Disinfection method for water in a central circulating hot water supply system

Abstract

To provide a method for disinfecting the system water within a central circulating hot water system whereby metal corrosion is less likely to occur, chemicals are less prone to breakdown even at high temperatures, and prolonged disinfecting effects are achieved.SOLUTION: A method for disinfecting the system water within a central circulating hot water system includes an addition step for adding either monochloramine, chloramine T, or a combination of both to the system water within the central circulating hot water system.SELECTED DRAWING: Figure 1

Description

Technical Field

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[0001] The present invention relates to a method for sterilizing the water in a central circulation type hot water supply system in a hot water supply system, and particularly relates to a method for sterilizing water in a water system that has an excellent bactericidal effect on Legionella bacteria over a long period and is less likely to corrode the metal pipes used.

Background Art

[0002] In the case of buildings such as hospitals, condominiums, hotels, etc. where there are multiple hot water supply points, a hot water supply system (central circulation type hot water supply system) using central circulation type hot water supply equipment that supplies hot water from a central storage tank to each hot water supply point and returns the unused hot water to the storage tank for heating and circulation is used.

[0003] Fig. 6 shows a schematic diagram of a central circulation type hot water supply system 1000. In the central circulation type hot water supply system 1000, water pumped from a water receiving tank or the like is stored in an elevated water tank 100, and this water is sent to a storage tank 300 through a water supply pipe 200. The water in the storage tank 300 is sent to a heater 400 through a pipe 210 and heated to become hot water, and the hot water is sent to the storage tank 300 through a pipe 220. A heating circulation pump 500 is provided in the middle of the pipe 210, and water can circulate through the storage tank 300, the heater 400, and the pipes 210 and 220. Part of the hot water is sent from the storage tank 300 through a hot water supply pipe 230 to a branch pipe 240 connected to a hot water faucet or a shower. Also, the hot water that has not been sent to the branch pipe 240 is returned to the storage tank 300 through a return hot water pipe 250.

[0004] Thus, in the central circulation type hot water supply system 1000, water circulates between the storage tank 300 and the heater 400, and also water circulates from the storage tank 300 back to the storage tank 300 by passing through the hot water supply pipe 230 and the return hot water pipe 250 from the storage tank 300.

[0005] It is known that various bacteria, including Legionella, can be detected in central circulating hot water supply systems. In particular, in central circulating hot water supply systems used in hospitals and elderly care facilities where many people have weakened immune systems, microorganisms such as bacteria and fungi, as well as protists such as amoebas, can proliferate. If a biofilm containing Legionella bacteria is formed, for example, people can develop pneumonia by inhaling aerosols containing Legionella bacteria. Therefore, it is necessary to take measures to prevent the growth of Legionella bacteria before they can be detected in hot water taps, showers, etc., so that Legionella bacteria are not detected.

[0006] Conventionally, measures have been taken to clean the biofilm generated by exposing the hot water supply piping system to high-concentration chlorinated water with a chlorine concentration of about 10-100 mg / L for several hours, or to maintain the free residual chlorine concentration of the piping system water at about 0.5 mg / L or higher, and in some cases to maintain the free residual chlorine concentration at the end of the hot water supply above a predetermined level by maintaining a concentration of over 1 mg / L in the circulating hot water supply system. Methods for generating hypochlorous acid by electrolysis are also known. (Patent Documents 1 and 2)

[0007] Furthermore, it is known that bacteria such as Legionella can be killed by heating the hot water in the storage tank to a sterilizable temperature, for example, 60°C or higher. (Patent Documents 3 and 4)

[0008] However, since hot water piping is often made of metal, such as copper pipes or SUS304 stainless steel pipes, there was a concern that if the above-mentioned Legionella countermeasures were implemented, corrosion would occur in the metal piping due to the relationship between water quality, temperature, and free residual chlorine concentration, potentially causing malfunctions in the entire system. Therefore, improvements were needed.

[0009] Furthermore, in central circulating hot water supply systems, the water temperature at the outlet from the storage tank to the hot water pipe (outlet temperature) is often around 50°C or higher, and the water temperature at the inlet from the return pipe to the storage tank (return temperature) is also around 35°C or higher. With conventionally used free chlorine agents such as hypochlorous acid, the free chlorine agent decomposes in the water system in a short period of time and becomes ineffective, requiring frequent addition of free chlorine agents, which could lead to metal corrosion of the equipment within the water system. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2001-99486 [Patent Document 2] Japanese Patent Publication No. 2006-317105 [Patent Document 3] Japanese Patent Publication No. 2004-150649 [Patent Document 4] Japanese Patent Publication No. 2015-38397 [Overview of the project] [Problems that the invention aims to solve]

[0011] The present invention aims to provide a method for disinfecting a central circulating hot water supply system that solves the above problems, specifically, a method for disinfecting the water system of a central circulating hot water supply system that uses a specific disinfectant to achieve low metal corrosivity, resistance to decomposition even at high temperatures, and long-lasting disinfection effects. [Means for solving the problem]

[0012] The inventors of this invention have conducted extensive research to solve the above problems and have found that by adding monochloramine or chloramine T alone or in combination to the water system of a central circulating hot water supply system, a superior antibacterial effect that could not be expected from conventional technology can be obtained over a long period of time, leading to the completion of the present invention as described in (1) to (5) below.

[0013] (1) A method for disinfecting water from a central circulating hot water supply system, comprising an addition step of adding monochloramine or chloramine T alone or in combination to the water from the central circulating hot water supply system.

[0014] (2) The method for disinfecting water from a central circulating hot water supply system according to (1), wherein the addition step is to add monochloramine or chloramine T alone or in combination so as to maintain the total chlorine concentration of the water system at 0.5 mg / L or more and 100 mg / L or less.

[0015] (3) A method for disinfecting water from a central circulating hot water supply system according to (1) or (2), wherein the piping through which the water from the system flows in the central circulating hot water supply system is made of metal.

[0016] (4) The method for disinfecting water from a central circulating hot water supply system according to (1) or (2), wherein the water temperature of the water supplied from the storage tank of the central circulating hot water supply system to the hot water supply pipe is 35°C to 65°C at the outlet from the storage tank to the hot water supply pipe.

[0017] (5) A method for disinfecting water from a central circulating hot water supply system as described in any one of (1) or (2) to (4), wherein the disinfection method is a method for removing Legionella bacteria. [Effects of the Invention]

[0018] The present invention relates to a method for disinfecting water in a central circulating hot water supply system, wherein by adding at least one of monochloramine or chloramine T to the water in the central circulating hot water supply system, it exhibits excellent disinfecting effects over a long period of time even in high-temperature water, and allows for improved metal corrosion compared to conventionally used disinfectants. [Brief explanation of the drawing]

[0019] [Figure 1] This figure shows the results of a temperature stability test of a chemical at 25°C. [Figure 2] This figure shows the results of a temperature stability test of a chemical at 35°C. [Figure 3]Figure showing the results of the temperature stability test of the chemical at 45°C. [Figure 4] Figure showing the results of the temperature stability test of the chemical at 55°C. [Figure 5] Figure showing the results of the corrosion voltage measurement of each chemical at 45°C as an evaluation of the corrosiveness of the chemical to stainless steel. [Figure 6] Schematic diagram of the central circulation type hot water supply system 1000.

Embodiments for Carrying Out the Invention

[0020] Hereinafter, an example of an embodiment of the present invention will be described. The method for sterilizing the water system water of the central circulation type hot water supply system of the present invention is a sterilization method including an addition step of adding monochloramine or chloramine T alone or in combination.

[0021] The monochloramine used in the present invention is monochloroamine, and can be prepared, for example, by reacting sodium hypochlorite with an ammonium salt. Also, chloramine T is N-chloro-4-methylbenzenesulfonamide. Monochloramine and chloramine T do not generate free residual chlorine in water, and the combined residual chlorine shows an oxidizing power as total residual chlorine, contributing to the killing and growth inhibition of various microorganisms.

[0022] The addition concentration of monochloramine or chloramine T alone or in combination to the water system water in the present invention is added so that the total chlorine concentration of the central circulation type hot water supply system water is maintained within the range of 0.5 mg / L to 100 mg / L, preferably 1 mg / L to 100 mg / L. Thereby, a continuous bactericidal effect against Legionella bacteria can be obtained in particular. For example, monochloramine or chloramine T can be added to the circulation system of the hot water supply to adjust the concentration.

[0023] Furthermore, regarding metal corrosion, in cases where at least some of the piping through which the water system water of a central circulating hot water supply system flows is made of metal, conventionally used free chlorine agents increase the corrosion potential of stainless steel and other materials, causing pitting corrosion. However, when monochloramine or chloramine T added in the present invention is added within the aforementioned concentration range, the corrosion potential is more than 100 mV lower than that of free chlorine agents, thus preventing corrosion of high-temperature hot water supply piping.

[0024] Furthermore, when using water supply and hot water for dialysis in hospitals and other facilities, the residual chlorine concentration in the water system can sometimes be a problem. However, monochloramine and chloramine T can be removed by an activated carbon filter (SV50 or less) built into a standard dialysis water production system, thus reducing the residual chlorine concentration through filtration.

[0025] Compared to free chlorine, monochloramine and chloramine T experience less concentration reduction at high temperatures, making it easier to maintain a high concentration. Even when stagnant water occurs in hot water piping such as hot water supply and return pipes, or when water remains stagnant for extended periods in infrequently used mixing faucets, residual chlorine is maintained for a long time, preventing microbial growth, biofilm establishment, and the establishment and proliferation of Legionella bacteria. Therefore, it is possible to use monochloramine or chloramine T at a lower addition frequency than when adding sodium hypochlorite. For example, when the water temperature is 55°C, if monochloramine or chloramine T is added to achieve a residual chlorine concentration of 3 mg / L in the water system, it will take more than 10 days for the residual chlorine concentration to reach 1.5 mg / L (Figure 4). Therefore, if a residual chlorine concentration of 1.5 mg / L is set as the lower limit maintenance concentration, monochloramine or chloramine T should be added to the water system at 10-day intervals. On the other hand, when sodium hypochlorite is added to aqueous water, the residual chlorine concentration becomes 1.5 mg / L in about two days under the same conditions, so it is necessary to add sodium hypochlorite at intervals of about two days (Figure 4).

[0026] As agents that can be used in combination with monochloramine or chloramine T of the present invention, to the extent that the effects of the present invention are not hindered, and for the purpose of further improving the disinfecting effect, known corrosion inhibitors that have been conventionally used in water treatment applications, such as nitrite, phosphoric acid, silicic acid, molybdic acid, tungstic acid, aluminic acid, boric acid, oxyacids, amino acids, aliphatic organic acids, aromatic carboxylic acids, lignin sulfonic acid, or salts thereof, as well as tannins, lignin, or azoles such as toltriazole, benzotriazole, mercaptobenzothiazole, etc., known corrosion inhibitors such as zinc salts, detergents such as anionic surfactants and nonionic surfactants, dispersants, etc., can be used individually or in combination as appropriate. The use of these corrosion inhibitors in combination is also included in the present invention. [Examples]

[0027] The following describes specific examples of the disinfection method of the present invention, but the present invention is not limited thereto.

[0028] [Temperature stability test of chemicals] (Test method) The following procedure was used to adjust the free residual chlorine concentration of the sodium hypochlorite aqueous solution, and the total residual chlorine concentration of the monochloramine or chloramine T aqueous solution, as well as to perform a stability test by storing the test water in a constant temperature chamber and measuring the chlorine concentration over time. (1) Sodium hypochlorite is added to dechlorinated water from Tsukuba City to prepare test water with a residual chlorine concentration (free residual chlorine concentration) of 3 mg / L. (2) Prepare test water with a residual chlorine concentration (total residual chlorine concentration) of 3 mg / L by adding monochloramine or chloramine T to Tsukuba City water that has been dechlorinated in the same manner as in (1). (3) Prepare several pouch bags with stoppers, fill them to the brim with the prepared test waters (1) and (2), and seal them tightly. This will put the test water (1) into four pouch bags, and similarly put the test water with monochloramine added and the test water with chloramine T added into four pouch bags each. (4) One pouch containing the test water from (1), one pouch containing the test water with monochloramine added, and one pouch containing the test water with chloramine T added were placed in a constant temperature chamber (dark place at 55°C) to maintain the temperature of the test water at 55°C. Every two to three days, the test water was removed from the constant temperature chamber and the residual chlorine concentration was measured. Similarly, one pouch was placed in each of three constant temperature chambers (dark places at 45°C, 35°C, and 25°C) and the residual chlorine concentration was measured in the same manner. Note that the test water with chloramine T added was not tested in the 25°C and 35°C tests. The residual chlorine concentration was measured using the DPD free chlorine concentration method and the DPD total chlorine concentration method.

[0029] Figures 1-4 show the results of measuring the change in total residual chlorine concentration over time when each test water was held at each temperature. In Figures 1-4, the results for "sodium hypochlorite" are those for test water (1).

[0030] The results shown in Figures 1-4 indicate that, compared to adding sodium hypochlorite, adding monochloramine and chloramine T resulted in a less significant decrease in residual chlorine concentration, allowing for longer intervals between chemical additions. In particular, monochloramine and chloramine T showed less decrease in residual chlorine concentration even at high temperatures of 45°C and 55°C, indicating that they are less prone to decomposition at high temperatures and have excellent storage stability.

[0031] [Corrosion testing of chemicals on stainless steel] The free chlorine concentration of the sodium hypochlorite aqueous solution and the total chlorine concentration of the monochloramine or chloramine T aqueous solution were adjusted and corrosiveness tests were performed according to the following procedure.

[0032] (Test method) 900 mL of dechlorinated water from Tsukuba City was placed in a plastic container and heated to 45°C. A SUS304 test piece was then partially immersed in the Tsukuba City water in the container, and the corrosion potential was measured over four days while the water was stirred at 300 rpm. Figure 5 shows the results for the "untreated" measurement, without any added chemicals.

[0033] Similarly, sodium hypochlorite was added to dechlorinated water from Tsukuba City to prepare test water with residual chlorine concentrations of 3 mg / L and 10 mg / L. 900 mL of each was placed in separate plastic containers and heated to 45°C. A SUS304 test piece was then partially immersed in the test water in the plastic container, and the corrosion potential was measured over four days while stirring the test water at 300 rpm. The results for the test water with a free residual chlorine concentration of 3 mg / L are labeled "Sodium Hypochlorite (3)," and the results for the test water with a free residual chlorine concentration of 10 mg / L are labeled "Sodium Hypochlorite (10)." These results are shown in Figure 5.

[0034] Similarly, monochloramine was added to dechlorinated Tsukuba city water to prepare test water with residual chlorine concentrations of 3 mg / L and 10 mg / L. 900 mL of each was placed in separate plastic containers and heated to 45°C. A SUS304 test piece was then partially immersed in the test water in the plastic container, and the corrosion potential was measured over four days while stirring the test water at 300 rpm. The results for the test water with a residual chlorine concentration of 3 mg / L are labeled "Monochloramine (3)," and the results for the test water with a residual chlorine concentration of 10 mg / L are labeled "Monochloramine (10)." These results are shown in Figure 5.

[0035] Similarly, chloramine T was added to dechlorinated water from Tsukuba City to prepare test water with residual chlorine concentrations of 3 mg / L and 10 mg / L. 900 mL of each was placed in separate plastic containers and heated to 45°C. A SUS304 test piece was then partially immersed in the test water in the plastic container, and the corrosion potential was measured over four days while stirring the test water at 300 rpm. The results for the test water with a residual chlorine concentration of 3 mg / L are labeled "Chloramine T(3)," and the results for the test water with a residual chlorine concentration of 10 mg / L are labeled "Chloramine T(10)." These results are shown in Figure 5.

[0036] As shown in Figure 5, the corrosion potential of the untreated water remained around -50 mVAg-AgCl, while that of the test water to which sodium hypochlorite was added remained around 150 mVAg-AgCl at a concentration of 3 mg / L and around 50 mVAg-AgCl at a concentration of 10 mg / L. On the other hand, the test water to which monochloramine was added and the test water to which chloramine T was added remained around 0 mVAg-AgCl at concentrations of 3 mg / L and 10 mg / L, respectively, indicating that the test water to which monochloramine and chloramine T were added exhibited superior corrosion resistance.

[0037] [Bactericidal effect against Legionella bacteria] The bactericidal effect against Legionella bacteria was assessed using the following procedure.

[0038] (Test method) (1) Legionella bacteria were inoculated into test water, which was concentrated by circulating water from Tsukuba City at 30°C using a circulation device, by taking a colony of Legionella cultured in a culture medium and suspending it in the test water. (2) After inoculating the test water with Legionella bacteria according to (1), each drug was added to the test water so that the residual chlorine concentration of the test water was 3 mg / L. The test water was then kept at a temperature of 30°C and allowed to stand. After 30 minutes, 1 hour, 3 hours, and 6 hours, the test water was collected and neutralized with sodium thiosulfate. Subsequently, the number of Legionella bacteria was measured by performing a culture test according to ISO 11731:2017.

[0039] For reference, Table 1 shows the water quality of concentrated water from Tsukuba City. In Table 1, the unit is mg / L, except for pH.

[0040] [Table 1]

[0041] Table 2 shows the number of Legionella bacteria after performing the Legionella culture test. Legionella bacteria were detected in untreated water without any added chemicals, but no Legionella bacteria were detected in the test water to which sodium hypochlorite was added, indicating a good bactericidal effect. In addition, no Legionella bacteria were detected in the test water to which monochloramine and chloramine T were added, indicating good bactericidal activity.

[0042] [Table 2] [Explanation of symbols]

[0043] 100 Elevated water tanks 200 Water supply pipe 210 Piping 220 Piping 230 Hot water pipe 240 branch piping 250 Return water pipe 300 Hot water storage tank 400 heating machine 500 Heating and Circulation Pump 1000 Central circulating hot water supply system

Claims

1. The process includes adding monochloramine or chloramine T, either alone or in combination, to the water system of a central circulating hot water supply system. A method for disinfecting water from a central circulating hot water supply system, wherein the addition step is to add monochloramine or chloramine T alone or in combination so as to maintain the total chlorine concentration of the water system water at 0.5 mg / L or more and 100 mg / L or less.

2. The method for disinfecting water in a central circulating hot water supply system according to claim 1, wherein the piping through which the water in the central circulating hot water supply system flows is made of metal.

3. A method for disinfecting water from a central circulating hot water supply system according to claim 1 or 2, wherein the water temperature of the water supply system supplied from the storage tank of the central circulating hot water supply system to the hot water supply pipe is 35°C to 65°C at the outlet from the storage tank to the hot water supply pipe.

4. A method for disinfecting water from a central circulating hot water supply system according to claim 1 or 2, wherein the disinfection method is a method for removing Legionella bacteria.

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