activated sludge process can not maintain normal operation due to sludge loss when treating this kind of sewage, and has achieved good results. In 1970s, with the appearance of honeycomb straight packing and three-dimensional corrugated plastic packing with large aperture and high specific surface area, the application scope of biological contact oxidation method was broadened. It can not only treat domestic sewage, but also treat high-concentration organic wastewater and toxic and harmful industrial wastewater. Compared with other biological treatment methods, it has shown its superiority. In 1970s, China began to study biological contact oxidation, and the first productive experimental device was used to treat municipal sewage. In terms of treatment effect and power consumption, compared with activated sludge process, biological contact oxidation process has the following main advantages: ① biological contact oxidation takes filler as carrier for biota to inhabit and grow, forming a stable ecosystem with high microbial concentration, generally reaching10 ~ 20g/L; The oxygen utilization rate is high, reaching 10%. Strong impact load resistance, strong adaptability to environmental changes, and less excess sludge. ② Biological contact oxidation can make full use of the strong oxidation ability of filamentous bacteria without sludge bulking. Moreover, it is not necessary to adjust the sludge volume and dissolved oxygen concentration by sludge reflux as in the activated sludge process, which is easy to manage and operate. After more than ten years of practice, the structural form of oxidation tank, the types and installation methods of fillers, and the types and layout forms of gas supply devices have been continuously innovated and optimized. At present, biological contact oxidation technology has been widely used to treat domestic sewage, domestic miscellaneous water and industrial wastewater with different organic concentrations.

Filler is the place where microorganisms inhabit and the carrier of biofilm. Biofilm grows on the surface of filler, and the metabolic process of biofilm benefits and purifies sewage. The performance of filler directly affects the effect and economic rationality of biological contact oxidation technology, so the selection of filler is the key of biological contact oxidation technology.

The characteristics of the filler depend on the material and structure of the filler. The filling material should have the characteristics of stable molecular structure, aging resistance, corrosion resistance and good biological stability. The structure of the filler should have the functions of large specific surface area, high porosity, high hardness, water and gas distribution and bubble cutting. The gap between fillers can change under the action of external force, which is conducive to the timely discharge of the fallen biofilm from the filler area. Moreover, the volume of fillers should be compressible and not deformed after recovery, which is convenient for transportation and installation.

Development of Immobilized Carrier

(1) fixed packaging

Fixed fillers are represented by honeycomb and corrugated fillers, and are mostly made of FRP and various plastic sheets. Recently, the ceramic honeycomb packing produced by direct sintering of pottery clay is hexagonal with a pore size of 20 ~ 100 mm, which has a small specific surface area, a small amount of biofilm, a smooth surface, easy shedding of biofilm, lateral non-circulation of the packing, uneven gas distribution, easy blockage and even failure to operate normally, and high cost. In recent years, this kind of filler has been gradually eliminated.

(2) Suspended filler

Suspended fillers include soft fillers, semi-soft fillers, combined packing and soft fillers, and the theoretical specific surface area and porosity are > large; 90%, fast film formation, low cost, convenient assembly, stable effluent and good treatment effect, and the removal rate of COD and BOD5 is over 80%. However, when the concentration of wastewater is high or the suspended matter in water is large, the filler wire will agglomerate, greatly reducing the actual specific surface area, and it is easy to break the wire and the central rope, which will affect the service life, and its service life is generally 1~2 years. Semi-soft filler, strong foam cutting performance, water and gas distribution ability again, good film hanging and peeling effect and no blockage; The removal rate of COD and BOD is 70-80%. The service life is longer than that of soft filler. However, its theoretical specific surface area is small (87-93m2/m3), and the total amount of biofilm is insufficient, which affects the sewage treatment effect and the cost is high.

Combined packing is a new type of filler designed for the above shortcomings of soft and semi-soft fillers. It absorbs the large specific surface area of soft fillers, is easy to film, and the semi-soft fillers are not caked, so it has good foam cutting performance. A semi-soft part is designed in the center of the filler to support the soft fiber bundles at the periphery, and its plane is like a shield, so it is also called shielding filler. Its specific surface area is 1000~2500 m2/m3, and its porosity is 98%-99%. It has the advantages of fast film formation, large biomass and no caking. The sewage treatment capacity is better than that of soft and semi-soft fillers. Under normal hydraulic load, COD removal rate is 70%-85%, and BOD5 removal rate is 80%-90%. Similarly, there are lanterns (or dragons) and YDT elastic solid fillers.

(3) Disperse fillers

Dispersed fillers include stacked fillers and suspended fillers, and there are many kinds. The utility model is characterized in that it does not need to be fixed or hung, but only needs to be placed in the processing device, which is convenient to use and simple to replace. Porous spherical suspended filler from Beijing Xiaoqing Environmental Protection Company and SNP suspended filler from Beijing Sander Company have the advantages of good oxygenation performance, fast film formation and long service life. The spherical lightweight ceramic packing newly developed by Jiangxi Pingxiang Canon Environmental Protection Engineering Company has a particle size of 2~4 mm and a huge specific surface area, which keeps the reactor at a high biomass per unit volume. Moreover, the biofilm on the filler is relatively thin and the activity is relatively high, which fully meets the international performance standards of biological aerated filter filler. It has been used in the Malanhe Sewage Treatment Plant in Dalian, China, which was built by French, and it is a major breakthrough in the development of new fillers in China.

4. Application of hydrolytic acidification-aerobic activated sludge process in domestic sewage treatment.

After anaerobic treatment, it takes a long time to reach the secondary effluent standard under the existing technical conditions. In this way, although anaerobic treatment has advantages in operation and management costs, it has lost its competitiveness in infrastructure investment. Therefore, from the microbial and chemical point of view, anaerobic treatment only provides a pretreatment, which generally needs post-treatment to meet the new sewage discharge standards. While India and South American countries actively promote the application of anaerobic domestic sewage treatment technology, they generally realize that it is necessary to further treat anaerobic effluent because nitrogen and phosphorus are basically not removed after anaerobic treatment. Lack of suitable post-treatment technology is one of the main reasons for the slow application of anaerobic biological treatment technology in the field of domestic sewage treatment. Although the existing small-scale experimental results show that the combination of two-stage anaerobic system can achieve good treatment effect. But at present, in actual production, anaerobic and aerobic combined systems are still the most widely used. In India, oxidation pond is the most commonly used post-treatment method. The removal rates of BOD5, CODcr and TSS in effluent treated by anaerobic pond and oxidation pond are 87%, 86% and 90% respectively. The domestic sewage treatment project for 7,000 people in Nova Vista, Brazil and Manga160,000 people in Bokar, Colombia all adopt facultative oxidation pond for post-treatment. In the anaerobic domestic sewage treatment project in Mexico, the post-treatment methods are diversified, such as secondary sedimentation tank+chlorine disinfection, submerged filter+secondary sedimentation tank+chlorine disinfection, oxidation ditch and so on. , and finally directly discharged into the urban sewage pipe network or used for agricultural irrigation. In Japan, municipal domestic sewage is generally treated by anaerobic digestion+aerobic activated sludge, anaerobic filter+aerobic filter and anaerobic filter+contact oxidation. The newly developed advanced JOHKASO small domestic sewage purification system with nitrogen and phosphorus removal function is widely used in decentralized treatment of domestic sewage. [7] The combination of anaerobic and aerobic biological treatment technology can effectively remove most organic and inorganic pollutants. Professor G Lettinga, an expert in anaerobic biology, asserts that anaerobic treatment biotechnology can become the core means of decentralized domestic sewage treatment mode if it is matched with suitable post-treatment methods. This model is more sustainable and vital than the traditional centralized treatment method, especially suitable for developing countries. [8]

Anaerobic-aerobic combined treatment technology has given full play to the energy-saving advantages of anaerobic technology and the high-efficiency advantages of aerobic technology, and has become the main trend of sewage treatment technology development at present. In foreign countries, the anaerobic-aerobic combined treatment process, which consists of upflow anaerobic sludge bed reactor (UASB) and aerobic biofilm reactor, has always been the focus of research. [9, 10, 1 1] The nitrification/denitrification performance and kinetic mechanism of the combined process were deeply studied. [12, 13] In recent years, Ricardo Francie Goncalves and others [14, 15] have carried out small-scale and pilot-scale research, and the research shows that the combined process of UASB and submerged biological aerated filter (BF) is used to treat domestic sewage. When the two-stage hydraulic retention time is 6h and 0. 17h respectively, the removal rates of CODcr, BOD5 and SS are all above 90%, and the effect of the combined system to stabilize the effluent quality is better than that of the single UASB sewage treatment system. When sludge from BF section flows back to UASB section, methanation capacity of organic matter in anaerobic reactor is improved, gas production is increased, excess sludge is reduced, and sludge concentration tank and digestion tank can be reduced or even omitted.

Because the anaerobic-aerobic combined treatment process with UASB as the main body is greatly affected by temperature, especially at low temperature, the performance of the system can not be fully exerted. Igor Bodik et al. [16] studied the denitrification performance of the combined process of anaerobic baffled biofilter and submerged aerated biofilter in treating domestic sewage at low temperature. After one year's operation, when the hydraulic retention time in anaerobic stage and aerobic stage is 15 h and 4h respectively, even if the ambient temperature is lower than 10℃ (average temperature is 5.9℃), the removal rate of CODcr, BOD5 and SS is about 80%. Low temperature affects nitrification activity to some extent. In the temperature range of 4.5 ~ 23℃, the removal rate of TKN is 46.4% ~ 87.3%, and the system also has a certain denitrification function, which provides a reference for the denitrification treatment of domestic sewage in low temperature environment.

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