What types of sludge retention anaerobic digesters are there?

The characteristic of the sludge retention anaerobic digester is that it uses various solid-liquid separation methods to retain the sludge in the digester, thereby improving the efficiency of the digester and reducing the size of the required digester. . This type of digester includes anaerobic contact processes, upflow anaerobic sludge beds, upflow solids reactors and baffled flow reactors.

(1) Anaerobic contact process

This process adds a sedimentation tank outside the complete mixing digester to collect the sludge and return it to the digester. The process flow is shown in Figure 8-9. The mixed liquid discharged from the complete mixing digester is first separated from solid and liquid in the sedimentation tank. The supernatant is discharged from the upper part of the sedimentation tank, and the settled sludge is returned to the digester, which reduces the solid matter in the effluent. content, and increases the sludge concentration in the digester, thereby improving the organic load rate and treatment efficiency of the equipment to a certain extent. Because the anaerobic contact process has these advantages, it is widely used in production.

Figure 8-9 Schematic diagram of the anaerobic contact process

Practice has shown that this process allows the sewage to contain higher suspended solids, is resistant to impact loads, and has greater buffering capacity. The operation process It is relatively simple and the process operation is relatively stable. The process has the same advantages as a fully mixed digester and can operate at higher loads. The disadvantage is that additional equipment is required for sedimentation and reflux of solids and microorganisms.

(2) Upflow anaerobic sludge bed

The upflow anaerobic sludge bed (UASB) is a new product successfully developed by Lettinga et al. from 1974 to 1978. The digester process is currently the fastest growing digester in the world. It is widely used due to its simple structure, low operating cost and high processing efficiency. This digester is suitable for treating soluble wastewater requiring low suspended solids content. UASB consists of three parts: sludge reaction zone, gas-liquid-solid three-phase separator (including sedimentation zone) and gas chamber. Its structure is shown in Figure 8-10.

Figure 8-10 Schematic structural diagram of UASB digester

A large amount of anaerobic sludge remains in the bottom reaction zone, and the sludge with good sedimentation and coagulation properties forms sewage in the lower part. Mud layer. The sewage to be treated flows from the bottom of the anaerobic sludge bed and mixes with the sludge in the sludge layer. The microorganisms in the sludge decompose the organic matter in the sewage and convert them into biogas. Biogas is continuously released in the form of tiny bubbles. During the rising process, the tiny bubbles continue to merge and gradually form larger bubbles. Due to the agitation of the biogas in the upper part of the sludge bed, the sludge concentration is thinner, and the sludge and water rise together into the three phases. Separator, when the biogas hits the reflecting plate at the bottom of the separator, it is deflected around the reflecting plate, and then passes through the water layer and enters the air chamber. The biogas concentrated in the air chamber is exported through a conduit, and the solid-liquid mixture enters the three-phase after reflection. In the sedimentation area of ??the separator, the sludge in the sewage flocculates, and the particles gradually increase in size and settle under the action of gravity. The sludge that settles on the sloping wall slides back into the anaerobic reaction zone along the sloping wall, causing a large amount of sludge to accumulate in the reaction zone. The treated effluent separated from the sludge overflows from the upper part of the overflow weir in the settling zone, and then the sewage is discharged. mud bed.

(3) Upflow solids reactor

The upflow solids reactor (USR) is a digester with a simple structure and suitable for high suspended solids raw materials (Figure 8 -11). The USR reactor adopts the upflow sludge bed principle. The raw materials enter the digester from the bottom. There is no need for sludge reflux in the digester, and no mechanical stirring is used. The gas production rate ranges from 0.4 to 1.2 depending on the temperature. Undigested biomass solid particles and biogas fermentation microorganisms are retained in the digester by passive sedimentation, and the supernatant is discharged from the upper part of the digester. In this way, a solid retention time and a microbial retention period that are much higher than the hydraulic retention time can be obtained. This improves the decomposition rate of solid organic matter and the efficiency of the digester.

Figure 8-11 Schematic diagram of USR digester

Using the USR fermentation process to process livestock and poultry manure raw materials, the biogas residue and biogas slurry produced has a high COD concentration and is not suitable for aerobic treatment. It meets emission standards and is generally used for farmland fertilization for ecological treatment. It is a typical energy-ecological biogas engineering process.

(4) Baffled flow reactor

The baffled flow reactor is shown in Figure 8-12. In this type of digester, due to the obstruction of the baffle, the sewage flows up and down through the sludge layer, so that each unit is equivalent to a reactor. The use of baffled flow reactors in my country has been poor in recent years.

The reasons are as follows: First, the baffled flow reactor divides a digester into several small chambers, and the feed load is all concentrated in the first small chamber. This causes the first small chamber to be severely overloaded, causing acidification of the fermentation liquid and reducing the production capacity. The activity of methanogens is inhibited, causing fermentation to fail. Second, in the baffled reactor, the feed liquid flows in a plug flow manner. The acidified feed liquid in the first chamber will gradually push out the sludge in the subsequent chambers and acidify it. In order to overcome the acidification phenomenon, some people use the return sludge method to send methanogenic bacteria into the first chamber. Since the first chamber is constantly feeding material, a small reflux flow will not prevent acidification. When the reflux flow is large, complete mixing will occur and acidification can be prevented. In this case, it is not as convenient as using a complete mixing type.

Figure 8-12 Schematic diagram of a baffled flow reactor

In the above types of sludge retention digesters, activated sludge exists in a suspended state. Various methods are used to make the sludge The mud is retained in the digester, thus achieving a longer solid retention time and microbial retention period, so the efficiency is significantly higher than that of conventional digesters. However, when subjected to shock loads or toxic substances, pollution is often caused by the rise of volatile acid. Mud loss. Therefore, the fermentation situation must be monitored regularly to maintain the normal operation of the digester.