Drinking water production method and drinking water production system

Abstract

To provide a method of producing potable water and a system of producing potable water, capable of sufficiently removing ammonia nitrogen in water to be treated and producing potable water of stable water quality at a low cost in a simple manner.SOLUTION: The method of producing potable water comprises: a membrane filtration step of treating water W1 to be treated including ammonia nitrogen using a reverse osmosis membrane 5 to obtain permeated water W2; a hypochlorite mixing step of mixing the permeated water W2 and a hypochlorite to obtain oxidatively treated water W3; a total chlorine removal step of removing total chlorine contained in the oxidatively treated water W3 to obtain total chlorine removed water W4; and a disinfectant mixing step of mixing the total chlorine removed water W4 and a disinfectant to obtain treated water W5. In the hypochlorite mixing step, hypochlorite is mixed with the permeated water W2 in an amount capable of making the free chlorine concentration in the oxidatively treated water w3 equal to or higher than 0.01 mg / L.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a drinking water production method and a drinking water production system. [Background technology]

[0002] Drinking water is sometimes produced by removing ammonia from treated water containing ammonia nitrogen. To produce safe drinking water, hypochlorite is used to meet water quality standards after removing the ammonia nitrogen from the treated water. Chlorine derived from this hypochlorite may be contained in the drinking water as residual chlorine. Residual chlorine can be broadly classified into free chlorine and combined chlorine.

[0003] Patent Document 1 describes a method for producing treated water in which groundwater is directly supplied to a reverse osmosis membrane and the permeate of the reverse osmosis membrane is supplied to a cation exchange resin. In this method, cations such as ammonia ions contained in the permeate of the reverse osmosis membrane are removed by the cation exchange resin. Thereafter, the ammonia in the permeate from which the cations have been removed by the cation exchange resin is decomposed by hypochlorite in an ammonia decomposition means, thereby obtaining treated water. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-107591 Summary of the Invention [Problem to be solved by the invention]

[0005] In the production of drinking water, if ammonia remains in the water when a disinfectant is mixed, the disinfectant, such as hypochlorous acid, reacts with ammonia and combined chlorine, causing the water quality, such as the residual chlorine concentration, to fluctuate and become unstable. Therefore, it is necessary to sufficiently remove ammoniacal nitrogen from the water to be treated. However, the method of Patent Document 1, which uses a cation exchange resin, has difficulty in adsorbing and completely removing ammonia in the water permeated through the reverse osmosis membrane. In particular, when the reverse osmosis membrane is clogged or restarted, the amount of ammonia in the permeated water may fluctuate, which may lead to fluctuations in the amount of ammonia leaking from the cation exchange resin. Therefore, the manufacturing method of Patent Document 1 also requires a control system that controls the amount of hypochlorite used based on the amount of ammonia in the cation exchange resin-treated water. As a result, the manufacturing method of Patent Document 1 makes the entire process of producing drinking water complicated.

[0006] The present invention provides a drinking water production method and drinking water production system that can sufficiently remove ammonia nitrogen from water to be treated and can inexpensively and easily produce drinking water of stable quality. [Means for solving the problem]

[0007] The present invention has the following aspects. [1] A method for producing drinking water, comprising: a membrane filtration step in which water to be treated containing ammonia nitrogen is treated using either a reverse osmosis membrane or a nanofiltration membrane, or both, to obtain permeate; a hypochlorite mixing step in which the permeate is mixed with hypochlorite to obtain oxidized water; a total chlorine removal step in which all chlorine contained in the oxidized water is removed to obtain total chlorine-removed water; and a disinfectant mixing step in which the total chlorine-removed water is mixed with a disinfectant to obtain treated water; wherein in the hypochlorite mixing step, the permeate is mixed with an amount of hypochlorite such that the free chlorine concentration of the oxidized water is 0.01 mg / L or more. [2] The method for producing drinking water according to [1], wherein the total chlorine removal step uses activated carbon to remove the total chlorine from the oxidized water. [3] The method for producing drinking water according to [1] or [2], wherein either or both of the reverse osmosis membrane and the nanofiltration membrane have a polyamide resin membrane. [4] The method for producing drinking water according to any one of [1] to [3], wherein hypochlorite is used as the disinfectant in the disinfectant mixing step. [5] A drinking water production system comprising: a membrane filtration means for treating water to be treated containing ammonia nitrogen using either or both of a reverse osmosis membrane and a nanofiltration membrane to obtain permeate; a hypochlorite mixing means for mixing the permeate with hypochlorite to obtain oxidized water; a total chlorine removal means for removing all chlorine contained in the oxidized water to obtain total chlorine-removed treated water; and a disinfectant mixing means for mixing the total chlorine-removed treated water with a disinfectant to obtain treated water; wherein the hypochlorite mixing means mixes the hypochlorite with the permeate in an amount such that the free chlorine concentration of the oxidized water is 0.01 mg / L or more. [6] The drinking water production system according to [5], wherein the total chlorine removal means removes the total chlorine from the oxidized water using activated carbon. [7] The drinking water production system according to [5] or [6], wherein either or both of the reverse osmosis membrane and the nanofiltration membrane have a polyamide resin membrane. [8] The drinking water production system according to any one of [5] to [7], wherein the disinfectant mixing means uses hypochlorite as the disinfectant. [Effects of the Invention]

[0008] According to the present invention, a drinking water production method and drinking water production system are provided that can sufficiently remove ammonia nitrogen from water to be treated and can inexpensively and easily produce drinking water of stable quality. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic configuration diagram showing an example of a drinking water production system according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] As used herein, the following terms have the following meanings: "Ammoniacal nitrogen" refers to nitrogen contained in water as an ammonium salt. It is also called ammoniacal nitrogen. "Mixing" is a term that encompasses the concept of "contact." The symbol "to" indicating a range of values ​​means that the values ​​before and after it are included as the lower and upper limits.

[0011] <Drinking water production system> The drinking water production system of this embodiment comprises: a membrane filtration means for treating water to be treated containing ammonia nitrogen using either a reverse osmosis membrane or a nanofiltration membrane, or both, to obtain permeate; a hypochlorite mixing means for mixing the permeate with hypochlorite to obtain oxidized water; a total chlorine removal means for removing all chlorine contained in the oxidized water to obtain total chlorine-removed treated water; and a disinfectant mixing means for mixing the total chlorine-removed treated water with a disinfectant to obtain treated water; and the hypochlorite mixing means mixes the permeate with hypochlorite in an amount such that the free chlorine concentration in the oxidized water is 0.01 mg / L or more. Hereinafter, an example of a drinking water production system according to this embodiment will be described with reference to the drawings.

[0012] FIG. 1 is a schematic diagram showing an example of a drinking water production system according to this embodiment. The drinking water production system 1 comprises a raw water tank 2 for storing raw water containing ammonia nitrogen; a raw water supply pump 3 for the raw water tank 2; a raw water pretreatment means 4; a membrane filtration means 6 for treating water to be treated W1 containing ammonia nitrogen using a reverse osmosis membrane 5 to obtain permeated water W2; a hypochlorite mixing means 7 for mixing the permeated water W2 with hypochlorite to obtain oxidized water W3; a total chlorine removal means 8 for removing all chlorine contained in the oxidized water W3 to obtain total chlorine-removed water W4; and a total chlorine removal treatment means 9 for treating the water W1. The system comprises a disinfectant mixing means 9 for mixing treated water W4 with a disinfectant to obtain treated water W5; a line L1 having a first end connected to the raw water tank 2 and a second end connected to the primary side of the membrane filtration means 6; a line L2 having a first end connected to the secondary side of the membrane filtration means 6 and a second end connected to the primary side of the total chlorine removal means 8; a water quality meter C1 provided on line L2; a line L3 having a first end connected to the secondary side of the total chlorine removal means 8; and a water quality meter C2 provided on line L3.

[0013] In the drinking water production system 1, a water supply pump 3 and a pretreatment means 4 are provided in this order on the line L1 from the primary side, i.e., from the side of the raw water tank 2. Raw water from the raw water tank 2 is supplied to the pretreatment means 4 by the water supply pump 3.

[0014] The raw water is not particularly limited as long as it contains at least ammonia nitrogen. Examples of raw water include groundwater, well water, lake water, river water, etc. The quality of these various raw waters may vary depending on the region, but this difference in water quality is acceptable in the present invention. From the viewpoint of securing a water source, groundwater is preferable as the raw water. Groundwater is water present underground, specifically, water present in pore spaces in geological strata.

[0015] Raw water may contain various substances other than ammonia nitrogen depending on the source and type of water. Examples include various minerals derived from minerals (e.g., calcium ions, sodium ions, potassium ions, magnesium ions, etc.), chloride ions, sulfate ions, iron ions, silica, etc. The soluble silicic acid concentration (silica concentration) is preferably within a range of, for example, 10 mg / L to 90 mg / L. The sodium ion concentration is preferably 6000 mg / L or less, more preferably 500 mg / L or less, and even more preferably 300 mg / L or less. When the soluble silicic acid concentration and sodium ion concentration are within the above ranges, the effects of applying the present invention are more pronounced. In addition, raw water may contain organic matter and various bacteria. Examples of the main components of organic matter include humic acid and fulvic acid. However, raw water may contain organic matter other than these exemplified components. The total organic carbon (TOC) concentration is preferably 50 mg / L or less, more preferably 30 mg / L or less, even more preferably 10 mg / L or less, and particularly preferably 5 mg / L or less. When the TOC is within the above numerical range, the effect of applying the present invention can be obtained more significantly.

[0016] In the pretreatment means 4, the quality of the raw water is adjusted to become water to be treated W1. The pretreatment means 4 is appropriately selected depending on the quality of the raw water and the quality required for the water to be treated W1, and examples thereof include sand filtration means, pH adjustment means, flocculation means, and decarbonization treatment means. Although the drinking water production system 1 of this embodiment is provided with a pretreatment means 4, the pretreatment means is not an essential component. In other embodiments, if the quality of the raw water is good, the pretreatment means does not need to be installed. In this embodiment, the "water to be treated" is water containing ammonia nitrogen, and can also be said to be water supplied to the primary side of the membrane filtration means.

[0017] The content of ammonia nitrogen in the water to be treated W1 is not particularly limited and is, for example, within the range of 0.1 to 60 mg / L. The ammoniacal nitrogen concentration is preferably 50 mg / L or less, more preferably 25 mg / L or less, even more preferably 20 mg / L or less, and particularly preferably 10 mg / L or less. When the ammoniacal nitrogen concentration in the water to be treated W1 is equal to or less than the upper limit, the ammoniacal nitrogen concentration in the permeate W2 is easily suppressed, and the amount of hypochlorite used can be easily reduced. The ammoniacal nitrogen concentration is preferably 0.5 mg / L or more, more preferably 1 mg / L or more, and even more preferably 2 mg / L or more. Within this range, the applicability of the present invention is further improved.

[0018] The membrane filtration means 6 includes a reverse osmosis membrane 5 and a line L4 through which concentrated water from the reverse osmosis membrane 5 flows. A first end of a line L2 is connected to the secondary side of the reverse osmosis membrane 5. Permeated water W2 from the reverse osmosis membrane 5 is led out to the line L2. A first end of line L4 is connected to the secondary side of the reverse osmosis membrane 5, and a second end thereof branches into lines L4A and L4B. The branched line L4A is connected to the middle of line L1 between the pretreatment means 4 and the membrane filtration means 6. According to the membrane filtration means 6, a portion of the concentrated water can be resupplied to the primary side of the reverse osmosis membrane 5 using line L4A, and the remainder of the concentrated water can be discharged outside the system using line L4B. A pH adjusting means for adjusting the pH of the permeate water W2 may be provided downstream of the membrane filtration means 6.

[0019] There is no particular limitation on the reverse osmosis membrane 5. In this embodiment, the membrane filtration means 6 has a reverse osmosis membrane 5, but in other embodiments, the membrane filtration means may have a nanofiltration membrane, or may have both a reverse osmosis membrane and a nanofiltration membrane. The shape of the reverse osmosis membrane 5 (or nanofiltration membrane in another embodiment) is not particularly limited, and examples thereof include a spiral membrane and a hollow fiber membrane. Commercially available reverse osmosis membranes and nanofiltration membranes may be used, such as spiral reverse osmosis membrane elements (manufactured by Nitto Denko Corporation, "ESPA2-LD").

[0020] Examples of materials for the reverse osmosis membrane 5 (or nanofiltration membrane in another embodiment) include polyamide, polysulfone, cellulose acetate, polyacrylonitrile, etc. A polyamide membrane is preferred because it is easy to ensure the amount of permeate water W2 and is readily available. Furthermore, by providing a membrane filtration means on the upstream side of the hypochlorite mixing means as in this embodiment, membrane deterioration caused by chlorination of the amide bond of polyamide by hypochlorite can be prevented, which is a major advantage as it allows polyamide membranes, which are easy to obtain and can easily ensure a sufficient permeate flow rate, to be used continuously for a long period of time.

[0021] The hypochlorite mixing means 7 includes a tank 10 for storing hypochlorite, a line L5 having a first end connected to the tank 10 and a second end connected to the middle of the line L2, and a pump 11 provided on the line L5. The hypochlorite mixing means 7 can mix the hypochlorite in the tank 10 with the permeated water W2 led out to the line L2.

[0022] In the hypochlorite mixing means 7, ammoniacal nitrogen remaining in the permeate W2 is decomposed by hypochlorite supplied from a tank 10 of the hypochlorite mixing means 7. When hypochlorite dissolves in water, it reacts with water to produce hypochlorous acid (HClO). Regarding the decomposition of ammoniacal nitrogen, the reaction between ammonia (NH3) and hypochlorous acid (HClO) is thought to proceed as shown in the following formulas (1) to (3). NH3+HClO→NH2Cl+H2O...Formula (1) NH2Cl+HClO→NHCl2+H2O...Formula (2) NHCl2+HClO→NCl3+H2O...Formula (3)

[0023] As shown in the above formulas (1) to (3), combined chlorine in monochloramine (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3) is sequentially produced, and hypochlorous acid (HClO) is consumed, resulting in the permeate water W2 becoming oxidized water W3. Therefore, in the drinking water production system 1, when free chlorine such as HClO is detected by the water quality meter C1, it can be determined that all ammonia nitrogen has been completely oxidized. Therefore, it is possible to determine whether the decomposition reaction of ammonia nitrogen has been completed based on the free chlorine concentration of the oxidized water W3.

[0024] Specifically, the hypochlorite mixing means 7 mixes hypochlorite with the permeated water W2 in an amount such that the free chlorine concentration in the oxidized water W3 becomes 0.01 mg / L or more. The amount of hypochlorite used may be calculated based on the molar equivalent of ammonia nitrogen in the permeated water W2. For example, the amount of hypochlorite required to completely react with all of the ammoniacal nitrogen in the permeate W2 can be calculated from the molar equivalent of the chemical reaction formula. To completely oxidize and decompose all of the ammoniacal nitrogen, a larger amount of hypochlorite than the molar equivalent of the ammoniacal nitrogen is used.

[0025] The free chlorine concentration in the present invention is measured by a polarographic method. The pH for this measurement is 1 to 10. If the pH of the solution to be measured exceeds 10, it is adjusted to 1 to 10 with sulfuric acid, and if the pH is less than 1, it is adjusted to 1 to 10 with sodium hydroxide solution. The free chlorine concentration can also be measured by the diethylparaphenylenediamine method (DPD method), but the current method (polarographic method) is preferred from the viewpoint of ease of remote monitoring of the water treatment system.

[0026] The amount of hypochlorite used in the hypochlorite mixing means 7 is preferably set appropriately according to the target value (0.01 mg / L or more) of the free chlorine concentration of the oxidation-treated water W3. The preferred numerical range of the free chlorine concentration of the oxidation-treated water W3 will be described later.

[0027] The hypochlorite in the tank 10 may be in a solid form (powder) or in a solution form. However, a solution form is preferred in terms of ease of mixing. Examples of hypochlorite include sodium hypochlorite, calcium hypochlorite, and magnesium hypochlorite. In terms of adjusting the amount used, an aqueous sodium hypochlorite solution is preferred. When using a sodium hypochlorite aqueous solution, the concentration of the sodium hypochlorite aqueous solution is preferably adjusted based on the ammoniacal nitrogen concentration of the permeated water W2. Specifically, for example, the ammoniacal nitrogen concentration of the permeated water W2 is measured at the beginning of the flow of the reverse osmosis membrane 5. Then, assuming that the rejection rate of the reverse osmosis membrane will decrease, the concentration and amount of the sodium hypochlorite aqueous solution to be used are set based on the amount of ammoniacal nitrogen in the permeated water W2 at the beginning of the flow of the water.

[0028] The water quality meter C1 is not particularly limited as long as it can measure the free chlorine concentration of the oxidized water W3 by the current method (polarography). The water quality meter C1 may additionally have the function of measuring, for example, pH, conductivity, water temperature, etc. in addition to the free chlorine concentration.

[0029] The total chlorine removal means 8 (e.g., a chlorine removal tower) uses activated carbon to remove all chlorine from the oxidation-treated water W3, converting the oxidation-treated water W3 into total chlorine-removed treated water W4. Here, the total amount of chlorine is the sum of the amount of combined chlorine and the amount of free chlorine. By removing all chlorine, particularly combined chlorine, from the oxidation-treated water W3, the removal of ammonia nitrogen contained in the water to be treated W1 is completed. In this embodiment, activated carbon is used as the total chlorine removal means 8, but any total chlorine removal material other than activated carbon can be used as long as it can remove all chlorine from the oxidized water W3. Examples of total chlorine removal materials other than activated carbon include reducing agents such as sodium thiosulfate and sodium hydrogen sulfite.

[0030] The disinfectant mixing means 9 includes a tank 12 for storing hypochlorite; a line L6 having a first end connected to the tank 12 and a second end connected to the middle of the line L3; and a pump 13 provided on the line L6. The disinfectant mixing means 9 mixes the hypochlorite in the tank 12 with the total chlorine-removed treated water W4 to disinfect the total chlorine-removed treated water W4, producing treated water W5 that meets the drinking water standards and tap water standards. The water quality meter C2 is not particularly limited as long as it can measure the free chlorine concentration of the treated water W5. The water quality meter C2 may additionally have the function of measuring, for example, pH, conductivity, water temperature, etc. in addition to the free chlorine concentration.

[0031] The treated water W5 is drinking water that meets drinking water standards and tap water standards. The water quality standards imposed on drinking water and tap water vary by region or country. Therefore, the disinfectant can be changed appropriately depending on the region or country. Examples of disinfectants include sodium hypochlorite, chlorine dioxide, chloramine, and chlorine (Cl2). In Japan, sodium hypochlorite is used as the disinfectant.

[0032] <Drinking water production method> The drinking water production method of this embodiment includes a membrane filtration process in which water to be treated containing ammonia nitrogen is treated using either a reverse osmosis membrane or a nanofiltration membrane, or both, to obtain permeate; a hypochlorite mixing process in which the permeate is mixed with hypochlorite to obtain oxidized water; a total chlorine removal process in which all chlorine contained in the oxidized water is removed to obtain total chlorine-removed water; and a disinfectant mixing process in which the total chlorine-removed water is mixed with a disinfectant to obtain treated water.In the hypochlorite mixing process, hypochlorite is mixed with the permeate in an amount such that the free chlorine concentration in the oxidized water is 0.01 mg / L or more. An example of the drinking water production method of this embodiment will be described below with reference to the drinking water production system 1.

[0033] First, in the drinking water production system 1, raw water from a raw water tank 2 is supplied to a pretreatment means 4 by a water supply pump 3. The pretreatment means 4 adjusts the quality of the raw water, and water to be treated W1 is obtained. However, pretreatment is not essential in this embodiment. For example, if the quality of the raw water is good, pretreatment can be omitted.

[0034] Next, in the membrane filtration step, the water to be treated W1 is passed through a reverse osmosis membrane 5, and permeate W2 is obtained from the secondary side of the reverse osmosis membrane 5. The amount of water to be treated W1 introduced into the reverse osmosis membrane 5 may be appropriately set depending on the treatment capacity of the reverse osmosis membrane 5. The membrane filtration flux is, for example, 0.1 to 2.0 m 3 / m 2 / Day is preferable, 0.2 to 0.6 m 3 / m 2 / Day is more preferable, 0.25 to 0.5m 3 / m 2 / Day is even more preferable.

[0035] Permeated water W2 from the reverse osmosis membrane 5 is discharged to line L2. Concentrated water from the reverse osmosis membrane 5 is discharged to line L4. A portion of the concentrated water is resupplied to the primary side of the reverse osmosis membrane 5 as treated water W1 via lines L4A and L1. The remainder of the concentrated water is discharged to the outside of the system via line L4B. In this embodiment, a reverse osmosis membrane 5 is used in the membrane filtration step, but in other embodiments, a nanofiltration membrane may be used in the membrane filtration step, or a reverse osmosis membrane and a nanofiltration membrane may be used in combination.

[0036] The pH of the permeated water W2 before mixing with hypochlorite is preferably 8.6 or less, more preferably 7 or less. If the pH of the permeated water W2 is below the upper limit, ammonium ions will be predominant among the ammoniacal nitrogen in the permeated water W2. Therefore, ammoniacal nitrogen can be easily removed efficiently by hypochlorite in the subsequent hypochlorite mixing step.

[0037] Next, in the hypochlorite mixing step, the permeated water W2 is mixed with hypochlorite to obtain oxidized water W3. Hypochlorite is mixed with the permeate W2 in line L2 from tank 10 via line L5 by pump 11. After mixing of hypochlorite, the oxidative decomposition reaction of ammonia nitrogen in the permeate W2 begins.

[0038] In the hypochlorite mixing step, hypochlorite is mixed with the permeate in an amount such that the free chlorine concentration of the oxidized water is 0.01 mg / L or more, thereby ensuring that the ammonia nitrogen remaining in the permeate W2 is oxidized and decomposed, thereby removing the ammonia nitrogen from the permeate W2. For example, by detecting free chlorine of 0.01 mg / L or more using the water quality meter C1, it can be determined that all ammonia nitrogen has been reliably oxidized and that the treatment of ammonia nitrogen has been completed.

[0039] The free chlorine concentration of the oxidized water W3 is preferably 0.1 mg / L or higher, more preferably 1 mg / L or higher, even more preferably 2 mg / L or higher, and particularly preferably 3 mg / L or higher. Even if the amount of ammonia in the permeate fluctuates when the reverse osmosis membrane is clogged or when the system is restarted, mixing hypochlorite with the permeate W2 in an amount that results in a free chlorine concentration above the lower limit can more reliably oxidize ammonia nitrogen. Therefore, when hypochlorite is used as a disinfectant in subsequent treatments, the adverse effects of residual ammonia nitrogen can be prevented, making it easier to produce safe drinking water with little fluctuation in water quality. The free chlorine concentration of the oxidized water W3 is preferably 10 mg / L or less, more preferably 8 mg / L or less, even more preferably 6 mg / L or less, and particularly preferably 4 mg / L or less. Mixing hypochlorite with the permeated water W2 in an amount that results in a free chlorine concentration below the upper limit makes it easier to prevent deterioration of the total chlorine removal means 8 installed downstream due to free chlorine. In addition, the amount of hypochlorite used can be reduced, which is advantageous in terms of cost. These upper and lower limits of the free chlorine concentration can be combined in any desired manner.

[0040] The amount of hypochlorite used in the hypochlorite mixing step can be appropriately set according to the target value of the free chlorine concentration of the oxidized water W3 (0.01 mg / L or more). The amount of hypochlorite used can be adjusted based on the amount of ammoniacal nitrogen in the permeated water W2. In order to reliably oxidize and decompose the ammoniacal nitrogen in the permeated water W2, it is preferable to measure the ammoniacal nitrogen concentration of the permeated water W2 at the beginning of its passage through the reverse osmosis membrane 5. This is because the amount of hypochlorite can be calculated and set so that the free chlorine concentration of the oxidized water W3 is more reliably 0.01 mg / L or more, and ammoniacal nitrogen can be more reliably and sufficiently removed.

[0041] Next, in the total chlorine removal step, all chlorine is removed from the oxidation-treated water W3 to obtain total chlorine-removed water W4. By removing all chlorine, especially combined chlorine, from the oxidation-treated water W3, the removal of ammonia nitrogen contained in the water to be treated is completed. In this embodiment, all chlorine is removed from the oxidation-treated water W3 using activated carbon in the total chlorine removal means 8. However, any total chlorine removal material other than activated carbon can be used as long as it can remove all chlorine from the oxidation-treated water W3.

[0042] The rate at which the oxidized water W3 is passed through the total chlorine removal means in the total chlorine removal step is not particularly limited. For example, the space velocity (SV) is 1 to 200 h -1 is preferable, and 3 to 100 hours -1 More preferably, 5 to 30 hours -1Furthermore, the linear velocity (LV) is preferably from 1 to 100 m / h, more preferably from 3 to 50 m / h, and even more preferably from 6 to 30 m / h. When the water flow rate is equal to or higher than the lower limit of the above range, the productivity of drinking water is excellent and sufficient treatment efficiency can be easily ensured.When the water flow rate is equal to or lower than the upper limit of the above range, the consumption of total chlorine removal material such as activated carbon can be reduced, which is advantageous in terms of cost.

[0043] In the disinfectant mixing process, the totally chlorine-removed treated water W4 is mixed with a disinfectant to obtain treated water W5. The type of disinfectant can be varied depending on the region or country. In Japan, sodium hypochlorite is used as the disinfectant. Japanese tap water standards require a free chlorine concentration of 0.1 mg / L or more, or a combined chlorine concentration of 0.4 mg / L or more. The amount of sodium hypochlorite used as a disinfectant is adjusted to meet these water quality standards.

[0044] <Mechanism of action> In the present embodiment described above, water to be treated containing ammoniacal nitrogen is treated using either or both of a reverse osmosis membrane and a nanofiltration membrane to obtain permeate. Ammoniacal nitrogen may remain in the permeate, and the amount of remaining ammoniacal nitrogen may vary depending on membrane blockage and the operating conditions of the membrane filtration means. Therefore, in this embodiment, a sufficient amount of hypochlorite is mixed with the ammonia nitrogen in the permeate to oxidize the ammonia nitrogen in the permeate. Specifically, an amount of hypochlorite is mixed with the permeate so that the free chlorine concentration of the oxidized water after oxidation is 0.01 mg / L or more. The amount of hypochlorite that results in a free chlorine concentration of 0.01 mg / L or more in the oxidized water is set to be larger than the reaction equivalent of hypochlorite that undergoes an oxidation reaction with the amount of ammonia nitrogen in the permeate. Therefore, even if the amount of ammonia nitrogen in the permeate fluctuates, the amount of hypochlorite used is set to be larger than the reaction equivalent of hypochlorite that undergoes an oxidation reaction. Therefore, even if the amount of ammonia nitrogen in the permeate fluctuates, the ammonia nitrogen can be reliably oxidized and decomposed because the amount of hypochlorite used is set to be larger than the reaction equivalent. According to this embodiment, all of the ammoniacal nitrogen in the permeate can be reliably oxidized, and therefore the ammoniacal nitrogen contained in the water to be treated can be sufficiently removed by removing all chlorine, i.e., combined chlorine and free chlorine, in the subsequent stage.

[0045] In addition, the calculation of the amount of hypochlorite to be mixed with the oxidized water does not require complex calculations as long as the amount of ammonia nitrogen in the permeate is known. Therefore, by mixing a sufficient amount of hypochlorite with the oxidized water, ammonia nitrogen can be sufficiently removed from the water to be treated without the need for complex control of the amount of hypochlorite used. Since no ammoniacal nitrogen remains in the oxidized water obtained in this way, it is possible to reliably prevent reaction with ammonia when a disinfectant such as hypochlorous acid is added. Furthermore, because total chlorine, i.e., combined chlorine and free chlorine, is removed before mixing with the disinfectant, the water quality, such as the residual chlorine concentration, of drinking water is stabilized after mixing with the disinfectant. Therefore, ammonia nitrogen can be sufficiently removed from the water to be treated without requiring complex control of the amount of hypochlorite used, and drinking water of stable quality can be easily produced.

[0046] To explain the mechanism of action of the present embodiment described above in more detail, in the drinking water production system 1, hypochlorite is mixed with the permeate W2 so that the free chlorine concentration of the oxidized water W3 is 0.1 mg / L or more. Therefore, with the drinking water production system 1, the quality of the treated water W5 is likely to be stable even if the rejection rate of the reverse osmosis membrane 5 changes when the system is restarted after a shutdown or when the membrane is clogged. In addition, since no ion exchange resin is required to remove ammoniacal nitrogen, the method is suitable for space saving and cost reduction compared to conventional methods such as the manufacturing method of Patent Document 1.

[0047] The drinking water production system 1 described above has the above-mentioned configuration, and therefore, by using the drinking water production system 1, the drinking water production method according to the present embodiment can be carried out and the above-mentioned mechanism of action can be exerted.

[0048] <Other Embodiments> Although the present embodiment has been described above by showing one example, the present invention is not limited to the embodiment disclosed in this specification and can be appropriately modified and implemented without departing from the spirit of the invention. The embodiment disclosed in this specification can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. [Example]

[0049] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following description.

[0050] <Raw water> Raw water 1: Raw water with ammonia nitrogen concentration of 4.9 mg / L, hardness (calcium and magnesium) of 17 mg / L, sodium ion concentration of 250 mg / L, TOC of 9.5 mg / L, and soluble silica concentration of 54 mg / L. Raw water 2: Raw water with an ammonia nitrogen concentration of 2 mg / L, hardness (calcium and magnesium) of 86 mg / L, sodium ion concentration of 164 mg / L, TOC of 2.3 mg / L, and soluble silica concentration of 45 mg / L. Raw water 3: Raw water with an ammonia nitrogen concentration of 0.9 mg / L, hardness (calcium ion and magnesium) of 63 mg / L, sodium ion concentration of 232 mg / L, TOC of 2.3 mg / L, and soluble silica concentration of 48 mg / L.

[0051] <Test system construction> A drinking water production system 1 shown in Figure 1 was constructed. A thread-wound filter (ZW-PB10-750L manufactured by Z Corporation) was installed on line L1 as the pretreatment means 4. A reverse osmosis membrane element (ESPA2-LD manufactured by Nitto Denko Corporation) was used as the membrane filtration means 6. A concentration measuring instrument (reagent-free free chlorine meter FC400G manufactured by Yokogawa Electric Corporation) was used as the water quality meter C1 and water quality meter C2. In Examples 1 to 3 and Comparative Example 3, this drinking water production system 1 was used.

[0052] After pretreating raw water 1, water to be treated W1 is passed through a reverse osmosis membrane 5. The pH of the water to be treated W1 is 7.0, and the membrane filtration flux is 0.30 m 3 / m 2 / Day, permeate flow rate 7.0m 3 / h to obtain reverse osmosis membrane permeate water containing residual ammonia nitrogen. Next, a 6% by mass aqueous solution of sodium hypochlorite was mixed with the membrane permeate water W2 at a flow rate of 21.3 mL / min. After that, activated carbon was used to remove all chlorine from the oxidation-treated water W3. Then, a 6% by mass aqueous solution of sodium hypochlorite was mixed with the total chlorine-removed treated water W4 as a disinfectant at a flow rate of 0.4 mL / min to obtain treated water W5.

[0053] <Example 2> The raw water 2 is pretreated, and the treated water W1 is passed through a reverse osmosis membrane 5 at a pH of 7.0 and a membrane filtration flux of 0.25 m 3 / m 2 / Day, permeate flow rate 6.0m 3 / h, and a reverse osmosis membrane permeate containing residual ammonia nitrogen was obtained. Next, a 6% by mass aqueous solution of sodium hypochlorite was mixed with the membrane permeate W2 at a flow rate of 12.1 mL / min. Then, activated carbon was used to remove all chlorine from the oxidation-treated water W3. Then, a 6% by mass aqueous solution of sodium hypochlorite was mixed with the total chlorine-removed treated water W4 as a disinfectant at a flow rate of 0.3 mL / min, and treated water W5 was obtained.

[0054] Example 3 The raw water 3 is pretreated, and the treated water W1 is passed through a reverse osmosis membrane 5 at a pH of 7.0 and a membrane filtration flux of 0.40 m 3 / m 2 / Day, permeate flow rate 22.0m 3 / h to obtain reverse osmosis membrane permeate water containing residual ammonia nitrogen. Next, a 6% by mass aqueous solution of sodium hypochlorite was mixed with the membrane permeate water W2 at a flow rate of 22.2 mL / min. After that, activated carbon was used to remove all chlorine from the oxidation-treated water W3. Then, a 6% by mass aqueous solution of sodium hypochlorite was mixed with the total chlorine-removed treated water W4 as a disinfectant at a flow rate of 1.1 mL / min to obtain treated water W5.

[0055] <Comparative Example 1> In Comparative Example 1, raw water 1 was used. A comparative test apparatus 1 was used, which was configured in this order after the pretreatment means, including a hypochlorite mixing means, a membrane filtration means, a total chlorine removal means, and a disinfectant mixing means. Treated water was obtained under the same water flow conditions as in Example 1, except that in the hypochlorite addition means of the comparative test apparatus 1, 6 mass % sodium hypochlorite was mixed with the water to be treated W1 at a flow rate of 86.9 mL / min.

[0056] <Comparative Example 2> In Comparative Example 2, raw water 1 was used. A comparative test apparatus 2 was used, which was configured in this order after the pretreatment means, including a hypochlorite mixing means, a total chlorine removal means, a membrane filtration means, and a disinfectant mixing means. Treated water was obtained under the same water flow conditions as in Example 1, except that in the hypochlorite addition means of the comparative test apparatus 2, a 6% by mass aqueous sodium hypochlorite solution was mixed with the water to be treated W1 at a flow rate of 89.5 mL / min.

[0057] <Comparative Example 3> In Comparative Example 3, raw water 1 was used. Treated water was obtained in the same manner as in Example 1, except that the amount of sodium hypochlorite aqueous solution used in the hypochlorite adding means was changed to 5.3 mL / min so that the free chlorine concentration of the oxidized treated water W3 was less than 0.01 mg / L.

[0058] <Measurement method> (ammonia nitrogen concentration) The ammonia nitrogen concentrations of the water to be treated W1 and the permeate W2 were measured by the Nessler method using a HACH multi-parameter water quality analyzer DR1900 (the measurement item was "ammonia nitrogen").

[0059] (free chlorine concentration) The free chlorine concentrations of the oxidation treated water W3 and treated water W5 were measured using water quality meters C1 and C2. Yokogawa Electric Corporation's "reagent-free free chlorine meter FC400G" was used for water quality meters C1 and C2, and free chlorine concentrations were measured using the amperometric method (polarographic method). The pH of oxidation treated water W3 was 5.0-5.5, and the pH of treated water W5 was 7.0-8.0.

[0060] Table 1 shows the measurement results of the ammonia nitrogen concentrations in the water to be treated W1 and the permeate W2, and the free chlorine concentrations in the oxidized water W3 and the treated water W5 in Examples 1 to 3 and Comparative Example 3.

[0061] [Table 1]

[0062] In Examples 1 to 3, ammonia nitrogen was sufficiently removed from the water to be treated without the need for complex control of the amount of hypochlorite used, and drinking water of stable quality was produced. In contrast, in Comparative Example 1, the membrane rejection rate of the reverse osmosis membrane was reduced to 70% compared to Example 1 due to the sodium hypochlorite used as the oxidizing agent. As a result, the quality of the water permeating through the reverse osmosis membrane deteriorated and no longer met the drinking water standards, making it necessary to replace the reverse osmosis membrane. In Comparative Example 2, the amount of aqueous sodium hypochlorite solution used in the hypochlorite adding means was about 4.2 times larger than in Example 1, and the amount of activated carbon consumed was accordingly larger. In Comparative Example 3, when the rejection rate changed due to membrane clogging, the amount of sodium hypochlorite used was insufficient, resulting in a free chlorine concentration of less than 0.01 mg / L in the oxidized treated water. Because ammonia nitrogen was not sufficiently removed from the treated water, when sodium hypochlorite was added as a disinfectant, the remaining ammonia nitrogen reacted with sodium hypochlorite, causing the free chlorine concentration of the treated water to fluctuate within a range of 0.3 to 1.5 mg / L. Thus, the quality of the treated water was highly unstable. Therefore, in order to meet the drinking water standards, a complicated process was required, such as controlling the amount of disinfectant (e.g., sodium hypochlorite) added depending on the amount of ammonia nitrogen. Furthermore, the free chlorine concentration of the treated water may increase due to fluctuations, which may corrode piping and equipment. [Explanation of symbols]

[0063] 1. Drinking water production system 2 Raw Water Tank 3. Water supply pump 4 Pretreatment methods 5 Reverse osmosis membrane 6 Membrane filtration means 7 Hypochlorite Mixing Method 8. Total chlorine removal method 9. Disinfectant Mixing Method

Claims

1. a membrane filtration step of treating the water to be treated containing ammonia nitrogen using either or both of a reverse osmosis membrane and a nanofiltration membrane to obtain permeate; a hypochlorite mixing step of mixing the permeated water with hypochlorite to obtain oxidized water; a total chlorine removal step of removing all chlorine contained in the oxidation treated water to obtain total chlorine removed treated water; a disinfectant mixing step of mixing the total chlorine-removed treated water with a disinfectant to obtain treated water; and In the hypochlorite mixing step, the hypochlorite is mixed with the permeate in an amount such that the free chlorine concentration of the oxidized water is 0.01 mg / L or more.

2. 2. The method for producing drinking water according to claim 1, wherein the total chlorine removal step uses activated carbon to remove the total chlorine from the oxidized water.

3. 3. The method for producing drinking water according to claim 1, wherein either or both of the reverse osmosis membrane and the nanofiltration membrane have a polyamide resin membrane.

4. The method for producing drinking water according to any one of claims 1 to 3, wherein hypochlorite is used as the disinfectant in the disinfectant mixing step.

5. a membrane filtration means for treating the water to be treated containing ammonia nitrogen using either or both of a reverse osmosis membrane and a nanofiltration membrane to obtain permeated water; a hypochlorite mixing means for mixing the permeated water with hypochlorite to obtain oxidized water; a total chlorine removal means for removing all chlorine contained in the oxidation-treated water to obtain total chlorine-removed treated water; a disinfectant mixing means for mixing the total chlorine-removed treated water with a disinfectant to obtain treated water; and The hypochlorite mixing means mixes the hypochlorite with the permeated water in an amount such that the free chlorine concentration of the oxidized water is 0.01 mg / L or more.

6. 6. The drinking water producing system according to claim 5, wherein the total chlorine removing means removes the total chlorine from the oxidized water using activated carbon.

7. 7. The drinking water production system according to claim 5 or 6, wherein either or both of the reverse osmosis membrane and the nanofiltration membrane have a polyamide resin membrane.

8. The drinking water production system according to any one of claims 5 to 7, wherein the disinfectant mixing means uses hypochlorite as the disinfectant.

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