JPS6319237B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved chlorine injection control method for water purification plants, and more particularly to a chlorine injection control method suitable for maintaining the residual chlorine concentration in treated water at a target value.

At water purification plants, chlorine injection treatment is performed for the purpose of removing organic matter, bacteria, etc., and further removing iron, manganese, etc. in raw water taken from the water. The safety of the quality of the treated water is ensured by allowing a predetermined concentration of chlorine to remain in the treated water after chlorine injection.

By the way, in order to perform this chlorine injection treatment satisfactorily, it is necessary to appropriately control the amount of chlorine injection in accordance with the raw water flow rate and raw water quality to be treated. Generally, the amount of chlorine injection is controlled by controlling the residual chlorine concentration (hereinafter simply referred to as residual salt) in the treated water at the outlet of the settling tank or at the outlet of the filter tank at the time when the reaction between chlorine and the oxidized substance is almost completed. is controlled so that it becomes a predetermined value. More specifically, the standard chlorine injection rate is set by adding the chlorine demand amount of raw water, the target value of residual salt, etc., and chlorine is injected at a chlorine injection amount according to the raw water flow rate. After chlorine injection, the residual salt in the treated water at the settling tank outlet (or filter pond outlet) is measured, and the chlorine injection rate is corrected according to the deviation between the measured value and the target value of residual salt at the measurement point. method is being adopted. However, since the residence time from the chlorine injection point to the residual salt measurement point is about 2 to 4 hours,
There is a delay of residence time after correcting the chlorine injection rate until the result is known. Therefore, in such a method, if the amount of chlorine consumed changes due to changes in raw water quality, etc., the residual salt at the target point will also change, resulting in a significant deviation from the target residual salt. Not only is water with a strong chlorine odor with residual salt exceeding the target value provided to consumers, but also the amount of chemicals consumed increases, making it uneconomical. On the other hand, if the residual salt falls below the target value, there is a strong possibility that bacteria etc. will be activated, making it insufficient to ensure the safety of the treated water quality.

The present invention has been made in view of the above-mentioned problems, and its purpose is to provide a chlorine injection control method for a water purification plant that can maintain the residual salt in treated water at a target value with high accuracy.

First, the basic idea of the present invention will be explained.

To solve the problem of residence time, so-called time lag, as soon as possible after chlorine injection,
For example, residual salt may be detected at the outlet of a chemical mixing pond, and the chlorine injection rate may be corrected according to the deviation from the target value at the detection point. By the way, the chlorine consumption characteristics by oxidizable substances exhibit a tendency as shown in FIG. It takes approximately 30 to 60 minutes for the oxidation reaction between chlorine and the oxidized substance to complete, after which the amount of residual salt becomes constant. Even if the residual salt is measured at the outlet of the chemical mixing pond or at a point earlier than that, the reaction between chlorine and oxidized substances is still in progress due to the short residence time, and the residual salt is not stable. No. For this reason, even if residual salt is detected early and the chlorine injection rate is corrected according to the deviation from the target value in order to eliminate the time delay problem, there is an element of uncertainty in the deviation. Therefore, highly accurate correction cannot be achieved. After all, simply detecting residual salt earlier after chlorine injection will cause control to become unstable.

In view of the above-mentioned problems, the inventors of the present invention conducted several experiments and as a result of repeated studies, they found that the reaction between the oxidized substance and chlorine after the injection of chlorine until the residual salt becomes constant is We found that it depends on the PH of the raw water at the time. In other words, as shown in the experimental results in Figure 2, when the pH of the raw water changes, the degree of reaction between the oxidized material and chlorine changes, and even if the chlorine injection rate remains the same, residual salt remains at the stage where the reaction is in progress. to differ greatly. When the chlorine consumption rate is determined from the relationship between the chlorine injection rate and the residual salt, as shown in FIG. 3, it is found that the chlorine consumption rate peaks at a predetermined pH and then decreases. This means that there is an optimum pH for the reaction between the oxidized substance and chlorine, and when the pH deviates from the optimum pH, the reaction tends to slow down. Therefore, as shown in Figure 4, even if residual salt is detected at the turning point of the contact time (residence time) T _o and compared with the target residual salt value K _p at that point, the PH will not change due to changes in raw water quality. PHw to PHv or PHx
If it changes, the residual salt will also change, so the target value
The deviation (±ΔRCL) from K _p will be inaccurate.

The present invention is based on the above-mentioned study, and the target value of residual salt is
The chlorine injection rate is corrected based on the deviation between the corrected target value and the residual salt after correction based on the pH. By doing so, the present invention solves the problem of early detection of residual salt,
This makes it possible to shorten the time delay.

An embodiment of the present invention will be described below.
The same reference numerals are used for the same components as those described above.

Figure 6 shows the treatment process flow of a water treatment plant, where 1 is the receiving well, 2 is the chemical mixing pond, and in this mixing pond,
A flocculant (not shown) such as PAC is injected and mixed with the raw water by the stirrer 3. In addition,
If necessary, an alkaline agent is injected before the flocculant is injected. 4 is a flocculation pond into which flocculated water containing flocs (not shown) from the chemical mixing pond 2 flows. At this point, the flocculator 5 performs slow stirring. 6 is a sedimentation tank where the flocs are sedimented and separated. 7 is Kagaike. 1 to 7 constitute the treatment process of a general water purification plant. 8 is a sampling pump, and raw water RW sampled by this pump 8 is sent to a chlorine demand meter 9. Chlorine demand meter 9 is the chlorine demand of raw water RW
K ₂ is measured and input to the injection rate calculation circuit 10. still,
The chlorine requirement K ₂ may be determined and given by manual analysis or the like. The injection rate calculation circuit 10 stores a target value K ₁ of residual salt at the outlet of the settling tank 6 or the outlet of the filter tank 7, etc.
is also input to calculate the basic chlorine injection rate CL. 11
is an adder circuit which inputs the basic chlorine injection rate CL and a correction value ΔRCL to be described later to obtain the chlorine injection rate DCL, which is added to the multiplier circuit 12. The multiplier circuit 12 multiplies the raw water flow rate F measured by the flow meter 13 by the chlorine injection rate DCL to obtain the multiplier chlorine injection amount FDCL. Reference numeral 14 denotes a chlorine injection machine, which is shown in a simplified manner, but injects chlorine into the raw water PW according to the chlorine injection amount FDCL.
In this embodiment, the water is injected into the landing well 1, but it goes without saying that the water may be poured before or after the water landing well 1, as long as it is before the flocculant and the alkaline agent are injected.

15 is a sampling pump that samples the treated water after chlorine has been injected. The point at which the treated water is sampled is not limited to the outlet of the chemical mixing pond 2, and may be any point after chlorine has been injected, for example, the inlet of the chemical mixing pond 2. 16 is a residual chlorine concentration meter that measures residual salt (hereinafter simply referred to as a residual salt meter).
Then, the treated water sampled by the sampling pump 15 is introduced. The residual salt meter 16 measures the residual salt RCL in the treated water after chlorine is injected. Residual salt total 16
The residual salt RCL in the treated water measured in Comparison circuit 2
It is input to 0.

Reference numeral 17 denotes a PH meter, into which the raw water RW is introduced by the sampling pump 8, and measures the PH of the raw water. The raw water PH measured by the PH meter is input to the consumption rate calculation circuit 18. The residence time T _o up to the residual salt RCL measurement point is input to the consumption rate calculation circuit 18 . Residence time
T _o is the time from injecting chlorine to measuring the residual salt RCL, and more specifically, the time from the chlorine injection point to the sampling point of the treated water, and the time from the sampling point to the residual salt meter 16. be. The consumption rate calculation circuit 18 calculates the raw water PH and residence time before time T _o
Calculate the chlorine consumption rate CCL based on T _o . FIG. 6 shows an example of the consumption rate calculation circuit 18, which calculates the chlorine consumption rate CCL in accordance with the measured raw water PH and the residence time T _o at that time. For example, if the residence time T _o is PH 7.5, the chlorine consumption rate characteristic curve
From the intersection with CLC, the chlorine consumption rate CCL becomes 50%.
The chlorine consumption rate characteristic curve shown in FIG. 6 may be obtained in advance by a jar test (water quality test), etc., and used as input. An example of a chlorine consumption rate characteristic curve is shown in FIG. This is a chlorine consumption rate characteristic curve determined by the present inventors through a jersey test. Reference numeral 19 denotes a residual salt command calculation circuit, into which the chlorine consumption rate CCL obtained by the consumption rate calculation circuit 18 and the chlorine injection rate DCLT before T _o time are input, and according to the following equation (1),
Find the chlorine command value _K3 .

K ₃ =DCLT−(DCLT×CCL) ……(1) Residual salt command value obtained by the residual salt command calculation circuit 19
K ₃ is applied to comparator circuit 20. Comparison circuit 20
is the residual salt command value _K3 and the actual residual salt value from the residual salt meter 16
RCL is compared with the polarity shown in the figure, and the deviation ΔRCL is output. This deviation ΔRCL is input to the addition circuit 11 to correct the chlorine injection rate CL.

Chlorine injection is controlled as described above, but if the raw water quality fluctuates and the pH changes, the reaction rate between the oxidized material and chlorine will differ, so the value of residual salt after chlorine injection will also differ. For this reason, the residual salt RCL
Assuming that the command value K ₀ of is fixed as before,
As shown in FIG. 4, the deviation ΔRCL between the residual salt RCL and the target value K _p becomes an inaccurate value due to a change in PH. By the way, in the present invention, the chlorine consumption rate is determined from the pH at the time of chlorine injection, and the command value K ₃ of residual salt after chlorine injection is determined from the relationship between the chlorine injection rate and the chlorine consumption rate at that time. . That is, the command value _K3 for residual salt is set taking into consideration that the residual salt after chlorine injection changes depending on the pH, and is set as the command value _K3 corresponding to the pH. Therefore, even if the raw water quality fluctuates, the PH changes, and the reaction rate between oxidized substances and chlorine changes, the command value K ₃ corresponding to the PH at that time changes.
Therefore, the deviation ΔRCL between the detected residual salt RCL and the command value _K3 at that time is accurate.

Therefore, according to the present invention, residual salt can be detected early and the chlorine injection rate can be corrected based on it, so that time delays can be shortened and chlorine injection control can be performed with even higher precision.

In the above-described embodiment, the command value of residual salt is corrected in accordance with the raw water PH, but when a flocculant and an alkaline agent are injected, the PH after chlorine injection changes further. Therefore, the PH of landing well water or mixing pond water after various chemicals are injected is measured and its PH is determined.
Of course, the residual salt command value may be corrected in accordance with the pH.

Further, although the amount of chlorine required is determined based on the quality of raw water, it is clear that the same effect can be achieved even if a fixed target value is provided.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram of residual chlorine, Figure 2 is a characteristic diagram of chlorine consumption, Figure 3 is a characteristic diagram of chlorine consumption rate, Figure 4 is a characteristic diagram showing the relationship between residual chlorine and PH, and Figure 5 is a characteristic diagram of chlorine consumption. FIG. 6, a block diagram showing one embodiment of the present invention, is a partially detailed view of FIG. 5. 1... Water landing well, 2... Chemical mixing pond, 10... Adder, 11... Arithmetic unit, 13... Flow meter, 16...
...Residual salt meter, 17...PH meter, 18...Consumption rate calculation circuit, 19...Residual salt command calculation circuit, 20...Comparison circuit.

Claims (1)

[Claims] 1 Detect the residual chlorine concentration in the treated water by injecting chlorine into the raw water at any point in the reaction process between the oxidizable substance and chlorine in the raw water, and inject chlorine so that the residual chlorine concentration at this detection point becomes the target value. The target value of the residual chlorine concentration is corrected according to the pH of the raw water, and the injection rate of chlorine to be injected into the raw water according to the deviation between the actual value of the residual chlorine concentration and the corrected target value of the residual chlorine concentration. A method for controlling chlorine injection in a water treatment plant, characterized by correcting.