JP2024065696A - Water treatment method - Google Patents

Abstract

To provide a water treatment method which can effectively suppress slime formation in white water in a paper making step.SOLUTION: Disclosed is a water treatment method for suppressing slime formation by adding a medicament to papermaking process water, wherein the water treatment method is characterized by that an oxidation reduction potential (ORP) of the papermaking process water and a dissolved oxygen concentration change rate (DO change rate, %) represented by the following numerical expression (1) of the papermaking process water are measured, and an addition amount of the medicament to the papermaking process water is controlled corresponding to the ORP and a value of the DO change rate after adding the medicament. [numerical expression 1]. In the numerical expression (1), DO0 shows a dissolved oxygen density at a time when the dissolved oxygen density change of the papermaking process water becomes stable, and DOt shows dissolved oxygen density after passage of time t since DO0. DO0>0.SELECTED DRAWING: None

Description

The present invention relates to a water treatment method, and more specifically, to a water treatment method that adds chemicals to the papermaking process water to suppress the generation of slime during the papermaking process.

It has long been known that process water in the paper and pulp industry and cooling water systems in various industries contain large amounts of organic matter such as starch, sizing agents, latex, and casein, making it easy for bacteria, fungi, and other microorganisms to grow. Slime derived from these microorganisms is likely to form in the circulating water system or on the surfaces of pipes and equipment, causing problems such as reduced product quality and reduced production efficiency.

Many bactericides have been used to prevent damage caused by these microorganisms. In the past, organic mercury compounds and chlorinated phenol compounds were used, but these agents are highly toxic to humans and fish and shellfish, and their use has been restricted due to environmental pollution. Recently, organic compounds with relatively low toxicity, such as organic nitrogen-sulfur compounds represented by methylene bisthiocyanate, 1,2-benzisothiazolin-3-one, and 5-chloro-2-methyl-4-isothiazolin-3-one, organic bromine compounds represented by 2,2-dibromo-2-nitroethanol, 2,2-dibromo-3-nitrilopropionamide, 1,2-bis(bromoacetoxy)ethane, 1,4-bis(bromoacetoxy)-2-butene, and bistribromomethylsulfone, and organic sulfur compounds represented by 4,5-dichloro-1,2-dithiol-3-one, have been widely used as industrial bactericides (see Non-Patent Document 1).

In addition, methods for preventing slime have been proposed using inorganic compounds such as chlorine agents such as sodium hypochlorite and combined chlorine such as chloramine, as well as the combined use of these inorganic compounds with organic compounds (Patent Documents 1 to 6).
However, in order to obtain final paper and pulp products of higher quality, there is a demand for a disinfectant that can further suppress slime and a disinfection method using said disinfectant.

Conventionally, the measurement of the viable bacteria count in white water was used as a guide for optimal slime control treatment, but since it took about one to three days for the results of a general viable bacteria count measurement to become available, the situation within the system had changed by the time the viable bacteria count measurement results became available, making it difficult to perform slime control treatment at the optimal time.
To address such problems, for example, Patent Documents 7 and 8 disclose methods for controlling slime in the papermaking process, which include a step of measuring the oxidation-reduction potential and/or the dissolved oxygen concentration (DO value).
However, the method of using the dissolved oxygen concentration (DO value) is significantly affected by water temperature, making it difficult to use it as a guide for slime control.

Japanese Patent Application Laid-Open No. 5-146785 JP 2000-256993 A JP 2002-336867 A JP 2003-105692 A JP 2003-306893 A JP 2008-43836 A JP 2010-518823 A JP 2017-110326 A

"Encyclopedia of Antibacterial and Antifungal Agents" published by the Japan Society of Antibacterial and Antifungal Agents on August 22, 1986, pages 24-30

In view of the above-mentioned conventional techniques, the present invention aims to provide a water treatment method that can effectively suppress the generation of slime in white water during the papermaking process.

The inventor of the present invention has conducted extensive research to solve the above problems, and as a result, has focused on the rate of change in dissolved oxygen concentration (DO rate of change) after adding a chemical agent to suppress slime generation in the papermaking process water. That is, it has been found that slime can be relatively appropriately controlled by controlling the slime in the papermaking process water so that the DO rate of change value is close to 0 (zero). However, if the amount of chemical agent is controlled using only the DO rate of change value as an index, there is a risk that the chemical agent will be added in excess.
As a result of further intensive research, the inventors of the present invention discovered that by adding chemicals while checking the oxidation-reduction potential (ORP) value of the papermaking process water in addition to the DO conversion rate value, it is possible to easily and effectively control slime in the papermaking process water, and thus completed the present invention.

なお、数式（１）中、ＤＯ_０は、前記製紙工程水の溶存酸素濃度変化が安定した時間における溶存酸素濃度を表し、ＤＯ_ｔは、ＤＯ_０から時間ｔ経過後の溶存酸素濃度を表す。但し、ＤＯ_０＞０である。 (1) The present invention is a water treatment method for suppressing the generation of slime by adding an agent to papermaking process water, characterized in that the oxidation-reduction potential (ORP) of the papermaking process water and the dissolved oxygen concentration change rate (DO change rate, %) of the papermaking process water, which is expressed by the following mathematical formula (1), are measured, and the amount of the agent to be added to the papermaking process water is controlled depending on the values of the ORP and the DO change rate after the agent is added.

In formula (1), DO ₀ represents the dissolved oxygen concentration at the time when the change in the dissolved oxygen concentration of the papermaking process water becomes stable, and DO _t represents the dissolved oxygen concentration after a time t has elapsed since DO ₀ , with the proviso that DO ₀ >0.

(2) The present invention is a water treatment method as described in (1), in which the amount of the chemical to be added to the papermaking process water is controlled so that the DO change rate after the addition of the chemical is 60% or less when t≦30 minutes.
(3) The present invention relates to a water treatment method according to (1) or (2), in which the chemical is an oxidizing agent having a halogen and/or hydrogen peroxide.

According to the present invention, when the DO value is used to calculate the DO change rate by, for example, taking the difference between the DO values upstream and downstream of a white water silo, the DO change rate is not due to the respiration of aerobic bacteria because of the effects of temperature and the increase in dissolved oxygen due to water flow. The present invention looks at the DO change rate over a specified period of time under sealed conditions for the same sampled water, so it is possible to accurately measure the amount of oxygen consumed by aerobic bacteria respiration. This makes it possible to provide a water treatment method that can effectively prevent slime from occurring in papermaking process water by making it possible to check the state of the papermaking process water in real time.

FIG. 1 is a graph showing the measurement results of the DO change rate and ORP in Test Example 1. FIG. 2 is a graph showing the measurement results of the DO change rate and ORP in Test Example 2. FIG. 3 is a graph showing the measurement results of the DO change rate and ORP in Test Example 3. FIG. 4 is a graph showing the measurement results of the DO change rate and ORP in Test Example 4. FIG. 5 is a graph showing a typical change over time in the dissolved oxygen concentration of sampled water collected from the papermaking process. FIG. 6 is a graph showing the measurement results of the DO change rate in Test Examples 5 to 8.

The following describes an embodiment of the present invention. However, the embodiment described below is merely an example of a typical embodiment of the present invention, and the scope of the present invention should not be interpreted narrowly as a result.

In this specification, "X to Y" means "X or more and Y or less."

The present invention is a water treatment method for suppressing the generation of slime by adding an agent to the papermaking process water.
Typically, the papermaking process includes a pulping process in which the raw material for paper, called pulp, is produced; a preparation process in which the pulp fibers are physically processed and chemicals are added before paper is made; and a papermaking process in which the pulp is spread into a sheet and subjected to dehydration, drying, surface treatment, etc.
The papermaking process water in the present invention includes white water and pulp slurry used in the pulping process, the preparation process and the papermaking process. The papermaking process water may include water circulating through each process in the papermaking process, and industrial water or recycled water supplied to the water circulating through each process in the papermaking process.

なお、数式（１）中、ＤＯ_０は、上記製紙工程水の溶存酸素濃度変化が安定した時間における溶存酸素濃度を表し、ＤＯ_ｔは、ＤＯ_０から時間ｔ経過後の溶存酸素濃度を表す。但し、ＤＯ_０＞０である。
上記ＯＲＰは、上記ＤＯ_０測定時における製紙工程水のＯＲＰ値であり、上記ОＲＰの測定方法としては特に限定されず、例えば、市販の酸化還元電位計（一例として、マイラナ社製ポータブル水質計Ｐ４０）を用いて測定することができる。
また、上記数式（１）におけるＤＯ_０は、上記薬剤を添加した製紙工程水の溶存酸素濃度変化が安定した時間における溶存酸素濃度であり、具体的には、任意の箇所で採取した上記製紙工程水（以下、サンプリング水）について、溶存酸素濃度変化が安定した時間における溶存酸素濃度を測定する。なお、上記製紙工程水から採取したサンプリング水の溶存酸素濃度は経時的に減少するが、図５に示したように、その減少の程度は一定ではなく採取後一定時間までは急激に減少し、その後緩やかな減少となる。上記「溶存酸素濃度が安定した時間」とは、溶存酸素濃度の急激な減少から緩やかな減少に変化した時間である。また、上記サンプリング水の溶存酸素濃度変化が安定する時間は対象系によって異なるため一義的に決定することはできないため、例えば、サンプリング水採取後３分や５分における溶存酸素濃度をＤＯ０と決めて測定を行ってもよい。
そして、上記ＤＯ変化率とは、上述した規定したＤＯ_０と、上記サンプリング水の所定時間（ｔ）経過後の溶存酸素濃度（ＤＯ_ｔ）の数値差を、ＤＯ_０で割って百分率で表示したものである。但し、ＤＯ_０＞０である。なお、上記数式（１）における時間（ｔ）としては特に限定されず適宜設定でき、例えば、１０秒以上、３０秒以上、１分以上、３分以上、５分以上、若しくは１０分以上、及び、６０分以下、４５分以下、３０分以下、若しくは２０分以下の組合せから任意に設定できる。
上記溶存酸素濃度の測定方法としては特に限定されず、例えば、市販の溶存酸素計（一例として、ＨＡＣＨ社製蛍光式溶存酸素計ＬＤＯ２）を用いることができる。 In the present invention, the oxidation-reduction potential (ORP) of the papermaking process water and the rate of change in dissolved oxygen concentration of the papermaking process water (DO rate of change, %), which is represented by the following mathematical formula (1), are measured.

In formula (1), DO ₀ represents the dissolved oxygen concentration at the time when the change in the dissolved oxygen concentration of the papermaking process water becomes stable, and DO _t represents the dissolved oxygen concentration after a time t has elapsed since DO ₀ , with the proviso that DO ₀ >0.
The ORP is the ORP value of the papermaking process water when the DO is measured _{at 0.} The method for measuring the ORP is not particularly limited, and it can be measured, for example, using a commercially available oxidation-reduction potential meter (for example, the Mylana Portable Water Quality Meter P40).
DO ₀ in the above formula (1) is the dissolved oxygen concentration at the time when the dissolved oxygen concentration change of the papermaking process water to which the agent has been added becomes stable. Specifically, the dissolved oxygen concentration of the papermaking process water (hereinafter, sampled water) collected at an arbitrary point is measured at the time when the dissolved oxygen concentration change becomes stable. The dissolved oxygen concentration of the sampled water collected from the papermaking process water decreases over time, but as shown in FIG. 5, the degree of decrease is not constant, and the dissolved oxygen concentration decreases rapidly for a certain time after collection, and then decreases gradually. The above "time when the dissolved oxygen concentration becomes stable" is the time when the dissolved oxygen concentration changes from a rapid decrease to a gradual decrease. The time when the dissolved oxygen concentration change of the sampled water becomes stable varies depending on the target system and cannot be determined uniquely, so for example, the dissolved oxygen concentration 3 minutes or 5 minutes after the sampled water is collected may be determined as DO 0 and measurement may be performed.
The DO change rate is the difference between the DO ₀ defined above and the dissolved oxygen concentration (DO _t ) of the sampled water after a predetermined time (t) has elapsed, divided by DO ₀ and expressed as a percentage, where DO ₀ > 0. Note that the time (t) in the above formula (1) is not particularly limited and can be set appropriately, and can be set arbitrarily from, for example, a combination of 10 seconds or more, 30 seconds or more, 1 minute or more, 3 minutes or more, 5 minutes or more, or 10 minutes or more, and 60 minutes or less, 45 minutes or less, 30 minutes or less, or 20 minutes or less.
The method for measuring the dissolved oxygen concentration is not particularly limited, and for example, a commercially available dissolved oxygen meter (for example, a fluorescent dissolved oxygen meter LDO2 manufactured by HACH) can be used.

In the present invention, the amount of the chemicals added to the papermaking process water is controlled according to the ORP and DO change rates.
In the present invention, the state of the papermaking process water (the ORP and DO change rates) flowing during the papermaking process can be checked in real time, making it possible to adjust the amount of chemicals added to the papermaking process water before it reaches a state where slime is generated.

Specifically, the ORP is preferably -400 mV to +600 mV, and the amount of the agent added to the papermaking process water is preferably controlled so that the DO change rate after the addition of the agent is 60% or less when t≦30 minutes. By adjusting in this way, the generation of slime can be more effectively suppressed.
As microbial contamination of the water in the papermaking process progresses and oxygen in the system is consumed, the ORP is likely to decrease, and if the ORP exceeds +600 mV, there is concern that the equipment in the system may become corrosive. It is more preferable that the ORP is -150 mV or higher.
In addition, as microbial contamination of the papermaking process water progresses and oxygen in the system is consumed, the DO rate of change is likely to increase. It is more preferable that the DO rate of change is 55% or less. When t = 20 minutes, the DO rate of change is preferably 50% or less. Furthermore, when t = 10 minutes, the DO rate of change is preferably 30% or less.

In the present invention, the location where the ORP and DO change rates are measured is not particularly limited, but suitable locations include downstream of the chemical addition point in the pulping process, the preparation process, or the papermaking process.
Specific locations in the pulping process where the ORP and DO change rates are measured include, for example, a raw material tank and a blow chest.
In addition, a specific location where the ORP and DO change rates are measured in the adjustment step is, for example, a whitewater adjustment pit.
In addition, a specific location where the ORP and DO change rates are measured in the papermaking process is, for example, a white water pit.
The ORP and DO change rates may also be measured in a dilution water line for chemicals (chemicals requiring dilution such as paper strength agents, sizing agents, and dyes) and a shower water line.

The number of live bacteria in the papermaking process water is not particularly limited, but is preferably 1 x 10 ⁵ CFU/mL or less immediately after the addition of the agent, and more preferably 1 x 10 ⁴ CFU/mL or less. If the number of live bacteria exceeds the upper limit, the generation of slime in the papermaking process water may not be suppressed. Under normal conditions other than immediately after the addition of the agent, it is preferably 1 x 10 ⁷ CFU/mL or less, and more preferably 1 x 10 ⁶ CFU/mL or less.
The viable cell count can be measured by a known method.

The chemical functions as a bactericide capable of suppressing the generation of slime in the papermaking process water.
Such an agent is preferably an oxidizing agent having a halogen and/or hydrogen peroxide.
The oxidizing agent having the halogen includes a bound halogen and/or a free halogen, the bound halogen includes a bound chlorine and a bound bromine, and the free halogen includes a chlorite, a hypochlorite, and a chlorine dioxide.

The above-mentioned bound halogen includes bound chlorine and bound bromine, and specifically, monochloramine, monobromamine, chlorosulfamate, and/or bromosulfamate are preferable.
The monochloramine and monobromamine are mild oxidizing agents produced by a reaction such as OCl ⁻ (Br ⁻ ) + NH ^{4 +} → NH ₂ Cl (Br) + H ₂ O. For example, monochloramine can be produced by mixing sodium hypochlorite with an ammonium compound, and specific examples of the ammonium compound include ammonium sulfate, ammonium bromide, ammonium chloride, and ammonium sulfamate, which can be used alone or in combination of two or more kinds.
In one or more embodiments, the molar ratio of the hypochlorite to the ammonium compound is preferably 1:1 to 1:2, calculated as the molar ratio of the residual chlorine amount to nitrogen.

The chlorosulfamates and bromosulfamates are reaction products of a chlorine-based or bromine-based oxidizing agent and a sulfamic acid compound or a salt thereof.
In one or a plurality of embodiments, the chlorine-based oxidizing agent includes hypochlorous acid or a salt thereof, and among these, sodium hypochlorite is preferable.
In one or a plurality of embodiments, the bromine-based oxidizing agent includes hypobromous acid or a salt thereof, and among these, sodium hypobromite is preferable.
In one or a plurality of embodiments, examples of the sulfamic acid compound include sulfamic acid, chlorosulfamic acid, and bromosulfamic acid.
In one or more embodiments, the chloro/bromosulfamate is, without being particularly limited thereto, a reaction product of "a reaction product of sodium hydroxide and sulfamic acid" and "hypochlorous acid/sodium hypobromite", and in one or more embodiments, the chloro/bromosulfamate can be obtained by blending sodium hydroxide, sulfamic acid, and sodium hypochlorite or sodium hypobromite.

The free halogens include chlorite, hypochlorite, chlorine dioxide, etc., and the chlorites specifically include sodium chlorite, potassium chlorite, calcium chlorite, etc., and in the present invention, these can be used alone or in combination of two or more.
Specific examples of the hypochlorite include sodium hypochlorite, potassium hypochlorite, and calcium hypochlorite. In the present invention, these can be used alone or in combination of two or more.

Since chlorine dioxide is an extremely unstable chemical substance, its storage and transportation are very difficult, and therefore it is preferable to produce (generate) chlorine dioxide on-site by a known method and adjust the concentration to be added before use.
For example, chlorine dioxide can be produced by the following reaction, and a commercially available chlorine dioxide generator (apparatus) can also be used.
(1) Reaction of sodium hypochlorite, hydrochloric acid, and sodium chlorite NaOCl + 2HCl + _2NaClO2 → _2ClO2 + 3NaCl + _H2O
(2) Reaction of sodium chlorite with hydrochloric acid: _5NaClO2 + 4HCl → _4ClO2 + 5NaCl + _2H2O
(3) Reaction with sodium chlorate, hydrogen _{peroxide ,} and sulfuric acid _{: 2NaClO3} + _H2O2 + _H2SO4 → 2ClO2 + _{Na2SO4 + O2} + _2H2O

The halogen-containing oxidizing agent used in the present invention is preferably a hypochlorite.

The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to these examples.

(Test Examples 1, 2, and 3)
Test Example 1
Chemicals were added to the white water silo in the papermaking process at a certain paperboard mill A. In the white water silo where chemicals were added, the ORP and DO change rates were constantly measured. A commercially available water quality measuring device was used to measure the water quality.
The DO change rate was calculated by taking the DO value 3 minutes after sampling water from the white water silo as DO ₀ and the DO value 10 minutes after sampling as DO _t . ORP was measured for the sampled water collected for DO measurement. The results are shown in Figure 1. In Figure 1, the ORP value was read from the top of the peak excluding the value immediately after the addition of the agent. The same applies to Figures 2 to 4 below.
The chemical used was a bound halogen (monochloramine). The amount of chemical added was adjusted so that the ORP between the addition of the chemical and the next addition was always between -50 mV and +100 mV, except immediately after the chemical was added, and so that the DO change rate was approximately 0.
As shown in FIG. 1, at a certain paperboard mill where tests were conducted, defects due to slime were occurring, but in this embodiment, the occurrence of such defects was suppressed by adding the agent for 15 minutes three times a day.
By using the DO conversion rate and ORP in combination, it was possible to prevent excess or deficiency of chemicals and add an appropriate amount to suppress the occurrence of slime.
The viable cell count measured before the start of the test was 1×10 ⁷ CFU/mL or more, whereas the viable cell count immediately after the addition of the agent was 1×10 ⁵ CFU/mL or less.

Test Example 2
Test Example 2 was carried out at a certain paperboard mill A in the same manner as Test Example 1 except for the amount of chemicals added.
The DO change rate was calculated by taking the DO value 3 minutes after sampling water from the white water silo as DO ₀ and the DO value 10 minutes later as DO _t . The ORP was measured for the water samples taken for DO measurement. The results are shown in Figure 2.
The chemical used was a bound halogen (monochloramine). The amount of chemical added was adjusted so that the ORP between the addition of the chemical and the next addition was always −50 mV to +100 mV, except immediately after the chemical was added, and the DO change rate was approximately 0 to 10%.
As shown in FIG. 2, defects caused by slime were observed in a certain paperboard mill where tests were conducted, but in this embodiment, the occurrence of such defects was suppressed by adding the agent for 15 minutes three times a day.
By using the DO conversion rate and ORP in combination, it was possible to prevent excess or deficiency of chemicals and add an appropriate amount to suppress the occurrence of slime.
The viable cell count measured before the start of the test was 1×10 ⁷ CFU/mL or more, whereas the viable cell count immediately after the addition of the agent was 1×10 ⁵ CFU/mL or less.

Test Example 3
Test Example 3 was carried out in a certain paperboard mill A in the same manner as Test Example 1, except for the amount of chemicals added. The results are shown in FIG.
In test example 3, the ORP between the addition of the chemical and the next addition of the chemical ranged from -30 mV to +50 mV, and the DO change rate was almost the same as when the rate was not after the addition of the chemical, so the occurrence of slime could not be suppressed and the occurrence of defects could not be suppressed.
By using DO conversion rate and ORP in combination, we discovered that the chemicals were insufficient to suppress the occurrence of slime.
The viable cell count measured before the start of the test was 1×10 ⁷ CFU/mL or more, whereas the viable cell count immediately after the addition of the agent was 1×10 ⁶ CFU/mL or more.

Test Example 4
At a certain paperboard mill B, chemicals were added to the white water silo in the papermaking process. In the white water silo where chemicals were added, the ORP and DO change rates were constantly measured. A commercially available water quality measuring device was used to measure the water quality. The results are shown in Figure 4.
The drug used was a bound halogen (monochloramine).
During the test period from day 1 to day 5, the agent was added so that the DO change rate measured in the same manner as in Example 1 was 15% to 55%. At this time, the ORP was always +0 mV to +100 mV except immediately after the agent was added. However, during this test period, the suppression of slime generation was not sufficient, and the occurrence of defects could not be suppressed.
Subsequently, the amount of the agent added was adjusted so that the DO change rate measured in the same manner as in Example 1 from 6 to 12 days was approximately 0. Moreover, in order to avoid excessive addition, the ORP was constantly adjusted to +100 mV to +300 mV except immediately after the agent was added.
In this test, the occurrence of the above defects was suppressed by adding the agent for 10 minutes four times a day.
The viable cell count before the start of the test was 1 x 10 ⁷ CFU/mL or more. On the other hand, the viable cell count excluding the time immediately after the drug addition on days 1 to 5 was 1 x 10 ⁷ CFU/mL, and the viable cell count immediately after the drug addition was 1 x 10 ⁵ CFU/mL. Furthermore, the viable cell count excluding the time immediately after the drug addition on days 6 to 12 was 1 x 10 ⁶ CFU/mL, and the viable cell count immediately after the drug addition was 1 x 10 ⁴ CFU/mL or less.

Test Example 5
Water from the papermaking process was collected from a domestic paper mill, and 20 mg/L of a chemical was added to the sampled water to measure the ORP and DO change rates. A commercially available water quality measuring device was used to measure the water quality.
The DO change rate was calculated by setting the DO value 3 minutes after the addition of the agent to the collected sample water as DO ₀ , and the DO value after time t (3 minutes, 5 minutes, 10 minutes, 15 minutes, 20 minutes, 30 minutes, 45 minutes, and 57 minutes) from DO ₀ as DO _t . The ORP was 340 mV as a result of measuring the sample water at DO ₀ measurement. The results of the DO change rate are shown in FIG. 6.
The agent used was a bound halogen (monochloramine). The ORP was measured 3 minutes after the sampling water was taken.
As shown in FIG. 6, the papermaking process water in Test Example 5 showed a rapid increase in DO change rate, indicating that defects caused by slime were not sufficiently suppressed.

Test Examples 6 to 8
Papermaking process water was collected from different paper mills from those in Test Example 5 to prepare sample water, and the ORP and DO change rate were measured in the same manner as in Test Example 5, except that 20 mg/L (Test Example 6), 9 mg/L (Test Example 7), and 11 mg/L (Test Example 8) of a chemical were added to the sample water. The ORP was 297 mV (Test Example 6), 377 mV (Test Example 7), and 412 mV (Test Example 8). The results of the DO change rate are shown in FIG. 6.
As shown in FIG. 6, the papermaking process water of Test Examples 6 to 8 had a DO change rate of 60% or less when t≦30 minutes in the above formula (1), and it was found that defects caused by slime were sufficiently suppressed.
The viable cell count of the sampled water after the test was measured, and it exceeded 1×10 ⁶ CFU/mL in Test Example 5, was 1×10 ⁵ CFU/mL or less in Test Example 6, and was 1×10 ⁴ CFU/mL or less in Test Examples 7 and 8.

Claims (3)

A water treatment method for suppressing the generation of slime by adding chemicals to papermaking process water,
the oxidation-reduction potential (ORP) of the papermaking process water and the dissolved oxygen concentration change rate (DO change rate, %) of the papermaking process water, which is expressed by the following mathematical formula (1), are measured, and the amount of the chemical to be added to the papermaking process water is controlled according to the values of the ORP and the DO change rate after the addition of the chemical.

In formula (1), DO ₀ represents the dissolved oxygen concentration at the time when the change in the dissolved oxygen concentration in the papermaking process water becomes stable, and DO _t represents the dissolved oxygen concentration after a time t has elapsed since DO ₀ , where DO ₀ >0. The water treatment method according to claim 1, in which the amount of the agent added to the papermaking process water is controlled so that the DO change rate after the agent is added is 60% or less when t≦30 minutes. The water treatment method according to claim 1 or 2, wherein the chemical is an oxidizing agent having a halogen and/or hydrogen peroxide.

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