JPS58119306A - Injection control of flocculant for water purifying plant - Google Patents

Abstract

PURPOSE:To achieve a better flocculation with a simple control system overcoming any sharp change in the quality of raw water due to rainfall or the like by injecting a flocculant after the control of quantity of an alkali agent injected based on two indicators of water quality that is the alkalinity of the raw water and the pH of a floculated water following the injection of the flocculant. CONSTITUTION:The alkalinity of a raw water 31 in a waterwell 4 is measured with an alkalinity meter 35 and a corrected alkalinity Al1 is determined with an arithmetic unit 37 from a chlorine injection rate DCl and an alkali agent injection rate DAl. An alkali agent injection rate DAl1 is determined with an arithmetic unit 43 from the turbidity Tu measured with a turbidimeter 34 and the corrected alkalinity Al1. On the other hand, the pH of a flocculated water is measured at the inlet of a floc formation pond 6 with a pH meter 41 and an alkali injection rate DAl2 is determined from the turbidity Tu with an arithmetic unit 42. The alkali injection rates DAl1 and DAl2 are compared with a comparator and after the larger value is choosed as the alkali injection rate DAl, an alkali injection rate Al is obtained from the value and the flow rate F of the raw water. The quantity CO of the flocculant injected is found from the flocculant injection rate DCO determined from the turbidity Tu and the corrected alkalinity Al1 and the flow rate F of the raw water.

Description

[Detailed description of the invention] The present invention relates to a method for controlling the injection of a flocculant in a water purification plant,
In particular, the present invention relates to a method for controlling the amount of coagulant to be injected, which is suitable for always obtaining stable, high-quality water regardless of fluctuations in raw water quality.

Raw water taken from water sources such as rivers and lakes contains various impurities, and in order to use it as drinking water or other domestic water, it is necessary to remove the impurities with T-rays. Impurities are mainly 6 colors of turbidity.

Iron, manganese, microorganisms, etc. generally exist as particles dispersed or suspended in raw water, or as substances adsorbed thereto. Removing these particles is extremely important for improving water quality. As a method for removing particles, generally a coagulant is added to coagulate the particles to form flocs, followed by sedimentation and filtration. Recently, interest in coagulant injection control has increased as water intake sources become wider, the water intake sources deteriorate, and the amount of water intake increases.

Conventional technology for coagulant injection control at water treatment plants I'i: A calculation formula for the coagulant injection rate using parameters such as turbidity of raw water and alkalinity of raw water is determined in advance, and the turbidity of raw water is calculated at one receiving well, etc. This method measures and inputs the alkalinity, etc. into the calculation formula, determines the injection rate, and performs injection. This flocculant injection control method #'
i. It is a method of simply calculating # drug collection injection rate and injecting, #
You're the ninth person who hasn't given any consideration to the water quality before injecting the concentrate.

Due to factors such as rainfall and seasonal changes, the alkalinity of the raw water decreases, making it impossible for the flocculant to perform its full function, and there is a high risk that flocculation will fail. In addition, the calculation formula for the flocculant injection rate is a nine-approximation formula that is obtained by analyzing past operation data, in which there are many cases where water quality changes are small, using mathematical methods such as regression analysis. If the water quality changes significantly, the error will increase and there is a high risk of failing to properly inject the flocculant.

If the alkalinity of the raw water is too low, or if the flocculant is improperly injected, the suspended particles in the raw water will not flocculate and flow out as is, making it impossible to obtain clear water, or the flocs will become small, causing problems in the filtration basin. It has the disadvantage of making me watch TV too often.

In order to improve this shortcoming, the present applicant has filed Japanese Patent Application No. 52-21562 (%Kokai No. 53-107148).
is proposed. This #i is raw water alkalinity.

Three water quality indicators: coagulated water pH and coagulated water alkalinity.

After controlling the injection amount of alkaline agent so that it falls within the effective flocculation area where the flocculant can effectively perform its function, this method is used to control the turbidity of the raw water and the water quality before the flocculant injection. Based on the measured values, the alkalinity is first maintained at an appropriate value, and then a flocculant is injected, making it possible to control chemical injection in response to changes in water quality due to rainfall, etc. However, this control method uses three water quality indicators to control the amount of alkaline agent injected, which makes the control system somewhat complicated, and water treatment plants that do not have coagulated water alkalinity needles will need to install a new one. However, there are some practical problems.

The present invention has been made to improve the drawbacks of the prior art described above, and its purpose is to provide a simple structure that can be used even when the quality of water such as turbidity and alkalinity of raw water changes suddenly due to rainfall etc. An object of the present invention is to provide a method for controlling the injection of a flocculant in a water purification plant, which can perform flocculation well.

The present invention has experimentally clarified that there is a water quality region defined as an effective flocculation region in which the flocculant can fully exert its function, and that this is expressed by two water quality indicators: raw water alkalinity and flocculated water 1) H. It was created as a motive.

Measuring instruments detect the ever-changing alkalinity of raw water and the pH of flocculated water.

After controlling the injection amount of the alkaline agent so that these two water quality indicators fall within the effective coagulation region, the coagulant is injected.

The present invention was conceived as a result of various experiments and repeated studies aimed at improving the prior art.

First, the overall configuration of the water purification process will be described using FIG. Nengen water is taken from a water source 1 such as a river, and wood chips, stones, etc. are removed by a screen 2. Next, sand settling pond 3
The sand with large grain size is removed and then led to the landing well 4 where it is mixed with pre-chlorine 12 which is separately injected.
Sterilization of raw water and fermentation of iron, manganese, etc. occur. After that.

An alkaline agent 14 is injected if necessary and enters the mixing basin 5. In the mixing basin 5, the flocculant 16 injected by the flocculant injector 15 and water are mixed. The water mixed with the flocculant then enters the floc formation pond 6. In the floc formation pond, sifloc is formed by the combination of fine particles in the raw water and flocculant.

The raw water containing cough fluid is then sent to settling tank 7.

Sedimentation separation of the flocs is carried out. Fine flocs that cannot be separated by sedimentation are separated by filtration and Ikezuki. The treated water from which turbidity has been removed is then sent to the disinfection pond 9, where it is injected with chlorine 18 and alkaline agent 20 to prevent the reoccurrence of bacteria and adjust the pH. The water is sent to the distribution reservoir.

Next, the theoretical background of the present invention will be described. Mechanism of flocculation Fi was previously thought to occur when negatively charged suspended particles were electrically neutralized by positively charged flocculants, and the neutral dangerous particles were bound together by van der Waals forces, etc. . However, according to recent aggregation theory, aggregation is not a physical phenomenon based on mere electrical neutralization, but is mainly caused by chemical reactions as shown below. That is, when injected into raw water, t
A flocculant such as PAC (polyaluminum chloride) or sulfuric acid is dissociated to produce aluminum ions Aj. As shown in the schematic diagram in Fig. 2, this aluminum ion At is more concentrated than the hydroxyl ion OH- which is free in the water and is adsorbed to the suspended particles I)C and has a higher concentration.
It chemically reacts preferentially with H″″ to form a bond and form a floc. When the number of bonds that bind suspended particles increases, the binding force increases and a strong and large floc is formed, but when there are few bonds, the force that binds suspended particles becomes weak.

Flocks tend to collapse due to external forces. Therefore, large flocs are not formed. As shown in Figure 2, the number of bonds that bind suspended particles does not depend only on the amount of flocculant, that is, the amount of aluminum ions At, but also on the surface of the suspended particles that are the binding partner. It also depends on the amount of hydroxyl ion OH- present. What is the amount of hydroxyl ions (OH) adsorbed on the surface of suspended particles?

It is in adsorption equilibrium with the free hydroxyl ions OH''' in the water. When a coagulant is injected into the raw water, the water shifts to the acidic side, and the free hydroxyl ions in the water are consumed. Since the number of hydroxyl ions adsorbed on the surface of suspended particles decreases, good flocs cannot be formed unless there is something in the water that consumes the hydroxyl ions. The alkalinity of raw water indicates the capacity to compensate for the consumption of acid ions.Therefore, in order to form large flocs with good sedimentation properties, it is necessary to increase the alkalinity of raw water beyond the limit.

On the other hand, aluminum hydroxide, which is produced by a chemical reaction between aluminum ions and hydroxide ions adsorbed on the surface of clay particles, is an amphoteric hydroxide and is described in (1) below.

As shown in formula (2), it dissolves in both acids and alkalis.

Aj(oH)s+3H′″→ Aj,””+3H!0
......(1) At (01(), +0)(-
→ At(OH); ・・・・・・(2) Therefore, the generated aluminum hydroxide is dissolved.

In other words, in order to prevent dissociation of flocs, it is necessary to maintain the pH of the flocculated water after floc formation at an appropriate value near neutrality.

Alkalinity indicates the amount of alkaline content such as heavy carbonate, carbonate, or hydroxide in water. The alkalinity of flocculated water indicates the alkalinity remaining after a flocculant is injected into #L water and alkaline content is consumed. The fact that flocs are formed and the flocculated water has an alkalinity value indicates that the hydroxyl ions adsorbed on the surface of the suspended particles react with the aluminum ions generated by the dissociation of the flocculant. It means that there is. Therefore, according to the recent flocculation mechanism described above, it is necessary that the alkalinity of the flocculated water does not fall below zero. However, the pH at zero alkalinity of the coagulated water
Fi4.8. A pH value of 4.8 is acidic and aluminum hydroxide will dissolve.

Therefore, the effective range of coagulated water pH is much narrower than the effective range of coagulated water alkalinity, so it can be said that attention should be paid to pH.

This will be explained specifically.

The fact that the pH of the flocculated water is 4.8 or higher indicates that there are rarely enough hydroxide ions adsorbed on the surface of the suspended substance that can react with the flocculant before injection of the flocculant.

Therefore, according to the nine flocculation mechanisms described above, the alkalinity of flocculated water is the alkalinity at pH=4.8,
In other words, it is sufficient if it is greater than or equal to zero.

However, the aluminum hydroxide that forms the flocs is amphoteric and dissolves in both acidic and alkaline conditions.

At pH=4.8, aluminum hydroxide dissolves. Therefore, if an alkaline agent is injected into the raw water to maintain the pH of the coagulated water in a range where the aqueous aluminum does not dissolve, the hydroxide ions adsorbed on the turbid surface before the coagulant injection will also be maintained at a sufficient concentration. be able to.

To summarize what has been said above, there are two water quality factors that greatly affect flocculation: raw water alkalinity and flocculating water pH.

Next, the present invention will be explained using specific experimental examples.

[Example 1] Suspended matter created using the camphor semi-turbidity adjustment method based on river sediment was suspended in actual river water to create raw water, and used as a sulfuric acid agglomerate. A series of flocculation experiments were conducted using a jar tester to investigate the effects of raw water alkalinity and flocculating water pH on flocculation. Note that caustic soda was used for alkaline y4. Figure 3 shows the experimental results. In the figure.

The heat mark indicates the 9th experimental point where the supernatant water turbidity, which is one of the criteria for determining whether good flocs have been formed, is 2 ppm or less, and the 0 mark indicates the 9 experimental point where the supernatant water turbidity is 2 ppm or less. There is. From the figure, it can be seen that the water quality region where good flocs are formed (effective coagulation region) is expressed using the two coordinate axes of raw water alkalinity and coagulated water pH, and that this region changes depending on the raw water turbidity. Outside this region, good flocs are not formed regardless of the amount of coagulant injected.

This is because strong flocs were not formed due to the low alkalinity and the aluminum hydroxide was dissolved due to the inappropriate pH.

[Example 2] Using the same suspended matter and the same apparatus as in Example 1, an experiment was conducted to examine the relationship between the alkalinity of the flocculated water and the pH of the flocculated water. The experimental results are shown in Figure 4. In the figure.

The heat mark indicates that a good floc is formed and the experimental point is solid.

From the figure, it can be seen that there is a linear relationship between the alkalinity of the coagulated water and the pH of the coagulated water. According to the flocculation mechanism described above, the alkalinity of flocculated water is not a water quality factor that greatly influences flocculation, but even if it were to be affected, the alkalinity of flocculated water is clear from the results of this figure. Therefore, it is not necessary to measure the three water qualities of raw water alkalinity, #collected water alkalinity, and flocculated water 1) H in order to display the effective flocculation area as in the past.

It can be said that it is sufficient to maintain two water qualities: raw water alkalinity and flocculated water 1) H.

[Example 3] Using the same turbidity and the same equipment as in Example 1, an experiment was conducted to examine the relationship between raw water turbidity and flocculant injection rate at which the supernatant water turbidity was 2 ppm or less. The experimental results are shown in Figure 5. As shown in the figure, it can be seen that the flocculant injection rate varies depending on the raw water turbidity and raw water alkalinity, and increases as the raw water turbidity and raw water alkalinity increase.

Next, one specific example for carrying out the present invention will be described.

The flocculant injection control method of the present invention controls the injection amount of alkali so that the alkalinity of the raw water and the pH of the flocculated water fall within the water quality range defined as the effective flocculation range. A flocculant is injected according to the conditions. The effective aggregation area is within the line shown in Example 1K. Therefore, it is sufficient that the alkalinity of the raw water is equal to or higher than the minimum value of the characteristic curve shown in FIG. 3, but at the minimum value, the amount of flocculated water is increased to provide a pHVC margin for the flocculated water.

FIG. 6 shows a schematic diagram of the effective flocculation area in the following case where the flocculated water 1)H is given a margin for control. The region shown in the figure and having an important meaning within the nine effective flocculation regions is the lower limit value of both the raw water alkalinity and the flocculated water pH. This is because although there is an upper limit for the pH of flocculating water, in the case of most river and lake water, the pH of the raw water is neutral and within the effective flocculation range, but the pH of the flocculating water is increased by the injection of flocculant or chlorine. This is because it shifts to the acidic side and deviates from the effective aggregation region.

The flocculant injection control method will be described in detail below using FIG. 7. The control system consists of two parts: an alkali injection control system and a flocculant injection control system. In the alkaline agent injection control system, first, the corrected alkalinity AL1 before the flocculant injection is determined. The corrected alkalinity here corresponds to the raw water alkalinity of 1% in Example.

Change the alkalinity of raw water 31 to 35 at landing well 4 etc.
Measured by This alkalinity is input to the corrected alkalinity calculator 37. The chlorine injection rate DC1 and the alkaline agent injection rate DAj are separately input to the corrected alkalinity calculator 37, and the addition and subtraction shown in FIG. 8 is performed to obtain the corrected alkalinity A.
11 is required. Here, K represents the alkalinity conversion coefficient of the alkaline agent, and 1 and 4 represent the alkalinity consumption rate per 1 ppm of chlorine. The corrected alkalinity At is output to the raw water alkalinity adjustment alkaline agent injection rate calculator 43. The raw water alkalinity adjustment alkaline agent injection rate calculator 43 uses the corrected alkali fAz and the turbidity Tu measured at the landing well 4 etc. by the turbidity meter 34 as shown in FIG. Injection rate DA4. is required. The relationship shown in the figure is obtained by a jar test, etc., and the limit value of the corrected alkalinity is the limit value of the raw water alkalinity shown in Figure 6.

b, c correspond to 4IC.

On the other hand, the pH of the coagulated water is 41.
is measured and output to the pH adjustment alkaline agent injection rate calculator 42. F in the pH adjustment alkaline agent injection rate calculator 42
i. Coagulated water 1) Based on H and raw water turbidity Tu, determine the alkaline agent injection rate nAzt by the method shown in FIG. The relationship shown in the figure is determined in advance by a jar test or the like. In the figure, the limit value of flocculated water 1) H changes depending on the raw water turbidity Tu, and the limit value is the limit value α of the flocculated water pH shown in FIG.

Corresponds to β and γ. The output value DAt from the raw water alkalinity adjustment alkaline agent injection rate calculator 43 and the output f*DA from the pHn adjustment alkaline agent injection rate calculator 42! The signal is input to the tVi comparison calculator 44. In the comparison calculator 44, as shown in FIG. 11, DAj.

and DAt are compared, and the larger one is output as the alkaline agent injection rate. The alkali injection amount calculator 45 performs the multiplication shown in FIG. 12 based on the DAI and the output value F from the flowmeter 32, and outputs the alkali injection amount At to the alkali injection machine 46. Alkaline injection machine 4
In step 6, alkali is injected into the raw water according to the injection amount At.

The flocculant injection rate DCO is determined by the flocculant injection rate calculator 38 based on the degree of lubricity At1 using the method shown in FIG. 813.

The relationship shown in this figure corresponds to FIG. 5 of Experimental Example 3, and is determined in advance by a jar test or the like. In the flocculant injection amount calculator 39.

Based on the DCO and the output value F from the flowmeter 32, the multiplication shown in FIG. 14 is performed and outputted to the flocculant injector 40. The flocculant injection amount 4o is based on the injection amount CO, and the flocculant is injected into the raw water.

As explained above, according to the present invention, there are two water quality indicators for control, which simplifies the control system.

It is possible to reduce the labor required for maintenance of the Lucariness needle.

[Brief explanation of the drawing]

@Figure 1 is a system diagram showing the water purification process, Figure 2 is a schematic diagram of flocculation, and Figure 3 is a characteristic diagram of experimental results showing the effective flocculation area.
Figure 4 is a characteristic diagram showing experimental results showing the relationship between flocculating water alkalinity and flocculating water pH, Figure 5 is a characteristic diagram showing experimental results showing the relationship between flocculant injection rate and turbidity, and Figure 6 is a characteristic diagram showing experimental results showing the relationship between flocculant injection rate and turbidity. is a schematic diagram of the effective aggregation area used for control, FIG. 7 is a configuration diagram showing an example of the agglomeration 11i11 injection control method according to the present invention, and FIGS. 8 to 14 are detailed explanations of the parts in FIG. 7. FIG. 8 is a diagram showing a calculation formula for corrected alkalinity. @Figure 9 is a diagram showing how to determine the injection rate of alkaline agent to adjust raw water alkalinity, Figure 1ILO is a diagram showing how to determine the injection rate of alkaline agent to adjust the pH of coagulated water, and Figure 11 is a calculation formula for the injection rate of alkaline agent. 12 is a diagram showing a calculation formula for the amount of alkali agent injection, FIG. 13 is a diagram showing a method for determining the flocculant injection rate, and FIG. 14 is a diagram showing a calculation formula for the flocculant injection rate. Tu... Raw water turbidity, A/? ... Raw water alkalinity, D
CA...Chlorine injection rate, DAA...Alkali injection rate,
Aβ1... Corrected alkalinity, DA, #, Raw water alkalinity adjustment alkali injection rate, DCO... Coagulant injection rate, A1... Alkali injection amount, CO... Coagulant injection amount, pH ...Agglomerated water pH, D A J *...Agglomerated water pH 1! Illustrated alkali injection rate, F... Raw water flow rate, 35... Alkaline needle, 34... Turbidity meter, 41
...pH meter, 42.43...alkali injection rate calculator, 38...flocculant injection rate calculator. 4... Water landing well, 5... Mixing pond, 6... Floc formation pond, 1g 3 Figure; Suspect drug 4 (F'H, (-ra'$ 4 Ward; Suspect collection!H( -] Part F Figure 0 20 4θ 60 80 A
1 Old Kairi Cow (f'l) 9Y Otsu Zuko Chestnut Noto; /'tpucha g Sencha q Figure lo Ill Figure 72 Figure $ 74 Figure

Claims (1)

[Claims] 1. At a water treatment plant where a coagulant is injected into raw water to coagulate and precipitate impurities suspended in the raw water, the alkalinity of the raw water and the pH of the coagulated water after the coagulant is injected are measured. A method for controlling the injection of a flocculant, characterized in that the injection amount of the alkaline agent is controlled so that these two water quality indicators fall within a specific water quality range, and then the flocculant is injected. 2. The flocculant injection control method for a water purification plant according to item 1, wherein the specific water quality range of the two water quality indicators is changed depending on the turbidity of the raw water.