1. About activated sludge method
Currently popular wastewater treatment processes are: AB method, SBR method, oxidation ditch method, ordinary aeration method, A / A / O method, A/O method, etc., which are derived from the activated sludge method, and each has its own characteristics.
① AB method (Adsorption-Biooxidation)?
This method was first developed by Prof. Bohuke of Germany. The process of aeration pool according to high and low loads in two levels of oxygen supply, A-level load is high, aeration time is short, producing a large amount of sludge, sludge load of 2.5kgBOD/(kgMLSS?d) or more, the pool volume load of 6kgBOD/(m3?d) or more; B-level load is low, the age of the sludge is longer. intermediate sedimentation tanks are set up between the A level and B level. Level 2 pools F/M (pollutant quantity and microbial quantity ratio) is different, the formation of different microbial groups. AB method, despite the advantages of energy saving, but is not suitable for low concentration of water quality, level A and B can also be constructed in stages.
② SBR method (Sequencing Batch Reactor)?
SBR method has been developed as early as the beginning of the 20th century, due to cumbersome artificial management has not been promoted. This method is a set of water intake, aeration, sedimentation, water in a pool to complete, often by four or three pools constitute a group, rotating operation, a pool of pools to run intermittently, it is called sequencing batch activated sludge method. Nowadays, we have developed some improved SBR process with continuous water intake and continuous water discharge, such as ICEAS method, CASS method, IDEA method and so on. This integrated process is characterized by the simplicity of the process, because there is only one reaction tank, no need for secondary sedimentation tanks, reflux sludge and equipment, generally do not set up a regulating tank, in most cases can be omitted primary sedimentation tank, so save space and investment, shock load resistance and flexible mode of operation, can be arranged from time to time aeration, anoxic and anaerobic different states, to achieve the purpose of phosphorus removal and nitrogen removal. However, because each pool needs to set up aeration and water transfer and distribution systems, the use of decanters and control systems, intermittent drainage head loss, the utilization rate of the pool capacity is not ideal, so, in general, is not very suitable for large-scale municipal wastewater treatment plants .
③ A/A / O method (Anaerobic-Anoxic-Oxic)
Since the effluent of municipal wastewater treatment has the requirement of removing nitrogen and phosphorus, the domestic 10 years ago to develop this anaerobic-anoxic- aerobic composition process. Aerobic composition of the process. The use of biological treatment to remove nitrogen and phosphorus, you can get high quality effluent, is a deep secondary treatment process. a / a / o method can be synchronized phosphorus and nitrogen removal mechanism consists of two parts: one is to remove phosphorus, phosphorus in the sewage in the anaerobic state (DO & lt; 0.3mg / L), the release of phosphorus-polymerizing bacteria, and in the aerobic condition will be more absorbed, in the form of residual sludge discharged from the system. Second, denitrification, anoxic section to control DO<0.7 mg/L, due to the role of parthenogenetic denitrification bacteria, the use of BOD in water as a hydrogen supplier (organic carbon source), nitrate and nitrite from the mixture of aerobic pools to nitrogen gas escaping into the atmosphere, to achieve the purpose of denitrification. In order to effectively remove nitrogen and phosphorus, for general municipal wastewater, COD/TKN 3.5 ~ 7.0 (complete denitrification COD/TKN & gt; 12.5), BOD/TKN 1.5 ~ 3.5, COD/TP 30 ~ 60, BOD/TP 16 ~ 40 (generally should be > 20). If reduce sludge concentration, compressed sludge age, control nitrification, to remove phosphorus, BOD5 and COD is the main, can be used A/O process.
Some municipal wastewater treatment effluent is not discharged into the lake, the use of large bodies of water for deep water discharge or irrigated farmland, nitrogen and phosphorus removal can be placed in the next step of the renovation and expansion of the time to consider, in order to save the near-term investment.
④ Ordinary aeration and its variations?
This process appeared the earliest, and still has a strong vitality. General aeration method of treatment is good, more experience, can adapt to the large amount of sewage, for large plants can be centralized to build a sludge digester, the resulting methane gas can be used as energy. The shortcomings of the traditional aeration method is only as a conventional secondary treatment, does not have the function of nitrogen and phosphorus removal.
In recent years, in engineering practice, by reducing the volume load of ordinary aeration pool, can achieve the purpose of denitrogenation; set up an anaerobic zone in front of the aeration pool, can remove phosphorus, can also be used to remove phosphorus by chemical method. The use of ordinary aeration method to remove BOD5, there are a variety of forms in the pool type (as described below oxidation ditch), engineering known as the ordinary aeration method of variations, can also be collectively referred to as the ordinary aeration method.
⑤ Oxidation ditch method?
This process was developed in the early 1950s, because of its simple structure, easy to manage, and soon be popularized, and continuous innovation, development prospects and competitiveness, currently can be called a popular process. Oxidation ditch in the application of the development of a variety of forms, more representative of:?
Pa type (Passveer) referred to as single ditch type, surface aeration using brush aeration, water depth is generally 2.5 ~ 3.5m, brush power efficiency of 1.6 ~ 1.8kgO2/(kW?h).
Ao type (Orbal) referred to as concentric circle type, the application of more than oval three-ring road composition, three ring road with different DO (such as the outer ring of 0, the middle ring of 1, the inner ring of 2), is conducive to denitrification and removal of phosphorus. The use of rotating disk aeration, water depth is generally 4.0 ~ 4.5m, power efficiency and turn brush close to, now in Weifang, Shandong, Beijing Huangcun and Hefei WangXiaoYing city sewage treatment plant applications.
If the oxidation ditch inlet can be designed in a variety of ways, can effectively resist the impact of heavy rainfall flow, for some of the combined system drainage system of municipal wastewater treatment is particularly applicable.?
Card type (Carrousel) referred to as circulation folding type, using inverted umbrella impeller aeration, from the process operation, the depth of water is generally about 3.0m, but the sludge is easy to deposit, the reason is that the oxygen supply and the flow rate of the contradiction.?
Three ditch oxidation ditch (T-type oxidation ditch), this type consists of three pools, the middle for the aeration pool, the left and right two pools are also used as a settling pool and aeration pool. T-type oxidation ditch structure is simple, the treatment effect is good, but its use of rotary brush aeration, water depth is shallow, covers an area of large, complex control instrumentation to increase the difficulty of operation and management. There is no anaerobic tank, does not have the function of phosphorus removal.
Oxidation ditch generally does not have a primary sedimentation tank, low load, shock resistance, less sludge. Construction costs and power consumption depending on the type of ditch and change, such as in the form of aeration in the turn of the dish and turn the brush, and then the introduction of microporous aeration, increase the depth of water, can effectively improve the utilization rate of oxygen (increased by 20%) and power efficiency [up to 2.5 ~ 3.0 kgO2 / (kW?h)].
2. Aerated biofilter
Aerated biofilter is essentially often referred to as biological contact oxidation tank, equivalent to the aeration of the tank to add for microorganisms attached to the filler (filter), the filler under the drum, is a biofilm with the characteristics of activated sludge method. Aerated biofilter (BAF) originated in mainland Europe in the late 70s, has been developed for the French, British and other countries equipment manufacturing company's technology and equipment products. Due to the selection of different fillers, as well as whether there is a requirement for nitrogen removal, the design of the process parameters are different, such as the requirements for the treatment of effluent BOD5, SS <20mg / L, the removal of BOD5 up to more than 90% of the process, its volumetric loading of 0.7 ~ 3.0 kgBOD5 / (m3?d), hydraulic retention time of 1 ~ 2h; nitrification (more than 90%) is the main process, the volumetric loading of 0.5～2.0kgBOD5/(m3?d), hydraulic retention time 2～3h.?
It is generally believed that the biofilm method of treatment of municipal wastewater in the country still need to accumulate experience, the scale of treatment should not be too large, about 5 × 104m3 / d or so appropriate. Foreign countries (mainly in Europe) to deal with water volume of 36 × 104m3 / d, which is related to its filler materials, automatic control means and advanced backwash device, but also with its long-term accumulation of operational management experience.
3. UNITANK process
UNITANK process and similar TCBS process, MSBR process, are the same as the SBR method is a new variant and development. It combines the advantages of "sequential batch method", "ordinary aeration tank method" and "three ditch oxidation ditch method", overcoming the "sequential batch method
Typical UNITANK process is three pools, three pools of hydraulic connectivity between the pools, each pool is equipped with an aeration system, the outer two pools are equipped with an outlet weir and sludge discharge, they alternate as aeration and sedimentation tanks. Sewage can enter any one of the three pools, using continuous water intake, cycle alternating operation. Under automatic control, the tanks are kept in aerobic, anoxic and anaerobic states to complete the removal of organic matter and nitrogen and phosphorus.
UNITANK process by the Belgian Seghers first built in China's Macao SAR, the volume of water treatment 14 × 104m3 / d (not rain when the average volume of water treatment is 7 × 104m3 / d), the pool is closed, designed to use the volumetric loading of 0.58kgBOD / (m3 ?d), the total volume of the reactor is 46800m3, aeration pool The hydraulic retention time is 8h, and the BOD5 and SS of the effluent are <20mg/L.
This kind of integrated process is a deformation of the traditional activated sludge process, and it can remove different pollutants by adopting the design method of the activated sludge process, such as the consideration of nitrification, and its loading is generally in the range of 0.05-0.10 kgBOD5/(kgMLSS?d), and the rate of nitrification varies according to the temperature of the effluent. The rate of nitrification varies depending on the temperature of the sewage. The sludge stabilization is required, and its sludge load and sludge age should be far more than the value of nitrification.
Low volume utilization is the main problem with this type of integrated process***, which means that about 50% of the tank capacity is used for sedimentation in an aeration system with a long residence time.
The success of the UNITANK process depends on the system using stable and reliable instrumentation and equipment, so the introduction of technology, digestion, absorption and development of advanced self-control system is the application of this process is the key issue. It is generally believed that the UNITANK process is not very suitable for large-scale (> 10 × 104m3 / d) of the municipal wastewater treatment plant.
4. Biological treatment method of new progress
Biological treatment method is currently more research, new technology emerging methods, both aerobic biological treatment technology, or anaerobic biological treatment technology has aroused great interest in researchers. Because the biological method uses the metabolism of microorganisms to feed on pollutants, metabolize them into stable small molecules such as CO2, H2O, NH3, SO2, etc., and its secondary pollution is small, the treatment of sewage and organic wastewater with similar properties has its unique advantages. Biological treatment method since its inception, its technology has gained great development, with the increasing standard of living of people, living in the sewage into an increasingly complex, so the purpose of biological treatment methods from the previous degradation of proteins, fats, carbohydrates, and other substances to also deal with synthetic detergents, denitrification, dephosphorization, and some other difficult to degrade the complexity of the organic matter. In the past, due to the unsatisfactory efficiency of anaerobic biological treatment and the slow treatment time, it has not attracted much attention and was only used for the pretreatment of sludge or highly concentrated organic wastewater, but nowadays, due to the energy constraints, the anaerobic biological treatment has attracted more and more attention due to the production of methane as an energy source, and a number of new processes have emerged. Many new processes have emerged as a result.
(1) New development of activated sludge method
So far, there is no big breakthrough in the operation mode of activated sludge method, and what is often done is some local improvement, but there are big achievements in the aeration mode, such as pure oxygen aeration, deep well aeration, jet aeration, and the use of microbubbles diffusers, etc., which have increased the oxygen transfer rate, improved the utilization of oxygen, and made the aeration tank oxygen concentration increase. Oxygen concentration in the aeration pool increased. Such as the United States and Japan developed an ultra-micro bubble diffuser, bubble diameter 50Lm, oxygen absorption rate of 90%, Reid Engineering Company of Frederick shurg and other development of oxidation ditch under the surface aeration is also an improvement in the aeration method, the brush aeration (Brush Aeration) to improve the turbine aeration (Turbine Aeration) to avoid the need for aeration. Turbine Aeration (Turbine Aeration) avoids the problems of aerosols, splashing, icing, etc. Another development trend of activated sludge method is the improvement of the aeration method. Another development trend of activated sludge method is to develop in the direction of multi-functionality, and the methods adopted include: cultivating and domesticating special bacteria, so that the object of activated sludge treatment is not only limited to domestic sewage, but also can deal with toxic organic substances such as phenol which are difficult to be degraded, and even domesticate the inorganic substances such as cyanide, which are highly poisonous; combining the activated sludge with other treatments, for example, activated carbon, activated sludge, activated sludge, activated sludge, activated sludge, activated sludge and other treatments. Activated sludge and other treatment methods, such as activated carbon - activated sludge method, which is actually a form of activated sludge method of activated carbon adsorption, biological oxidation method of integrated treatment method; fixed activated sludge method is to provide microorganisms attached to surfaces, such as synthetic fibers, plastics, sand, clay coke, etc., so that the aeration tank at the same time there are attached to the phase and suspended phase of the organisms; these are to improve the purification efficiency of the activated sludge to improve the ability of shock loads, such as toxic substances. These all improve the purification efficiency of activated sludge, improve the resistance to toxic substances and other impact load capacity, but also has the effect of decolorization, denitrogenation, cut foam, foreign countries have been used in synthetic fibers, chemical printing and dyeing, oil refining, coking and other industrial production of wastewater treatment; activated sludge method and anaerobic process to combine to denitrogenation, dephosphorization, etc., the most typical process is A-O (anaerobic-oxic) process. Activated sludge can also be combined with chemical methods to improve the purification of PCBs, organic phosphorus removal effect.
(2) New progress in biofilm treatment
The first biofilm process is the emergence of the British in 1893, the sewage will be sprinkled on the coarse filter media to be purified by the ordinary biofilter, which is the earliest to appear and still in the improvement and development of artificial biological treatment equipment. On its basis, the emergence of high-load biofilter, tower biofilter, biological carousel and biological contact oxidation. In the past two or three decades, there are some new biofilm treatment technology, such as biofluidized bed, it is sand, coke, activated carbon and other granular materials as a carrier, the surface of the carrier attached to the growth of biofilm, oxygenated sewage to a certain flow rate of flow from bottom to top so that the carrier is in a state of fluidization, the carrier of biofilm on the wastewater can be fully in contact with the sewage, so that the purification efficiency is increased, it is the process of the air flow bed, Pure oxygen fluidized bed, three-phase fluidized bed and anaerobic and fluidized bed process. Activated biofilter is a new type of sewage treatment process that combines biofilter, aeration tank and two-settlement tank into one, which is characterized by part of the effluent of the biofilter back to the two-settlement tank with the reflux sludge into the biofilter, and the experimental results of the activated biofilter for treating domestic wastewater and food processing wastewater show that: the system has the advantages of good treatment effect, high efficiency, high BOD volumetric loading, non-expansion and shock load resistance, etc.. The system has the advantages of good treatment effect, high efficiency, large BOD volume load, no sludge expansion and shock load resistance. There are also air-driven bio-turntables, bio-turntables combined with aeration tanks, algae turntables and so on. As the ecological environment of biofilm method is different from that of activated sludge method, the ecosystem of biofilm method can be used for nitrogen removal and so on, such as growth of algae, post-growth animals, and even growth of nitrifying bacteria and denitrifying bacteria, and so on.
(3) The new development of anaerobic biological treatment
Anaerobic biological treatment method also has a history of more than 100 years, it is the use of anaerobic microorganisms in the anaerobic conditions of organic decomposition technology. Due to the low processing efficiency, slow speed, and methane bacteria on the environment requirements of strict and difficult to control and other shortcomings, anaerobic biological treatment method for a long time is generally only used for sludge treatment, its main process is septic tanks, digesters and so on. However, due to the energy crisis and increased environmental pollution in recent years, anaerobic biological treatment has been emphasized due to its products with energy substances, and a large number of new anaerobic biological treatment technologies have been born one after another. In order to improve the concentration of anaerobic microorganisms, there are anaerobic microorganisms adhering to the surface of the carrier of anaerobic biofilm treatment methods, such as anaerobic biofiltration, anaerobic rotary discs, anaerobic expansion beds, anaerobic contact oxidation, anaerobic There are anaerobic biofilm treatment methods such as anaerobic biofilters, anaerobic rotors, anaerobic expansion beds, anaerobic contact oxidation, anaerobic stall reactors, anaerobic fluidized beds, and methods such as the upflow anaerobic sludge bed reactor (UASB) that rely on cohesive granulation of microorganisms and their own immobilization. There are also artificially fixed microorganisms embedded immobilization method, which is artificially slow proliferation rate of anaerobic microorganisms in high concentrations in the treatment system, improve the treatment speed, reduce the size of the treatment equipment and can be used for the treatment of organic wastewater in low concentrations. For example, Japan Honda et al. in 1988 using embedded fixed anaerobic microorganisms to deal with TOC for 150mg?L artificial water, TOC removal rate of up to 95% or more. In anaerobic treatment, methane proliferation rate is slow to become the decision step of gas production, so in order to maintain a high concentration of microorganisms in the methane fermentation, the emergence of solid-liquid separation using membranes, such as Kashiwa points et al. 1988, the use of ultrafiltration membranes (UF) to carry out methane fermentation test, the results show that: improve the concentration of methane in the reactor, TOC volumetric loading of 2g?L? day, and the removal rate of up to 98.4%. The removal rate can be more than 98.4%. The current development trend of anaerobic biological treatment is to be used in conjunction with other biological treatment methods, such as anaerobic - aerobic composite process, etc., which has a series of advantages, such as saving investment, saving energy, low sludge production, good effluent quality and so on. Anaerobic biological treatment method is towards the treatment of low concentration of organic wastewater, can be dephosphorization and denitrogenation and convenient operation and maintenance of economic development.
5. The development trend of activated sludge process
Through decades of research and practice, activated sludge process has become a relatively perfect process. It has been difficult to have a large development in the pool shape, operation mode, aeration mode, carrier and so on. It has been difficult to make a breakthrough in biology by conventional means. Some scholars believe that the process of the two future directions are membrane separation technology and the application of molecular biology technology.
(1) The application of membrane separation technology
The use of membrane separation instead of sedimentation for mud-water separation can bring about the following changes in the activated sludge process:
① There is no longer a problem of sludge expansion. When regulating the activated sludge system, it is no longer necessary to consider the problem of sludge settlement performance, thus greatly simplifying the process control;
② The sludge concentration in the aeration tank will be greatly increased (MLSS can be greater than 20,000mg/L) so that the system can be operated in the super-large sludge age, ultra-low load state, to fully meet the need to remove a variety of pollutants;
③ In the same processing In the same treatment requirements, the aeration tank volume can be greatly reduced, saving the footprint of the treatment plant;
④ The increase in sludge concentration will require a higher aeration rate, and thus pure oxygen aeration will be used in large quantities with the membrane separation.
Although membrane separation is still easy to clog and other aspects of the problem, but these problems are gradually being solved. In fact, there are a number of membrane separation activated sludge system in operation, such as the city of Hiroshiwa, Japan's Higashi wastewater treatment plant membrane separation system has been running continuously for three years.
(2) Application of molecular biotechnology
At present, molecular biotechnology has begun to be applied to the field of sewage treatment. In order to clarify the biochemical mechanism of phosphorus removal by polyphosphorus bacteria, has begun to use molecular diagnostic techniques to obtain the genetic information of polyphosphorus bacteria. Now from the activated sludge has been found in more than 30 kinds of filamentous bacteria, only 4 kinds of accurate naming and taxonomic positioning, because most of these filamentous bacteria can not be isolated pure culture. Molecular diagnostic techniques are being used to biologically localize these filamentous bacteria for further accurate characterization.
The large number of applications of molecular diagnostic techniques, the establishment of activated sludge microbial gene pool, on the basis of which gene technology to cultivate sludge strains with high efficiency and activity, to further improve the treatment effect, is the direction of future development.