JPH027720B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for nitrifying and denitrifying organic wastewater using a complete mixing type aeration tank, and more specifically, the present invention relates to a method for nitrifying and denitrifying organic wastewater using a complete mixing type aeration tank. This article relates to a method for nitrification and denitrification of organic wastewater that can increase the rate of nitrification and denitrification of organic wastewater. [Prior Art] One of the methods for treating N component-containing organic wastewater such as human waste and septic tank sludge is a method of nitrification and denitrification using a complete mixing type aeration tank. In the above method, for example, organic sewage (hereinafter simply referred to as sewage) is introduced into a complete mixing type aeration tank having relatively deep downflow pipes and upflow pipes, and a circulating flow is created by a circulation pump installed outside the tank. However, the DO of sewage in the tank
is a condition suitable for simultaneous nitrification and denitrification (DO: 0.2~
This method removes N components by blowing air into the tank to promote nitrification and denitrification reactions such that the nitrogen content is 1.0 mg/). In this method, a DO sensor is installed in the tank to detect DO, and based on the detected value, the number of operating blowers is increased or decreased, or the blower air volume is controlled by PID. Internal DO adjustments are being made. However, when controlling the DO mentioned above, ○a DO that allows nitrification and denitrification reactions to proceed in a well-balanced manner.
Since the level is very low at 1 mg/or less, the sensitivity and accuracy of the DO sensor is unstable in this area, and malfunctions are likely to occur. In order to do so, it is necessary to change the standard DO (DO with which the DO measurement value is compared) while checking it with the sewage quality analysis value.
DO control is complicated. ○C At the above DO level, the strength of air blowing into the tank (aeration strength) is not that high, so a large amount of gas that causes errors in the DO sensor is not degassed and exists. Due to some circumstances, it was not possible to perform DO control accurately. As a result, the nitrification and denitrification reactions did not proceed well, and a satisfactory denitrification rate could not be obtained. Therefore, in the nitrification/denitrification method using a completely mixed aeration tank, intermittent aeration is unavoidably performed to provide an anaerobic time period and an aerobic time period (see Figure 4), but with intermittent aeration, continuous Because the aeration intensity per aeration time increases, the air dissolution efficiency decreases and the amount of foam generated increases, but this method requires aeration stop time, and the difference between anaerobic and aerobic conditions. There is a drawback that satisfactory sewage treatment efficiency cannot be obtained due to reasons such as a decrease in reaction efficiency at the time of switching. The inventors of the present invention were concerned about such a situation, and conducted repeated studies to provide a method that would improve the N removal rate without reducing sewage treatment efficiency. In other words, since the intermittent aeration method shown in Figure 4 has the advantage of a fairly high denitrification rate, we wanted to maintain this advantage while improving treatment efficiency, and in intermittent aeration, the nitrification reaction and denitrification reaction are separated. We focused on the fact that this is already being done, and looked into ways to push this approach further. As a result, the nitrification and denitrification reactions that take place in the fully mixed aeration tank are controlled over time (
Denitrification priority process, () Simultaneous nitrification and denitrification process,
By functionally dividing the process into four stages: () nitrification priority process and () nitrification completion process, and by appropriately adjusting the DO in each process, the de-N rate can be maintained at a high level while the We came up with the idea of suppressing the decline in processing efficiency by reducing the difference in DO between processes, smoothing the transition between processes, and eliminating non-aerated conditions. Also, among the above four steps, the DO is high ()
We thought that DO measurement accuracy could be improved by measuring DO at the nitrification completion stage. [Problems to be solved by the invention] The present invention was completed as a result of further research in order to embody such an idea, and it is possible to smoothly carry out each of the reactions () to () above in the aeration tank. The aim is to improve the N removal rate without reducing treatment efficiency by appropriately controlling the DO in the tank over time so that it progresses. [Means for Solving the Problems] The present invention detects DO at the completion stage of nitrification when nitrifying and denitrifying organic wastewater using a complete mixing type aeration tank, and compares the detected DO with standard DO. The gist is to find the required total amount of blown air from the relationship, distribute the total amount of blown air over time into two or more stages, and supply the plant to the premises so that the amount increases in the second half. [Operation] The prerequisite for properly controlling DO is
The objective is to understand DO more accurately, and for this purpose, in the present invention, the DO detection timing is set to the nitrification completion stage. That is, when the DO is low, there is a problem that the accuracy and sensitivity of the DO sensor itself is low, and moreover, the DO sensor is easily affected by other gases.For these reasons, in the present invention, the DO detection timing is set at nitrification when the DO is relatively high. The completion stage was selected. Next, in order to proceed with the nitrification/denitrification reaction efficiently, each process is functionally divided as described above () ~
It is necessary to appropriately adjust the DO in (), but according to the research conducted by the present inventors, it was found that the role of each step and the suitable DO differ as shown below. () Denitrification priority process: () to () period 1
In the case of a cycle, this corresponds to the first period of the cycle, in which the denitrification reaction proceeds preferentially. D, is 0.1
~0.3mg/ () Simultaneous nitrification and denitrification process: The process continues the longest during the intermediate stage, where both nitrification and denitrification reactions progress. DO is 0.2 to 0.5 mg/ () Nitrification priority process: Nitrification reaction proceeds preferentially in the latter half of the cycle. DO is 0.8~1.2
mg/ () Nitrification completion process: At the final stage of the cycle, all NH ₄ -N in the wastewater is nitrified and becomes only NO _x -N. There is no consumption of O ₂ and DO is rapidly increasing. DO is 1.5~3.5mg/ Also, the time allocation for each process from () to () is as follows:
():():():()=10~25%:50~70%
:
10-20%: It is desirable to set it to 1-5%, especially when the wastewater (BOD/T-N) is small ()
It is desirable that the ratio is even higher than the above. Therefore, if the above-mentioned preferred DO distribution over time is graphed, a pattern approximately as shown in FIG. 1 (hereinafter referred to as an ideal pattern) can be obtained. On the other hand, in order to make the DO distribution of one cycle match or approximate the ideal pattern, it is necessary to change the amount of blown air in accordance with the reaction state of each step. That is, in () the denitrification priority process, the amount of blown air must be reduced, and in the () nitrification priority process and () the nitrification completion process, the amount of blown air must be increased. That is, the present invention forms the ideal pattern.
By adjusting the amount of blown air so that it increases in the latter half of the cycle and controlling the total blown air required for one cycle, the
The aim is to make the denitrification reaction proceed efficiently. In other words, the standard DO for each process can be determined from the ideal pattern, so normally it would be sufficient to detect the DO for each process and increase or decrease the amount of blown air based on the detected value, but for each process of () to (), DO
Since the value of is low, accurate detection of DO is difficult.
However, since the relative ratio between the standard DOs for each process is approximately fixed, the ratio of the amount of blown air required for each process can also be considered to be approximately constant in relation to each other. That is, considering the total amount of blown air required throughout one cycle, it can be considered that the distribution ratio is constant. Therefore, it is necessary to know the difference between the value detected in the nitrification completion process where the DO is high and the standard DO at the detection time, and to know whether the total amount of air blown in the cycle is excessive or insufficient based on the relationship between the two, taking into account the distribution ratio. be able to. By the way, in the above-mentioned sewage treatment, activated sludge has a very high oxygen consumption potential and can easily follow changes in the oxygen supply rate, so the response to DO is very small and it is necessary to change the blown air level for a short time. There is a situation in which the DO of sewage hardly changes even if the air is blown, but in the present invention, the total amount of blown air required for the next cycle is determined from the above excess or deficiency.
Since this is distributed evenly within each process according to the distribution ratio, the amount of blown air in each process increases and decreases stably, making it possible to stably control DO in each period. As a result, the DO of each period can be brought closer to the ideal pattern, and as a result, the N removal rate can be increased. The present invention will be explained in more detail below. In the present invention, the time for one cycle from () to () is preferably 2 to 8 hours,
If this time is too short, the blown air level will change to the next period before each period becomes stable, making it difficult to clearly differentiate the four processes. On the other hand, if the above time is too long, the number of times the total blown air volume level is changed by DO detection will be reduced, so even if the water quality changes significantly within one cycle, the operation will continue without being able to correct this, and the water quality will be controlled appropriately. become unable. In addition, DO is detected during the DO stabilization period at the completion stage of nitrification.
Moreover, it is desirable to carry out the measurement over a certain amount of time; for example, it is desirable to measure DO from 5 to 10 minutes before the end of one cycle. Then, in calculating the required total amount of blown air for the next cycle from the measured DO, calculations are performed, for example, according to equation (A). F _o ; Total amount of blown air in the current cycle F _o+1 ; Total amount of blown air in the next cycle C _s ; Saturated DO (mg/) DO; DO measurement value DO ^* ; DO standard value m; Coefficient a; Measurement start time b: Measurement end time In other words, in equation (A ₎ , calculate the difference between C _s and ^DO and integrate it over the measurement time from a to b. F _o+1 can be obtained by dividing by the value integrated over the measurement time, calculating the mth root of the obtained value, and multiplying this by F _o . The total amount of air required for the next cycle obtained in this way
F _o+1 in the same ratio as the current cycle () ~
It is sufficient to distribute it to each process in (). In addition, when distributing it, it is necessary to increase the supply amount in the second half (specifically, to increase the aeration intensity taking into account the time distribution of each period), but it is more preferable ()
It is desirable that the aeration intensity is ≧() aeration intensity≧() aeration intensity≧(). [Example] Example 1 Fig. 2 is a flow explanatory diagram showing an example of a complete mixing type aeration apparatus for carrying out the method of the present invention. ), denitrification tank 2, re-aeration tank 3, settling tank 4
It is composed of etc. When performing nitrification/denitrification treatment on wastewater L, wastewater L and seed sludge S are put into the aeration tank 1, and the wastewater L drawn out by the circulation pump P is descended through the circulation line 5. Flow tube 6
reflux into the tank. Next, the sewage descends inside the downflow pipe 6 as shown by the arrow and reaches the lower end, where it changes direction and ascends the channel sandwiched between the aeration tank 1 and the downflow pipe 6, reaching the upper part of the tank and then overflowing. It is passed over the flow weir 7 and subjected to recirculation. On the other hand, air blowing piping 8 is provided in the circulation flow path 5.
is connected, and there is DO in the wastewater above the aeration tank 1.
The sensor 9 is immersed, and air is blown into the waste water L according to the control method of the present invention (see FIG. 3 for the control method, which will be described later). In addition, 10 is a controller,
11 indicates an electric valve. The sewage treated in this manner is post-treated in a denitrification tank 2 and then in a re-aeration tank 3, and the treated water L separated from the sludge _Sp in the settling tank 4 is discharged to the outside of the system. A portion of the sludge _Sp is returned as seed sludge S, and the remainder is treated as surplus sludge _S1 . FIG. 3 is a schematic diagram showing an example of an air blowing amount control device, in which an electric valve 11 and a differential pressure oscillator 12 are interposed in the air supply pipe 8 leading to the waste water circulation flow path 5. Then, the data detected by the DO sensor 9 is input to the controller 10, and the required total amount of blown air for the next cycle is distributed to each process from () to () to set the required air amount for the relevant process. Then, the opening degree of the electric valve 11 is adjusted accordingly to supply air to the wastewater circulation channel 5. Further, the amount of air flowing through the air supply pipe 8 is detected by a differential pressure oscillator 12 to finely adjust the set amount of air. Incidentally, if necessary, the amount of sewage input may be controlled by a controller. When sewage treatment was carried out under the following treatment conditions according to the method of the above example, treated water having the quality shown in Table 1 was obtained. Table 1 also shows the results obtained when intermittent aeration was performed in the aeration device A shown in FIG. 1 (comparative example). Processing conditions DO measurement: It was carried out for 5 minutes from 5 minutes before the end of the current cycle (nitrification completion stage) to the end of the current cycle. Time allocation of () to () (): (): (): () = 45:90:40:5 Amount of human waste processed: 40Kl/day Amount of returned sludge: 140m ³ /day Amount of sewage circulation: 30m ³ /min Sewage capacity in aeration tank: 200m ³ , depth 10m

【table】

〔Effect of the invention〕

The present invention is configured as described above, and calculates the required total amount of blown air for the next cycle based on the DO value detected at the nitrification completion stage, and distributes the total amount of blown air over time into two or more stages. Since it is supplied into the tank,
Appropriate air blowing can be performed in any period from () to (), and a DO distribution that is equivalent to or similar to the ideal pattern can be formed. As a result, the N removal rate can be dramatically increased.

[Brief explanation of drawings]

Figure 1 is a graph showing DO changes in an ideal pattern and the amount of blown air for each process, Figure 2 is a flow diagram showing an example of an aeration device for carrying out the method of the present invention, and Figure 3 is the amount of blown air. A schematic diagram showing an example of the control method, Fig. 4 is a diagram showing an example of the control method in the intermittent aeration method.
It is a graph showing changes in DO and changes in the amount of blown air. 1: Aeration tank, 6: Downflow pipe, 7: Overflow weir, 8:
Air introduction pipe, 9: DO sensor, 10: Controller, 11: Electric valve.

Claims (1)

[Claims] 1 When performing nitrification and denitrification of organic wastewater using a completely mixed aeration tank, DO
is detected, the total amount of blown air required for the next nitrification/denitrification cycle is determined from the relationship between the detected DO and the standard DO, and the total amount of blown air is divided over time into two or more stages to determine the second half. A method for nitrification and denitrification of organic wastewater, which is characterized by supplying organic wastewater into a tank in increasing amounts.