Sewage treatment system

Abstract

The utility model provides a sewage treatment system which comprises a pretreatment tank group, a biochemical tank group, a sedimentation tank group and a disinfection tank group which are communicated in sequence, the biochemical tank group comprises a hydrolysis acidification tank, an anaerobic tank, a front anoxic tank, a front aerobic tank, a rear anoxic tank and a rear aerobic tank which are communicated in sequence, the hydrolysis acidification tank is communicated with the pretreatment tank group, and the front anoxic tank is communicated with the front aerobic tank. And the rear aerobic tank is communicated with the sedimentation tank group. The sewage treatment device can be used for effectively treating organic matters in sewage and reducing the ammonia nitrogen content in the sewage.

Description

Technical Field

[0001] The utility model relates to the technical field of sewage treatment, in particular to a sewage treatment system. Background Art

[0002] With the rapid development of the economy and society and the acceleration of the industrialization process, water demand is increasing day by day, and water resources have become an important factor restricting social development. However, pollution sources such as chemical industry, brewing, domestic sewage in villages and towns, and aquaculture wastewater have problems such as small discharge volume and scattered discharge points, which makes centralized treatment difficult. This type of sewage generally has high content of organic matter and ammonia nitrogen. Direct discharge of this type of sewage into rivers and lakes will directly lead to eutrophication of river water, which is an important factor in inducing blue algae and red tides in rivers and lakes. For the above pollution sources, micro-powered anaerobic biochemical treatment and artificial wetland treatment are mainly used at this stage. These two processes occupy a large area, and the facilities need to build reinforced concrete foundation structures, with large one-time investments and long construction periods.

[0003] Therefore, it is necessary to provide a sewage treatment system to effectively treat organic matter in sewage and reduce the ammonia nitrogen content in sewage. Utility Model Content

[0004] The utility model aims to provide a sewage treatment system to solve the problem of high content of organic matter and ammonia nitrogen in sewage.

[0005] In order to solve the above technical problems, the utility model provides a sewage treatment system, comprising a pretreatment tank group, a biochemical tank group, a sedimentation tank group and a disinfection tank group connected in sequence, the biochemical tank group comprises a hydrolysis acidification tank, an anaerobic tank, a front anoxic tank, a front aerobic tank, a rear anoxic tank and a rear aerobic tank connected in sequence, the hydrolysis acidification tank is connected to the pretreatment tank group, and the rear aerobic tank is connected to the sedimentation tank group.

[0006] Optionally, the sewage treatment system further comprises a carbon source adding room for adding carbon source to the front anoxic tank and the rear anoxic tank.

[0007] Optionally, the front aerobic tank flows back to the front anoxic tank, and the rear aerobic tank flows back to the rear anoxic tank.

[0008] Optionally, the sedimentation tank group includes a secondary sedimentation tank and a high-efficiency sedimentation tank which are connected in sequence, the secondary sedimentation tank is connected to the post-aerobic tank, and the high-efficiency sedimentation tank is connected to the disinfection tank group.

[0009] Optionally, the sedimentation tank group further includes an intermediate lift pump room, which is located between the secondary sedimentation tank and the high-efficiency sedimentation tank and is connected to the secondary sedimentation tank and the high-efficiency sedimentation tank respectively.

[0010] Optionally, the sedimentation tank group further includes a deep bed filter, which is located between the disinfection tank group and the high-efficiency sedimentation tank, and is connected to the disinfection tank group and the high-efficiency sedimentation tank respectively.

[0011] Optionally, it also includes a sludge storage tank and a sludge pump room for receiving sludge discharged from the secondary sedimentation tank. The sewage produced by the sludge pump room is discharged into the anaerobic tank, the sludge produced by the sludge pump room is discharged into the sludge storage tank, and the sludge produced by the high-efficiency sedimentation tank is discharged into the sludge storage tank.

[0012] Optionally, the disinfection tank group includes an ozone contact tank and a contact disinfection tank which are connected in sequence, and the ozone contact tank is connected to the sedimentation tank group.

[0013] Optionally, the ozone contact tank includes a tank body, an ozone dosing device, a gas explosion device and a gas collecting hood, the input end of the gas explosion device is connected to the output end of the ozone dosing device, the gas explosion device is arranged in the tank body, the gas collecting hood is located above the gas explosion device, and the output end of the gas collecting hood is connected to the ozone dosing device.

[0014] Optionally, the pretreatment tank group includes a coarse grid, a fine grid and a cyclone grit chamber which are connected in sequence, and the cyclone grit chamber is connected to the biochemical tank group.

[0015] The utility model provides a sewage treatment system, which has the following beneficial effects:

[0016] By setting up a hydrolysis and acidification tank, the organic matter in the wastewater can be effectively degraded. By setting up an anaerobic tank, nitrogen and phosphorus in the sludge can be effectively removed and the chemical oxygen demand can be reduced. By setting up a pre-anoxic tank, nitrate nitrogen can be removed by denitrification and the chemical oxygen demand can be reduced. By setting up a pre-aerobic tank, organic matter can be effectively degraded. By setting up a post-anoxic tank, nitrate nitrogen can be further removed by denitrification. By setting up a post-aerobic tank, organic matter can be further effectively degraded. In this way, organic matter, nitrogen and phosphorus in the sewage can be effectively removed. The hydrolysis and acidification tank is set upstream of the biochemical tank, and the two-stage anaerobic and aerobic tank is set downstream, which can further improve the treatment effect of organic matter, nitrogen and phosphorus. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a structural block diagram of the sewage treatment system in the first embodiment of the utility model;

[0018] Figure 2 This is a schematic diagram of the overall structure of the ozone contact tank provided in Example 2 of the utility model;

[0019] Figure 3 for Figure 2 A in the enlarged view;

[0020] Figure 4 for Figure 2 The enlarged view of point B in the figure;

[0021] Figure 5 This is a schematic diagram of the structure of the residual ozone detection device provided in Example 2 of the present utility model.

[0022] Description of reference numerals:

[0023] 1-tank body; 101-water inlet area; 1021-first reaction area; 1022-second reaction area; 103-water outlet area; 2-ozone dosing system; 201-dosing support frame; 202-branch pipe; 3-explosion device; 301-support rod; 302-bottom nozzle; 4-demist system; 401-demister; 402-tail gas destruction device; 7-gas collecting hood; 701-overflow hole; 8-upper baffle; 9-lower baffle; 10-water inlet weir; 12-sampling tube; 13-residual ozone meter; 14-filter device. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical scheme and advantages of the embodiments of the utility model clearer, the technical scheme in the embodiments of the utility model will be clearly and completely described below in conjunction with the drawings in the embodiments of the utility model. Obviously, the described embodiments are part of the embodiments of the utility model, not all of the embodiments. Generally, the components of the embodiments of the utility model described and shown in the drawings here can be arranged and designed in various different configurations.

[0025] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the present invention to be protected, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0027] In the description of the present utility model, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inside", "outside", etc. indicate the orientation or position relationship based on the orientation or position relationship shown in the accompanying drawings, or the orientation or position relationship in which the utility model product is usually placed when in use, which is only for the convenience of describing the utility model and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present utility model. In addition, the terms "first", "second", "third", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0028] In addition, the terms "horizontal", "vertical" and the like do not mean that the components are required to be absolutely horizontal or suspended, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0029] In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0030] Embodiment 1

[0031] refer to Figure 1 , Figure 1 It is a structural block diagram of the sewage treatment system in Example 1 of the utility model. This embodiment provides a sewage treatment system, including a pretreatment tank group, a biochemical tank group, a sedimentation tank group and a disinfection tank group connected in sequence, wherein the biochemical tank group includes a hydrolysis acidification tank, an anaerobic tank, a front anoxic tank, a front aerobic tank, a rear anoxic tank, and a rear aerobic tank connected in sequence, the hydrolysis acidification tank is connected to the pretreatment tank group, and the rear aerobic tank is connected to the sedimentation tank group.

[0032] By setting up a hydrolysis and acidification tank, the organic matter in the wastewater can be effectively degraded. By setting up an anaerobic tank, nitrogen and phosphorus in the sludge can be effectively removed and the chemical oxygen demand can be reduced. By setting up a pre-anoxic tank, nitrate nitrogen can be removed by denitrification and the chemical oxygen demand can be reduced. By setting up a pre-aerobic tank, organic matter can be effectively degraded. By setting up a post-anoxic tank, nitrate nitrogen can be further removed by denitrification. By setting up a post-aerobic tank, organic matter can be further effectively degraded. In this way, organic matter, nitrogen and phosphorus in the sewage can be effectively removed. The hydrolysis and acidification tank is set upstream of the biochemical tank, and the two-stage anaerobic and aerobic tank is set downstream, which can further improve the treatment effect of organic matter, nitrogen and phosphorus.

[0033] refer to Figure 1 The sewage treatment system also includes a carbon source dosing room for adding carbon source to the front anoxic tank and the rear anoxic tank. The carbon source plays a vital role in the metabolism of microorganisms, can promote the balance of aerobic and anaerobic bacteria, and is more conducive to the degradation of organic matter, nitrogen and phosphorus.

[0034] refer to Figure 1 , the front aerobic tank flows back to the front anoxic tank, and the rear aerobic tank flows back to the rear anoxic tank. In this way, the organic matter degraded in the aerobic tank can be facilitated to undergo nitrification reaction in the anoxic tank, thereby achieving a better nitrogen removal effect. After the sewage is anaerobicly treated in the anaerobic tank, it passes through anoxic-aerobic-anoxic-aerobic in sequence, and the sewage after aerobic treatment is diverted. For example, part of the sewage in the front aerobic tank flows back to the front anoxic tank, and the other part flows downstream to the rear anoxic tank; and part of the sewage in the rear aerobic tank flows back to the rear anoxic tank, and the other part flows to the sedimentation tank group. For this reflux method, the reflux flow rate can be controlled according to the actual detection situation. In this way, the organic matter degraded in the aerobic tank can be facilitated to undergo nitrification reaction in the anoxic tank, thereby achieving a better nitrogen removal effect.

[0035] refer to Figure 1 The sedimentation tank group includes a secondary sedimentation tank and a high-efficiency sedimentation tank which are connected in sequence. The secondary sedimentation tank is connected to the post-aerobic tank, and the high-efficiency sedimentation tank is connected to the disinfection tank group. The sewage after biochemical treatment enters the secondary sedimentation tank for static sedimentation, wherein the upper layer of clean water enters the high-efficiency sedimentation tank, and the sludge deposited at the bottom enters the sludge pump room through the sludge pump. For the high-efficiency sedimentation tank, according to the actual situation, such as timing or according to the test results, flocculants are added into it to remove suspended matter in the water by flocculation and sedimentation.

[0036] refer to Figure 1The sedimentation tank group also includes an intermediate lifting pump room, which is located between the secondary sedimentation tank and the high-efficiency sedimentation tank and is connected to the secondary sedimentation tank and the high-efficiency sedimentation tank respectively. Generally speaking, the secondary sedimentation tank and the high-efficiency sedimentation tank are both sedimentation tanks with relatively high water levels, and there is a certain water level difference between the two. The upper clear water in the secondary sedimentation tank is pumped into the high-efficiency sedimentation tank through the lifting pump room.

[0037] refer to Figure 1 The sedimentation tank group also includes a deep bed filter, which is located between the disinfection tank group and the high-efficiency sedimentation tank, and is connected to the disinfection tank group and the high-efficiency sedimentation tank respectively.

[0038] refer to Figure 1 The sewage treatment system also includes a flocculation dosing room for adding flocculants to the high-efficiency sedimentation tank.

[0039] refer to Figure 1 The sewage treatment system also includes a sludge storage tank and a sludge pump room for receiving sludge discharged from the secondary sedimentation tank. The sewage produced by the sludge pump room is discharged into the anaerobic tank, the sludge produced by the sludge pump room is discharged into the sludge storage tank, and the sludge produced by the high-efficiency sedimentation tank is discharged into the sludge storage tank. In this way, sludge diversion can be achieved, part of the sewage is returned to the anaerobic tank, and the remaining sludge is stored in the sludge storage tank.

[0040] refer to Figure 1 The disinfection tank group includes an ozone contact tank and a contact disinfection tank which are connected in sequence. Both are used to disinfect the filtered clean water. The ozone contact tank is used to blow ozone into the clean water after filtering through the deep bed filter to sterilize and disinfect it, while the contact disinfection tank is used to inject disinfectant into the clean water after ozone treatment. The clean water treated by the contact disinfection tank can be directly discharged.

[0041] refer to Figure 1 The pretreatment tank group includes a coarse grid, a fine grid and a cyclone grit chamber which are connected in sequence, and the cyclone grit chamber is connected to the biochemical tank group.

[0042] refer to Figure 1 The sewage treatment system also includes a sludge comprehensive treatment workshop, the sludge storage tank is connected to the sludge comprehensive treatment workshop, the water produced by the sludge comprehensive treatment workshop enters the coarse screen, and the sludge produced by the sludge comprehensive treatment workshop is transported out.

[0043] Embodiment 2

[0044] refer to Figure 2 , Figure 3 , Figure 4 and Figure 5 , Figure 2This is a schematic diagram of the overall structure of the ozone contact tank provided in Example 2 of the utility model. Figure 3 for Figure 2 A in the enlarged view, Figure 4 for Figure 1 The enlarged view of point B in the figure, Figure 5 This is a structural schematic diagram of the residual ozone detection device provided in Example 2 of the utility model. This embodiment provides an ozone contact tank, including a tank body 1, an ozone dosing device 2, a gas explosion device 3 and a gas collecting hood 7. The input end of the gas explosion device 3 is connected to the output end of the ozone dosing device 2. The gas explosion device 3 is arranged in the tank body 1. The gas collecting hood 7 is located above the gas explosion device 3. The output end of the gas collecting hood 7 is connected to the ozone dosing device 2.

[0045] By arranging an air collecting hood 7 above the aeration device 3 and connecting the output end of the air collecting hood 7 with the ozone dosing device 2, the sewage entering the tank body 1 from the aeration device and part of the ozone overflowing from the sewage can be collected by the air collecting hood, and the ozone dosing device 2 can recover the ozone collected by the air collecting hood, thereby avoiding ozone waste and improving the utilization rate of ozone.

[0046] Specifically, refer to Figure 2 and Figure 4 The pool body 1 is provided with an upper baffle 8 and a lower baffle 9, the upper baffle 8 extends from the top to the bottom of the pool body 1, and the lower baffle 9 extends from the bottom to the top of the pool body 1, and the upper baffle 8 and the lower baffle 9 are alternately arranged at intervals, the explosion device 3 is arranged between the upper baffle 8 and the lower baffle 9, and the explosion device 3 is located at one of the two positions downstream or upstream of the lower baffle 9, the gas collecting hood 7 is located above the explosion device 3 and is connected to the lower baffle 9 on one side and connected to the upper baffle 8 on the other side, so that the sewage entering the pool body 1 from the explosion device and part of the ozone overflowing from the sewage can be more comprehensively collected, and the utilization rate of ozone can be further improved.

[0047] For example, when the explosion device 3 is located downstream of the lower baffle plate 9, the partial interval between the upper baffle plate 8 and the lower baffle plate 9 and located downstream of the lower baffle plate 9 can be called the first reaction zone 1021, and the partial interval between the upper baffle plate 8 and the lower baffle plate 9 and located upstream of the lower baffle plate 9 can be called the second reaction zone 1022.

[0048] Preferably, reference Figure 2The distance between the lower baffle plate 9 and the adjacent upper baffle plate 8 located upstream of the lower baffle plate 9 is a first distance, the distance between the lower baffle plate 9 and the adjacent upper baffle plate 8 located downstream of the lower baffle plate 9 is a second distance, the first distance is greater than the second distance, and the explosion device 3 is located downstream of the lower baffle plate 9. In this way, the explosion reaction can be more concentrated, and it is more convenient for the gas collecting hood 7 to collect ozone overflowing from the sewage.

[0049] refer to Figure 4 A water overflow hole 701 is provided on one side where the gas collecting hood 7 is connected to the lower baffle plate 9. The height difference between the highest point and the lowest point of the water overflow hole 701 is 0.5 to 0.7 m. Figure 4 As shown, the overflow hole 701 on the air collecting hood 7 is completely submerged in water, ensuring the water flow inside and outside the air collecting hood 7 and maintaining a consistent water level.

[0050] Preferably, the gas collecting hood 7 is also fixedly connected to the pool wall of the pool body 1 so that the gas collecting hood 7 covers the space above the liquid surface of the first reaction zone 1021 as much as possible.

[0051] The ozone dosing device 2 includes a dosing support frame 201 and a distribution pipeline arranged on the dosing support frame 201. The input end of the distribution pipeline is connected to an ozone delivery pipe, and the output end of the distribution pipeline is connected to the explosion device 3. Figure 2 As shown, valves and flow meters are provided on the distribution pipeline and the ozone delivery pipe. The ozone delivery pipe is connected to a power device for realizing gas delivery. The dosing support frame 201 serves as a centralized management point for ozone dosing and distribution. The distribution pipeline is connected to the ozone supply source through the ozone delivery pipe. The operator can adjust the various valves and flow meters on the distribution pipeline near the dosing support frame 201, start the power device, and enable the ozone delivery pipe to transport the gas containing ozone from the ozone supply source to the distribution pipeline.

[0052] Specifically, the dosing support frame 201 is a door-type support, which is convenient for fixing on the ground above the pool body 1 or next to the pool body 1. The distribution pipeline includes a transverse connecting pipe and multiple branch pipes 202. The transverse connecting pipe is arranged on the dosing support frame 201 and is closed at both ends to form a semi-closed gas storage chamber. Multiple branch pipes 202 are arranged in parallel on the transverse connecting pipe and are all connected to the interior of the transverse connecting pipe. The other end of the branch pipe 202 is connected to the explosion device 3, which is convenient for managing the gas supply of multiple first reaction zones 1021.

[0053] The aeration device 3 includes a support rod 301 and a plurality of aeration heads arranged on the support rod 301. The support rod 301 is obliquely arranged in the middle of the first reaction zone 1021. The plurality of aeration heads are circumferentially arranged on the support rod 301. Figure 2As shown, the inclined support rods 301 and the multiple aeration heads arranged circumferentially enable the aeration device 3 to release ozone circumferentially in the first reaction zone 1021, thereby increasing the contact range with the water body and improving the effect of ozone oxidation.

[0054] In order to further improve the oxidation effect, the explosion device 3 also includes a bottom nozzle 302, and the bottom nozzle 302 is directed toward the gap between the upper baffle 8 and the bottom surface of the tank body 1. Figure 2 As shown, the bottom nozzle 302 sprays ozone into the bottom of the second reaction zone 1022, and the ozone contacts the water in the second reaction zone 1022, further increasing the contact range with the water.

[0055] The ozone contact tank further comprises a defogger system 4 , which comprises a defogger 401 disposed on the top of the tank body 1 and a tail gas destruction device 402 connected to the defogger 401 , wherein the tail gas destruction device 402 is disposed on the tank body 1 near the water outlet area 103 .

[0056] Specifically, the demister 401 is disposed above the second reaction zone 1022, and cooperates with the tail gas destruction device 402 to process a small amount of ozone escaping from above the second reaction zone 1022 to avoid pollution caused by ozone leakage.

[0057] The ozone contact tank further comprises a water inlet area 101 and a water outlet area 103 , and the water flows through the water inlet area 101 , the tank body 1 and the water outlet area 103 in sequence.

[0058] In order to stabilize the inlet water flow, a transition zone is set between the water inlet zone 101 and the pool body 1, and an inlet weir 10 is set between the transition zone and the first reaction zone 1021. The top side of the inlet weir 10 is inclined toward the first reaction zone 1021, thereby ensuring stable water inflow from the transition zone to the first reaction zone 1021.

[0059] In order to further understand the ozone dissolution in the water body, the ozone contact tank also includes a residual ozone detection device, which includes a sampling tube 12, a residual ozone meter 13 and a filtering device 14 connected in sequence. Figure 5 As shown, the sampling tube 12 is arranged in the second reaction zone 1022, and part of the water in the middle of the second reaction zone 1022 is extracted for detection. The sampling tube 12 is connected to the residual ozone meter 13 to detect ozone in the water. The operator or the system can control the opening and closing of the aeration head and / or the bottom nozzle 302 according to the actual detection data to control the process of ozone dissolution and oxidation in the pool.

[0060] The above description is only a description of the preferred embodiment of the utility model, and is not any limitation on the scope of the utility model. Any changes and modifications made by ordinary technicians in the field of the utility model based on the above disclosure shall fall within the scope of protection of the claims.

Claims

1. A sewage treatment system, comprising a pretreatment tank group, a biochemical tank group, a sedimentation tank group and a disinfection tank group connected in sequence, characterized in that: The biochemical tank group includes a hydrolysis acidification tank, an anaerobic tank, a front anoxic tank, a front aerobic tank, a rear anoxic tank, and a rear aerobic tank which are connected in sequence. The hydrolysis acidification tank is connected to the pretreatment tank group, and the rear aerobic tank is connected to the sedimentation tank group.

2. The sewage treatment system according to claim 1, characterized in that: The sewage treatment system also includes a carbon source adding room for adding carbon source to the front anoxic tank and the rear anoxic tank.

3. The sewage treatment system according to claim 1, characterized in that: The front aerobic tank flows back to the front anoxic tank, and the rear aerobic tank flows back to the rear anoxic tank.

4. The sewage treatment system according to claim 1, characterized in that: The sedimentation tank group includes a secondary sedimentation tank and a high-efficiency sedimentation tank which are connected in sequence. The secondary sedimentation tank is connected to the post-aerobic tank, and the high-efficiency sedimentation tank is connected to the disinfection tank group.

5. The sewage treatment system according to claim 4, characterized in that: The sedimentation tank group also includes an intermediate lifting pump room, which is located between the secondary sedimentation tank and the high-efficiency sedimentation tank and is connected to the secondary sedimentation tank and the high-efficiency sedimentation tank respectively.

6. The sewage treatment system according to claim 5, characterized in that: The sedimentation tank group further includes a deep bed filter, which is located between the disinfection tank group and the high-efficiency sedimentation tank, and is communicated with the disinfection tank group and the high-efficiency sedimentation tank respectively.

7. The sewage treatment system according to claim 5, characterized in that: It also includes a sludge storage tank and a sludge pump room for receiving sludge discharged from the secondary sedimentation tank. The sewage produced by the sludge pump room is discharged into the anaerobic tank, the sludge produced by the sludge pump room is discharged into the sludge storage tank, and the sludge produced by the high-efficiency sedimentation tank is discharged into the sludge storage tank.

8. The sewage treatment system according to claim 1, characterized in that: The disinfection tank group includes an ozone contact tank and a contact disinfection tank which are connected in sequence, and the ozone contact tank is connected to the sedimentation tank group.

9. The sewage treatment system according to claim 8, characterized in that: The ozone contact tank comprises a tank body, an ozone dosing device, an explosion device and a gas collecting hood, wherein the input end of the explosion device is connected to the output end of the ozone dosing device, the explosion device is arranged in the tank body, the gas collecting hood is located above the explosion device, and the output end of the gas collecting hood is connected to the ozone dosing device.

10. The sewage treatment system according to claim 1, characterized in that: The pretreatment tank group comprises a coarse grid, a fine grid and a cyclone grit chamber which are connected in sequence, and the cyclone grit chamber is connected with the biochemical tank group.

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