JP2000107786A - Control method for intermittent aeration activated sludge process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control method for a sewage water treatment process capable of preventing removal ratio of phosphorus from being deteriorated by solving a problem wherein a large amount of nitrate nitrogen is contained in a returned sludge due to decrease of denitrification speed to lower the removal ratio of phosphorus due to hindrance to phosphorus emission in an operation under low water temperature conditions, in a biological dephosphorus process such as dual tank intermittent aeration process or the like. SOLUTION: From the time of detecting an inflection point A of ORP by a first ORP meter in an agitation step of a first aeration tank until a finish time of the agitation step, flow rate of a return sludge is made lower than a set value, and from the time of starting an aeration step in the first aeration tank to the time of detecting the inflection point of the ORP in the agitation step by the first ORP meter, the flow rate of the return sludge is raised so as to make the total amount of the return sludge in one cycle equal to the set value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for biologically treating sewage and domestic wastewater, and more particularly to a method for controlling a process for removing nitrogen and phosphorus in wastewater.

[0002]

2. Description of the Related Art The treatment of sewage and domestic wastewater mainly involves the removal of organic substances, and biological treatment represented by the activated sludge method has been generally used. However, in recent years, eutrophication has become a major problem in closed water bodies such as lakes and marshes, and it has become important to remove nitrogen and phosphorus which cause this problem. Therefore, a treatment method capable of removing nitrogen and phosphorus in addition to organic substances has been developed as an improved method of the activated sludge method. Typical methods include the A ₂ O method (anaerobic-anoxic-aerobic method) and the batch method. Activated sludge method, intermittent aeration type activated sludge method (hereinafter, abbreviated as intermittent aeration method) and the like can be given. In these methods, microorganisms are alternately subjected to aerobic conditions and anaerobic conditions to remove organic substances, nitrogen, and phosphorus.

Here, the principle of sewage treatment for removing nitrogen and phosphorus will be briefly described. Organic matter in the sewage becomes food for microorganisms constituting the activated sludge and is decomposed and removed. Nitrogen is oxidized from NH ₄ -N (ammonia nitrogen) to NO ₃ -N (nitrate nitrogen) by the action of nitrifying bacteria under aerobic conditions, and then by the action of denitrifying bacteria under anaerobic conditions. NO ₃ -N are removed is reduced to N ₂ (nitrogen gas). The relationship between nitrification and denitrification is summarized as follows. Reaction Nitrogen form change Reaction conditions Microorganism Nitrification reaction Ammonium nitrogen → Nitrate nitrogen Aerobic (with dissolved oxygen) Nitrifying bacteria Denitrification reaction Nitrate nitrogen → Nitrogen gas Anaerobic (without dissolved oxygen) Denitrification bacteria By alternately changing the operating conditions to aerobic and anaerobic, activated sludge having a property of accumulating a large amount of phosphorus in cells is produced, and the activated sludge is removed using the activated sludge. In other words, this activated sludge has the property of releasing phosphorus under anaerobic conditions and absorbing phosphorus under aerobic conditions. Therefore, it absorbs phosphorus under aerobic conditions and converts activated sludge that has absorbed a large amount of phosphorus into excess sludge. To remove phosphorus from the processing system. This relationship can be summarized as follows. Phosphorus concentration in the reaction tank Reaction conditions Phosphorus removal Phosphorus release increase Anaerobic (without dissolved oxygen)-Phosphorus absorption decrease Aerobic (with dissolved oxygen) Extraction of activated sludge Although two conditions are indispensable, strictly speaking, the anaerobic conditions for denitrification and the anaerobic conditions for dephosphorization are different. In the intermittent aeration method, denitrification is completed and NO _{3 is} contained in the tank. Only after the disappearance of oxygen molecules due to -N, phosphorus is released from the activated sludge, which leads to the absorption of phosphorus in the next aeration step.

The intermittent aeration method has attracted attention because it can set the ratio of aerobic conditions and anaerobic conditions temporally, and can be applied to existing facilities relatively easily. As a method of greatly improving the intermittent aeration method, two aeration tanks, a first aeration tank into which wastewater flows, and a second aeration tank connected in series to the first aeration tank, are used. The device provided and its control method (hereinafter referred to as 2
A tank-type intermittent aeration method) is disclosed in JP-A-6-55190.

[0005] The outline is shown below in FIGS.
This will be described with reference to FIG. FIG.
It is a schematic diagram which shows the principal part structure of the sewage treatment apparatus for demonstrating the intermittent aeration method and control system of the 90th publication. In this figure, the paths of water and air are represented by solid arrows, and the control signal system is represented by dotted arrows. This device mainly receives sewage 1 and organic matter, nitrogen,
A first aeration tank 2a from which phosphorus is removed, and a second aeration tank 2b
And a final sedimentation basin 4 from which activated sludge is separated by gravity sedimentation to obtain treated water 3, and a return sludge pump 5 for returning the settled activated sludge to the first aeration tank 2a. The volume ratio of the first aeration tank 2a and the second aeration tank 2b is approximately 1 to 1, and the total residence time of the treated water is equal to the final settling tank 4
16 to 32 hours, including The control system includes a first ORP meter (oxidation-reduction potential meter) 6a for measuring the oxidation-reduction potential in the first aeration tank 2a and a second ORP meter 6b for measuring the oxidation-reduction potential in the second aeration tank 2b. And a control device 9 for outputting control signals to the first aeration blower 7a, the second aeration blower 7b, the first stirring pump 8a, and the second stirring pump 8b based on those values.

[0006] The basic concept of operation control in such an apparatus system is as follows: a first aeration tank into which drainage flows, and a first aeration tank.
Using two aeration tanks of a second aeration tank connected in series to the aeration tank, the first aeration tank 2a controls nitrification and denitrification to a certain time, thereby ensuring the phosphorus release time. 2
In the aeration tank 2b, nitrification and denitrification are performed, and one cycle of control is maintained at a predetermined time while preventing the release of phosphorus, thereby obtaining a high nitrogen and phosphorus removal rate.

The specific method will be described with reference to FIGS. 3 (a) and 3 (b) together with a change in ORP due to control. FIGS. 3A and 3B show that the aeration start time is set to zero at an arbitrary timing during the control, and the OR of each aeration tank is set.
FIG. 3A is a diagram showing the relationship between the time change of P and the ORP of the first aeration tank, and FIG. 3B is a relationship diagram of the ORP of the second aeration tank with respect to time.

First, the control method of the first aeration tank 2a will be described. The aeration time for nitrification and phosphorus absorption is Te, the denitrification time is Tf, and the time Tg, which is the sum of Te and Tf, is a predetermined time Tgs. The aeration time Te is adjusted to be equal to Looking at the change in the ORP of the first ORP meter 6a, the inflection point A appears after the end of the denitrification. By detecting the inflection point A, the time Tg is measured, and Tgs and Tgs are measured.
The aeration time Te is adjusted based on the difference in g. as a result,
As will be described later, one cycle is maintained substantially at the time Tds, so that the phosphorus release time is secured as Tds-Tgs.

A method of controlling the second aeration tank 2b will be described.
The aeration time for nitrification and phosphorus absorption is Tb, and the stirring time during which denitrification proceeds is Tc, and the time Td, which is the sum of Tb and Tc,
Is adjusted to match the preset time Tds, and the first aeration tank 2a and the second aeration tank 2b are simultaneously returned to the aeration state assuming that one cycle has ended after the time Td. This is performed by detecting the inflection point B from the change in the ORP of the second ORP meter 6b, measuring the time Td, and adjusting the aeration time Tb based on the difference between Tds and Td. As a result, the aeration state is established immediately after the denitrification is completed, so that phosphorus is not released in the second aeration tank 2b, and a high nitrogen and phosphorus removal rate can be obtained.

[0010]

As described above, the present inventors have described the two-tank intermittent aeration method described in Japanese Patent Application Laid-Open No. 6-55190. However, biological dephosphorization including this method has the following problems that must be solved.
That is, when the returned activated sludge contains a large amount of nitrate nitrogen, the removal of phosphorus is reduced due to the inhibition of phosphorus release. In the case of this method, when the operation is performed under a low water temperature condition of 15 ° C. or less in winter or the like, the denitrification rate is reduced, the denitrification becomes insufficient, and a large amount of nitrate nitrogen may remain in the treated water. . Under such low water temperature conditions, it is sufficient to take a long time for the nitrification and denitrification steps.However, since the operation is performed so as to secure a constant phosphorus release time, the nitrification and denitrification times should be extended to some extent or more. And the denitrification becomes insufficient. In this case, the second operation tank is forcibly shifted to the next operation cycle in a state where the denitrification is not completed. When the amount of the remaining nitrate nitrogen increases, the concentration of the nitrate nitrogen in the sludge returned from the final sedimentation tank to the first aeration tank also increases. In the state of normal operation, the return of the nitrate nitrogen component is at a low concentration level, so it does not become a hindrance to phosphorus release, but the sludge return rate is operated at a high sludge return rate under the above low water temperature conditions. In such a case, phosphorus removal may be deteriorated.

The present invention has been made in view of the above points, and an object of the present invention is to provide a method for controlling a sewage treatment process by a two-tank intermittent aeration method capable of preventing a decrease in a phosphorus removal rate. is there.

[0012]

In order to solve the above-mentioned problems, the operation of the two-tank intermittent aeration method of the present invention comprises first and second intermittent aeration methods.
And the intermittent aeration method using two aeration tanks, the first O
After performing aeration for a predetermined time in the first aeration tank in which the RP meter is installed, the process shifts to the stirring step, and after detecting the ORP bending point of the first ORP meter appearing in the stirring step of the first aeration tank,
That is, in the step in which phosphorus release proceeds, the amount of activated sludge returned to the first aeration tank is reduced.

In the first aeration tank, a predetermined time, for example, 3
After performing aeration for 0 minutes, the process proceeds to the stirring step. In this stirring step, a denitrification reaction occurs first, and phosphorus is released after the ORP bending point of the first ORP meter installed in the first aeration tank appears. For example, if the detection of the ORP bending point is 60
When one cycle of minute, aeration, and stirring is 120 minutes, phosphorus release proceeds for 60 minutes, which is the difference between the time of one cycle and the time of the ORP inflection point. Only during this phosphorus release step will the amount of sludge returned from the final settling basin be reduced, reducing the nitrate nitrogen returned with this sludge.

Now, the reason why the return sludge flow rate is reduced after the first ORP meter detects the ORP bending point will be described. As described above, in the first aeration tank of the two-tank intermittent aeration method, phosphorus is absorbed in the aerobic step, and phosphorus is released after the first ORP inflection point appears in the anaerobic step. In general, nitrate nitrogen is known as a component that inhibits phosphorus release. When phosphorus release is inhibited and the amount of released phosphorus is reduced, a phenomenon occurs in which phosphorus absorption is weakened and the phosphorus removal rate is reduced. Therefore, such as when operating under low water temperature conditions where it is expected that the return of nitrate nitrogen may be large, the phosphorus release progresses or
After detecting the P inflection point, the returned sludge flow rate is reduced, the returned nitrate nitrogen is reduced, and phosphorus release is not hindered.

In the case of normal operation, the returned nitrate nitrogen is very small and does not greatly affect the removal of phosphorus. However, the returned nitrate nitrogen such as an operation under a low water temperature condition of 15 ° C. or less is used. Even when the concentration of nitrogen is high, the application of the present operation method can prevent deterioration of phosphorus removal. As described in the related art, in the two-tank intermittent aeration method, which is the present method, the timing of phosphorus release can be clearly distinguished using the ORP, and therefore, such control of the returned sludge amount becomes possible. .

According to the above operation method, the inhibition of phosphorus release by the returned nitrate nitrogen can be reduced, the deterioration of the phosphorus removal rate can be prevented, and stable phosphorus removal can be achieved.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is a schematic diagram showing a main configuration of a sewage treatment apparatus including a two-tank intermittent aeration method apparatus and a control system to which the present invention is applied. 1 and 2 are denoted by the same reference numerals, and the handling of arrow lines is the same as in FIGS. 1 and 2. The apparatus of the present invention shown in FIG. 1 is basically the same as the conventional apparatus shown in FIG. 2, but is different from the conventional apparatus shown in FIG.

The control method of the present invention using this device is performed as follows. From the detection time of the ORP inflection point of the first ORP meter 6a appearing at the end of the denitrification in the stirring step of the first aeration tank 2a to the time until the next cycle starts, that is, the period during which the phosphorus release step proceeds Then, the return sludge flow rate is reduced at a constant rate (r) with respect to a preset return sludge flow rate (Q). The amount reduced in the phosphorus release step is set before the detection of the ORP inflection point, that is, in the period of the aeration and denitrification steps, so that the returned sludge amount (TdsQ) set in one cycle of aeration and stirring can be secured. , Increase return sludge flow. Specifically, O of the first ORP total 6a
The method of calculating the return sludge flow rate before and after the detection of the RP inflection point is based on the following equations (1) and (2).

[0019]

(Equation 1)

[0020]

(Equation 2) Qa: Return sludge flow rate before detection of the first ORP inflection point r: Decrease coefficient of return sludge flow rate after detection of the first ORP inflection point (0 to 1) Tds: Setting of second ORP meter inflection point detection time Value (= one cycle of aeration and stirring) Tgs: Set value of first ORP meter bending point detection time Q: Set value of returned sludge flow rate Qb: Returned sludge flow rate after detection of first ORP bending point

As a result, the total value Qt of the returned sludge sent in one cycle is given by the following equation (3), and the total value of the returned sludge is a constant value Tds irrespective of the reduction coefficient r of the returned sludge flow rate.
Q.

[0022]

(Equation 3) This calculation is performed by the control device 9. The return sludge can be adjusted according to the following concept. When the return of nitrate nitrogen is small, such as under normal temperature conditions, the operation is performed with the set return sludge amount Q. At this time, a nitrate nitrogen upper limit concentration C that inhibits phosphorus release is previously experimentally obtained,
When the nitrate nitrogen becomes equal to or more than the value C in the treated water quality analysis, the return sludge flow control based on the calculations of the above equations (1) and (2) is performed by the return sludge pump and the return sludge flow meter 10. And the control device. When the treated water quality analysis falls below the upper limit concentration C, the operation is returned to the normal operation of the returned sludge flow rate Q.

In the return flow rate setting calculation, the set value Tgs of the first ORP meter bending point detection time and the set value Tds of the second ORP meter bending point detection time are used.
Since the RP inflection point may be detected with a slight deviation due to the inflow load, the amount of sludge returned in one cycle may slightly deviate from the set value, but the two-tank intermittent aeration method is described in the prior art. As described above, since the operation control is performed so as to approach the detection time set value of each ORP inflection point, the deviation from the set value is very small, so that the returned sludge amount almost as set can be secured.

[0024]

The operation control method of the two-tank intermittent aeration method according to the present invention has been described above. Conventionally, when the returned activated sludge contains a large amount of nitrate nitrogen, such as when operating under a low water temperature condition of 15 ° C. or lower, there has been a problem that the removal of phosphorus is reduced due to inhibition of phosphorus release. . The method of the present invention has been made to address this problem and has the following advantages.

According to the method of the present invention, after performing aeration for a predetermined time in a first aeration tank in which a first ORP meter is installed, the process shifts to a stirring step, and the first first aeration tank appears in the first aeration tank. After the ORP inflection point of the ORP meter is detected, that is, the amount of activated sludge returned to the first aeration tank is reduced in a step in which phosphorus release proceeds. As a result, the inhibition of phosphorus release due to returned nitrate nitrogen is reduced, the deterioration of the phosphorus removal rate can be prevented, and a stable and high phosphorus removal rate can be maintained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a main configuration of a sewage treatment apparatus to which a control method according to the present invention is applied.

FIG. 2 is a schematic diagram showing a main configuration of a sewage treatment apparatus to which a control method of an intermittent aeration method applied by the present inventors is applied.

FIG. 3 shows changes in ORP of a first aeration tank and a second aeration tank in a control method of an intermittent aeration method which the present inventors filed,
(A) ORP of the first aeration tank, (b) O of the second aeration tank
FIG. 4 is a relationship diagram of the RP with respect to the passage of time.

[Explanation of symbols]

 1: sewage 2a: first aeration tank 2b: second aeration tank 3: treated water 4: final sedimentation basin 5: return sludge pump 6a: first ORP meter 6b: second ORP meter 7a: first aeration blower 7b : Second aeration blower 8a: First stirring pump 8b: Second stirring pump 9: Control device 10: Return sludge flow meter

Continued on front page F term (reference) 4D028 AA08 BB07 BC18 BC26 BD08 BD10 CA11 CA12 CC11 CD00 CD01 CD02 4D040 BB07 BB22 BB33 BB63 BB66 BB73 BB91 BB92

Claims (2)

[Claims] A first aeration tank provided with a first ORP meter;
A second aeration tank connected downstream of the second ORP meter and having a second ORP meter installed therein, a final sedimentation basin connected downstream of the second sedimentation tank, a control device, a return sludge pump, a return sludge flow meter, and an aeration blower were provided. In the sewage treatment device, the wastewater is allowed to flow into the first aeration tank,
In the two aeration tanks, an aerobic state in which aeration is performed, and an anaerobic state in which aeration is stopped and stirring is performed alternately and repeatedly, and then the treated water is discharged from the final sedimentation basin. The activated sludge settled and accumulated in the sludge is returned to the first aeration tank and extracted as surplus sludge, and the intermittent aeration-type activated sludge method for removing nitrogen and phosphorus in the wastewater is used. The amount of activated sludge returned to
From the start time of the aeration step in the first aeration tank to the first
The adjustment is made according to conditions such as temperature between the detection time of the ORP bending point of the ORP meter and the detection time of the ORP bending point of the first ORP meter in the stirring process to the end time of the stirring process. Control method of intermittent aeration activated sludge method. 2. The method for adjusting the amount of activated sludge returned from the final sedimentation tank to the first aeration tank according to claim 1, wherein the operation is performed at a preset amount of returned sludge under normal temperature conditions. On the other hand, under conditions such as low water temperature conditions in which the return of nitrate nitrogen that hinders phosphorus release becomes large, the time from the detection of the ORP inflection point of the first ORP meter to the end of the stirring in the stirring of the first aeration tank may be reduced. The return sludge flow rate is reduced at a fixed rate with respect to a preset return sludge flow rate, and the ORP inflection point of the first ORP meter in the stirring step from the start time of the aeration step in the first aeration tank. A method for controlling an intermittently aerated activated sludge method, wherein the return sludge flow rate is increased until the detection time so that the amount of returned sludge in one cycle is equal to a predetermined amount as a whole.