JPH031080B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a biological denitrification process, and particularly to organic carbon source supply control that appropriately maintains the organic carbon source necessary for the denitrification reaction and provides a good denitrification treatment solution. Regarding the method.

Biological denitrification process produces ammonia nitrogen (hereinafter referred to as _NH3 -N) through the action of nitrifying bacteria.
a nitrification tank that oxidizes the nitrogen into nitrite nitrogen (NO ₂ -N) or nitrate nitrogen (NO ₃ -N) under aerobic conditions;
It has a denitrification tank that reduces NO ₃ -N and NO ₂ -N (hereinafter collectively referred to as NO _x -N) to nitrogen gas under anaerobic conditions using denitrifying bacteria. The biological reaction in both tanks can be expressed as a chemical formula as follows.

Nitrification tank NH ₄ ⁺ +2O ₂ →NO ₃ ^- +H ₂ O+2H ⁺ (1) Denitrification tank 2NO ₃ ^- 5 (H ₂ ) → N ₂ +4H ₂ O+2OH ^- (2) Hydrogen as a reducing agent for NO _x -N in the denitrification tank A donor is required and an organic carbon source is often used. However, if the amount of organic carbon source supplied is excessive, it is not only uneconomical but also causes deterioration of treated water due to an increase in residual organic matter. In addition, insufficient supply will not only reduce denitrification efficiency but also cause residual
Sludge floats up due to NO _x -N, which deteriorates the quality of treated water. Therefore, proper regulation of the organic carbon source in the denitrification tank is an important operating factor in the biological denitrification process.

Traditionally, organic carbon source feed flows into a denitrification tank
Proportional to NO _x -N amount. Alternatively, there are methods to measure residual organic carbon sources, but none of these methods has been put into practical use because online measurement of NO _x -N or organic carbon sources is impossible.

The present invention provides an organic carbon source supply control method that improves processing efficiency and reduces operating costs by appropriately supplying an organic carbon source.

The present inventors determined the oxidation-reduction potential (ORP) in the denitrification tank.
It was clarified that the method appropriately expresses the NO _x -N and organic matter concentration (COD _cr ) in the liquid. Furthermore, since the daily fluctuations in the inflow nitrogen concentration were small, it was found that the nitrogen load was almost equal to the fluctuation in the amount of inflow water. For this reason, two organic carbon supply parts are provided, the first injection part is proportional to the flow rate, and the second injection part is made to correspond to the amount of residual NO _x -N determined from ORP, and _the excess COD
was detected using the outflow ORP, and it was revealed that both could be maintained below the allowable value.

Next, the content of the present invention will be explained in detail. First, the basic principle that made it possible to carry out the present invention will be explained.

Figure 1 shows disturbance conditions at two treatment plants exhibiting typical fluctuation patterns of inflow water volume. A is a merging ceremony.
The amount of inflow water at both the treatment plant and the separate B treatment plant fluctuates greatly from day to day. On the other hand, in the inflow water
The NH ₃ -N concentration is stable with almost no fluctuation at both treatment plants. This suggests that if NH ₃ -N is completely nitrified in the nitrification tank located before the denitrification tank, the fluctuation in the concentration of NO _x -N flowing into the denitrification tank is also small. Therefore, the inflow to the denitrification tank
Since the amount of NO _x -N is proportional to the amount of inflow water, it can be seen that the amount of organic carbon source supplied with respect to the amount of NO _x -N, which accounts for most of the organic carbon source consumption, can be controlled according to the amount of inflow water.

On the other hand, the present inventors conducted a denitrification experiment and found that the
The relationship between NO _x −N and COD _cr with respect to ORP was clarified. FIG. 2 shows the relationship between the residual NO _x -N concentration and ORP when the residual COD _cr is 20 mg/or less.
When ORP is −100 mV or less, the change in NO _x -N concentration is warm, indicating that NO _x -N concentration can be estimated from ORP. Figure 3 shows that the residual NO _x −N is 1
This is the relationship between residual COD _cr and ORP at mg/or less. In this way, residual COD _cr also correlates with ORP and concentration can be estimated.

From the above, the organic carbon supply amount has the function of determining the denitrification state by comparing it with the inflow water amount, which expresses the amount of inflow NO _x -N, which is the main consumption factor of organic carbon.
Effective denitrification can be performed by correcting with ORP.

An embodiment of the present invention will be described with reference to FIG. FIG. 4 shows an example of a general biological denitrification process in which a nitrification tank 1, a denitrification tank 2, and a settling tank 3 are in series. Inflow water 5 containing NH ₃ -N and return sludge 7 containing nitrifying bacteria and denitrifying bacteria flow into the nitrification tank 1, and NH ₃ -N is nitrified to NO _x -N. NO _x
The nitrifying solution 6 containing -N and denitrifying bacteria is supplied to the head of the denitrifying tank 2. Organic carbon serving as a reducing agent is applied from two different locations. First injection pipe 8
is located at the head of the denitrification tank 2, the second injection pipe 9 is located at the rear downstream of the first injection pipe 8, and the supply device 11 is located at the rear of the first injection pipe 8.
and 12 to adjust the amount of organic carbon. The feeding operation is performed as follows. First, the organic carbon supply amount C ₁ from the first injection pipe 8 is determined by the actual measurement value q ₁ of the flow meter 13 installed in the middle of the inflow water 5 pipe.
and calculate the manipulated variable at the current time using equation (3).

C ₁ =k ₁・q ₁ (3) Here, the proportionality coefficient k ₁ is in the range of 0.02 to 0.12Kg/m ³ when organic carbon is methanol, and is NH ₃ −N based on the experience of each treatment plant. Concentration and nitrification efficiency are the setting criteria. Equation (3) shows that the inflow NO _x − to the denitrification tank 2 changes in the same way as the inflow water 5 changes.
This shows that the amount of organic carbon supplied corresponds to the amount of N. The control circuit 21 further calculates the deviation ΔC ₁ between the current and previous manipulated variables and outputs it to the adjustment circuit 26 . The adjustment circuit 26 adjusts the supply device 11 according to the deviation ΔC ₁ to supply an amount of organic carbon corresponding to the amount of inflow NO _x -N. On the other hand, the amount _C2 of organic carbon supplied from the second injection pipe 9 is determined as follows. First, the signal of the actual measurement value p ₁ of the ORP meter 15 installed just before the second injection pipe 9 is sent to the arithmetic circuit 2.
2 and is incorporated into the circuit, e.g.
NO _x −N concentration corresponding to p ₁ according to the characteristic diagram in Figure 2
Estimate n _x . Furthermore, the arithmetic circuit 23 integrates the actual measurement value q ₂ signal of the flow meter 14 installed in the middle of the nitrification liquid 6 pipe and the NO _{x −N} concentration n
Calculate the immediately preceding residual NO _x −N amount N _x . Residual NO _x
-N amount _Nx , nitrification liquid flow rate _q2 , and signals of the actual measurement value _p2 of the ORP meter 16 installed at the outlet of the denitrification tank 2 are input to the control circuit 24, and the operation shown in FIG. 5 is performed to supply Obtain the quantity C ₂ . Figure 5 shows the amount of residual NO _x -N
The main loop is the manipulated variable C' corresponding to N _x , and the adjustment loop is the organic carbon amount C'' corresponding to the excess COD _cr amount at the outflow section.The manipulated variable C' is the amount of residual NO _x -N.
The amount required to reduce N _x is given by equation (4).

C′=k ₂・N _x (4) Here, k ₂ is a proportionality coefficient that changes depending on the type of organic carbon. The excess amount C'' is obtained by calculating the deviation ΔP ₂ between the outflow ORP lower limit value P ^* and the actual measured value P ₂ using the following formula, and calculating the residual amount at the value of ΔP ₂ .
Estimate the COD _cr concentration C _r . The relationship of COD _cr to ΔP ₂ ΔP ₂ =P ^* −P ₂ (5) is shown in FIG. 6 when the ORP lower limit value in FIG. 3 is set to, for example, −100 mV.

Therefore, if the measured value _P2 is less than the lower limit P ^* , it is determined that the amount of organic carbon from the second injection pipe 9 onward is excessive. Incorporating this relationship into Figure 5, the residual from ΔP ₂
Estimate COD _cr . Organic carbon generally has a proportional relationship with COD _cr , and the residual COD _cr concentration C _r and nitrification liquid flow rate
Excess organic carbon amount from residual COD _cr amount determined by the product of q ₂
C″ is determined by the following formula. Here, the proportionality coefficient k ₃ is the phase C _″ of _the organic carbon source used and COD _cr ( ₆ ). The difference between the calculated manipulated variable C′ and excess amount C″ is given as the supply amount C ₂ , and the control circuit 2
Output from 4. The regulating circuit 25 is, for example, a C ₂ =C'-C'' PI controller, and regulates the supply device 12 in accordance with the controlled variable C ₂ .

In this example, the manipulated variable C ₁ of the first injection part is the inflow water flow rate q ₁ , and the manipulated variable C ₂ of the second injection part is the nitrifying liquid flow rate q ₂ which is the total of the inflow water flow rate q ₁ and the return sludge flow rate q _r was targeted for estimation, but since the returned sludge contained almost no NO _x -N in the target process, the amount of NO _x -N flowing into denitrification tank 2 corresponded to the flow rate of inflow water q ₁ . value, and the residual amount in denitrification tank 2
Since the detected NO _x -N concentration is the value in the mixed liquid passing through the tank, the NO _x -N amount is determined by the nitrifying liquid flow rate q ₂
must be targeted.

On the other hand, it is expected that the amount of organic carbon from the first injection point will be excessive and the _ORP value just before the second injection point will already be below the lower limit. Since it is also consumed by dissolved oxygen and the microorganisms themselves, there is no risk of the amount becoming excessive if the operation corresponds to the amount of inflow NO _x -N.

By the above-described operations, organic carbon can be supplied with high precision in accordance with the process conditions, and denitrification treatment can be performed well and organic carbon can be saved.

According to the present invention, it is possible to supply organic carbon corresponding to the amount of nitrogen load without using any means for directly measuring NO _x -N. In addition, by providing an organic carbon supply point, the delay in retention time can be overcome, and by performing operations to suppress the organic carbon from the second injection point, the amount of residual _NO This provides a denitrification solution with low concentrations of NO _x -N and organic matter, contributing to maintaining denitrification efficiency and reducing operating costs.

[Brief explanation of the drawing]

Figure 1 is a time-series characteristic diagram of inflow water volume and nitrogen concentration at an actual treatment plant, Figure 2 is a characteristic diagram of redox potential and residual oxidizing nitrogen concentration, and Figure 3 is a characteristic diagram of redox potential and residual organic matter concentration. , FIG. 4 is a block diagram showing an embodiment of the present invention, FIG. 5 is a block diagram showing a method of supplying organic carbon from the second injection point of the present invention, and FIG. 6 is a diagram showing excess organic carbon with respect to redox potential. FIG. 3 is a characteristic diagram showing concentration. 1... Nitrification tank, 2... Denitrification tank, 5... Inflow water,
6... Nitrification liquid, 8, 9... Injection tube, 11, 12...
...Supply device, 13, 14...Flow meter, 15, 16
... ORP meter, 21, 24 ... Control circuit, 22,
23... Arithmetic circuit.

Claims (1)

[Claims] 1. A nitrification tank that oxidizes nitrogen compounds in influent wastewater under aerobic conditions, and a dehydration tank that reduces nitrate nitrogen or nitrite nitrogen in the nitrified liquid flowing out from this nitrification tank to nitrogen gas in the presence of organic carbon. In a biological denitrification process having a nitrification tank, a first injection part is provided at the head of the denitrification tank and a second injection part is provided downstream thereof, and the flow rate of the inflow wastewater and the nitrification solution is detected. means for detecting an oxidation-reduction potential at the outflow section of the denitrification tank and immediately before the second injection section; The amount of organic carbon supplied from the second injection section was made proportional to the product of the residual nitrate nitrogen concentration or the estimated nitrite nitrogen concentration obtained from the redox potential measurement immediately before the second injection section and the nitrifying solution flow rate. A method for controlling a biological denitrification process, comprising correcting the amount of organic carbon supplied by the deviation between the actual value of the redox potential of the outflow section and a preset lower limit value when the deviation is negative.