Upflow anaerobic sludge bed reactor is an anaerobic biological method for treating sewage. Invented by Professor Lettinga in the Netherlands in 1977.
Sewage passes through the UASB from the bottom up. There is a high-concentration, high-activity sludge bed at the bottom of the reactor. Most of the organic pollutants in the sewage are degraded into methane and carbon dioxide by anaerobic fermentation.
Due to the agitation of water flow and air bubbles, there is a suspended sludge layer above the sludge bed.
The upper part of the reactor is provided with a three-phase separator for separating digested gas, digested liquid and sludge particles. The digested gas is led out from the top of the reactor; the sludge particles automatically slide down and settle to the sludge bed at the bottom of the reactor;
UASB has a large load capacity and is suitable for the treatment of high-concentration organic wastewater. A well-run UASB has a high removal rate of organic pollutants, does not require stirring, and can adapt to large-scale load shocks, temperature and pH changes.
The anaerobic reaction process in the UASB reactor is the same as other anaerobic biological treatment processes, including hydrolysis, acidification, acetic acid and methane production. The substrates are converted into the final product by different microorganisms participating in the substrate conversion process. The anaerobic microorganisms that participate in the reaction during the anaerobic digestion of inorganic substances such as biogas and water are mainly the following: ① hydrolysis-fermentation ( (Acidification) bacteria, which hydrolyze and ferment substrates with complex structures into various organic acids, ethanol, sugars, hydrogen, and carbon dioxide; (2) acetic acid bacteria, which convert the products of the first hydrolysis and fermentation to hydrogen, acetic acid, and carbon dioxide; ③ Methanogens, they convert simple substrates such as acetic acid, methanol and carbon dioxide, hydrogen, etc. into methane 
The UASB consists of a sludge reaction zone, a gas-liquid-solid three-phase separator (including a sedimentation zone) and an air chamber. A large amount of anaerobic sludge remains in the bottom reaction zone, and sludge with good sedimentation and coacervation properties forms a sludge layer in the lower part. The sewage to be treated flows from the bottom of the anaerobic sludge bed and comes into contact with the sludge in the sludge layer for mixed contact. The microorganisms in the sludge decompose the organic matter in the sewage and convert it into biogas. The biogas is continuously emitted in the form of micro-bubbles. During the ascent process, the micro-bubbles continuously merge to gradually form larger bubbles. In the upper part of the sludge bed, due to the agitation of the biogas, a sludge with a relatively thin sludge concentration and water rises into the Phase separator, when the biogas hits the reflecting plate at the lower part of the separator, it is folded around the reflecting plate, and then passes through the water layer to enter the gas chamber. The biogas is concentrated in the gas chamber, and is led out by a duct. The solid-liquid mixed liquid enters the three phases after reflection. In the sedimentation area of the separator, the sludge in the sewage flocculates, the particles gradually increase and settle under the action of gravity. The sludge settled 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 discharges the sewage Mud bed.
The structure of UASB is a compact anaerobic reactor, which integrates biological reaction and precipitation. The reactor is mainly composed of the following parts.
Inlet water distribution system
Its main functions are:
1. The raw wastewater entering the reactor is evenly distributed to the entire cross section of the reactor and rises evenly;
2. Play the role of hydraulic stirring.
These are the key links for the efficient operation of the reactor.
It is the main part of UASB, including granular sludge area and suspended sludge area. A large amount of anaerobic sludge remained in the reaction zone, and sludge with good agglomeration and sedimentation properties formed a granular sludge layer at the bottom of the tank. The wastewater flows from the bottom of the sludge bed, and is mixed with the granular sludge. The microorganisms in the sludge decompose organic matter, and at the same time, the tiny biogas bubbles are continuously emitted. During the rise of the tiny bubbles, they are constantly merged and gradually form larger bubbles. In the upper part of the granular sludge layer, due to the agitation of the biogas, a suspended sludge layer with a small sludge concentration is formed.
It consists of sedimentation zone, return joint and air seal. Its function is to separate three phases such as gas (biogas), solid (sludge) and liquid (wastewater). The biogas enters the gas chamber, and the sludge is precipitated in the sedimentation zone, and then returned to the reaction zone through the reflux slit. The clarified wastewater is discharged out of the reactor as treated water.
The separation effect of the three-phase separator will directly affect the treatment effect of the reactor.
Also known as gas hood, its function is to collect the generated biogas and send it out of the gas chamber to the biogas cabinet.
Treatment water drainage system
The function is to uniformly collect the treated water on the water surface of the precipitation zone and discharge it out of the reactor.
In addition, a sludge removal system and a scum removal system should be set up in the reactor as needed.
Compared with other types of anaerobic reactors, it has the following advantages:
1. There is more biomass in the sludge bed, and the reduced concentration can reach 20 ~ 30g / L;
2. The volume loading rate is high. Under medium temperature fermentation conditions, it can generally reach about 10kgCOD / (m³ · d), and even up to 15 ~ 40kgCOD / (m³ · d). The hydraulic retention time of wastewater in the reactor is short. Therefore, the required pool capacity is greatly reduced.
3. The equipment is simple and convenient to operate. There is no need to set up a sedimentation tank and sludge return device, no filling is required, and no mechanical stirring device is required in the reaction zone.
The main contents are: ① choose the pool type according to water quality characteristics, water volume, removal rate, etc., determine the main size; ② design the water inlet, distribution and outlet systems; ③ choose the type of three-phase separator, biogas recovery equipment.
The design parameters should be determined through experiments. In the unconditional test, the design can be performed with reference to empirical parameters.
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