1. Basic principles
A/O stands for anoxic/aerobic. Its advantage is that it not only degrades organic pollutants, but also has a certain function of nitrogen and phosphorus removal. It uses anaerobic hydrolysis technology as pretreatment of activated sludge, so A/O method is an improved activated sludge method.
In the A/O process, the first anoxic stage and the second aerobic stage are connected in series, the DO in the A stage is not more than 0.2mg/L, and the DO in the O stage is 2 ~ 4 mg/L. In the anoxic stage, heterotrophic bacteria hydrolyze suspended pollutants such as starch, fiber, carbohydrate and soluble organic matter in sewage into organic acids, so that macromolecular organic matter is decomposed into micromolecule organic matter and insoluble organic matter is converted into soluble organic matter. These products after anoxic hydrolysis enter an aerobic tank for aerobic treatment, which can improve the biodegradability and oxygen efficiency of sewage. In the anoxic stage, heterotrophic bacteria ammoniate pollutants such as protein and fat (N in organic chain or amino in amino acid) and release ammonia (NH3 and NH4+). Under the condition of sufficient oxygen supply, the nitrification of autotrophic bacteria oxidizes NH3-N(NH4+) into NO3-, and returns to the A pool through reflux control. Under anoxic conditions, denitrification of heterotrophic bacteria reduces NO3- to molecular nitrogen (.
2. Characteristics of A/O internal circulation biological nitrogen removal process
According to the above description of the basic process of biological denitrification, combined with years of experience in denitrification of coking wastewater, we come to the conclusion that (A/O) biological denitrification process has the following advantages:
(1) High efficiency. This process has a high removal effect on organic matter and ammonia nitrogen in wastewater. When the total residence time is more than 54 hours, the effluent after biological denitrification can be coagulated and precipitated, the COD value can be reduced to below 100mg/L, other indicators can meet the discharge standards, and the total nitrogen removal rate is above 70%.
(2)
Simple process, less investment and low operation cost. In this process, the organic matter in wastewater is used as the carbon source for denitrification without adding expensive carbon sources such as methanol. Especially, after the ammonia distillation tower is equipped with a device to remove fixed ammonia, the ratio of carbon to nitrogen is increased, and the alkalinity produced in the denitrification process correspondingly reduces the alkali consumption required in the nitrification process.
(3)
The anoxic denitrification process has high degradation efficiency for pollutants. For example, the removal rates of COD, BOD5 and SCN- in anoxic zone are 67%, 38% and 59% respectively, and the removal rates of phenol and organic matter are 62% and 36% respectively, so denitrification is the most economical and energy-saving degradation process.
(4)
High capacity load. Because the enhanced biochemistry is adopted in the nitrification stage and the high-concentration sludge membrane technology is adopted in the denitrification stage, the sludge concentration of nitrification and denitrification is effectively improved, and compared with similar processes abroad, it has a higher volume load.
(5)
The anoxic/aerobic process has a strong ability to resist load shock. When the influent water quality fluctuates greatly or the pollutant concentration is high, the process can maintain normal operation, so the operation management is also very simple. Through the comparison of the above processes, it is not difficult to see that the biological nitrogen removal process itself is nitrogen removal, and at the same time, it also degrades organic substances such as phenol, cyanide and COD. Combined with the characteristics of water quantity and water quality, we recommend anoxic/aerobic (A/O) biological nitrogen removal.
(internal circulation) process, so that the sewage treatment device can not only meet the requirements of nitrogen removal, but also meet the emission standards of other indicators.
3. Shortcomings of A/O process
1. Because there is no independent sludge reflux system, it is impossible to cultivate sludge with unique functions, and the degradation rate of refractory substances is low;
2. In order to improve the nitrogen removal efficiency, the internal circulation ratio must be increased, thus increasing the operation cost. In addition, the internal circulating liquid comes from the aeration tank and contains a certain amount of DO, which makes it difficult to maintain the ideal anoxic state in section A and affects the denitrification effect, and the denitrification rate is difficult to reach 90%.
3. Influencing factors
Hydraulic retention time (nitrification > 6h, denitrification < 2h) sludge concentration mlss (> 3000mg/l) sludge age (> 30d )N/MLSS loading rate (
< 0.03) influent total nitrogen concentration (< 30mg/l)
Second, A2/O process
1. Basic principles
A2/O process is the abbreviation of anaerobic-anoxic-aerobic, and it is the abbreviation of anaerobic-anoxic-aerobic biological nitrogen and phosphorus removal process. The treatment efficiency of this process can generally reach: BOD5 and ss are 90%~95%, total nitrogen is over 70%, and phosphorus is about 90%. It is generally suitable for large and medium-sized urban sewage plants that need nitrogen and phosphorus removal. However, the capital construction cost and operation cost of A2/O process are higher than those of ordinary activated sludge process, which requires high operation management. Therefore, according to the present situation of our country, A2/O process is only adopted when the treated sewage is discharged into closed water body or slow water body, resulting in eutrophication and thus affecting the water supply source.
2. Characteristics of A2/O process:
(1) High pollutant removal efficiency, stable operation and good impact load.
(2) The sludge sedimentation performance is good.
(3) Organic matter, nitrogen and phosphorus can be removed simultaneously under three different environmental conditions: anaerobic, anoxic and aerobic, and with the organic cooperation of different kinds of microbial flora.
(4) The effect of nitrogen removal is affected by the reflux ratio of mixed liquor, while the effect of phosphorus removal is affected by DO and nitrate oxygen entrained in the returned sludge, so the efficiency of nitrogen and phosphorus removal cannot be very high.
(5) In the process of simultaneous deoxidation, dephosphorization and removal of organic matter, this process is the simplest, and the total hydraulic retention time is less than other similar processes.
(6) Under anaerobic-anoxic-aerobic alternate operation, filamentous bacteria will not multiply in large numbers, and SVI is generally less than 100, so sludge bulking will not occur.
(7) The phosphorus content in sludge is high, generally above 2.5%.
3. The shortcomings of A2/O process
The volume of the reaction tank is larger than that of the A/O denitrification process;
Large internal reflux of sludge and high energy consumption;
Small and medium-sized sewage treatment plants have high cost;
The economic benefit of biogas recycling is poor;
Sludge leachate needs chemical dephosphorization.
Third, oxidation ditch.
1 oxidation ditch technology
Oxidation ditch, also known as continuous loop reactor, is a variant of activated sludge process. Oxidation ditch sewage treatment worker
Art was successfully developed by Dutch Institute of Health Engineering in 1950s. Since 1954 was first put into use in the Netherlands. Because of its good effluent quality and stable operation,
Convenient management and other technical characteristics have been widely used in the treatment of domestic sewage and industrial sewage at home and abroad [1]. Up to now, oxidation ditch technology has gone through more than half a century.
Development, continuous innovation in structural form, aeration mode and operation mode. , leading to a wide variety of oxidation ditches, each with its own characteristics [2].
From the operation mode, the development of oxidation ditch technology mainly has two aspects: on the one hand, sewage treatment is mainly arranged in chronological order; On the other hand, according to spatial order.
Discharge is the main way of sewage treatment. There are alternating and semi-alternating oxidation ditches belonging to the former; Belonging to the latter, there are continuous divided oxidation ditch and combined oxidation ditch [3], as shown in figure 1.
Classification of oxidation ditch process.
At present, the widely used types of oxidation ditch are Pasveer oxidation ditch, Carrousel oxidation ditch and Orbal oxidation ditch.
T-type oxidation ditch (three-ditch oxidation ditch), DE-type oxidation ditch and integrated oxidation ditch.
2. Application of oxidation ditch process in sewage treatment
Theoretically speaking, oxidation ditch has the characteristics of plug flow reaction and the advantage of complete mixing reaction; The former makes it have excellent effluent conditions, while the latter makes it impact-resistant.
Load capacity. It is precisely because of this cycle and energy division that it has many advantages that other wastewater biological treatment technologies have, the most obvious of which is.
The advantage of this book is its stability and reliability. Oxidation ditch technology is different from the traditional activated sludge process because of its good effluent quality, stable operation and convenient management.
Technology has been widely used in sewage treatment. According to incomplete statistics [4], there are currently more than 2,000 oxidation ditch sewage treatment plants in Europe and more than 800 in North America. oxygen
The treatment capacity of Huagou has increased from the initial service population of only 360 people to the population equivalent of 5 million to100000. Not only the number of oxidation ditches is increasing, but also the treatment scale is increasing.
With the continuous expansion of the scale, the treatment targets have also developed to treat urban sewage and industrial wastewater at the same time, such as petroleum wastewater, chemical wastewater, papermaking wastewater, printing and dyeing wastewater and food processing wastewater.
. This technology has been applied in China since 1980s. With the great development of sewage treatment, oxidation ditches of different sizes and types have been built all over the country.
Sewage treatment plant. At present, there are nearly 100 sewage treatment plants that use oxidation ditch to treat urban sewage and industrial wastewater in China, as shown in table 1 (typical types and applications of oxidation ditch in China and
Table 2 (Type and scale of some domestic oxidation ditch sewage treatment plants).
New development of oxidation ditch technology
By analyzing the time-space relationship of various continuous flow biological phosphorus and nitrogen removal processes, and combining with the new theory of phosphorus and nitrogen removal, we continue to implement the idea of simple sewage treatment of Chongqing University Wang.
Tao [5], Zhong [6], Liu [7], Mai Songbing [8] and others improved the oxidation ditch process.
3. 1 principle of improved oxidation ditch pool construction
The construction of improved oxidation ditch pool is based on the idea of integrated simple sewage treatment technology, relying on Carrousel oxidation ditch, integrated oxidation ditch and Ober oxidation ditch.
Stand. It is in the form of continuous flow, and there is no special time and space distribution. Through spatial partition, spatial order and optimal control of dissolved oxygen, sewage is purified (C, N and P are removed).
In addition, the reactor with mechanical internal reflux is changed into hydraulic internal reflux by integrating with solid-liquid separation function. The general principle of architecture is continuous flow and not harmonious.
Complete the simultaneous removal of c, n, p and SS in the principle space.
3.2 Improved oxidation ditch pool type
According to the above construction principles, an improved oxidation ditch model as shown in Figure 2 is proposed. Sewage flows into the outer ditch, flows through the middle ditch and the inner ditch after passing through the backflow regulating gate, and circulates in each ditch for dozens of times.
Several times to hundreds of times, and finally the solid-liquid separator separates the mud and water. The outer-middle-inner channels are aerobic/anoxic alternating zone, anaerobic zone and aerobic zone respectively to complete the treatment of organic matter.
Degradation and simultaneous nitrogen and phosphorus removal.
The model mainly retains the advantages of nitrification and denitrification in Aubert oxidation ditch, and overcomes the shortcomings of this process, which covers a large area. Drawing lessons from the configuration and structure of runway channel in Carrousel oxidation ditch
Hydraulic internal reflux mode reduces the mechanical equipment with large reflux ratio; Consider expanding the concentric canal of Aubert oxidation ditch to remove the invalid land occupation of the central island and keep it at the same time.
The flow characteristics of three channels in series and step by step propulsion. In addition, the side ditch solid-liquid separator technology in the integrated oxidation ditch is combined, and a separate secondary sedimentation tank is not set.
Pump-free automatic return of fresh sludge.
3.3 Optimization Analysis of Improved Oxidation Ditch
(1) The improved oxidation ditch adopts the characteristics that three oxidation ditches in Aubert are connected in series, and each partition is regarded as series, which is beneficial to the removal of refractory organic matter and can reduce pollution.
The occurrence of mud swelling [9].
(2) The improved oxidation ditch draws lessons from the gradient distribution of dissolved oxygen in Aubert oxidation ditch, and it has a good denitrification function. Alternating aerobic and large-area anoxic environment is formed in the outer tank.
At the same time, "simultaneous nitrification/denitrification" occurs highly, and good denitrification effect can be obtained even without internal reflux. Because the average value of dissolved oxygen in the external channel is very low
Low, oxygen transfer is carried out in the absence of oxygen, so the oxygen transfer efficiency is improved, which has a certain energy-saving effect, and generally saves energy consumption 15%~20%. Jiawaigou
The unique simultaneous nitrification/denitrification function in the tunnel has more obvious energy-saving effect. As the key to the final effluent, the inner channel should generally maintain a high dissolved oxygen, but the inner channel capacity
Minimum product, relatively low energy consumption.
(3) The improved oxidation ditch designed a circular or elliptical ditch with relatively difficult layout of oxidation ditch in Aubert as an annular runway, which reduced the occupied area and the project cost. Take at the same time
Eliminate the central island with invalid land occupation, and further save land occupation and cost.
(4) The improved oxidation ditch draws lessons from the hydraulic conditions of Carroll oxidation ditch, so that the aerobic zone in the inner ditch can return to the anoxic zone in the outer ditch to realize hydraulic internal reflux, which simplifies the treatment process.
Thereby saving equipment and energy consumption.
(5) The improved oxidation ditch draws lessons from the advantages of integrated oxidation ditch, which integrates aeration purification and solid-liquid separation. The secondary sedimentation tank and sludge return pumping station are not built separately, and sludge automatically returns.
, simple, energy-saving, saving land and infrastructure investment.
4 conclusion
(1) oxidation ditch is widely used in sewage treatment plants in China because of its good effluent quality, stable operation and convenient management.
(2) The improved oxidation ditch model draws lessons from the configuration and internal reflux mode of Carroll oxidation ditch, and introduces the solid-liquid separation technology of side ditch integrated oxidation ditch, while maintaining
It is the integration of many advanced technologies and the new progress of oxidation ditch technology research, and maintains the water flow characteristics of Aubert oxidation ditch with three ditches in series and promoted layer by layer.
(3) The improved oxidation ditch process has the advantages of simple system, convenient management, energy saving, land saving and reduced capital investment.
The following are several common types and structural schematic diagrams of oxidation ditch:
Multi-ditch alternating oxidation ditch Carrousel oxidation ditch integrated oxidation ditch
Aubert oxidation ditch
1. Basic principles
Oxidation ditch, also known as oxidation ditch, is named after its closed annular ditch. It is a variant of activated sludge process. Because sewage and activated sludge are constantly circulating in the aeration tank, some people call it "circulating aeration tank" and "non-terminal aeration tank" Oxidation ditch has long hydraulic retention time and low organic load, which is essentially a delayed aeration system. Oxidation ditch generally consists of ditch body, aeration equipment, water inlet and outlet devices, diversion and mixing equipment. The plane shape of the ditch body is generally annular, and it can also be rectangular, L-shaped, circular or other shapes. The end faces of ditches are mostly rectangular and trapezoidal.
2. Technological characteristics of oxidation ditch
Diversity of (1) structural forms
The aeration tank of the basic form oxidation ditch is a closed ditch, but the shape and structure of the ditch are various, and the ditch can be round and oval. Can be a single channel system or a multi-channel system; Multi-ditch system can be a group of concentric interconnected ditches or a group of parallel ditches with the same size. There are oxidation ditches built separately from the secondary sedimentation tank, and there are also oxidation ditches built together, which can be divided into in-vivo and in-vitro types, and so on. Various structural forms endow the oxidation ditch with flexible operation performance, which can be operated according to any activated sludge operation mode and combined with other process units to meet different effluent quality requirements.
(2) Diversity of aeration equipment
Commonly used aeration equipment includes rotary brush, turntable, surface aerator and jet aeration. Different aeration devices lead to different types of oxidation ditches, such as Carrousel oxidation ditch with surface aeration and Pasville oxidation ditch with rotating brush. Different from other activated sludge processes, aeration devices are only installed in one or several places in the ditch, and the number should be determined according to the scale of the treatment plant, the quality of raw sewage and the structure of the oxidation ditch. The function of aeration device is to provide enough oxygen, and at the same time, it is necessary to provide a water flow speed of not less than 0.3m/s in the ditch to maintain circulation and activated sludge.
(3) the aeration intensity can be adjusted
There are two ways to adjust the aeration intensity of oxidation ditch. First, through the adjustment of the overflow weir: by adjusting the height of the overflow weir, the water depth in the ditch is changed, and then the submerged depth of the aeration device is changed, so that it can be oxygenated to meet the operation needs. The change of submerged depth will also affect the driving force of aeration equipment, and the inflow velocity can be adjusted to some extent; Second, by directly adjusting the rotating speed of the aerator: due to the development of electromechanical equipment and automatic control technology, the rotating speed of the aerator in the oxidation ditch can be adjusted at present, thus adjusting the driving force of aeration intensity.
(4) Pretreatment and sludge treatment are simplified
The hydraulic retention time and sludge age of the oxidation ditch are longer than those of the general biological treatment method, and the suspended organic matter and dissolved organic matter are completely stable at the same time, so the ancient oxidation ditch can not be provided with a primary sedimentation tank. Due to the long sludge age and low load of oxidation ditch process, the discharged excess sludge has been highly stable and the amount of excess sludge is very small. So anaerobic digestion is no longer needed, only concentration and dehydration are needed.
3. Disadvantages of oxidation ditch process:
(1) Sludge bulking problem When there are many carbohydrates in wastewater, the contents of N and P are unbalanced, the pH value is low, the sludge load in oxidation ditch is too high, the concentration of dissolved oxygen is insufficient, and the sludge discharge is not smooth, filamentous sludge bulking is easy to occur; Non-filamentous sludge bulking mainly occurs in the case of low wastewater temperature and high sludge load. The load of microorganisms is high, and bacteria absorb a lot of nutrients. Due to the low temperature and slow metabolism, a large number of high-viscosity polysaccharides are accumulated, which greatly increases the attached water on the surface of activated sludge and causes the sludge to swell.
(2) Foam problem Because the influent contains a lot of grease, the treatment system cannot completely and effectively remove it. The sludge is rich in some oil, which is stirred with oxygen rotary brush to produce a lot of foam; Sludge age is too long, sludge aging, but also easy to produce foam.
(3) Sludge Float Problem When the oil content in the wastewater is too high, the sludge quality of the whole system becomes lighter, and its residence time in the secondary sedimentation tank cannot be well controlled during operation, which is easy to cause hypoxia and produce septic tank sludge floating; If the aeration time is too long, high nitrification will occur in the tank, which will lead to high nitrate concentration, and denitrification will easily occur in the secondary sedimentation tank, which will generate nitrogen and make the sludge float. In addition, the oil content in wastewater is too high, and sludge may float with the oil.
(4) The speed is uneven and sludge is deposited in the oxidation ditch. In order to obtain its unique mixing and treatment effect, the mixed liquid must circulate in the ditch at a certain speed. Generally speaking, the minimum flow rate should be 0. 15m/s, and the average flow rate without sedimentation should reach 0.3 ~ 0.5m/s. The aeration equipment in oxidation ditch is generally aeration rotary brush and aeration rotary table, with the immersion depth of rotary brush being 250~300mm and rotary table being 480~.
530mm. Compared with the water depth of oxidation ditch (3.0~3.6m), the rotary brush only accounts for110 ~12 of the water depth, and the rotary table only accounts for1/6 ~12 of the water depth. There is almost no velocity of mixed liquid), which leads to a large amount of sludge accumulation at the bottom of the ditch (sometimes the thickness of sludge accumulation reaches 1.0m), which greatly reduces the effective volume of the oxidation ditch, reduces the treatment effect and affects the effluent quality.
Fourth, SBR process
1. Process principle
A certain amount of activated sludge is pre-cultured and domesticated in the reactor. When wastewater enters the reactor to contact with activated sludge and coexist with oxygen, microorganisms use the organic matter in wastewater for metabolism, degrade the organic matter and proliferate microbial cells. Microbial cell matter is precipitated and separated from water, and wastewater is treated. The treatment process is mainly completed by several purification processes such as initial removal and adsorption, microbial metabolism, floc formation and flocculation and sedimentation performance.
2.SBR process characteristics
(1) The ideal plug-flow process increases the driving force and efficiency of biochemical reaction. Anaerobic and aerobic conditions in the tank are alternating, and the purification effect is good.
(2) The operation effect is stable, and the sewage is precipitated in an ideal static state, which requires short time, high efficiency and good effluent quality.
(3) Impact load. There is stagnant treated water in the pool, which can dilute the buffered sewage and effectively resist the impact of water and organic pollutants.
(4) Each process in the process can be adjusted according to the water quality and quantity, and the operation is flexible.
(5) Less processing equipment, simple structure and convenient operation, maintenance and management.
(6) There is a concentration gradient of DO and BOD5 in the reaction tank, which effectively controls the expansion of activated sludge.
(7)SBR system itself is also suitable for combined construction, which is conducive to the expansion and transformation of sewage treatment plants.
(8) Nitrogen and phosphorus removal, properly controlling the operation mode, realizing the alternation of aerobic, anoxic and anaerobic states, which has good nitrogen and phosphorus removal effect.
(9) The process flow is simple and the cost is low. There is only one sequencing batch batch reactor as the main equipment, and there is no secondary sedimentation tank and sludge reflux system, and the regulating tank and primary sedimentation tank can also be omitted, so the layout is compact and the occupied area is saved.
3. Shortcomings of 3.SBR process
(1) runs intermittently and periodically, which requires high automation control;
(2) The power consumption increases when the water level changes;
(3) The efficiency of nitrogen and phosphorus removal is not too high;
(4) The stability of sludge is not as good as that of anaerobic nitrification.
Verb (abbreviation for verb) casting process
Process principle of 1 and CAST
CASS biological treatment is short for circulating activated sludge process, and CASS tank is divided into pre-reaction zone and main reaction zone. In the pre-reaction zone, microorganisms quickly absorb most of the soluble organic substances in sewage through the rapid transfer mechanism of enzymes, and experience a rapid accumulation process of high-load substrates, which plays a good buffer role on the water quality, water quantity, PH and toxic and harmful substances, and inhibits the growth of filamentous bacteria, which can effectively prevent sludge bulking; Subsequently, it undergoes a low-load substrate degradation process in the main reaction zone. CASS process integrates reaction, precipitation, drainage and function. The degradation of pollutants is a push-flow process in time, and microorganisms are in the periodic changes of aerobic, anoxic and anaerobic, thus realizing the removal of pollutants and also having good nitrogen and phosphorus removal functions.
2, casting process characteristics
(1) flexible and reliable operation.
According to the quality of sewage, the biological selector can operate in aerobic, anoxic and anaerobic modes. The selector can be operated at a constant volume or a variable volume.
● The state can be adjusted at will, giving play to the physiological characteristics of different microorganisms.
The volume of the selector is variable, which avoids sludge bulking and improves the reliability of the system.
Strong impact load resistance, suitable for industrial wastewater and urban sewage treatment.
(2) Less treatment structures and simple process.
● The total volume of the pool is reduced, and the construction cost is low.
● There is no need to set up a secondary sedimentation tank and its sludge scraping equipment, and there is no need to set up a return sludge pumping station.
(3) phosphorus and nitrogen removal can be realized
● Adjust the aeration and non-aeration sequence of the variable volume biological selector to improve the biological phosphorus and nitrogen removal effect.
(4) Saving investment
● Less structure and smaller floor space.
● Simple equipment and control system
Low aeration intensity, no need for air supply equipment with large volume.
Low operating cost
3. Technical defects
(1) runs intermittently and periodically, which requires high automation control;
(2) The power consumption increases when the water level changes;
(3) Low volume utilization rate;
(4) The stability of sludge is not as good as that of anaerobic nitrification.