A multi-grid oxidation ditch activated sludge method for sewage treatment

Abstract

The patent of the present invention proposes a multi-grid oxidation ditch activated sludge method for sewage treatment. Its reaction tank, regulating tank and secondary sedimentation tank are all composed of vertical grids adjacent to each other in a multi-grid shape. The upper part of each grid is a regulating tank, and the lower part is a reaction tank; the sewage passes through each vertical grid in turn in a multiple circulation mode of alternating downward flow and upward flow, and forms a closed and terminalless flow channel through a circulating pump, and forms a plug flow state in the reaction tank. The multi-grid oxidation ditch itself can have an anaerobic section, which can greatly reduce the cost of phosphorus removal; the circulating pump can be operated in an intermittent opening and closing mode according to the water level of the regulating tank, and is also more suitable for small equipment. The use of this patent can promote the oxidation ditch method to an integrated sewage treatment device with a minimum capacity of 500 liters / day.

Description

Technical Field

[0001] The invention discloses a multi-grid oxidation ditch activated sludge method for treating sewage (hereinafter referred to as "multi-grid oxidation ditch method"), which belongs to the field of water treatment. Background Art

[0002] According to the national environmental protection standard HJ-578-2010, the oxidation ditch activated sludge method for sewage treatment (hereinafter referred to as the "oxidation ditch method") refers to a closed, non-terminal circulating flow channel arrangement of the reaction pool, equipped with oxygenation and plug flow. Obviously, it is a method of achieving AAO through continuous spatial circulation.

[0003] The overall structure and control of the oxidation ditch method are simple, but it requires the sewage in the flow channel to reach a certain flow rate (generally not less than 0.3 m / s) to prevent sludge precipitation, and requires at least one AO ​​or AAO process cycle to be completed in a closed flow channel cycle. Therefore, the entire flow channel is required to have a considerable length and aspect ratio. According to experience, in an open closed flow channel, it takes about 5 to 10 minutes for sewage to complete an AO dissolved oxygen process. Based on this calculation, the ditch length of an oxidation ditch should be at least 90 to 180 meters. Based on this, general oxidation ditches cannot be designed for large-scale projects.

[0004] Traditional oxidation ditches are all flat, that is, the entire flow channel is arranged at the same elevation, and only a small amount of power is needed to achieve flow in the entire flow channel. Generally, a surface aerator is used to provide aeration and flow power at the same time. It has a simple structure, but occupies a large area, and the cross-sectional size of the flow channel is often large. The flow velocity in the same cross-section presents a large gradient, which is easy to form structural precipitation. Generally, the flow channel adopts a concrete structure, which is suitable for large-scale sewage treatment projects, but not suitable for integrated sewage treatment devices.

[0005] Our company proposed a leap-type oxidation ditch activated sludge method and equipment in patents 202020388695.6, which divides the flow channel into two obvious water level stages, upper and lower, and uses a pump to achieve lifting and circulation from low water level to high water level. The leap-type oxidation ditch method is conducive to the configuration of zero-power and zero-emission filtration, which can be realized with concrete structure or steel structure, and can be made into an integrated sewage treatment device, but the structure is relatively complex and the unit power consumption of operation is relatively large.

[0006] The water flow of decentralized and small integrated sewage projects often varies greatly, and even stops at night. At present, it is necessary to develop an oxidation ditch process and equipment that can avoid the above problems and automatically start and stop according to the water flow, thereby reducing the actual power consumption per ton of water. Summary of the invention

[0007] The present invention proposes a multi-grid oxidation ditch method, including a regulating tank, a reaction tank and a secondary sedimentation tank. The reaction tank is equipped with an oxygenation and flow-pushing device, and is characterized in that: the reaction tank, the regulating tank and the secondary sedimentation tank are composed of a plurality of adjacent vertical grids, the secondary sedimentation tank adopts an upper speed-limited gravity outflow method, and the part of each vertical grid above the outlet of the secondary sedimentation tank is the regulating tank, and the part below is the reaction tank and the secondary sedimentation tank; each vertical grid in the reaction tank adopts a downward flow and an upward flow multiple times in alternating manner to form a closed and terminalless circulating flow channel.

[0008] The vertical multi-grid reaction tank of this patent is adopted, and each vertical grid in the reaction tank is adjacent to each other, and a connecting hole is opened alternately at the top or bottom, so that the sewage adopts an upward flow and a downward flow (or a downward flow and an upward flow) to form a closed and terminalless circulation channel. The length of the flow channel through which the sewage flows is equivalent to the sum of the heights of each vertical grid in the reaction tank. Generally, the aspect ratio of a single vertical grid is relatively large. Therefore, this design can make the reaction tank have a large "length" to width ratio, and can make the sewage in the reaction tank as a whole in a plug flow state.

[0009] Since the length and width of the cross-section of the multi-grid are relatively small, and the hydrophilicity of structural materials such as steel and plastic is generally much worse than that of concrete (due to structural pores, material, etc.), the flow velocity gradient of sewage on the cross-section will be very small. It is understood that the average flow velocity of 0.3 m / s in the oxidation ditch flow channel stipulated in the relevant technical specifications is based on hydraulic tests to prevent sludge sedimentation under large concrete structures, which is called the scouring and silting flow velocity. When structural materials such as steel and plastic are used, and the cross-section is very small, this average flow velocity can be much smaller (according to the analysis of hydraulic experts from the China Institute of Water Resources and Hydropower Research, this average flow velocity can be reduced to less than 0.1 m / s), so the length requirement for the entire oxidation ditch flow channel is also much smaller.

[0010] For example, the smallest submersible sewage pump in my country is 370W, with a head of 1.5 meters and a flow rate of 3 cubic meters per hour under the water ejector. If we use the existing nine-grid PVC pipe on the market, the cross-sectional size of each grid is 5X5cm, then the flow rate is 3 / 3600 / (5*5 / 10^4)=0.32 meters per second, which is higher than the required flow rate of 0.1 meters per second. Calculated based on a cycle of 10 minutes, the required flow channel length = 10*60*0.32=192 meters; calculated based on the height of each grid tube reaction pool of 1.5 meters, 128 grids are required. The volume of the multi-grid tube is calculated as 128*5*5 / 10^4*1.5=0.48 cubic meters. From this perspective, the multi-grid oxidation ditch method can be used for almost 500 liters / day sewage treatment plants. That is, using this multi-grid oxidation ditch method, the smallest sewage treatment plant that can be designed in theory is 500 liters / day. If the cycle period is multiplied, such as 20 or 30 minutes, and multiple aeration points are used, the processing capacity will be increased accordingly, and there is no upper limit to the processing capacity it can adapt to.

[0011] From another perspective, the number of cycles (the ratio of the circulation flow to the average water inlet flow) is 3*1000 / 500*24=144, which is higher than the general design requirements. If the volumetric air addition of water jet aeration is 30%, the air-water ratio of each cycle is 0.3, and the air-water ratio of 144 cycles reaches 43.2, while the air-water ratio of a general oxidation ditch is 4-8. Therefore, the circulation pump can be turned on and off intermittently, and the ratio of the opening and closing time can reach 1: (5-10). In order to ensure the intermittent operation of the water pump, the rest time when it is intermittently turned on is not less than 30 minutes, and the corresponding operation time of the circulation pump can be set to 3-6 minutes.

[0012] For larger equipment, steel multi-grids can also be used. Since steel multi-grids need to be welded and painted, the cross-sectional dimensions of each grid are generally larger. In this case, it is difficult to require the circulation flow rate in each grid to be greater than the flushing and silting flow rate. For example, in order to weld and paint the multi-grids, the cross-sectional dimensions of each grid should generally not be less than 50*50 cm. Even if calculated at a flushing and silting flow rate of 0.1 m / s, the circulation flow rate = 0.5*0.5*0.1*3600 = 90 tons / hour.

[0013] In fact, in order to prevent sludge from accumulating, the above requirements can be broken down into two aspects: (1) the upward and downward flow rate of the circulating water in the grid should not be lower than the sludge settling speed of 6 m / h, so that the sludge can be removed by water flow; (2) the speed of the circulating water when passing through the connecting holes at the bottom of the grid should not be lower than the flushing speed, so as to prevent the sludge settled at the bottom of the grid from settling for a long time. If this requirement is met, the required circulation flow rate will be much smaller.

[0014] Assuming that the scouring and silting flow rate through the bottom connecting hole is 0.3 m / s, the head difference between the grids is h≥v2 / 2g=0.3^2 / 2 / 9.8*1000=4.6mm; if there are 20 grids, the head difference required for circulation is less than 100mm. Because the operating head requirement of the circulation pump is very low, it is a waste to use a general submersible pump (the head is generally not less than 7 meters), and a sludge return pump can be used. The sludge return pump is a recently developed ultra-low head, large flow sludge pump.

[0015] Generally, the secondary sedimentation tank adopts a high-level water discharge method. The gravity water discharge methods of the secondary sedimentation tank are generally divided into free gravity water discharge and limited gravity water discharge. The former has a water discharge diameter not less than the water inlet diameter of the sewage treatment device, and the sewage treatment device generally has the same water inlet flow rate as the water discharge flow rate. The latter sets the water discharge of the secondary sedimentation tank to a certain flow rate, such as the average flow rate of the sewage treatment device. This patent adopts the latter, that is, the upper speed-limited gravity water discharge method.

[0016] When the sewage flow rate entering the reaction tank is higher than the set flow rate within a period of time, the sewage water level in each vertical grid will rise, and the sewage will automatically enter the regulating tank area of ​​each vertical grid. Therefore, the aeration and flow-pushing power device can be turned on and off according to the water level in the regulating tank.

[0017] When the sewage flow rate entering the reaction tank is lower than the set flow rate or even stops entering the water for a period of time, the water level in the reaction tank will remain stable and the sewage will not enter the regulation area. Although the water mixing ratio of the chemical ditch reaction tank is very large, and the inlet and outlet of the reaction tank are generally kept as far apart as possible, the newly entered sewage has little effect on its effluent, but when the duration reaches nearly 50% of the total hydraulic retention time of the reaction tank, it is possible that the newly entered sewage will reach the outlet. For this reason, it is necessary to set a maximum period for the aeration and plug flow power device to stop running. When this time is reached, even if the water level switch is not turned on, the aeration and plug flow power device should be started. Obviously, this time is related to the set effluent flow rate. Generally, when the effluent flow rate is set to the average flow rate of the sewage treatment plant, this time should not exceed 1 / 4 of the total hydraulic retention time of the reaction tank. In fact, setting this maximum shutdown time limit is also conducive to the maintenance of bacterial species in the reaction tank, which is equivalent to a seed preservation operation.

[0018] Since the hydraulic retention time required for the secondary sedimentation tank is relatively long, generally longer than the hydraulic retention time of a vertical grid, the secondary sedimentation tank can use multiple vertical grids, or a space equivalent to multiple vertical grids. In order to facilitate the extraction of sludge from the bottom of the secondary sedimentation tank, it is generally appropriate to use a space equivalent to multiple vertical grids.

[0019] The aeration and flow-pushing power device can be a sludge submersible pump plus water jet aeration, and the submersible pump can be directly arranged in its vertical grid. It can also be a surface aerator, but there is no such small surface aerator on the market. It is also possible to use an aeration fan and a circulation pump to share the aeration function with the circulation and flow-pushing function; when this method is used, if necessary, the MBR membrane can be installed in the oxygenation section where the aeration device is installed to further improve the effluent water quality; and when the MBR membrane is not required, it is not necessary to install the aeration device at the bottom of the grid, which can reduce the pressure requirements for the aeration fan.

[0020] The top of the traditional flat oxidation ditch is open, and the sewage is constantly mixed with air during the circulation process, so it is difficult for traditional oxidation ditches to form an anaerobic section; when phosphorus removal is required, chemical phosphorus removal or a pre-anaerobic tank with sludge return is required. The multi-grid oxidation ditch can be conveniently closed at the top or partially sealed, so an anaerobic section can be formed in the oxidation ditch, thereby realizing the AAO cycle in the entire oxidation ditch cycle. Therefore, compared with the traditional oxidation ditch, the multi-grid oxidation ditch only needs to appropriately increase the cycle time of the oxidation ditch to increase the phosphorus removal effect, and its phosphorus removal cost is much lower than that of the traditional oxidation ditch.

[0021] Generally speaking, the oxidation ditch does not have high requirements for influent water, and does not require a pre-primary sedimentation tank; the required aeration gas-water ratio is also relatively low. The use of a multi-grid oxidation ditch can also bring a certain amount of regulating tank, so it is suitable for lifting pump water supply when installed on the ground, or automatically adjusting the incoming water flow when installed underground. The regulating tank and the grid tank in front of the AAO method can also be omitted (the inlet grid of the multi-grid oxidation ditch can be designed as a coarse grid tank), so the multi-grid oxidation ditch can greatly save the investment and floor space of supporting facilities. On the other hand, the multi-grid oxidation ditch has a large aspect ratio, especially when it is not necessary to add an MBR membrane, a higher structure can be used, thereby reducing the floor space required for the oxidation ditch itself. In general, the multi-grid oxidation ditch saves much more floor space than traditional oxidation ditches and AAO.

[0022] In general, the multi-grid oxidation ditch method of the present invention can be applied to sewage treatment plants with a minimum capacity of 500 liters per day, greatly reducing the application area of ​​the oxidation ditch method and providing a new approach for decentralized rural sewage treatment. The multi-grid oxidation ditch can greatly reduce the investment in supporting facilities and the phosphorus removal cost of sewage treatment, and can be automatically opened and closed intermittently according to the water flow, which helps to reduce operating costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Attached Figure 1 , 2 3 and 4 are schematic diagrams of an embodiment of a multi-grid oxidation ditch method. Figure 1 It is a top view without cover, with Figure 2 is an external front view, with Figure 3 It is a summary diagram of the cross-sectional views of each vertical grid along the direction of water flow. DETAILED DESCRIPTION

[0024] As shown in the attached drawings, the processing capacity of the integrated sewage treatment device is 500 liters / day, the overall size is 71*71*14.5 cm, the reaction tank is 12 cm high, the volume is 588 liters, and the inlet mixing ratio is 28.2; the sedimentation time of the secondary sedimentation tank is 3.1 hours, and the adjustment time is 5.9 hours; the cross-sectional size of the circulating flow channel is 8.5*8.5 cm, and the total length of the flow channel is 54 meters; a 370W sludge submersible pump is used as a circulating pump, and the flow rate after the water ejector is equipped with a water pressure of 1.5 meters is about 3 cubic tons / hour, the average flow rate of the flow channel is 0.115 meters / second, the cycle time is 7.8 minutes, and the average number of hydraulic cycles is 217.1 times; the water jet aeration volume flow rate is about 30% of the circulating water volume, and the full-time gas-water ratio is 65; the circulating pump adopts an intermittent opening and closing mode of opening for 10 minutes and stopping for 30 minutes, the actual gas-water ratio is 16.3, and the actual number of cycles is 54 .2 times. The maximum continuous stay time is 4 hours.

[0025] The actual example of multi-grid oxidation ditch integrated sewage treatment device consists of 1 to 13 vertical grids (groups). The first vertical grid and the 2 to 12 vertical grid groups form a circle, and the middle forms the 13th vertical grid; in order to increase the water flow rate and avoid sludge accumulation, the 2 to 12 vertical grid groups are divided into 4, each with a cross-sectional size of about 8.5*8.5 cm; the area of ​​the 13th vertical grid is equivalent to the other 4 vertical grids, so it is used as the second sedimentation tank 30; the outlet 15 of the second sedimentation tank 30 is set at the upper position of the 13th vertical grid, and a manual valve 16 is used to set its outlet flow. A circulating sludge pump 20 is placed in the first vertical grid; the lower part of the 2 to 12 vertical grid groups is the reaction tank 25, and the upper part of the 1 to 13 vertical grids (groups) is the regulating tank 27, and the water level float switch 28 is installed in the regulating tank 27 at the upper part of the 13th vertical grid.

[0026] The entire device is integrated in a barrel 17, and a maintenance hole 18 and a maintenance hole 19 are provided on the top of the barrel.

[0027] Each vertical grid in each vertical grid group 2 to 12 is alternately connected through the upper connecting hole 26 or the bottom connecting hole 22, so that the sewage alternately forms a downward flow and an upward flow between each vertical grid (as shown by the arrow 31 in the figure). On the whole, the flow direction shown in 33 is formed between each vertical grid (group) 1 to 13. Among them, in order to ensure that the second sedimentation tank 30 of the 13th vertical grid is filled with water from the bottom, a lower elbow 32 is added at the last outlet of the 12 vertical grid groups to guide the water inlet of the second sedimentation tank 30 to the bottom.

[0028] The sewage enters from the water inlet 14, flows downward in the first vertical grid, and then passes through the circulation pump 20 and the water jet aeration device 21 to enter the upper part of the second vertical grid group. The water jet aeration device 21 adds air through the air inlet 23; the sewage first flows downward and then upward in the four vertical grids in the second vertical grid group, and after two repetitions, enters the third vertical grid group; ... and so on, and finally is divided into two parts from the last vertical grid of the 12th vertical grid group, the main part enters the upper part of the first vertical grid, forming a closed and terminalless flow channel; a small part enters the bottom of the secondary sedimentation tank 30 through the lower bend pipe 32; after sedimentation in the secondary sedimentation tank 30, it is discharged from the water outlet 15.

[0029] In the reaction tanks of the 1st to 12th vertical grids (groups), the sewage undergoes 22 cycles of upward flow and downward flow, and the aspect ratio of each vertical grid is very large, so that the sewage presents an almost complete plug flow state in the entire reaction tank 25.

[0030] The circulating pump 20 of the entire sewage treatment device is controlled according to the water level of the water level switch 28, and after the water level switch is turned on, it is operated in an intermittent opening mode. If the water level switch is not turned on for more than 4 hours in a row, it will be turned on for 2 hours in an intermittent operation mode. If the outlet flow rate is adjusted to the average flow rate of the sewage treatment device through the manual valve 16, when the inlet flow rate exceeds this set flow rate, the water level in the entire barrel will rise, the sewage will enter the regulating tank 27, and the water level switch 28 will be turned on. Since the volume of the reaction tank 25 is large enough and the inlet mixing ratio reaches 28.2, even if the circulating pump 20 is not turned on for a short time, the proportion of new water in the sewage entering the secondary sedimentation tank 30 is still very low, and the overall impact on the outlet water quality is not too large.

[0031] When the water inlet flow rate is lower than or equal to the set drainage flow rate, the water level in the entire barrel is maintained at an elevation of 15 at the outlet, and the water level switch is not turned on; when this state lasts for more than 4 hours, the aeration fan (omitted) will also be turned on and run in a circulating AAO mode for 2 hours to ensure that this low-flow water inlet is effectively treated, or when there is no water inlet, the bacterial strain is maintained.

[0032] The above design complies with the national environmental protection standard HJ-578-2010 and relevant experience, and the effluent is stable at Class B of the national urban domestic sewage discharge standard. The power consumption per ton of water = 370 / 1000*10 / 40*24 / 0.5 / 4 = 0.555KWh / ton of water (considering the use of chlorine tablets for disinfection), which is much lower than the relevant provisions of the "Household Domestic Sewage Treatment Device" (CJ / T441-2013) promulgated by the national urban construction department (2.4KWh / ton of water), and it can be automatically turned on and off according to the water flow, that is, the power consumption per ton of water is basically not affected or changed by the actual water flow.

Claims

1. A multi-grid oxidation ditch activated sludge method for sewage treatment, comprising a regulating tank, a reaction tank and a secondary sedimentation tank, wherein the reaction tank is equipped with an oxygenation and flow plugging device, and is characterized by: The reaction tank, regulating tank and secondary sedimentation tank are composed of multiple adjacent vertical grids. The secondary sedimentation tank adopts the upper speed-limited gravity water discharge method. The part of each vertical grid above the outlet of the secondary sedimentation tank is the regulating tank, and the part below is the reaction tank and the secondary sedimentation tank; each vertical grid in the reaction tank adopts the method of alternating downward flow and upward flow multiple times. Driven by the circulating pump, the water circulates between the reaction tanks to form a closed and endless circulating flow channel; the speed of the circulating water when passing through the connecting hole at the bottom of the grid is not lower than the scouring and silting flow rate; the upward flow rate of the circulating water is not lower than the sedimentation rate of the sludge.

2. A multi-grid oxidation ditch activated sludge method for sewage treatment as claimed in claim 1, characterized in that: Oxygenation and flow pushing are carried out using a micro surface aerator.

3. A multi-grid oxidation ditch activated sludge method for sewage treatment as claimed in claim 1, characterized in that: Oxygenation and flow propagation are carried out by using a submersible pump and water jet aeration.

4. A multi-grid oxidation ditch activated sludge method for sewage treatment as claimed in claim 1, characterized in that: Oxygenation is carried out by fans and aeration devices, and flow is carried out by water pumps.

5. A multi-grid oxidation ditch activated sludge method for sewage treatment as claimed in claim 4, characterized in that: The oxygenation section is equipped with MBR membrane.

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