JPH0411279B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a biological denitrification process, and particularly to a method of controlling organic carbon supply suitable for achieving a good denitrification state.

[Conventional technology]

In closed water bodies, water pollution due to eutrophication is significant and has become a major social problem. Nitrogen compounds in wastewater are considered to be a contributing factor to eutrophication, and their removal is being considered.

The most common method for removing nitrogen at sewage treatment plants is biological denitrification, which utilizes the biological nitrogen cycle. The process configuration of this denitrification method is characterized by having two microbial reactors with different management conditions. One is called a nitrification tank, and has the role of oxidizing NH ₃ -N in wastewater and NH ₃ -N, which is converted from organic nitrogen through the catabolic metabolism of oxidizing bacteria, into NO ₃ -N by nitrifying bacteria.

NH ₄ ⁺ +2O ₂ →NO ₃ ^- +H ₂ O + 2H ⁺ ...(1) The other is called the denitrification tank and is the amount of water produced in the nitrification tank.
It has the function of reducing NO ₃ -N to N ₂ gas using denitrifying bacteria.

2NO ₃ ^- +5(H ₂ )→N ₂ +4H ₂ O+2OH ^- ...(2) In formula (2), H ₂ required as a reducing agent is generally donated from nitrogen-free organic carbon such as methanol. be done. Therefore, the denitrification tank requires additional organic carbon.

In such a biological denitrification process, proper management of these two microbial reactors is extremely important from the viewpoint of water quality and economy. In particular, in denitrification tanks, it is important to manage the amount of organic carbon supplied. If the amount of organic carbon supplied is small, denitrification will be insufficient and water quality will deteriorate; if it is in excess, the residue will become a source of organic matter, which will not only deteriorate water quality but also become uneconomical. Therefore, adequate supply of organic carbon has become an operational issue.

To solve this problem, it is necessary to supply new organic carbon in accordance with the nitrate nitrogen concentration and organic carbon concentration that represent the denitrification state. However, since no measuring instruments have been developed to measure these denitrification tank management indicators online, no organic carbon control device using a direct detection method has been proposed. Therefore, at present, we have no choice but to rely on a fixed-quantity injection method that constantly supplies a constant amount of organic carbon. In general, the amount of nitrogen flowing into sewage treatment plants changes greatly from moment to moment, reflecting the life cycle of humans. If a fixed amount of organic carbon is injected in response to such an amount of inflowing nitrogen, an excess or deficiency of organic carbon will inevitably occur, and water quality will inevitably deteriorate. In addition, according to the conventional method, the amount of nitrogen gas generated in the denitrification reaction and the formula (1)
An organic carbon control method has been proposed that uses the amount of alkaline agent required to neutralize H ⁺ generated during the nitrification reaction as an index. However, it is difficult to accurately estimate the nitrate nitrogen concentration and organic carbon concentration in the denitrification tank from these indicators, so this cannot be considered a practical control method. As described above, it is impossible to appropriately control organic carbon corresponding to the denitrification state with the conventionally proposed methods.

[Problem to be solved by the invention]

An object of the present invention is to provide an organic carbon injection control method and apparatus that can accurately represent the denitrification state and is not restricted by process configuration.

[Means to solve the problem]

The present invention provides a biological denitrification method in which nitrate nitrogen or nitrite nitrogen in wastewater is reduced to nitrogen gas in the presence of organic carbon. The PH relationship is set in advance, the PH value and oxidation-reduction potential in the mixed liquid during the denitrification process are detected, and the preset oxidation-reduction potential target value is corrected based on the PH value. The present invention relates to a biological denitrification method characterized in that the amount of organic carbon supplied is adjusted according to the difference between a potential target value and the detected redox potential value.

〔Example〕

The present inventors have experimentally discovered that the redox potential is effective as an indicator of the state of denitrification treatment, and that the redox potential is correlated with PH, which led to the present invention. Ivy. The basic principle of the present invention based on the experiment will be explained below.

The treatment status of the denitrification tank is _NO
It can be expressed by the remaining amount of organic matter including N (generic term for NO ₃ -N and NO ₂ -N) and newly supplied organic carbon. FIG. 1 shows the results of measuring the relationship between the denitrification state and the oxidation-reduction potential when a steady state was reached by varying the inflow water amount, inflow nitrogen concentration, and organic carbon injection amount. In the experiment, the pH was set at 7.0, ammonium chloride, the nitrogen source, was nitrified and then led to a denitrification tank, and methanol was used as the organic carbon source. From this figure, it can be seen that if the residual organic carbon increases, that is, methanol is excessive, the redox potential decreases,
It can be seen that when NO _x -N increases, that is, methanol becomes insufficient, the redox potential tends to increase, and there is a clear correlation between the two. Furthermore, the presence of a minimum amount of organic carbon is a condition for achieving complete denitrification. According to this result, it is possible to find a range of redox potential that forms a good denitrification state in which methanol is not excessive and NO _x -N is at a low concentration. This range is approximately -100m
Exists between V and -200mV. The results shown in FIG. 1 show that the redox potential can be applied as a control index for methanol, which is a reducing agent.

On the other hand, the redox potential is expected to be influenced not only by NO _x -N and organic carbon but also by many factors. As a result of experimental studies on various factors, it was found that the influence of PH cannot be ignored, as shown in Figure 2. In FIG. 2, the amount of change in oxidation-reduction potential is expressed as the amount of deviation from the reference value, with the value at PH=7.0 as the reference. From this figure, the redox potential is linearly correlated with PH, and the correlation coefficient is approximately −60
It was mV/PH.

The above results can be explained theoretically. The denitrification reaction mediated by microorganisms can be divided into two stages. 1st
The step is an oxidation reaction of organic carbon, and when methanol is used as the organic carbon, the following equation is obtained.

0.5CH ₃ OH=0.5CH ₂ O+H ⁺ +e:K ₁ ...(3) 0.5CH ₂ O+0.5H ₂ O =0.5CO ₂ +2H ⁺ +2e:K ₂ ...(4) Redox potential E of these reactions Calculating _H yields the following formula.

E _H =2.303/3FR・T {logK ₁ +logK ₂ +log(H ⁺ ) ³ −log(CH ₃ OH) ^0.5 } ...(5) Here, K ₁ and K ₂ are equilibrium constants, F is Faraday's constant, R is the gas constant and T is the absolute temperature. Second
The step is the reduction of NO _X −N, most of _which is
Since NO ₃ -N, this reaction formula is: 0.2NO ₃ ^- +1.2H ⁺ +e = 0.1N ₂ +0.6H ₂ O; K ₃ ...(6) The redox potential E _H of this reaction is as follows. The formula becomes

E _H = 2.303/1.2FRT {logK ₃ + log (H ⁺ ) ^1.2 + log (NO ₃ ^- ) ^0.2 } ...(7) Calculate the redox potential E _H of the denitrification reaction from equation (5) and equation (7) And E _H = 2.303/FRT {(logK-PH) + 1/nlog (NO ₃ ^- )/(CH ₃ OH)} ...(8) Here, logK = 1/2 {1/3 logK ₁ + logK ₂ + 1 /1.2logK ₃ ), n is the reaction coefficient. (8) shows that the redox potential in the denitrification reaction depends on the ratio of NO ₃ -N to methanol and decreases as the pH increases. This relationship theoretically supports FIGS. 1 and 2. Figure 3 summarizes the above results. In other words, this indicates that the range of redox potential when a good denitrifying state is formed changes in correlation with pH.

By the way, it is known that the flow rate and water quality of wastewater vary greatly even during a day. FIG. 4 shows an example. This change in the inflow nitrogen concentration becomes an influencing factor for H ⁺ and OH ⁻ production in equations (1) and (2), and changes the pH of the nitrification tank and denitrification tank. Therefore, we attempted an experiment where the inflow water conditions were the same as in Figure 4, the pH of the nitrification tank was adjusted to 7.0, and methanol, which was capable of complete denitrification, was supplied to the denitrification tank. As a result, the pH of the denitrification tank was found to fluctuate, which is expected to be affected by changes in the amount of denitrification.

In order to maintain good denitrification conditions in such a denitrification tank, according to equation (8), the pH should be adjusted to a constant value, and then the amount of organic carbon supplied should be adjusted to maintain the redox potential at a constant value. One possible method is to control the

A conventional method for solving this problem is to adjust the pH to a constant value and then adjust the amount of organic carbon supplied so as to maintain the redox potential at a constant value. By the way, since the concentration of NH ₃ -N and organic nitrogen in wastewater fluctuates over time, the NO ₃ -N produced
It is known that the concentration also changes in conjunction. This means that the amount of OH ^-, a by-product during denitrification, changes, and the PH value changes accordingly.
Therefore, in order to maintain the pH at a constant value in such a denitrification tank, it is necessary to inject an alkaline agent or acid depending on the state of the denitrification reaction. This is 1
This is a complicated control method in which two manipulated variables are adjusted using one index, which is not practical. Furthermore, in the nitrification tank, in order to prevent the pH from decreasing due to H ⁺ production, as shown in equation (1), an alkaline agent is usually injected to adjust the pH. In addition to this, a denitrification tank
Adjusting the pH will increase operating costs and is not a good idea. Furthermore, it is produced during the denitrification reaction.
In the denitrification tank-nitrification tank method, which uses OH ^- as a neutralizing agent for H ⁺ generated during the nitrification reaction, the
The amount of OH ^- produced is suppressed and the amount of alkaline agent in the nitrification tank increases, leading to a rise in operating costs.

Therefore, without using new drugs, and
Organic carbon supply control methods that are independent of process configuration are important in wastewater treatment plants. An embodiment of the present invention is shown in FIG. In Figure 5, in denitrification tank 1
Inflow water 6 containing NO _x -N and denitrifying bacteria is introduced, and a fluid state is maintained by a stirring device 4 .

On the other hand, organic carbon 8, which is a reducing agent, is injected from the storage tank 2 into the denitrification tank 1 via the supply device 3. The effluent water 7 that has completed the denitrification reaction becomes a mixed liquid from which NO _x -N has been removed, and is led to the next step. In such a denitrification tank 1, an oxidation-reduction potential meter 11 and a PH meter 12 are installed.
are installed to detect the redox potential and PH. Among these, the PH detection value p is input to the arithmetic circuit 14,
A redox potential target value e ^* corresponding to p is set. This setting method first sets the oxidation-reduction potential reference value e ₀ that forms a good denitrification state at the PH reference value p ₀ .
is set in advance, and a relational expression between the PH deviation and the amount of change in redox potential Δe is given. In equation (9), Δe=f(p−p ₀ ) (9) If p>p ₀ , Δe becomes negative, and if p<p ₀ , Δe takes a positive value. Using the calculated Δe,
The oxidation-reduction potential reference value e ^* is set by the following formula.

e ^* = e ₀ + Δe ...(10) Therefore, e ^* takes a value higher than e ₀ if p>p ₀ , and takes a lower value if p<p ₀ , ensuring a good denitrification state. The value of the oxidation-reduction potential to be formed is always set. The oxidation-reduction potential target value e ^* output from the arithmetic circuit 14 is input to the comparison circuit 15. The comparison circuit 15 compares the actually measured oxidation-reduction potential detection value e and the target value e ^* , and outputs the deviation ε ε=ee−e ^* to the adjustment circuit 16. The adjustment circuit 16 operates the supply device 3 according to the deviation ε to adjust the amount of organic carbon supplied. The organic carbon supply amount decreases if the deviation ε is negative, that is, the target value e ^* is lower than the detected value e, and increases if the deviation is negative.

In this way, by performing the setting operation of the oxidation-reduction potential target value based on the PH detection value, a good denitrification state is always established.

〔Effect of the invention〕

According to the present invention, the denitrification state can be accurately expressed,
In addition, it becomes possible to control organic carbon injection without being restricted by the process configuration.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the redox potential and the denitrification state in the denitrification tank, Figure 2 is a characteristic diagram that experimentally determined the change in redox potential with respect to PH, and Figure 3 is a diagram showing the relationship between the redox potential and the denitrification state in the denitrification tank. FIG. 4 is a characteristic diagram showing the relationship between the oxidation-reduction potential and PH, FIG. 4 is a diagram showing the daily fluctuation pattern of the inflow water flow rate and nitrogen concentration, and FIG. 5 is a block diagram of an embodiment of the present invention. 1...Denitrification tank, 3...Organic carbon supply device, 6...
... Denitrification tank inflow water, 11 ... Oxidation-reduction potentiometer, 12
...PH meter, 14...Arithmetic circuit, 15...Comparison circuit, 16...Adjustment circuit.

Claims (1)

[Claims] 1. In a biological denitrification method in which nitrate nitrogen or nitrite nitrogen in wastewater is reduced to nitrogen gas in the presence of organic carbon, oxidation within a range that forms a good denitrification state The relationship between reduction potential and PH is set in advance, the PH value and oxidation-reduction potential in the mixed liquid during the denitrification process are detected, and the preset oxidation-reduction potential target value is corrected based on the PH value. A biological denitrification method, characterized in that the amount of organic carbon supplied is adjusted according to the difference between a corrected redox potential target value and the detected redox potential value. 2. Predetermining a PH reference value for the oxidation-reduction potential target value, determining the oxidation-reduction potential change amount from the deviation between this PH reference value and the PH value, and correcting the oxidation-reduction potential target value using this oxidation-reduction potential change amount. The biological denitrification method according to claim 1, characterized in that: 3 Set an allowable PH range with respect to the PH reference value, determine whether the detected PH value is within the PH allowable range, and if the detected PH value deviates from this allowable range, set the oxidation-reduction potential target. The biological denitrification method according to claim 1, characterized in that the value is corrected.