1. About activated sludge process

At present, the popular sewage treatment processes include AB method, SBR method, oxidation ditch method, ordinary aeration method, A/A/O method, A/O method and so on. These processes all come from activated sludge process, and each has its own characteristics. ?

① AB method (adsorption-biological oxidation)?

This method was first invented by Professor Bohuk in Germany. In this process, oxygen is supplied to the aeration tank in two stages according to high load and low load. Class A load is high, aeration time is short, and sludge production is large. Sludge load is 2.5kgBOD/(kgMLSS? D) above, the tank volume load is 6kgBOD/(m3? D) above; Low grade B load and long sludge age. There is an intermediate sedimentation tank between Grade A and Grade B. The F/M (ratio of pollutants to microbial biomass) of the secondary sedimentation tank is different, forming different microbial communities. Although AB method has the advantage of energy saving, it is not suitable for low-concentration water quality, and Grade A and B can also be built by stages. ?

② sequencing batch reactor?

SBR method was developed as early as the early 20th century, but it has not been popularized because of the tedious manual management. This method integrates water inlet, aeration, sedimentation and effluent in one tank, and usually consists of four or three tanks, which run intermittently from one tank to another in turn, so it is called sequencing batch activated sludge process. At present, some improved SBR processes, such as ICEAS method, CASS method and IDEA method, have been developed. This integrated process is characterized by simple process. Because there is only one reaction tank, there is no need for secondary sedimentation tank, return sludge and equipment, and there is generally no regulating tank, the primary sedimentation tank can be omitted in most cases, so it saves land occupation and investment, resists impact load, has flexible operation mode, and can arrange different states of aeration, anoxia and anaerobic in time to achieve the purpose of phosphorus and nitrogen removal. However, because each pond needs to be equipped with aeration, water distribution system, decanter and control system, the head loss of intermittent drainage is large and the capacity utilization rate of the pond is not ideal. So it is generally not suitable for large-scale urban sewage treatment plants. ?

③ A/A/O method (anaerobic-anoxic-aerobic)

Due to the need of nitrogen and phosphorus removal from municipal sewage treatment effluent, this anaerobic-anoxic-aerobic process was developed in China 10 years ago. Biological treatment of nitrogen and phosphorus is a deep secondary treatment process, which can obtain high quality effluent. A/A/O synchronous phosphorus and nitrogen removal machine consists of two parts: one is phosphorus removal, and the phosphorus in sewage is in anaerobic state (do 20). If the sludge concentration is reduced, the sludge age is compressed, nitrification is controlled, and phosphorus, BOD5 and COD are mainly removed, then A/O process can be adopted.

In some cities, the effluent from sewage treatment is not discharged into lakes. If large water bodies are used for deep water discharge or farmland irrigation, nitrogen and phosphorus removal can be considered in the next reconstruction and expansion to save short-term investment. ?

④ Ordinary aeration method and its improvement?

This process first appeared and still has strong vitality. The general aeration method has good treatment effect and more experience, and can adapt to the large amount of sewage. For large factories, sludge digesters can be built centrally, and the generated biogas can be used as energy. The disadvantage of traditional general aeration method is that it can only be used as a conventional secondary treatment, and it has no function of nitrogen and phosphorus removal. ?

In recent years, in engineering practice, nitrogen removal can be realized by reducing the volume load of ordinary aeration tanks; Phosphorus can be removed by setting an anaerobic zone in front of a general aeration tank or by chemical methods. There are many ways to remove BOD5 by ordinary aeration (oxidation ditch is as follows), which is called the variation of ordinary aeration in engineering, and can also be collectively called ordinary aeration.

⑤ oxidation ditch method?

Developed in the early 1950s, this technology has been rapidly popularized because of its simple structure, easy management, continuous innovation, development prospect and competitiveness, which can be described as a popular technology at present. Oxidation ditch has developed into various forms in application, and the representative ones are:

Passveer is the abbreviation of single ditch type, and the surface aeration adopts rotating brush aeration, the water depth is generally 2.5~3.5m, and the dynamic efficiency of rotating brush is 1.6 ~ 1.8kgo2/(kW? h).？

Orbal, referred to as concentric circle type for short, is mostly composed of elliptical third ring road in application, and the third ring road adopts different DO (for example, the outer ring is 0, the middle ring is 1, and the inner ring is 2), which is beneficial to nitrogen and phosphorus removal. Rotating disc aeration is adopted, the water depth is generally 4.0 ~ 4.5 m, and the dynamic efficiency is close to that of rotating brush. At present, it has been applied in the municipal sewage treatment plants in Weifang, Beijing Huangcun and Hefei Wang Xiaoying.

If the intake of oxidation ditch can be designed in various ways, it can effectively resist the impact of storm water, especially suitable for urban sewage treatment of some combined drainage systems. ?

Carrousel, the abbreviation of circulating baffle flow, adopts inverted umbrella impeller for aeration. From the point of view of process operation, the water depth is generally about 3.0m, but due to the contradiction between oxygen supply and flow rate, sludge is easy to deposit. ?

Three-ditch oxidation ditch (T-shaped oxidation ditch) is composed of three tanks, the middle of which is an aeration tank, and the left and right tanks are also used as sedimentation tanks and aeration tanks. T-shaped oxidation ditch has simple structure and good treatment effect, but its rotating brush aeration has shallow water depth, large area, complex control instruments and difficult operation and management. There is no anaerobic tank, so there is no phosphorus removal function. ?

Generally, there is no primary sedimentation tank in oxidation ditch, which has low load, impact resistance and less sludge. The construction cost and power consumption vary depending on the type of trench used. For example, in the form of rotary table and rotary brush aeration, introducing microporous aeration to increase the water depth can effectively improve the oxygen utilization rate (by 20%) and power efficiency (up to 2.5 ~ 3.0 kGo2/(kW? h)の.

2. About biological aerated filter

Biological aerated filter (BAF) is essentially a biological contact oxidation tank, which is equivalent to adding a filler (filter) for microorganisms to live in the aeration tank and blowing air under the filler. This is a biofilm process with the characteristics of activated sludge. Biological aerated filter (BAF) originated in the European continent in the late 1970s, and has now developed into the technology and equipment products of equipment manufacturing companies in France, Britain and other countries. Due to the different fillers selected and the need for denitrification, the designed process parameters are also different. If the effluent BOD5 and SS are required to be less than 20 mg/L, the BOD5 removal rate is greater than 90%, and the volume load is 0.7 ~ 3.0 kg BOD5/(m3? D) The hydraulic retention time is1~ 2h; The process volume load of nitrification (above 90%) is 0.5 ~ 2.0 kg BOD5/(m3? D) Hydraulic retention time is 2-3 hours. ?

Generally speaking, we still need to accumulate experience in the treatment of urban sewage by biofilm process in China, and the treatment scale should not be too large, about 5× 104m3/d is appropriate. The amount of treated water abroad (mainly in Europe) reaches 36× 104m3/d, which is related to its packing, automatic control means and advanced backwashing device, as well as its long-term accumulated operation and management experience.

3. About UNITANK process

UNITANK process, like similar TCBS process and MSBR process, is a new variant and development of SBR process. It combines the advantages of sequencing batch method, ordinary aeration tank method and three-ditch oxidation ditch method, and overcomes the shortcomings of sequencing batch method, three-ditch oxidation ditch method and ordinary aeration tank method, which occupy a large area and have many equipment. ?

The typical UNITANK process consists of three tanks, which are hydraulically connected, each tank is equipped with an aeration system, and the two tanks on the outside are equipped with water weirs and mud outlets, which are alternately used as aeration tanks and sedimentation tanks. Sewage can enter any one of the three ponds, and it can be operated by continuous water inflow and alternating circulation. Under automatic control, each pond is in aerobic, anoxic and anaerobic state, thus completing the removal of organic matter and nitrogen and phosphorus. ?

UNITANK process was first built by Belgian Seghers Company in China Macao Special Administrative Region, and its water treatment capacity is 14× 104 m3/d (the average water treatment capacity is 7×104m3/d when it doesn't rain). The tank type is closed, and the design volume load is 0.58kgBOD/(m3? D) The total volume of the reaction tank is 46800m3, the hydraulic retention time of the aeration tank is 8h, and the effluent BOD5 and SS are less than 20mg/L. ..

This integrated process is a variant of the traditional activated sludge process, and different pollutants can be removed through the design method of activated sludge process. If nitrification is considered, its load is generally 0.05 ~ 0. 10 kg BOD 5/(kg MLSS? D) Nitrification rate varies with sewage temperature. However, sludge stabilization is needed, and its sludge load and sludge age far exceed the values in the nitrification process. ?

The low volume utilization rate is the main problem of this integrated process, that is, in the aeration system with long residence time, about 50% of the tank capacity is used for precipitation. ?

The success of UNITANK process depends on the use of stable and reliable instruments and equipment in the system. Therefore, introducing technology, digesting, absorbing and developing advanced automatic control system are key issues. Generally speaking, UNITANK process is not suitable for large-scale (>: 10× 104m3/d) urban sewage treatment plants.

4. New progress in biological treatment methods

Biological treatment is a method that has been studied a lot and new technologies are emerging one after another. Both aerobic biological treatment technology and anaerobic biological treatment technology have aroused great interest of researchers. Because the biological method uses the metabolism of microorganisms to metabolize pollutants into stable small molecules such as CO2, H2O, NH3, SO2, etc., the secondary pollution is small, and it has its unique advantages in treating domestic sewage and organic sewage with similar properties. Since the advent of biological treatment, its technology has been greatly developed. With the improvement of people's living standards, the generation of domestic sewage is becoming more and more complicated. Therefore, the purpose of biological treatment has been improved from being able to deal with the degradation of protein, fat, carbohydrates and other substances to being able to deal with complex refractory organic substances such as synthetic detergents, nitrogen and phosphorus removal. This inevitably requires people to reform the process. In the past, anaerobic biological treatment was not paid attention to because of its unsatisfactory efficiency and slow treatment time. It was only used for the pretreatment of sludge or high concentration organic sewage. But now, due to energy shortage, anaerobic biological treatment is favored by more and more people because it can produce energy substance-methane, and many new processes have appeared.

New development of (1) activated sludge process

Up to now, there is no big breakthrough in the operation mode of activated sludge process, and some local improvements are often made, but great achievements have been made in the aeration mode, such as pure oxygen aeration, deep well aeration, jet aeration and the use of microbubble diffuser. The oxygen transfer rate, oxygen utilization rate and oxygen concentration in the aeration tank are increased. For example, the United States, Japan and other countries developed an ultra-miniature bubble diffuser with a bubble diameter of 50Lm and an oxygen absorption rate of 90%. The aeration under the oxidation ditch developed by Reid Engineering Company of Frederick Shurg is also an improvement on the aeration mode. Brush aeration is changed into turbine aeration, which avoids problems such as aerosol, splash and icing. Another development trend of activated sludge process is to develop in the direction of multi-function. The methods adopted include: cultivating and domesticating special bacteria, so that the object of activated sludge treatment is not limited to domestic sewage, but also toxic organic substances such as phenol and even toxic inorganic substances such as cyanide; Combining activated sludge with other treatment methods, such as activated carbon-activated sludge method, is actually a comprehensive treatment method of activated carbon adsorption and biological oxidation in the form of activated sludge method; The fixed activated sludge method is to provide the surface to which microorganisms attach, such as synthetic fiber, plastic, fine sand, clay coke and so on. So that organisms with attached phase and suspended phase exist in the aeration tank at the same time; All these have improved the purification efficiency of activated sludge, improved the ability to resist the impact load of toxic substances, and also have the functions of decoloration, denitrification and defoaming, which have been used in sewage treatment of synthetic fiber, chemical printing and dyeing, oil refining, coking and other industrial production abroad; Activated sludge process combined with anaerobic process to remove nitrogen and phosphorus, the most typical process is A-O (anaerobic-aerobic) process. Activated sludge method can also be combined with chemical method to improve the removal effect of PCBs and organophosphorus.

(2) New progress in biofilm treatment.

The earliest process of biofilm process is 1893, which appeared in the common biological filter in Britain, and was purified by spraying sewage on the coarse filter material. It is the earliest artificial biological treatment equipment, and it is still being improved and developed. On this basis, high-load biofilter, tower biofilter, biological turntable and biological contact oxidation appeared. In the past two or three decades, some new biofilm treatment technologies have emerged, such as biological fluidized bed, in which granular materials such as sand, coke and activated carbon are used as carriers and biofilm grows on the surface of the carriers. The oxygenated sewage flows from bottom to top at a certain flow rate, which makes the biofilm on the carrier fully contact with the sewage and improves the purification efficiency. Its process includes air fluidized bed, pure oxygen fluidized bed, three-phase fluidized bed and anaerobic fluidized bed process. Active biofilter is a new sewage treatment process which combines biofilter, aeration tank and secondary sedimentation tank. It is characterized in that part of the effluent reflux of the biofilter enters the biofilter together with the sludge from the secondary sedimentation tank. The experimental results of treating domestic sewage and food processing wastewater by active biological filter show that the system has the advantages of good treatment effect, high efficiency, large BOD volume load, no sludge bulking and impact load resistance. In addition, there are pneumatic biological turntable, the combination of biological turntable and aeration tank, algae turntable and so on. Because the ecological environment of biofilm process is different from activated sludge process, algae, metazoa and even nitrifying bacteria and denitrifying bacteria can grow in the ecosystem of biofilm process, so it can be used for nitrogen removal.

(3) New development of anaerobic biological treatment.

Anaerobic biological treatment has a history of 100 years. It is a technology that uses anaerobic microorganisms to decompose organic matter under anaerobic conditions. Due to the shortcomings of low efficiency, slow speed and strict requirements on the environment, anaerobic biological treatment has been generally only used for sludge treatment for a long time, and its main processes are septic tanks and digesters. However, due to the intensification of energy crisis and environmental pollution in recent years, anaerobic biological treatment has attracted people's attention because its products contain energy substances, and a large number of new anaerobic biological treatment technologies have been born one after another. In order to increase the concentration of anaerobic microorganisms, there are anaerobic biofilm treatment methods, such as anaerobic biofilter, anaerobic rotary table, anaerobic expanded bed, anaerobic contact oxidation, anaerobic baffled reactor, anaerobic fluidized bed method and upflow anaerobic sludge bed reactor (UASB reactor). There is also an artificial immobilized microorganism embedding immobilization method, which artificially keeps anaerobic microorganisms with slow proliferation rate at a high concentration in the treatment system, improves the treatment speed and reduces the treatment equipment, and can be used to treat low-concentration organic sewage. For example, Honda et al. in Japan used embedded immobilized anaerobic microorganisms to treat TOC in 1988, 150mg? The removal rate of l and TOC can reach above 95%. In anaerobic treatment, the slow growth rate of methane becomes the decisive step of gas production. Therefore, in order to maintain the high concentration of microorganisms in biogas fermentation, the method of solid-liquid separation by membrane appeared, such as the experiment of biogas fermentation by ultrafiltration membrane in 1988 by Bai Fen et al. The results show that the methane concentration in the reactor increases and the TOC volume load is 2g? l？ Day, its removal rate can reach more than 98.4%. At present, the development trend of anaerobic biological treatment is to combine with other biological treatment methods, such as anaerobic-aerobic composite process, which has a series of advantages such as saving investment, saving energy, less sludge output and good effluent quality. Anaerobic biological treatment is developing towards the treatment of low-concentration organic wastewater, phosphorus and nitrogen removal, convenient and economical operation and maintenance.

5. Development trend of activated sludge process

After decades of research and practice, activated sludge process has become a relatively perfect process. It is difficult to make great progress in pool type, operation mode, aeration mode and carrier. It is also difficult to make a breakthrough in biology by conventional means. Some scholars believe that the two major directions of this process in the future are the application of membrane separation technology and molecular biology technology.

Application of (1) membrane separation technology

Using membrane separation instead of sedimentation to separate sludge water can bring the following changes to the activated sludge process:

① The problem of sludge bulking no longer exists. When adjusting the activated sludge system, it is not necessary to consider the settling performance of sludge, which greatly simplifies the process control;

(2) The sludge concentration in the aeration tank will be greatly increased (MLSS can reach more than 20,000 mg/L), so that the system can run under the conditions of super sludge age and ultra-low load, and fully meet the needs of removing various pollutants;

(3) Under the same treatment requirements, the volume of the aeration tank can be greatly reduced, saving the occupied area of the treatment plant;

④ The increase of sludge concentration requires higher aeration rate, so pure oxygen aeration will be widely used in membrane separation.

Although there are still some problems in membrane separation, such as easy blockage, these problems are being solved step by step. In fact, several membrane separation activated sludge systems have been put into operation at present, such as the membrane separation system of Hiroshi Chuandong Sewage Treatment Plant in Japan, which has been running continuously for 3 years.

(2) Application of molecular biotechnology

At present, molecular biotechnology has been applied to the field of sewage treatment. In order to understand the biochemical mechanism of phosphorus removal by phosphorus accumulating bacteria, people used molecular diagnosis technology to obtain the genetic information of phosphorus accumulating bacteria. Among more than 30 kinds of filamentous fungi found in activated sludge, only 4 kinds were named and located accurately, because most of these filamentous fungi could not be isolated and cultured. At present, molecular diagnosis technology is being used to locate these filamentous bacteria in order to further understand their characteristics accurately.

It is the future development direction to widely apply molecular diagnosis technology, establish activated sludge microbial gene bank, and cultivate high activated sludge strains through gene technology on this basis to further improve the treatment effect.