What are the characteristics of UASB? 、

The anaerobic biological treatment process has low energy consumption. The organic volume load is high, generally 5- 10 kgcod/m3.d, and the highest can reach 30-50 kgcod/m3.d; Less excess sludge; Anaerobic bacteria have low nutritional requirements, strong toxicity resistance and large molecular weight of degradable organic matter; Strong impact load resistance; The generated biogas is a clean energy source. Today, when the whole society advocates circular economy and attaches importance to the recycling of industrial wastes, anaerobic biological treatment is obviously the first choice for sewage recycling. In recent years, the sewage anaerobic treatment process has developed very rapidly, and various new technologies and methods have emerged constantly, including anaerobic contact method, upflow anaerobic sludge bed, baffled anaerobic method, anaerobic biofilter, anaerobic expanded bed and fluidized bed, as well as the third generation anaerobic process EGSB and ic anaerobic reactor, which have developed very rapidly. UASB (Upflow Anaerobic Sludge Bed) process has the dual characteristics of anaerobic filtration and anaerobic activated sludge process, and it is a technology to transform pollutants in sewage into renewable clean energy-biogas. Strong adaptability to sewage with different solid content, relatively simple structure, operation, maintenance and management, and relatively low cost. Mature technology, more and more attention by the sewage treatment industry, widely welcomed and applied. This paper attempts to briefly explain the operation mechanism and process characteristics of UASB, as well as the design and start-up of UASB. The Origin of UASB 197 1 Professor Radinger of Vakhnin Root Agricultural University, the Netherlands, invented a three-phase separator through the design of physical structure and the difference of gravity field acting on substances with different densities. The retention time of activated sludge was separated from that of wastewater, and the prototype of upflow anaerobic sludge bed (UASB) reactor was formed. 1974 When CSM Company of the Netherlands treated beet sugar wastewater in its 6m3 reactor, it found the biological aggregate structure formed by the immobilization mechanism of activated sludge itself, namely granular sludge. The appearance of granular sludge not only promoted the application and development of the second generation anaerobic reactor represented by UASB, but also laid the foundation for the birth of the third generation anaerobic reactor. Edit the third paragraph. Basic principle of UASB The working principle of UASB consists of sludge reaction zone, gas-liquid-solid three-phase separator (including precipitation zone) and gas chamber. A large amount of anaerobic sludge remains in the bottom reaction zone, and sludge with good settling performance and coagulation performance forms a sludge layer in the lower part. The sewage to be treated flows out from the bottom of the anaerobic sludge bed and is mixed with the sludge in the sludge layer, and microorganisms in the sludge decompose the organic matter in the sewage and convert it into biogas. Biogas is continuously released in the form of tiny bubbles, and the tiny bubbles are continuously merged in the rising process, gradually forming larger bubbles. Due to the agitation of biogas, sludge with relatively thin sludge concentration is formed on the upper part of the sludge bed, and it rises with water into the three-phase separator. When biogas touches the reflector at the lower part of the separator, it bends around the reflector, and then enters the gas chamber through the water layer, where biogas is concentrated and led out by the conduit. The solid-liquid mixed solution enters the precipitation area of the three-phase separator by reflection, and the sludge in the sewage is flocculated. The sludge deposited on the inclined wall slides back to the anaerobic reaction zone along the inclined wall, so that a large amount of sludge accumulates in the reaction zone, and the treated water separated from the sludge overflows from the upper part of the overflow weir in the sedimentation zone, and then is discharged from the sludge bed. The basic requirements are: (1) to provide favorable physical, chemical and mechanical conditions for sludge flocculation, so that anaerobic sludge can obtain and maintain good settling performance; (2) A good sludge bed can often form a fairly stable biological phase, maintain a specific micro-ecological environment, resist strong disturbance, and the larger flocs have good settling performance, thus increasing the sludge concentration in the equipment; (3) By setting a sedimentation area in the sludge bed equipment, sludge fine particles are further flocculated and precipitated in the sludge layer in the sedimentation area, and then flow back to the sludge bed. Edit the fourth paragraph. Analysis and introduction of UASB flow pattern and sludge distribution principle The flow pattern of UASB is quite complicated, and the flow pattern in the reaction zone is highly related to gas production and the reaction zone. Generally speaking, in the sludge layer at the lower part of the reaction zone, as a result of gas production, more gas passes through some sections, forming an updraft, which drives some mixed liquids (referring to sludge and water) to move upward. At the same time, this gas and the medium around the water flow move downward, causing reverse mixing, thus causing short flow of water. It is easy to form a dead corner away from this updraft and water flow. These dead corners also have a certain gas production, forming a slow and weak mixing of sludge and water, so different degrees of mixing zones are formed in the sludge layer, and the size of these mixing zones is related to the degree of short flow. The mixed liquid in the suspended layer, due to the movement of gas coins, drives the liquid to rise and fall at high speed, forming a strong mixing. In the case of less gas production, sometimes there is an obvious boundary between sludge layer and suspended layer, but in the case of more gas production, this boundary is not obvious. Relevant tests show that the water flow in the sedimentation area is plug flow, but there are still dead zones and mixed zones in the sedimentation area. The sludge concentration of UASB is related to the organic loading rate of the equipment. This is the relationship between sludge distribution and load when UASB treats sugar wastewater. As can be seen from the figure, the sludge concentration in the sludge layer is higher than that in the suspended layer, and the difference between the sludge concentration in the upper and lower parts of the suspended layer is small, indicating that it is close to the complete mixed flow state, and the boundary between the sludge layer and the suspended layer is not obvious when the organic load is high. Experiments show that when the sewage passes through the bottom of the sewage at a height of 0.4-0.6m, 90% of the organic matter has been transformed ... This shows that the anaerobic sludge has high activity and has changed the long-standing concept that the anaerobic treatment process is slow. Among anaerobic sludge, the accumulation of a large number of highly active anaerobic sludge is the main reason for the huge treatment capacity of the equipment, which is attributed to the good settling performance of sludge. UASB has a high volumetric organic loading rate, which is mainly due to the existence of a large number of anaerobic sludge in the equipment, especially in the sludge layer. The stability and high efficiency of this process largely depend on the production of sludge with excellent settling performance and high methane activity, especially granular sludge. On the contrary, if the sludge in the reaction zone exists in the form of loose flocs, it will often float up and run away, making it impossible for UASB to operate stably under high load. According to the formation form of UASB sludge and the achieved COD volume load, the sludge granulation process can be roughly divided into three operation periods: (1) inoculation start-up period: from the start of sludge inoculation until the COD volume load of sludge bed reaches about 5 kg COD/m3.d, the sludge settling performance in this operation period is average; (2) Granular sludge formation period: this operation period is characterized by the appearance of small granular sludge, and ends when the total SS and VSS in the sludge bed are minimized, and the sludge settling performance in this operation period is not very good; (3) Maturation of granular sludge: This operation period is characterized by the formation of a large number of granular sludge, which gradually fills the whole UASB from bottom to top. When the volumetric load of the sludge bed reaches above 16 kg COD/m3.d, it can be considered that the granular sludge has been cultivated and matured. During this operation, the sludge settleability is very good. Edit paragraph 5. External sedimentation tank to prevent sludge loss. Although UASB has a gas-liquid-solid three-phase separator, the gas has been separated before the mixed liquid enters the precipitation area. Because the sludge in the precipitation area still has high methane production activity, it continues to produce gas in the precipitation area. Or due to impact load and sudden change of water quality, the sludge in the reaction zone may swell, resulting in poor solid-liquid separation in the precipitation zone and sludge loss, which affects the water quality and sludge concentration of the sludge bed. In order to reduce the inflow of suspended solids from effluent into water, a sedimentation tank is set outdoors, and the settled sludge flows back to the sludge bed. The advantages of setting external sedimentation tank are: (1) Sludge reflux can accelerate sludge accumulation and shorten start-up period; (2) removing suspended matters to improve the effluent quality; (3) When a large amount of floating mud occasionally appears, the visibility is improved, and the sludge can be recovered in time to maintain the process stability; (4) The returned sludge can be further decomposed, and the residual sludge can be reduced. Edit paragraph 6. The basic design of UASB The process design of UASB is mainly to calculate the balance of volume, gas production, excess sludge and nutritional requirements of UASB. UASB's swimming pools are round, square and rectangular. The height of sludge bed is generally 3-8m, and most of them are reinforced concrete structures. When the concentration of organic matter in sewage is high, the volumes of precipitation zone and reaction zone are relatively small, and the area of reaction zone can be the same as that of precipitation zone and pool. When the concentration of organic matter in sewage is low, the required precipitation area is large. In order to ensure a certain height of the reaction zone, if the area of the reaction zone cannot be too large, the area of the reaction zone can be smaller than the sedimentation zone, that is, the upper area of the sludge bed is larger than the lower pool shape. The gas-liquid-solid three-phase separator that meets the requirements is an important part of UASB, which plays a very important role in the normal operation of sludge bed and obtaining good effluent quality, so special attention should be paid in design. According to experience, the three-phase separator should meet the following requirements: 1. When the mixed liquid enters the precipitation zone, the bubbles in it must be removed to prevent the bubbles from entering the precipitation zone and affecting the precipitation; 2. The angle of the inclined wall of the precipitator can be greater than 45 degrees; 3. Before entering the sedimentation area, the surface hydraulic load of the sedimentation area should be lower than 0.7m3/m2.h, and the flow through the low joints of the sedimentation tank should not be greater than 2m/m2.h; 4. The sludge on the gas-liquid interface of the gas collector should be well immersed in water; 5, should prevent a large number of bubbles in the gas collector. The second and third conditions can be satisfied by properly selecting the depth-area ratio of the precipitator. For low-concentration sewage, it is mainly controlled by limiting the surface hydraulic load; For medium and high concentration sewage, under extremely high load, the amount of gas released per unit section may become a critical index. But so far, the results obtained at home and abroad show that the height of UASB is not greater than 10m as long as the loading rate is less than 20kgCOD/m3.d, except for the third generation anaerobic reactor. The separation of sludge from liquid is based on flocculation, sedimentation and filtration of sludge. Therefore, in the process of operation, we should create hydraulic conditions for sludge to form flocculation and sedimentation as much as possible, so that sludge has good flocculation and sedimentation performance, which is not only of great significance to the work of separator, but also of great significance to the removal rate of the whole organic matter. Special attention should be paid to prevent bubbles from entering the precipitation zone. Before entering the precipitation zone, solid and liquid should be well separated from bubbles. Scum formed on the gas-liquid surface can force some bubbles into the precipitation zone, so it must be considered in advance in the design: (1) Take appropriate technical measures to avoid the formation conditions of scum as much as possible and prevent the formation of scum layer; (2) There must be facilities or devices to disperse scum. Once scum appears in the sludge reaction zone, it can destroy the formation of scum layer or eliminate scum in time. As mentioned above, the mixing of sewage and sludge in UASB is completed by rising water flow and bubbles generated during fermentation. Therefore, multi-point water inflow is generally used to make the water inflow evenly distributed on the bed surface. The key is to be uniform-uniform and uniform. The calculation of UASB volume is generally based on organic volume load or hydraulic retention time. The parameters can be determined by experiments or refer to the design and operation parameters of similar wastewater. Edit this paragraph VII. Start of UASB 1. Domestication of Sludge The difficulty in starting UASB equipment is to obtain a large number of anaerobic granular sludge with good settling performance. The best domestication usually takes 3-6 months. If the equipment itself has accumulated, the longest commissioning date can be as long as 1-2 years. The practice shows that adding a small amount of carrier is beneficial to the adhesion of anaerobic bacteria and promotes the formation of initial granular sludge. Specific gravity flocculent sludge is easier to granulate; Anaerobic sludge with higher activity than methane can shorten the start-up period. 2. Key points of start-up operation (1) It is best to add enough inoculation sludge at one time; (2) The sludge flowing out of the sludge bed at the initial stage of start-up can't flow back, especially light fine sludge and suspended matter can be continuously discharged from the sludge bed, so that heavier activated sludge can accumulate in the bed and promote its proliferation to reach granulation gradually; (3) When the COD concentration of wastewater is low at start-up, the sludge granulation speed may not be accelerated; (4) The initial sludge loading rate is generally about 0.1-0.2 kg COD/kg TSS.d; (5) Before all kinds of volatile acids existing in sewage and anaerobic decomposition fail to be effectively decomposed, the organic volume load shall not be arbitrarily increased, and follow-up observation and water sample test are needed; (6) When the removal rate of degradable COD reaches about 70-80%, the organic volume loading rate can be gradually increased; (7) In order to promote sludge granulation, the lowest empty tower velocity in the reaction zone should not be lower than 1m/d, and the adoption of higher surface hydraulic load is beneficial to the separation of small-particle sludge from sludge, so that small-particle sludge can be condensed into large particles. Edit paragraph 8. Advantages and disadvantages of UASB process The main advantages of UASB are: 1, high sludge concentration and average sludge concentration of 20-40gvss/1; 2. High organic load and long hydraulic retention time. When medium temperature fermentation is used, the volume load is generally around 10kgCOD/m3.d; 3. Without mixing and stirring equipment, the biogas generated in the fermentation process moves upward to suspend the sludge on the upper part of the sludge bed, and the sludge layer on the lower part is also stirred to a certain extent; 4, the sludge bed is not filled with carriers, which saves the cost and avoids the problem that the filler blocks the runway; 5.UASB has a three-phase separator, generally without a sedimentation tank, and the sludge separated from the sedimentation area returns to the sludge bed reaction area, generally without sludge reflux equipment. The main disadvantages are: 1, the suspended solids in the influent need to be properly controlled, not too high, and generally controlled below 100mg/l; 2. Short flow exists in the sludge bed, which affects the treatment capacity; 3. It is sensitive to the sudden change of water quality and load, and has poor impact resistance. In recent years, UASB technology has developed rapidly at home and abroad, with a wide range of applications, and has been applied to various industries with different production scales. Practice has proved that realizing sewage recycling is a mature and feasible sewage treatment process, which not only solves the environmental pollution problem, but also achieves good economic benefits and has broad application prospects.