JPS5851979A - Method and apparatus for controlling ph in water-purifying plant - Google Patents

Abstract

PURPOSE:To suppress the formation of trihalomethane, by adding an acid to raw water to lower pH below the first estimated value, injecting chlorine, and then injecting an alkali agent to raise pH above the second estimated value before the addition of a flocculant. CONSTITUTION:A pH value (e.g. pH below 5) is set by a pH-setting instrument 12 to enlarge a pasteurizing effect while suppressing the formation of trihalomethane in raw water. Said value and a value from a pH meter 11 for the raw water are inputted into an arithmetic circuit 13 for calculating an amount of sulfuric acid to be injected. On the basis of the calculation result, sulfuric acid is injected to adjust pH of the raw water in a water-collecting well 1. A fixed amount of chlorine is then injected from a chlorine injector 7. A pH value at the point of injecting a flocculant is calculated from the pH of the raw water, the amount of injected sulfuric acid and the amount of injected chlorine by an arithmetic circuit 15 for calculating an amount of an alkali agent to be injected. The alkali agent is injected from a means 10 in response to a difference between said calculated pH value and a pH value from an instrument 14 for setting pH of coagulated water.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a PH control method and a control device for a water purification plant, and is capable of suppressing the production of tri-lomethane during chlorine injection while ensuring the original water treatment effect of the water purification plant. The present invention relates to a method and apparatus for controlling PF1 in a water purification plant, which performs chlorine injection while controlling pH.

FIG. 1 is a block diagram showing an outline of conventional water purification plant equipment. In the figure, 1 is a water landing well, 2 is a mixing pond, 3 is a floc formation pond, 4 is a sedimentation pond, 5 is a filtration cell, 6 is a water purification pond,
7 is a chlorine injection device, and 8 is a flocculant injection device.

The raw water flooded from the secondary water source is led to the landing well 1, where it is sterilized by being added with chlorine by the chlorine injection device it7.

Further, in the mixing pond 2, a flocculant is injected from the flocculant injection bag '#8. As a result, the suspended matter in the water aggregates in the floc-forming pond 5 to form flocs, which are then settled and removed in the settling basin 4.

Further, suspended solids that are not removed in the sedimentation tank 4 are removed by a filter device 51 in a filter cell 5. As mentioned above,
Water from which turbidity has been removed is stored in a water purification pond 6 and distributed to consumers.

However, chlorine, which is used to remove bacteria contained in raw water and oxidize and remove metal ions such as iron and manganese, reacts with organic matter contained in water to produce trihalomethane.

Trilomethane is known to be toxic to the central nervous system, liver, kidneys, etc., and is also reported to be carcinogenic. Therefore, in the raw water treatment process at water purification plants, preventing the generation of trihalomethanes is an important issue.

To remove this trihalomethane, methods such as (1) filtration with activated carbon and (2) air aeration have been proposed. However, in order to implement these measures, a new pond is required.
The problem was that it was expensive to renovate the equipment.

An object of the present invention is to provide a water treatment plant that can perform chlorine injection without impairing the water treatment effect of the water treatment plant, that is, the sterilization effect and the flocculating starch effect, while suppressing the amount of trihalomethane generated by chlorine injection. An object of the present invention is to provide a control method and a control device.

The present invention focuses on the fact that the amount of trihalomethane produced by chlorine injection, the sterilization effect, and the flocculating starch effect of the flocculant are all influenced by the - of the raw water, and the - of the raw water is set to the planned value of 1.
By injecting chlorine after the cm adjustment, the amount of trihalomethane produced is lowered and the sterilization effect and the flocculating starch effect are increased.

First, Figure 51!2 shows the relationship between the Vl of raw water and the amount of trihalomethane generated when chlorine is added to it. The horizontal axis is the negative value, and the vertical axis is the amount of trihalomethane generated. According to this, the lower -4 of the raw water is, the more trihalomethane will be generated.
If the Fi is low and the pi of the raw water is less than 5, the amount of trihalomethane generated will hardly change.

Therefore, it can be seen that it is desirable to keep the pH of the raw water at 5 or less during chlorine injection from the viewpoint of suppressing the generation of trihalomethane.

Chlorine is converted into hypochlorous acid (HOI) by the following reaction in water.
O) and hypochlorite ion (010-), which is called free residual chlorine.

01* +HIO→HOI +HOIOHC,lo#H
+010- Next, Figure 6 shows water... and hypochlorous acid (HC) in water.
10) and the ratio of hypochlorite ion (010-). In the figure, the horizontal axis represents the negative value of water,
The vertical axis shows the ratio of hypochlorous acid and hypochlorite ions in water.

Of these two, hypochlorous acid (HOIO) has the most disinfecting power.
is stronger than hypochlorite ion (010-'').

From the above considerations, it can be seen that pi (during chlorine injection) is desirably a low value from the viewpoint of the sterilizing effect of chlorine.

Next, FIG. 4 shows the influence of PH on the flocculating starch effect. In the figure, the vertical axis shows the ratio between the turbidity of supernatant water and the turbidity of raw water after settling the suspended solids by injecting a flocculant, and the horizontal axis shows the negative value. That is, a ratio of 100 is when no suspended solids are removed, and a ratio of 0- is when suspended solids are completely removed.

According to FIG. 4, it can be seen that when - is less than &5, the effect of removing turbidity due to cohesive starch is not so great, and when . . . is 6.7 or more, a good turbidity removal effect is obtained.

As is clear from the above considerations, a treatment method that effectively combines -1 control is required in order to perform chlorine injection that takes into account the following three aspects: suppression of Torino-lomethane generation, sterilization effect, and flocculating starch effect. It is.

In other words, before injecting chlorine, it is necessary to lower the - level of the raw water to obtain good results in suppressing the generation of lyinlomethane and improving the sterilization effect of chlorine. For this reason, when the raw water level is high, it is desirable to lower the raw water level by injecting an acid such as sulfuric acid.

However, since the iJH of the raw water decreases due to the injection of sulfuric acid or the like into the raw water and the subsequent injection of chlorine, the flocculating effect does not work effectively even if a flocculant is directly added to such water.

To prevent this, it is necessary to inject an alkaline agent after chlorine is injected and before injecting the flocculant to raise the pH before injecting the flocculant.

The present invention has been made based on the above considerations.

FIG. 5 shows a block diagram of a control device according to the present invention.

In FIG. 5, the same symbols as in FIG. 1 represent the same or equivalent parts.

In the figure, 9 is a sulfuric acid injection device, 10 is a caustic soda injection device, 11 is a raw water pH meter, 12 is a raw water setting device, 13
1 is a sulfuric acid injection amount calculation circuit, 14 is a coagulated water setting device, and 15 is an alkali agent injection amount calculation circuit.

The pi-1 setting device 12 is used to set a value of -1 (for example, 5 or more -F) that suppresses the production of trihalomethane and provides a perfect sterilization effect. This value and the raw water 1.+1, a total of 11 values, are input to the sulfuric acid injection amount calculation circuit 13, where the sulfuric acid injection amount is determined.

Then, based on this calculation result, sulfuric acid is injected by the sulfuric acid injection device 9.

Thereby, il of the raw water in the receiving well 1 is adjusted to a value suitable for sterilization. Then, the raw water is sterilized by injecting a predetermined amount of chlorine from the chlorine injection device 7. At this time, as can be seen from FIG. 2, the generation of trihalomethane is suppressed to a minimum.

Next, from the raw water PH, sulfuric acid injection, and salt purple injection density,
The pLI of the flocculant injection point is calculated in the alkaline agent injection amount calculation circuit 15. Furthermore, the amount of alkaline agent to be injected is calculated according to the difference between this pi (direct) and the value of FJl, which is desirable for flocculating starch and is set by the flocculating water PH setting device 14.

The above calculation is similarly performed by the alkaline agent injection amount calculation circuit 9-15, and the alkaline agent (caustic soda) is injected by the alkaline agent injection device 10. and,
A predetermined amount of flocculant is injected by the flocculant injection device 8,
Suspended matter is removed.

As is clear from the above description, according to the present invention,
・It suppresses the production of lomethane, increases the coagulating starch effect at water treatment plants, and exhibits the sterilization effect of chlorine.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing an overview of water treatment plant equipment, Figure 2 is a diagram showing an example of the relationship between raw water and the amount of trihalomethane produced.
Figure 3 is a diagram showing an example of the relationship between raw water - and HOIo, 010-, Figure 4 is a diagram showing an example of the relationship between raw water - and the turbidity removal ratio by flocculating starch, and Figure 5 is a diagram showing an example of the relationship between raw water - and HOIo, 010-. FIG. 2 is a block diagram of an embodiment of a controllable chlorine injection control device. 1... Water landing well, 2... Mixing pond, 3... Floc formation pond, 4... Sedimentation pond, 5... Filtration cell, 6... Water purification pond, 7... Chlorine injection device , 8... Coagulant injection device, 9... Sulfuric acid-1〇- injection device, 10... Caustic soda injection device, 11...
Raw water pH meter, 12... Raw water PH setting device, 13... Sulfuric acid injection amount calculation circuit, 14... Coagulated water pH setting device, 15
...Alkali agent injection amount calculation circuit Patent attorney Michito Hiraki 11-years old 2 years old Figure □ Figure 3 Figure 4 Figure 3

Claims (1)

[Claims] (4) In a water treatment plant control method in which chlorine is injected into the raw water taken to sterilize it, and then a flocculant is further injected to remove turbidity, the raw water is A step of injecting acid to lower - to below the first planned value, and a second step of injecting - of the raw water after injecting chlorine and before injecting the flocculant.
A control method for a water purification plant, comprising the step of injecting an alkaline agent to raise the water level above a scheduled value. (2) A patented control method characterized in that the first expected value of PH is 5 or less. (3) A method for controlling a water purification plant according to claim 1, wherein the second predetermined value of IIJ'{ is 6.7 or more. (4) A control device for a water treatment plant that injects chlorine into the raw water taken to sterilize it, and then further injects a flocculant to remove turbidity. means for setting a first scheduled value; means for calculating the amount of acid to be injected before chlorine injection based on the deviation of the raw water from the first scheduled value; means for injecting the raw water into the raw water; means for calculating 1 at the flocculant injection point from the amount of acid injected as described above, the amount of chlorine injected, and the raw water PH; and a second predetermined value of the PH at the flocculant injection point. means for calculating the amount of alkaline agent to be injected from the deviation of the first value calculated as described above from the second expected value; and means for injecting the alkaline agent based on the calculation result. 11. A control device for a water purification plant, characterized by comprising: (5) A control device for a water purification plant according to claim 4, wherein the first predetermined pH value is 5 or less. (6) The control device for a water purification plant according to claim 4, wherein the second predetermined pH value is &7 or more.