With the increasing environmental requirements of the country, water treatment processes are constantly innovating and upgrading. The UASB anaerobic treatment reactor is a newly developed process, and the process introduction is as follows:

The basic feature of UASB reactor is that it can form granular sludge with good settling performance without adsorption carrier, maintain high concentration of microorganisms in the reactor, and thus can withstand high COD load (up to 30-50kgCOD/(m3? D) or more), with a COD removal rate of over 90%. In aerobic biological treatment, the most effective aerobic pure biological fluidized bed is used. The COD load of processes such as deep well aeration is only about 10kgCOD/(m3? D), and the COD removal rate is 70%~80%. Compared with other anaerobic bioreactors, the characteristics of UASB are as follows.

(1) Simple and ingenious construction

The sedimentation zone is located at the top of the reactor, and the wastewater enters from the bottom of the reactor and flows upward through the sludge bed area to come into contact with a large number of anaerobic bacteria. The organic matter in the wastewater is decomposed into biogas (mainly composed of CH4 and CO2) by anaerobic bacteria, and the wastewater carries biogas and anaerobic bacteria solids during the upward flow process. Biogas undergoes solid-liquid separation in the gas chamber area, and the treated purified water is discharged from the top of the reactor, completing the entire process of wastewater treatment. Most of the sludge in the sedimentation zone can be returned to the sludge bed area, which can maintain sufficient biomass in the reactor. From this, it can be seen that the entire upper half of the reactor integrates biological reaction and precipitation, without mechanical stirring or packing. The structure is relatively simple and the operation and management are convenient.

(2) Anaerobic granular sludge can be cultured in the reactor

When treating most organic wastewater, the UASB reactor can generally cultivate anaerobic granular sludge in the reactor as long as the operating method is correct. The characteristics of anaerobic granular sludge are high organic matter removal activity, higher density than floc sludge, and good sedimentation performance, which can maintain a high biomass in the reactor.

(3) The separation of sludge age (SRT) and hydraulic retention time (HRT) has been achieved

Due to the high biomass that can be maintained in the reactor, the sludge age is long, and the HRT of the wastewater in the reactor is short, with SRT greater than HRT. Therefore, the reactor has a high volumetric load rate and good operational stability, which is the biggest difference between modern anaerobic reactors and traditional anaerobic reactors.

(4) UASB reactors have great adaptability to various types of wastewater

The UASB reactor can not only produce high concentration organic wastewater, such as alcohol, molasses, citric acid and other production wastewater, but also medium concentration organic wastewater, such as beer, slaughter, soft drink and other production wastewater, as well as low concentration organic wastewater, such as domestic sewage, urban sewage, etc. The UASB reactor can operate at high temperatures (around 55 ℃) and medium temperatures (around 35 ℃), and can operate stably at low temperatures (around 20 ℃). In addition to organic wastewater containing toxic and harmful substances, UASB reactors can almost adapt to various types of organic wastewater discharged from different industries.

(5) Low energy consumption and low sludge production

Due to the fact that UASB reactors do not require oxygen supply, stirring, or heating, while achieving high efficiency, they also achieve low energy consumption and can provide a large amount of bioenergy biogas. Therefore, UASB reactors are a production capacity wastewater treatment equipment. Due to the long SRT, not only is the generated sludge stable, but the sludge production is also minimal, thereby reducing the cost of sludge treatment.

(6) Cannot remove nitrogen and phosphorus from wastewater

The disadvantage of UASB reactor, like other anaerobic treatment equipment, is that it cannot remove nitrogen and phosphorus from wastewater. This is determined by the nature of anaerobic biochemical reactions. When treating high and medium concentration wastewater, anaerobic aerobic cascade process is adopted, which uses UASB reactor to remove most of the carbon containing organic matter in the wastewater as pretreatment, and uses aerobic treatment equipment to remove residual carbon containing organic matter and nitrogen, phosphorus and other substances. This is the best wastewater treatment process choice, which has great energy-saving significance and can greatly save infrastructure investment and reduce operating costs. Therefore, it has good economic and environmental benefits.

Due to the continuous growth of microorganisms during anaerobic digestion or the accumulation of non degradable suspended solids in the influent, the effluent quality will improve with the increase of sludge concentration in the reactor. However, if the sludge exceeds a certain height, it will be washed out of the reactor along with the effluent. Therefore, when the sludge in the reactor reaches a predetermined maximum height, it is necessary to discharge the sludge. Generally, sludge discharge should follow pre established regulations and discharge a certain volume of sludge at certain time intervals (such as weekly), which is equal to the amount accumulated during this period. A more reliable method is to determine the sludge concentration distribution curve for sludge discharge. In principle, there are two sludge discharge methods: ① discharge directly from the desired elevation; ② Use a pump to discharge the sludge.

The height of sludge discharge is important, as it should discharge low activity sludge and retain the best high activity sludge in the reactor. Generally, thick sludge will form at the bottom of the sludge bed, while thin flocculent sludge will form at the top. The remaining sludge should be discharged from the upper part of the sludge bed. The "thick" sludge at the bottom of the reactor may become less active due to the accumulation of particles and small sand particles. It is recommended to occasionally discharge sludge from the bottom of the reactor to avoid or reduce the accumulation of sand particles in the reactor.

① Suggest a height of 0.5-1.5m in the clear water area.

② Sludge discharge can be carried out at regular intervals, usually 1-2 times a week.

③ A sludge level monitoring device needs to be installed, which can determine the discharge time based on the height of the sludge surface.

④ The residual sludge discharge point should be located in the middle and upper part of the sludge area.

⑤ For rectangular pool sludge discharge, multiple points should be discharged along the longitudinal direction of the pool.

⑥ Due to the possible accumulation of particulate matter and small sand particles at the bottom of the reactor, the possibility of bottom sludge discharge should be considered to avoid or reduce the accumulation of sand particles inside the reactor.

⑦ For a porous water pipe, the inlet pipe can be considered to also serve as a sludge discharge or vent pipe.

Is it generally believed that the location for discharging excess sludge is in the reactor? At the height. However, most designers recommend installing the sludge discharge equipment near the bottom of the reactor, and some people also install a sludge discharge pipe 0.5m below the three-phase separator to remove the remaining flocculent sludge on the upper part of the sludge bed without discharging the granular sludge. The UASB reactor sludge discharge system must consider the installation of sludge discharge equipment at different positions, including upper, middle, and lower. The actual sludge discharge requirements should be considered based on the specific situation during production and operation to determine where to discharge sludge.

1、 UASB principle

The wastewater from the UASB reactor is introduced as evenly as possible into the bottom of the reactor, and the sewage flows upward through a sludge bed containing granular sludge or flocculent sludge. Anaerobic reactions occur during the process of contact between wastewater and sludge particles. The biogas produced under anaerobic conditions (mainly methane and carbon dioxide) causes internal circulation, which is beneficial for the formation and maintenance of granular sludge. Some gases formed in the sludge layer adhere to the sludge particles, and the attached and unattached gases rise towards the top of the reactor. The sludge rising to the surface collides with the bottom of the gas emitter in the three-phase reactor, causing the sludge flocs with attached bubbles to degas. After the release of bubbles, the sludge particles will settle on the surface of the sludge bed, and the attached and unattached gases will be collected in the gas collection chamber of the three-phase separator at the top of the reactor. The function of the baffle placed under the extreme unit gap is to act as a gas emitter and prevent biogas bubbles from entering the sedimentation zone, otherwise it will cause flocculation in the sedimentation zone and hinder particle sedimentation. The liquid containing some remaining solids and sludge particles enters the sedimentation zone through the separator gap.

Due to the increased flow area of the inclined sedimentation zone of the separator near the water surface, the upward flow velocity decreases near the discharge point. Due to the decrease in flow rate, sludge flocs can coagulate and settle in the sedimentation zone. The accumulated sludge flocs on the three-phase separator will to some extent exceed the frictional force it maintains on the inclined wall, and it will slide back to the reaction zone, where this part of the sludge will react with the incoming organic matter.

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